NRLF B ^ Ebl 7MD LJ N 1VERSITY OF CALIFORNIA. G-IFT OF THE ''CLASS OF 1883. X DISCOVERIES AND INVENTIONS OF THE NINETEENTH CENTURY. Who saw what ferns and palms were pressed Under the tumbling mountain's breast, In the safe herbal of the coal? But when the quarried means were piled. All is waste and worthless, till Arrives the wise selecting Will, And, out of slime and chaos, Wit Draws the threads of fair and fit. Then temples rose, and towns, and marts, The shop of toil, the hall of arts; Then flew the sail across the seas To feed the North from tropic trees ; The storm-wind wove, the torrent span, Where they were bid the rivers ran ; New slaves fulfilled the poet's dream, Galvanic wire, strong-shouldered steam. EMERSON. PLATE II. JKON IN ARCHITECTURE- THE CRYSTAL PALACE, SYPENHAM DISCOVERIES AND INVENTIONS OF THE NINETEENTH CENTURY. BY ROBERT ROUTLEDGE, B.Sc., F.O.S., Assistant Examiner in Chemistry and in Natural Philosophy to tke University of London. WITH NUMEROUS ILLUSTRATIONS. FIFTH EDITION. LONDON: GEORGE ROTJTLEDGE AND SONS, THE BROADWAY, LUDGATE. NEW YORK : 416 BROOME STREET. 1881. SAME AUTHOR. A POPULAR HISTORY OF SCIENCE. BY ROBERT ROUTLEDGE, .Sc. (LondJ, f.C.S. WITH MORE THAN THREE HUNDRED PORTRAITS, ILLUSTRATIONS, AND DIAGRAMS. '2/Z^'d SCIENCE IN SPORT MADE PHILOSOPHY IN EARNEST. EDITED BY ROBERT ROUTLEDGE, .Se. (LondJ, F.C.S. BEING AN ATTEMPT TO ILLUSTRATE SOME ELEMENTARY PRINCIPLES OF PHYSICAL KNOWLEDGE BY MEANS OF TOYS AND PASTIMES. WITH NUMEROUS ILLUSTRATIONS. PREFACE. IN the following pages an attempt has been made to present a popular account of remarkable discoveries and inventions which characterize the present century. From so large a field, selection was, of course, necessary; and the instances selected have been those which appeared to some extent typical, or those which seemed to have the most direct bearing on the general progress of our age. The topics comprise chiefly those great applications of mechanical, engineering, physical, and chemical science, in which every intelligent person feels concerned; and a few articles only are devoted to certain purely scientific discoveries which are exciting general interest. The Author has aimed at giving a concise, but clear, description of the several subjects ; and he has endeavoured to indicate, if not to ex- plain, the principles involved in each discovery and invention; and that Without assuming on the part of the reader any knowledge not usually possessed by young persons of either sex who have received an ordi- nary education. The design has been to treat the subjects as fami- liarly as might be consistent with a desire to impart real information; while the popular character of the book has not been considered a reason for regarding accuracy as unnecessary. On the contrary, pains have been taken to consult the best authorities ; and it is only because the sources of information to which the Author is under obligation are so many, that he cannot acknowledge them in detail. A book on a plan somewhat similar to, but by no means identical with, that of the present work, was projected two or three years ago, by Mr. J. H. PEPPER ; and, in fact, several sheets were in type, when its progress was permanently interrupted by that gentleman's removal to a distant part of the world. The matter in type, which had become viii PREFACE. the property of the Publishers, and also a number of woodcuts (re- lating chiefly to warlike inventions) which had been prepared under Mr. PEPPER'S direction, were placed at the Author's disposal. He has found it convenient to make use of some portion of this material; hence, the article on " The Suez Canal," the sections on " Shells and Explosive Bullets," and on "Sand Experiments," and the paragraphs giving numerical particulars regarding the Martini-Henry rifle, are from the pen of Mr. PEPPER. The articles in this work are, in general, independent of each other ; although in a few cases an acquaintance with preceding pages may be found to render the subject more easy for the general reader. The nature of the subjects treated of allows little opportunity for any regularly classified arrangement ; it may, however, be stated that the earlier part of the book deals with matters having reference mainly to mechanical and engineering achievements; these are followed by accounts of discoveries and inventions having some relation to the so-called physical forces ; while the chapters involving chemical con- siderations are placed in the latter part of the work ; and these last articles include a glimpse of some theories necessary for a real com- prehension of the subject. In the final article the Author has essayed to attract attention to a grand discovery, which, as yet, has seldom been introduced into books intended for the widest circle of readers. In some of the articles the reader is recommended to try certain easily performed and inexpensive experiments, which will give him or her an otherwise unattainable grasp of the principles involved. CONTENTS. Page INTRODUCTION... ... ... ... ... ... ... i STEAM ENGINES ... ... ... ... ... ... 3 THE LOCOMOTIVE ... ... ... ... ... ... 14 PORTABLE ENGINES... ... ... ... ... ... 2O THE STEAM HAMMER ... ... ... ... ... 21 IRON ... ... ... ... ... ... ... ... 25 TOOLS... ... ... ... ... ... ... ... 43 THE BLANCHARD LATHE ... ... ... ... ... 54 SAWING MACHINES ... ... ... ... ... ... 56 RAILWAYS ... ... ... ... ... .., ... 59 THE METROPOLITAN RAILWAYS ... ... ... ... 72 THE PACIFIC RAILWAY ... ... ... ... ... 74 STEAM NAVIGATION ... ... ... ... ... ... 83 THE BESSEMER CHANNEL STEAMER ... ... ... ..." 93 THE CASTALIA ... ... ... ... ... .,. 96 SHIPS OF WAR ... ... ... ... ... ... 99 FIRE-ARMS 117 RIFLED CANNON ... ... ' 118 BREECH-LOADING RIFLES ... 131 MITRAILLEURS, OR MACHINE GUNS ... 136 SHELLS AND EXPLOSIVE BULLETS* ... 142 TORPEDOES 147 THE SUEZ CANAL* 162 SAND ... 179 SAND EXPERIMENTS* 179 THE SAND BLAST ... 184 IRON BRIDGES ... 187 GIRDER BRIDGES 191 SUSPENSION BRIDGES 195 PRINTING MACHINES ... 201 LETTERPRESS PRINTING 202 PATTERN PRINTING... 217 HYDRAULIC POWER 22O PNEUMATIC DISPATCH ... ... 2 3 6 ROCK BORING ... 245 THE MONT CENIS TUNNEL ... - 247 ROCK-DRILLING MACHINES ... THE CHANNEL TUNNEL - 251 260 LIGHT... 26 7 SOME PHENOMENA OF LIGHT 269 VELOCITY OF LIGHT 271 By Mr. J. H. PEPPER CONTENTS. LIGHT continued. p a ge REFLECTION OF LIGHT ... ... ... ... ... 275 REFRACTION ... ... .. ... ... ... 283 DOUBLE REFRACTION AND POLARIZATION ... ... ... 285 CAUSE OF LIGHT AND COLOUR ... ... ... ... 294 THE SPECTROSCOPE ... ... ... ... ... ... 302 CELESTIAL CHEMISTRY AND PHYSICS ... ... ... ... 322 SIGHT... ... ... ... ... ... ... ... 332 THE EYE ... ... ... ... ... ... ... 334 VISUAL IMPRESSIONS ... ... ... ... ... 348 ELECTRICITY ... ... ... ... ... ... ... 359 ELEMENTARY PHENOMENA OF ELECTRICITY AND MAGNETISM ... 361 THEORY OF ELECTRICITY ... ... ... ... ... 365 ELECTRIC INDUCTION ... ... ... ... ... 366 DYNAMICAL ELECTRICITY ... ... ... ... ... 368 INDUCED CURRENTS ... ... ... ... ... 380 MAGNETO-ELECTRICITY ... ... ... ... ... 384 THE GRAMME MAGNETO-ELECTRIC MACHINE ... ... ... 388 THE ELECTRIC TELEGRAPH ... ... ... ... ... 397 TELEGRAPHIC INSTRUMENTS... ... ... ... ... 403 TELEGRAPHIC LINES ... ... ... ... ... 422 LIGHTHOUSES ... ... ... ... ... ... ... 432 PHOTOGRAPHY ... ... ... ... ... ... ... 446 PRINTING PROCESSES ... ... ... ... ... ... 459 STEREOTYPING ... ... ... ... ... ... 459 LITHOGRAPHY ... ... ... ... ... ... 463 OTHER PROCESSES ... ... ... ... ... ... 467 RECORDING INSTRUMENTS ... ... ... ... ... 472 AQUARIA ... ... ... ... ... ... ... 484 THE CRYSTAL PALACE AQUARIUM ... ... ... ... 486 THE BRIGHTON AQUARIUM ... ... ... ... ... 491 GOLD AND DIAMONDS ... ... ... ... ... ... 496 GOLD ... ... ... ... ... ... ... 496 DIAMONDS >.. ... ... ... ... ... ... 501 NEW METALS ... ... ... ... ... ... ... 505 INDIAN-RUBBER AND GUTTA-PERCHA ... ... ... ... 513*- INDIAN-RUBBER ... ... ... ... ... ... 513 GUTTA-PERCHA ... ... ... ... ... ... 517 ANAESTHETICS ... ... ... ... ... ... ... 520 EXPLOSIVES ... ... ... ... ... ... ... 529 MINERAL COMBUSTIBLES ... ... ... ... ... 537 COAL ... ... ... ... ... ... ... 537 . PETROLEUM ... ... ... ... ... ... 543 PARAFFIN ... ... ... ... ... ... ... 547 COAL-GAS ... ... ... ... ... ... ... 550 COAL-TAR COLOURS ... ... ... ... ... ... 561 THE GREATEST DISCOVERY OF THE AGE... .. ... ... 579 LIST OF ILLUSTRATIONS. Fif. Page Heading Wind, Steam, and Speed (after Turner) i Portrait of James Watt 3 2. Newcomen's Steam Engine 4 3. Watt's Double-action Steam Engine 5 4. Governor and Throttle-Valve 6 4a. Watt's Parallel Motion 8 5. Slide Valve 9 6. Section of GifFard's Injector n 7. Bourdon's Pressure Gauge 12 8. Steam Generator 13 9. Section of Locomotive 15 10. Stephenson's Link Motion 17 ii. Explosion of Boiler 19 1 2. Hancock's Steam Omnibus 19 13. Nasmyth's Steam Hammer 23 14. Merryweather's Steam Fire-Engine 24 15. A Foundry 25 1 6. Aerolite in the British Museum 26 17. Blast Furnace 27 18. Section of Blast Furnace 28 19. Plan of Blast Furnace 29 20. Cup and Cone 32 21. Rolling Mill 33 22. Fibrous Fracture of Wrought Iron... 35 23. Experiments at Baxter House ^7 24. Bessemer Converter 39 25. Apparatus for Making Bessemer Steel 40 26. Cupola Furnace 42 27. Portrait of Sir Joseph Whitworth ... 43 28. Whitworth's Screw Dies and Tap ... 44 29. Screw-cutting Lathe 45 30. Whitworth's Measuring Machine ... 47 31. Whitworth's Drilling Machine 49 32. Whitworth's Planing Machine 51 33. Pair of Whitworth's Planes or Surface Plates 52 34. Interior of Engineer's Workshop ... 53 35. Blanchard Lathe t 54 36. Vertical Saw 56 37. Circular Saw 57 38. Pit-Saw 58 39. Box Tunnel 59 40. Coal-Pit, Salop 60 41. Sankey Viaduct 61 42. Rails and Cramp-gauge 62 43. Fish-plate 63 44. Section of Rails and Fish-plates 64 45. Conical Wheels 65 46. Centrifugal Force 65 47- Points 66 48. Signal-Box on North London Rail- way 67 49. Post Office Railway Van 69 50. Gower Street Station, Metropolitan Railway 73 51. Map of the Route of Pacific Railway 75 52. Trestle Bridge 76 53. American Canyon 77 54. Cape Horn 79 55. Snow Plough 80 56. First Steam Railroad Train in America 81 Fig. Page 57. Railway Embankment 82 58. The Great Eastern at Anchor 83 59. Casting Cylinder of a Marine Steam Engine 85 60. Screw-Propeller 86 61. Section of Great Eastern amidships 88 62. The Great Eastern in course of Con- struction 89 63. The Great Eastern ready for Launch- ing 90 64. Comparative sizes of Steamships ... 91 65. The Great Eastern at Night 92 66. Saloon of the Bessemer Steamer 93 67. The Castalia in Dover Harbour 97 68. The same. End View 98 69. H.M.S. Devastation in Queenstown Harbour 99 70. Section of H.M.S. Hercules 101 71. Section of H.M.S. Inconstant 103 72. Section, Elevation, and Plan of Tur- ret of H.M.S. Captain 104 73. H.M.S. Captain 105 74. Diagram of H.M.S. Captain 108 75 Ditto 109 76. H.MS. Glatton 112 77. H.M.S. Thunderer 113 78. The Koiue Wilhelm 115 Firing at Floating Battery 116 79. Krupp's Works at Essen, Prussia ... 117 80. Section of 9 in. Fraser Gun 119 81. The 35-ton Fraser Gun 123 82. Mill wall Shield after being battered with Heavy Shot. Front View... 124 83. Rear View of the Millwall Shield ... 124 84. Comparative sizes of 35 and 8i-ton Guns 125 85. The 7-pounder Rifled Steel Gun 126 86. The 1 10 - pounder Breech - loading Krupp's Gun ; open, ready to load 126 87. The same, ready for firing 127 88. The 32 - pounder Prussian Krupp Siege Gun 127 89. Appearance of the Deckofa Shipafter the Bursting of a large Gun 128 90. Another view of the same disaster... 128 91. The Citadel of Strasburg after the Prussian Bombardment 129 92. Moncrieff *s Gun - Carriage ; Gun lowered for loading 130 93. The same, raised and ready for firing 130 94. 6oo-pounder Muzzle -loading Arm- strong Gun 131 95. Section of Martini-Henry Lock ....... 132 96. Martini-Henry Rifle 134 97. The Chassepot Rifle. Section of the Breech 135 98. The Catling Battery Gun. Rear View 137 99. The same. Front View 139 loo. The Montigny Mitrailleur 141 loi. Mallet's Mortar 143 102. The Shrapnel and Segment Shells... 143 XI Xll LIST OF ILLUSTRATIONS. Fig. Page 103. Norton's Explosive Bullets and Rifle Shells 145 104. General John Jacob's Explosive Bul- lets 146 105. Major Fosbery's Explosive Bullets 146 106. Henry's Torpedo. Working the Brakes 147 10 7 Submerged Torpedo 148 108. Mode of Firing Torpedo 150 109. Explosion of Whitehead's Torpedo... 151 no. Effect of the Explosion of White- head's Torpedo 152 in. Experiment with a Torpedo charged with 10 Ibs. Gun-Cotton 153 112. Explosion of Torpedo containing 67 Ibs. Gun-Cotton 154 113. Explosion of 432 Ibs. Gun-Cotton in 37ft. Water 155 114. The same in 27 ft. Water 155 115. Section of Priming-Cas* and Explod- ing Bolt 156 116. Harvey's Torpedo 157 117. The same 158 118. The same 159 119. Official Trial of "Harvey's Sea Tor- pedo" 159 120. Model of Submarine Guns 160 121. The Warner Experiment offBrighton 161 Portrait of M. Lesseps 162 122. Port Said 165 123. One of the Breakwaters at Port Said 166 124. Bird's-eye View of Port Said 166 125. Map of the Suez Canal 167 126. A Group of Egyptian Fellahs and their Wives 168 127. Dredges and Elevators at Work 170 128. Railway Station at Ismailia 171 129. Lake Timsah and Ismailia 172 130. The Viceroy of Egypt cutting Em- bankment 174 131. Apparatus for showing Sand Experi- ments 180 132. Model marked like Chevrons 181 133. Iron Cylinder, Sand, and Eggs 181 134. The Three Cylindrical Vessels and Tubes 182 135. Framework to represent Pail 182 136. Tube, Sand, and Sledge-Hammer .. 182 137. Hour-Glass on Screen at Polytechnic 183 138. Britannia Bridge, Menai Straits 187 139. Diagram showing Strains 189 140. Ditto 190 141. Girder 190 142. Ditto 190 143. Ditto 191 144. Section of a Tube of the Britannia Bridge 192 145. Albert Bridge, Saltash itfe 146. Clifton Suspension Bridge ,... 196 147. Section of Shaft 197 148. Newspaper Printing-Room 201 149. Inking Balls 202 150. Inking Roller 202 151. Diagram of Single Machine 204 152. Diagram of Perfecting Machine ... 205 153. Cowper's Double Cylinder Machine 205 154. Tapes of Cowper's Machine 206 155. Hopkinson and Cope's Perfecting Machine 207 156. Section of Casting Apparatus 210 157. Diagram of the Walter Press 211 158. Hoe's Type Revolving Cylinder Machine 213 Fig. Page 159. Hoe's "Railway" Machine 215 160. Napier's Platen Machine 216 161. Roller for Printing Wail-Papers 218 162. Machine for Printing Paper-Hang- ings 219 163. Chain-Testing Machine 220 164. Pascal's Principle 221 165. Collar of Hydraulic Cylinder 222 166. Hydraulic Press 223 167. Section of Hydraulic Lift Graving Dock 227 168. Section of Column 228 169. SirW. Armstrong's Hydraulic Crane 231 170. Raising Tubes of Britannia Bridge 232 171. Press for Raising the Tubes 233 172. Head of Link- Bars 234 173. Apparatus to prove Transmission of Pressure 235 174. Pneumatic Tubes and Carriages 236 175. Diagram of Tubes, &c 238 176. Sending and Receiving Apparatus... 239 177. Section of Receiving Apparatus 240 178. Sommelier Boring Machines 245 179. Transit by Diligence over Mont Cenis 249 180. Burleigh Rock Drill on Tripod 252 181. The same on Movable Column 254 182. The same Mounted on Carriage 255 183. Diamond Drill Crown 256 184. Diamond Drill Machinery 259 185. Chart of the Channel Tunnel 263 186. Section of the Channel Tunnel 264 187. View of Dover 265 188. Contrasts of Light 2.67 189. Rays 269 190. Diagram 270 191. Telescopic Appearance of Jupiter and Satellites 271 192. Diagram 273 193, 194, 195. Diagrams 275 196. Diagram 276 197. Polemoscope 277 198. Apparatus for Ghost Illusion 278 199. Illusion produced by Mirrors 280 200. A Stage Illusion 281 201. View of Venice Reflections 282 202. Refraction ; , 283 203. Diagram 284 204, 205. Diagrams of Crystals 286 206. Diagram 287 207. Diagram 289 208. Diagram 290 209. Polariscope 292 210. Section showing Polarization 293 211. Iceland Spar, showing Double Re- fraction 293 212. Diagram 294 213. Diagram 296 214. Diagram 298 215. Portrait of Professor Kirchhoff 302 216. Diagram 303 217. Newton's Experiment 304 218. Bunsen's Burner on Stand 307 219. Spectroscope with One Prism 309 220. Miniature Spectroscope 312 221. The Gassiot Spectroscope 313 222. Browning's Automatic Adjustment of Prisms 315 223. Apparatus for Spark Spectra :.. 316 224. The Sorby-Browning Micro-Spectro- scope 319 225. Section of Micro-Spectroscope, with Micrometer 320 226. Diagram 321 LIST OF ILLUSTRATIONS. Xlll Fig. Page 227. Section of Micro-Spectroscope 322 228. Solar Eclipse, 1869 325 229. The Planet Saturn 326 230. Solar Prominences, No. i 328 231. Ditto, No. 2 329 232. Section of Amateur Star Spectro- scope 330 233. Portrait of Professor Helmholtz 332 234. Vertical Section of the Eye 334 235. Section of Retina 336 236. Diagram 337 237. Muscles of Eyes 339 238. Diagram 341 239. Diagram 344 240. Diagram 345 241. Ruete's Ophthalmoscope 346 242. Diagram 347 243. Wheatstone's Reflecting Stereoscope 349 244. Diagram 350 245. Diagram 351 246. The Telestereoscope 353 247. Lines 355 248, 249. Diagrams 356 250, 251. Diagrams 357 252. Portrait of SirW. Thompson 359 253. A simple Electroscope 363 254. The Gold-leaf Electroscope 367 255. The Leyden Jar 368 256. A Voltaic Element 369 257. Ampere's Rule 370 258. Galvanometer 371 259. Daniell's Cell and Battery 373 260. Grove's Cell and Battery 373 261. Wire Ignited by Electricity 374 262. Duboscq's Electric Lantern and Regulator 375 263. Decomposition of Water 376 264. Electro-plating 377 265. A Current producing a Magnet 378 266. An Electro-magnet 379 267. Ruhmkorff's Coil 381 268. Discharge through Rarefied Air 382 269. Appearance of Spark on Looking- glass 384 270. Magneto-electric Spark 385 271. A Magnet producing a Current 386 272. Clarke's Magneto- electric Machine 386 273. Magneto-electric Light 387 274. Diagram 388 275. Gramme Machine 389 276. Insulated Coils 390 277. Hand Gramme Machine 390 278. Gramme Machine, with Eight Verti- cal Electro-magnets 393 279. Gramme Machine, with Horizontal Electro-magnets 395 280. Gramme Machine 395 281. Portrait of Professor Morse 397 282. Double-Needle Instrument 404 283. Electro-magnetic Bells 405 284. Portable Single-Needle Instrument 406 285. Connections of Telegraph Line 408 286. Morse Recording Telegraph 409 287. Morse Transmitting Key 411 288. Morse Transmitting Plate 412 289. Step-by-step Movement 417 290. Froment's Dials 417 291. Wheatstone's Universal Dial Tele- graph 418 292. Mirror Galvanometer 420 293. Telegraph Post and Insulators 423 294. Ditto ; 423 Fig. 295. Wire Circuit 424 296. Wire and Earth Circuit 424 297. Submarine Cable 425 298. Making Wire for Atlantic Cable 427 299. Instrument-Room at Valentia 428 300. Breaking of the Cable 429 301. Atlantic Telegraph Cable, 1866 430 302. Diagram 430 Lig-hthouse (heading) 432 303. Eddystone Lighthouse 433 304. Eddystone in a Storm 434 305. Revolving Light Apparatus 440 306. Stephenson's Holophotal Light 443 307. Camera 446 308. Camera and Slide 454 309. Folding Camera 455 310. Lenses 456 311. Bath 458 312. Portrait of Aloysius Senefelder 459 313. Press for Stereotyping by Clay pro- cess 460 314. Recording Anemometer 472 315. Registration of Height of Barometer and Thermometer 474 316. Electric Chronograph 476 317. Negretti's Deep-Sea Thermometer... 480 318. Ditto, General Arrangement 481 319. Atmospheric Recording Instrument 482 320. Domestic Aquarium 484 321. The Opelet 488 322. Viviparous Blenny 489 323. The Lancelet 490 324. Sea-Horses 492 325. Proteus anguinus 493 326. Mud-fish 494 327. The Axolotl 495 328. Sorting, Washing, and Digging at the South African Diamond-Fields ... 496 329. Gold Miners' Camp 498 330. Gold in Rocks 499 331. "Cradle "for Gold-washing 499 332. Pniel, from Jardine's Hotel 502 333. Sifting at the " Dry Diggings" 503 334. Vaal River, from Spence Kopje 504 335. Portrait of Sir Humphrey Davy 505 336. Apparatus 508 337. Portrait of Mr. Thomas Hancock ... 513 338. Portrait of Sir James Young Simp- son, M.D .'.: 520 339. Railway Cutting 529 340. View on the Tyne 537 341. Fossil Trees in a Railway Cutting... 538 342. Impression of Leaf in Coal Measures 539 343. Possible Aspect of the Forests of the Coal Age 540 344. The Fireside 542 345. View on Hyde and Egbert's Farm, Oil Creek 547 2^6. View of City of London Gas-works... 550 347 . Section of Gas-making Apparatus... 551 348. The Retort 553 34Q . The Gas Governor 555 350. Bunsen's Burner 557 3 5 j. Faraday's Ventilating Gas-burner ... 558 352. Apparatus for making Magenta 561 353. Iron Pots for making Nitro-Benzol... 564 354. Section of Apparatus for making Nitro-Benzol 565 355. Apparatus for making Aniline 56* 35 6. Section of Hollow Spindle 567 357. Portrait of J. Prescott Joule, F.R.S. 569 Tailpiece , 588 LIST OF PLATES. PLATE I. Drawn by THE GREAT STEAM HAMMER Royal) p Tr tivntp . v Gun Factory, Woolwich J P< HUNE PLATE II. IRON IN ARCHITECTURE The Crystal "> Palace, Sydenham J PLATF. III. INTERIOR OF A PULLMAN CAR on the") Midland Railway J PLATE IV. CLIFTON SUSPENSION BRIDGE, Niagara ,, PLATE V. THE GHOST ILLUSION PLATE VI. SPECTRA (Coloured Plate) '. From The Object To face ... Page 24 PLATE VII. LARGE INDUCTION COIL at the Polytechnic > Institution ) PLATE VIII. INTERIOR OF THE BRIGHTON AQUARIUM PLATE IX. RETORT HOUSE OF THE IMPERIAL GAS-") WORKS, King's Cross, London $ ... A Photograph ... Title page The Object ... Page 70 A Design by \ the Author $ ... 198 ,, 278 ... 308 The Object A Photograph The Object .. 384 49* 554 Wind, Steam, and Speed (after TURNER). INTRODUCTIONltn* I? 7 INLY by knowledge of Nature's laws can man subjugate her powers and appropriate her materials for his own purposes. The whole history of arts and inventions is a continued comment on this text ; and since the knowledge can be obtained only by observa- tion of Nature, it follows that Science, which is the exact and orderly summing-up of the results of such observation, must powerfully contribute to the well-being and progress of mankind. Some of the services which have been rendered by science in promoting human welfare are thus enumerated by an eloquent writer : " It has length- ened life ; it has mitigated pain ; it has extinguished diseases ; it has in- creased the fertility of the soil ; it has given new securities to the mariner ; it has furnished new arms to the warrior ; it has spanned great rivers and estuaries with bridges of form unknown to our fathers ; it has guided the thunderbolt innocuously from heaven to earth ; it has lighted up the night with the splendour of the day; it has extended the range of the human vision ; it has multiplied the power of the human muscles ; it has accele- rated motion ; it has annihilated distance ; it has facilitated intercourse, correspondence, all friendly offices, all dispatch of business ; it has enabled \nan to descend to the depths of the sea, to soar into the air, to penetrate securely into the noxious recesses of the earth, to traverse the land in cars which whirl along without horses, to cross the ocean in ships which run ten knots an hour against the wind. These are but a part of its fruits, and of its first-fruits ; for it is a philosophy which never rests, which has never attained, which is never perfect. Its law is progress. A point which yes- terday was invisible is its goal to-day, and will be its starting-point to- morrow." MACAULAY. INTRODUCTION, Thus every new invention, every triumph of engineering skill, is the embodiment of some scientific idea ; and experience has proved that dis- coveries in science, however remote from the interests of every-day life they may at first appear, ultimately confer unforeseen and incalculable benefits on mankind. There is also a reciprocal action between science and its application to the useful purposes of life ; for while no advance is ever made in any branch of science which does not sooner or later give rise to a corresponding improvement in practical art, so on the other hand every advance made in practical art furnishes the best illustration of scientific principles. The enormous material advantages which this age possesses, the cheap- ness of production which has placed comforts, elegancies, and refinements unknown to our fathers within the reach of the humblest, are traceable in "a high degree to that arrangement called the " division of labour," by which it is found more advantageous for each man to devote himself to one kind of work only ; to the steam engine and its numerous applications ; to in- creased knowledge of the properties of metals, and of the methods of extracting them from their ores ; to the use of powerful and accurate tools; and to the modern plan of manufacturing articles by processes of copy- ing, instead of fashioning everything anew by manual labour. Little more than a century ago everything was slowly and imperfectly made by the tedious toil of the workman's hand ; but now marvellously perfect results of ingenious manufacture are in every-day use, scattered far and wide, so that their very commonness almost prevents us from viewing them with the attention and admiration they deserve. Machinery, actuated by the forces of nature, now performs with ease and certainty work that was formerly the drudgery of thousands. Every natural agent has been pressed into man's service : the winds, the waters, fire, gravity, electricity, light itself. But so much have these things become in the present day matters of course, that it is difficult for one who has not witnessed the revolution pro- duced by such applications of science to realize their full importance. Let the young reader who wishes to understand why the present epoch is worthy of admiration as a stage in the progress of mankind, address himself to some intelligent person old enough to ^m^mber the century in its teens: let him inquire what wonderful changes in the aspect of things have been comprised within the experience of a single lifetime, and let him ask what has brought about these changes. He will be told of the railway, and the <:team-ship, and the telegraph, and the great guns, and the mighty ships of war " The armaments which thunderstrike the walls Of rock-built cities, bidding nations quake, And monarchs tremble in their capitals." He will be told of a machine more potent in shaping the destinies of our race than warlike engines the steam printing-press. He may hear of a chemistry which effects endless and marvellous transformations ; which from dirt and dross extracts fragrant essences and dyes of resplendent hue. He may hear something of a wonderful instrument which can make a faint beam of light, reaching us after a journey of a thousand years, unfold its tale and reveal the secrets of the stars. Of these and of other inven- tions and discoveries which distinguish the present age it is the purpose of this work to give some account. STEAM ENGINES. *T* O track the steps which led up to the invention of the Steam Engine, -* and fully describe the improvements by which the genius of the illus- trious Watt perfected it at least in principle, are not subjects falling within the province of this work, which deals only with the discoveries and inven- tions of the present century. But as it does enter into our province to describe some of the more recent developments of Watt's invention, it may be desirable to give the reader an idea of his engine, of which all the more recent applications of steam are modifications, with improvements of detail rather than of principle. Watt took up the engine in the condition in which it was left by Newcomen; and what that was may be seen in Fig. 2, which represents Newcomen's atmospheric engine the first practically useful engine in which a piston moving in a cylinder was employed. In the cut, the lower part of the cylinder, c, is remove^, or supposed to be broken off, in order that the piston, h, and the openings of the pipes, d, e,f, connected with the cylinder, may be exhibited. The steam was admitted beneath the piston by the attendant turning the cock /&, and as the elastic force of the steam was only equal to the pressure of the atmosphere, it was not employed to raise the piston, but merely filled the cylinder, the ascent of the piston being caused by the weight attached to the other side of the beam, which at the same time sent down the pump-rod, m; and when this was at its lowest position, the piston was nearly at the top of the cylinder, which was open. The attendant then cut off the communication with the boiler by closing the cock, k, at the same time opening another cock which allowed a jet 12 STEAM ENGINES. of cold water from the cistern, ^, to flow through the opening, d, into the cylinder. The steam which filled the cylinder was, by contact with the cold fluid, instantly condensed into water; and as the liquefied steam would take up little more than a two-thousandth part of the space it occupied in the gaseous state, it followed that a vacuum was produced within the cylinder ; and the weight of the atmosphere acting on the top of the piston, having no longer the elastic force of the steam to counteract it, forced the piston down, and thus raised the pump-bucket attached to FIG. 2. Ncwcomeris Steam Engine. the rod, m. The water which entered the cylinder from the cistern, toge- ther with that produced by the condensation of the steam, flowed out of the cylinder by the opening, /, the pipe from which was conducted down- wards, and terminated under water, the surface of which was at least 34 ft. below the level of the cylinder ; for the atmospheric pressure would cause the cylinder to be filled with water had the height been less. The improve- ments which Watt, reasoning from scientific principles, was enabled to effect on the rude engine of Newcomen, are well expressed by himself in the specification of his patent of 1769. It will be observed that the machine was formerly called the " fire engine." " My method of lessening the consumption of steam, and consequently fuel, in fire engines, consists of the following principles : First. That vessel in which the powers of steam are to be employed to work the engine (which is called the cylinder in common fire engines, and which I STEAM ENGINES. FlG. 3. Watfs Double-action Steam Engine. call the steam-vessel), must, during the whole time the engine is at work, be kept as hot as the steam that enters it ; first, by enclosing it in a case of wood, or any other materials that transmit heat slowly ; secondly, by surrounding it with steam or other heated bodies ; and thirdly, by suffering neither water nor any other substance colder than the steam to enter or touch it during that time. Secondly. In engines that are to be worked either wholly or partially by condensation of steam, the steam is to be condensed in vessels distinct from the steam- vessels or cylinders, although occasionally communicating with them, these vessels I call condensers ; and whilst the engines are working, these condensers ought to be kept at least as cold as the air in the neighbourhood of the engines by the appli- cation of water or other cold bodies. Thirdly. Whatever air or other elastic vapour is not condensed by the cold of the condenser, and may impede the working of the engine, is to be drawn out of the steam-vessels or condensers by means of pumps, wrought by the engines themselves or STEAM ENGINES. otherwise. Fourthly. I intend in many cases to employ the expansive force of steam to press on the pistons, or whatever may be used instead of them, in the same manner in which the pressure of the atmosphere is now employed in common fire engines. In cases where cold water cannot be had in plenty, the engines may be wrought by this force of steam only, by discharging the steam into the air after it has done its office. Lastly. Instead of using water to render the pistons and other parts of the engines air and steam-tight, I employ oils, wax, resinous bodies, fat of animals, quicksilver, and other metals in their fluid state." From the engraving we give of Watt's double-action steam engine, Fig. 3, and the following description, the reader will realize the high degree of per- fection to which the steam engine was brought by Watt. The steam is con- veyed to the cylinder through a pipe, B, the supply being regulated by the FIG. ^.Governor and Throttle- Valve. throttle-valve, acted on by rods connected with the governor, D, which has a rotary motion. This apparatus is designed to regulate the admission of steam in such a manner that the speed of the engine shall be nearly uni- form ; and the mode in which this is accomplished may be seen in Fig. 4, where D D is a vertical axis carrying the pulley, d, which receives a rotary motion from the driving-shaft of the engine, by a band not shown in the figures. Near the top of the axis, at e, two bent rods work on a pin, crossing each other in the same manner as the blades of a pair of scissors. The two heavy balls are attached to the lower arms of these levers, which move in slits through the curved guides intended to keep them always in the same vertical plane as the axis, D D. The upper arms are jointed at// to rods hinged at h h to a ring not attached to the axis, but allowing it to revolve freely within it. To this ring at F is fastened one end of the lever connected with the throttle-valve in a manner sufficiently obvious from STEAM ENGINES. the cut. The position represented is that assumed by the apparatus when the engine is in motion, the disc-valve, z, being partly open. If from any cause the velocity of the engine increases, the balls diverge from increased centrifugal force, and the effect is to draw down the ring at F, and, through the system of levers, to turn the disc in the direction of the arrows, and diminish the supply of steam. If, on the other hand, the speed of the engine is checked, the balls fall towards the axis, and the valve is opened wider, admitting steam more freely, and so restoring its former speed to the engine. On one side of the cylinder are two hollow boxes, E E, Fig. 3, com- municating with the cylinder by an opening near the middle of the box. Each of these steam-chests is divided into three compartments by conical valves attached to rods connected with the lever, H. These valves are so arranged that when the upper part of the cylinder is in communication with the boiler, the lower part is open to the condenser. I, and vice versa. The top of the cylinder is covered, and the piston-rod passes through an air and steam-tight hole in it; freedom of motion, with the necessary close fitting, being attained by making the piston-rod pass through a stuffing-box, where it is closely surrounded with greased tow. The piston is also packed, so that, while it can slide freely up and down in the cylinder, it divides the latter into two steam-tight chambers. In an engine of this kind, the elastic force of the steam acts alternately on the upper and lower surfaces of the piston ; and the condenser, by removing the steam which has performed its office, leaves a nearly empty space before the piston, in which it advances with little or no resistance. On the rod which works the air-pump, two pins are placed, so as to move the lever, H, up and down through a certain space, when one pin is near its highest and the other near its lowest posi- tion, and thus the valves are opened and closed when the piston reaches the termination of its stroke. In the condenser, I, a stream of cold water is constantly playing, the flow being regulated by the handle,^ The steam, in condensing, heats the cold water, adding to its bulk, and at the same time the air, which is always contained in water, is disengaged, owing to the heat and the reduced pressure. Hence it is necessary to pump out both the air and the water by the pump, J, which is worked by the beam of the engine. In his engines Watt adopted the heavy fly-wheel, which tends to equalize the movement, and render insensible the effects of those variations in the driving power and in the resistance which always occur. In the action of the engine itself there are two positions of the pistori, namely, where it is changing its direction, in which there is no force what- ever communicated to the piston-rod by the steam. These positions are known as the " dead points," and in a rotatory engine occur twice in each revolution. The resistance also is liable to great variations. Suppose, for example, that the engine is employed to move the shears by which thick plates of iron are cut. When a plate has been cut, the resistance is re- moved, and the speed of the engine increases ; but this increase, instead of taking place by a sudden start, takes place gradually, the power of the engine being in the meantime absorbed in imparting increased velocity to the fly-wheel. When another plate is put between the shears, the power which the fly-wheel has gathered up is given out in the slight diminution of its speed occasioned by the increased resistance. But for the fly-wheel, such changes of velocity would take place with great suddenness, and the shocks and strains thereby caused would soon injure the machine. This expedient, in conjunction with that admirable contrivance, the " governor/' renders it possible to set the same engine at one moment to forge an 8 STEAM ENGINES. anchor, and at the next to shape a needle. One of the most ingenious of Watt's improvements is what is termed the " parallel motion," consisting of a system of jointed rods connecting the head of the piston-rod, R, with the end of the oscillating beam. As, during the motion of the engine, the former moves in a straight line, while the latter describes a circle, it would be impossible to connect them directly. Watt accomplished this by hinging rods together in form of a parallelogram, in such a manner that, while three of the angles describe circles, the fourth moves in nearly a straight line. Watt was himself surprised at the regularity of the action. "When I saw it work for the first time, I felt truly all the pleasure of novelty, as if I 'was examining the invention of another man" A B is half the beam, A being the main centre ; B E, the main links, connecting the piston-rod, F.with the end of the beam ; c D, the air-pump links, from the centre of which the air- pump-rod is sus- pended; c D moves about the fixed centre, c, while D E is mov- able about the centre D, itself moving in an arc, of which c is the centre. The dot- ted lines show the position of the links and bars when the beam is at its highest position. FIG. 4a. Watt's Parallel Motion. Many improvements in the details and fittings of almost every part of the steam engine have been effected since Watt's time. For example, the opening and closing of the passages for the steam to enter and leave the cylinder is commonly effected by means of the slide-valve (Fig. 5). The steam first enters a box, in which are three holes placed one above the other in the face of the box opposite to the pipe by which the steam enters. The uppermost hole is in communication with the upper part of the cylinder, and the lowest with the lower part. The middle opening leads to the condenser, or to the pipe by which the steam escapes into the air. A piece of metal, which may be compared to a box without a lid, slides over the three holes with its open side towards them, and its size is such that it can put the middle opening in communication with either the uppermost or the lowest opening, at the same time giving free passage for the steam into the cylinder by leaving the third opening uncovered. In A, Fig. 5, the valve is admitting steam below the piston, which is moving upwards, the steam which had before propelled it downwards now having free exit. When the piston has arrived at the top of the cylinder, the slide is pushed down by the rod connecting it with the eccentric into the posi- tion represented at B, and then the opposite movement takes place. The slide-valve is not moved, like the old pot-lid valves, against the pressure of the steam, and has other advantages, amongst which may be named the readiness with which a slight modification renders it available for using the steam " expansively" This expansive working was one of Watt's in- STEAM ENGINES. ventions, but has been more largely applied in recent times. In this plan, \ when the piston has performed a part of its stroke, the steam is shut off. and the piston is then urged on by the expansive force of the steam enclosed in the cylinder. Of course as the steam expands its pressure decreases ; but as the same quantity of steam performs a much larger amount of work when used expansively, this plan of cutting off the steam is attended with great economy. It is usually effected by the modification of the slide-valve, shown at C, Fig. 5, where the faces of the slides are made of much greater width than the openings. This excess of width is called the " lap" and by properly adjusting it, the opening into the cylinder may be kept closed during the interval required, so that the steam is not allowed to enter the cylinder after a certain length of the stroke has been performed. The slide-valve is moved by an arrangement termed the eccentric. A circular FIG. 5. Slide Valve. disc of metal is carried on the shaft of the engine, and revolves with it. The axis of the shaft does not, however, run through the centre of the disc, but towards one side. The disc is surrounded by a ring, to which it is not attached, but is capable of turning round within it. The ring forms part of a triangular frame to which is attached one arm of a lever that com- municates the motion to the rod bearing the slide. Expansive working is often employed in conjunction with superheated steam, that is, steam heated out of contact with water, after it has been formed, so as to raise its tem- perature beyond that merely necessary to maintain it in the state of steam, and to confer upon it the properties of a perfect gas. Experience has proved that an increased efficiency is thus obtained. The actual power of a steam engine is ascertained by an instrument called the Indicator, which registers the amount of pressure exerted by the steam on the face of the piston in every part of its motion. The indicator con- sists simply of a very small cylinder, in which works a piston, very accu- rately made, so as to move up and down with very little friction. The piston is attached to a strong spiral spring, so that when the steam is admitted into the cylinder of the indicator the spring is compressed, and its elasticity resists the pressure of the steam, which tends to force the piston up. When the pressure of steam below the piston of the indicator i o STEAM ENGINES. is equal to that of the atmosphere, the spring is neither compressed nor extended ; but when the steam-pressure falls below that of the atmosphere, as it does while the steam is being condensed, ,then the atmospheric pres- sure forces down the piston of the indicator until it is balanced by the tension of the now stretched spring. The extension or compression of the spring thus measures the difference between the pressure of the atmo- sphere and that of the steam in the cylinder of the engine, with which the cylinder of the indicator freely communicates. From the piston-rod of the indicator a pencil projects horizontally, and its point presses against a sheet of paper wound on a drum, which moves about a vertical axis. This drum is made to move backwards and forwards through a part of a revolution, so that its motion may exactly correspond with that of the piston in the cylinder of the steam engine. Thus, if the piston of the indicator were to remain stationary, a level line would be traced on the paper by the movement of the drum ; and if the latter did not move, but the steam were admitted to the indicator, the pencil would mark an upright straight line on the paper. The actual result is that a figure bounded by curved lines is traced on the paper, and the curve accu- rately represents the pressure of the steam at every point of the piston's motion. The position of the point of the pencil which corresponds with each pound of pressure per square inch is found by trial by the maker of the instrument, who attaches a scale to show what pressures of steam are indicated. If the pressure per square inch is known, it is plain that by multiplying that pressure by the number of square inches in the area of the piston of the engine, the total pressure on the piston can be found. The pressure does not rise instantly when the steam is first admitted, nor does it fall quite abruptly when the sted.ni is cut off and communication opened with the condenser. When the steam is worked expansively, the pressure falls gradually from the time the steam is shut off. Now, the amount of work done by any force is reckoned by the pressure it exerts multiplied into the space through which that -pressure is exerted. Therefore the work done by the steam is known by multiplying the pressure in pounds on the whole surface of the piston into the length in feet of the piston's motion through which that pressure is exerted. The trace of the pencil on the paper /. marking the position of the index when known pressures are applied. The amounts of pressure, when the gauges are being graduated, are known by the compression produced in air contained in another apparatus. Gauges constructed on Bourdon's principle are applied to other purposes, and can be made strong enough to measure very great oressures, such as several FIG. 7. Monrdotfs Pressure Gauge. STEAM ENGINES. FIG. 8. Steam Generator. thousand pounds on the square inch ; they may also be made so delicate as to measure variations of pressure below that of the atmosphere. The simplicity and small size of these gauges, and the readiness with which they can be attached, render them most convenient instruments wherever the pressure of a gas or liquid is required to be known. A point to which great attention has been directed of late years is the construction of a boiler which shall secure the greatest possible economy in fuel. Of the total heat which the fuel placed in the furnace is capable of supplying by its combustion, part may be wasted by an incomplete burning of the fuel, producing cinders or smoke or unburnt gases, another part is always lost by radiation and conduction, and a third portion is carried off by the hot gases that escape from the boiler-flues. Many con- trivances have been adopted to diminish as much as possible this waste of heat, and so obtain the greatest possible proportion of available steam power from a given weight of fuel. Boilers wholly or partially formed of tubes have recently been much in favour. An arrangement for quickly i 4 THE LOCOMOTIVE. generating and superheating steam is shown in Fig. 8, in connection with a high-pressure engine. Steam engines are constructed in a great variety of forms, adapted to the purposes for which they are intended. . Distinctions are made accord- ing as the engine is fitted with a condenser or not. When steam of a low pressure is employed, the engine always has a condenser, and as in this way a larger quantity of work is obtainable for a given weight of fuel, all marine engines and all stationary engines, where there is an abundant supply of water and the size is not objectionable are provided with con- densers. High-pressure steam may be used with condensing engines, but is generally employed in non-condensing engines only, as in locomotives and agricultural engines, the steam being allowed to escape into the air when it has driven the piston to the end of the stroke. In such engines the beam is commonly dispensed with, the head of the piston-rod moving between guides and driving the crank directly by means of a connecting- rod. The axis of the cylinder may be either vertical, horizontal, or in- clined. A plan often adopted in marine engines, by which space is saved, consists in jointing the piston-rod directly to the crank, and suspending the cylinder on trunnions near the middle of its length. The trunnions are hollow, and are connected by steam-tight joints, one with the steam-pipe from the boiler, and the other with the eduction-pipe. Such engines have fewer parts than any others ; they are lighter for the same strength, and are easily repaired. The trunnion joints are easily packed, so that no leak- age takes place, and yet there is so little friction that a man can with one hand move a very large cylinder, whereas in another form of marine engine, known as the side-lever engine, constructed with oscillating beams, the friction is often very great. THE' LOCOMOTIVE. / T % HE first locomotive came into practical use in 1804. Twenty years -1 before, Watt had patented but had not constructed a locomotive engine, the application of steam to drive carriages having first been sug- gested by Robinson in 1759. The first locomotives were very imperfect, and could draw loads only by means of toothed driving-wheels, which engaged teeth in rack-work rails. The teeth were very liable to break off, and the rails to be torn up by the pull of the engine. In 1813, the impor- tant discovery was made that such aids are unnecessary, for it was found that the " bite " of a smooth wheel upon a smooth rail was sufficient for all ordinary purposes of traction. But for this discovery, the locomotive might never have emerged from the humble duty of slowly dragging coal-laden \vaggons along the tramways of obscure collieries. The progress of the locomotive in the path of improvement was, however, slow, until about '825, when George Stephenson applied the blast-pipe, and a few years /ater adopted the tubular boiler. These are the capital improvements which, at the famous trial of locomotives, on the 6th of October, 1829, enabled Stephenson's "Rocket" to win the prize offered by the directors of the Liverpool and Manchester Railway. The "Rocket" weighed 4| tons, and at the trial drew a load of tenders and carriages weighing 12 ;; tons. Its average speed was 14 miles an hour, and its greatest, 29 miles THE LOCOMOTIVE. FIG. 9. Section of Locomotive. an hour. This engine, the parent of the powerful locomotives of the pre- sent day, may now be seen in the Patent Museum at South Kensington. Since 1829, numberless variations and improvements have been made in 16 THE LOCOMOTIVE. the details of the locomotive. Its weight may now be 50 tons, its load from 50 to 500 tons, and its speed from 10 to 60 miles an hour. Fig. 9 represents the section of a locomotive as now constructed. The boiler is cylindrical ; and at one end is placed the fire-box, partly enclosed in the cylindrical boiler, and surrounded on all sides by the water, except where the furnace door is placed, and at the bottom, where the fuel is heaped up on bars which permit the cinders to drop out. At the other end of the boiler, a space beneath the chimney called the smoke-box is connected with the fire-box by a great number of brass pipes, open at both ends, firmly fixed in the end plates of the boiler. These tubes are from \\ in. to 2 in. in diameter, and are very numerous usually about one hundred and eighty, but sometimes nearly double that number. They therefore present a large heating surface to the water, which stands at a level high enough to cover them all and the top of the fire-box. The boiler of the locomotive is not exposed to the air, which would, if allowed to come in contact with it, carry off a large amount of heat. The outer surface is therefore protected from this cooling effect by covering it with a substance which does not permit the heat to readily pass through it. Nothing is found to answer better than felt ; and the boiler is accordingly covered with a thick layer of this substance, over which is placed a layer of strips of wood f in. thick, and the whole is surrounded with thin sheet iron. It is this sheet iron alone that is visible on the outside. The level of the water in the boiler is indicated by a gauge, which is merely a very strong glass tube ; and the water carried in the tender is forced in as required, by a pump or a Giffard's Injector. The steam leaves the boiler from the upper part of the steam-dome, A, where it enters the pipe, B ; the object being to prevent water from passing over with the steam into the pipe. The steam passes through the regulator, C, which can be closed or opened to any extent required by the handle, D, and rushes along the pipe, E, which is wholly within the boiler, but divides into two branches when it reaches the smoke-box, in order to conduct the steam to the cylinders. Of these there are two, one on each side, each having a slide-valve, by means of which the steam is admitted before and behind the pistons alter- nately, and escapes through the blast-pipe, F, up the chimney, G, increasing the draught of the fire by drawing the flame through the longitudinal tubes in proportion to the rush of steam ; and thus the rate of consumption of fuel adjusts itself to the work the engine is performing, even when the loads and speeds are very different. Though the plane of section passing through the centre of boiler would not cut the cylinders, one of them is shown in section. H is the piston ; K the connecting-rod jointed to the crank, L, the latter being formed by forging the axle with four rectangular angles, thus, , |~ ] ; and the crank bendings for the two cylinders are placed in planes at right angles to each other, so that when one is at the " dead point," the other is in a position to receive the full power of the piston. There are two safety valves, one at M, the other at N ; the latter being shut up so that it cannot be tampered with. Locomotives are fitted with an ingenious apparatus for reversing the engines, which was first adopted by the younger Stephenson, and is known as the " link motion." The same arrangement is employed in other engines in which the direction of rotation has to be changed ; and it serves another important purpose, namely, to provide a means by which steam may be employed expansively at pleasure. The link motion is represented in Fig. 10, where A, B, are two eccentrics oppositely -placed on the driving- THE LOCOMOTIVE, shaft, and their rods joined to the ends of the curved bar or link, CD. A slit extends nearly the whole length of this bar, and in it works the stud E, forming part of the lever, F, G, movable about the fixed joint, G, and having its extremity, F, jointed to the rod H, that moves the slide-valve. The weight FIG. io.Stephensotfs Link Motion. of the link and the eccentric rods is counterpoised with a weight, K, at- tached to the lever, I K, which turns on the fixed centre, L. This lever forms one piece with another lever,- L M, with which it may be turned by pulling the handle of O P, connected with it through the system of jointed rods. When the link is lowered, as shown in the figure, the slide-valve rod will follow the movement of the eccentric, B, while the backward and forward movement of the other eccentric will only be communicated to the end of C, and will scarcely affect the position of the stud E at all. By drawing the link up to its highest position, the motion due to eccentric A only will be communicated to the slide-valve rod, which will therefore be drawn back at the part of the revolution where before it was pushed for- ward, and vice 'versa; hence the engine will be reversed. When the link is so placed that the stud is exactly in the centre, the slide-valve will re- receive no motion, and remain in its middle position, consequently the engine is stopped. By keeping the link nearer or farther from its central position, the throw of the slide-valve will be shorter or longer, and the steam will be shut off from entering the cylinder when a smaller or larger portion of the stroke has been performed. !8 THE LOCOMOTIVE. The power of a locomotive, of course, depends on the pressure of the steam and the size of the cylinder, &c. ; but a very much lower limit than is imposed by these conditions is set to the power of the engine to draw loads by the adhesion between the driving wheels and the rails. By the term " adhesion," which is commonly used in this case, nothing more is really meant than the friction between surfaces of iron. When the resist- ance of the 7 oad drawn is greater than this friction, the wheels turn round and slip on the rails without advancing. The adhesion depends upon the pressure between the surfaces, and upon their condition. It is greater in proportion as the weight supported by the driving-wheels is greater, and when the rails are clean and dry it is equal to from 1 5 to 20 per cent, of that part of the weight of the engine which rests on the driving-wheels ; but when the rails are moist, or, as it is called, " greasy," the tractive power may be only 5 per cent, of the weight ; about one-tenth may be taken as an average. Suppose that 30 tons of the weight of a locomo- tive are supported by the driving-wheels, that locomotive could not be employed to drag a train of which the resistance would cause a greater pull upon the coupling-links of the tender than they would be subject to if they were used to suspend a weight of 3 tons. The number of pairs of wheels in a locomotive varies from two to five ; most commonly there are three pairs ; and one, two, or all, are driven by the engine, the wheels being coupled accordingly ; very often two pairs are coupled. The pressure at which the steam is used in the locomotive is sometimes very considerable. A pressure equal to I2olbs. on each square inch of the Surface of the boiler is quite usual. The greater economy obtained by the employment of high-pressure steam acting expansively in the cylinder, points to the probability of much higher pressures being adopted. There is practically no limit but the power of the materials to resist enormous strains, and there is no reason, in the nature of things, why steam of even 500 Ibs. per square inch should not be employed, if it were found otherwise desirable. It need hardly be said that locomotives are invariably con- structed of the very best materials, and with workmanship of the most perfect kind. The boilers are always tested, by hydraulic pressure, to several times the amount of the highest pressure the steam is required to have, and great care is bestowed upon the construction of the safety- valves, so that the steam may blow off when the due amount of pressure is exceeded. The explosion of a locomotive is, considering the number of engines in constant use, a very rare occurrence, and is probably in all cases owing to the sudden generation of a large quantity of steam, and not to an excessive pressure produced gradually. Among the causes capable of producing explosive generation of steam may be mentioned the deposition of a hard crust of stony matter, derived from the water ; this crust allows the boiler to be over-heated, and if water should then find its way into contact with the heated metal, a large quantity of steam will be abruptly generated. Or should the water in the boiler become too low, parts of the boiler may become so heated that on the admission of fresh water it would be suddenly converted into steam. When an explosion does take place, the enormous force of the agent we are dealing with when we bottle up steam in an iron vessel, is shown by the effects produced. Fig. 1 1 is from a photograph taken from an exploded locomotive, where we may see how the thick plates of iron have been torn like paper, and the tubes, rods, and levers of the engine twisted in inextricable confusion. Locomotive engines for propelling carriages on common roads were THE LOCOMOTIVE. FIG. II. Explosion of Boiler. invented many years ago, by Gurney, Anderson, Scott Russell, Hancock, and others. One designed by Hancock is represented in Fig. 12. Such engines do not appear to have found much favour, though the idea has FIG. 12. Hancock's Steam Omnibus. been successfully realized in the traction engines lately introduced. Pro- bably the application of steam power to the propulsion of vehicles along common roads fell into neglect on account of the superior advantages of railways, but the common road locomotive is at present receiving some attention. In the tramways which are now laid along the main roads in most large cities we see one-half of the problem solved. It is not so much mechanical difficulties that stand in the way of this economical system of 2 2 20 PORTABLE ENGINES. locomotion, as the prejudices and interests which have always to be over- come before the world can profit by new inventions. The engines can be made noiseless, emitting no visible steam or smoke, and they are under more perfect control than horses. But vestries and parochial authorities offer such objections as that horses would be frightened in the streets, if the engine made a noise ; and if it did not, people would be liable to be run over, and the horses be as much startled as in the other case. But horses would soon become accustomed to the sight of a carriage moving without equine aid, however startling the matter might appear to them at first ; and the objection urged against the noiseless engines might be alleged against wooden pavements, india-rubber tires, and many other improvements. It is highly probable that in the course of a few years the general adoption of steam-propelled vehicles wiil displace horses, at least upon tramways. The slowness with which inventions of undeniable utility and of proved advantage come into general use may be illustrated by the fact of the city of Manchester, a great centre of engineering industry, not having as yet a single tramway, while in all the populous cities of the United States, and in almost every European capital, tramways have been in successful operation for many years. PORTABLE ENGINES. nnHE application of steam power to agricultural operations has led to *- the construction of engines specially adapted by their simplicity and portability for the end in view. The movable agricultural engines have, like the locomotives, a fire-box nearly surrounded by the water, and horizontal tubes, and are set on wheels, so that they may be drawn by horses from place to place. , There is usually one cylinder placed hori- zontally on the top of the boiler ; and the piston-rod, working in guides, is, as in the old locomotive, attached by a connecting-rod to the crank of a shaft, which carries a fly-wheel, eccentrics, and pulleys for belts to com- municate the motion to the machines. Engines of this kind are also much used by contractors, for hoisting stones, mixing mortar, &c. These engines are made with endless diversities of details, though in most such simplicity of arrangement is secured, that a labourer of ordinary intelligence may, after a little instruction, be trusted with the charge of the engine; while their economy of fuel, efficiency, and cheapness are not exceeded in any other class of steam engine. Besides the steam engines already described or alluded to, there are many interesting forms of the direct application of steam power. There are, for example, the steam roller and the steam fire-engine. The former is a kind of heavy locomotive, moving on ponderous rollers, which support the greater part of the weight of the engine. When this machine is made to pass slowly over roads newly laid with broken stones, a few repetitions of the process suffice to crush down the stones and consolidate the mate- rials, so as at once to form a smooth road. Steam power is applied to the fire engine, not to propel it through the streets, but to work the pumps which force up the water. The boilers of these engines are so arranged that in a few minutes a pressure of steam can be obtained sufficient to throw an effective jet of water. The cut at the end of this chapter repre- THE STEAM HAMMER. 21 sents a very efficient engine of this kind, which will throw a jet 200 feet high, delivering 1,100 gallons of water per minute. It has two steam cylinders and two pumps, each making a stroke of two feet. These are placed horizontally, the pumps and the air reservoir occupying the front part of the engine, while the vertical boiler is placed behind. The steam cylinders, which are partly hid in the cut by the iron frame of the engine, are not attached to the boiler, which by this arrangement is saved from injurious strains produced by the action of the moving parts of the mechanism. There are seats for eight firemen, underneath which is a space where the hose is carried. A first-class steam fire-engine of this kind, completely fitted, costs upwards of ^1,300. There can be little doubt that before long it will be quite common to have such domestic work as grinding coffee, mincing meat, cleaning knives, &c., done by steam power. Already an efficient little engine has been introduced for working drawing-room sewing-machines. This miniature engine (for it is small enough to be carried in the pocket) is very simply constructed, with an oscillating cylinder, and can be instantly attached to a sewing machine, small lathe, &c. It is connected with its boiler merely by a flexible india-rubber tube, while another similar tube conducts the waste steam from the apartment. The boiler, which is somewhat orna- mental in construction, is heated by gas or paraffin, and the little engine is capable of working a sewing machine at double the ordinary speed, at the cost, it is said, of only one halfpenny per hour. The speed is regulated by a lever, which controls the admission of steam. When this little motor is used to drive a sewing machine, the lever is attached by a cord to the pedal of the machine in such a manner that when the pedal is held down steam is admitted in proportion to the pressure, while the removal of the foot shuts off the steam. This little engine is so cheap, portable, and easily applied that it will probably be largely used for many other purposes where a rotatory movement of small power is required. THE STEAM HAMMER. BEFORE the invention of the steam hammer, large forge hammers had been in use actuated by steam, but in an indirect manner, the ham- sner having been lifted by cams and otha expedients, which rendered the apparatus cumbersome, costly, and very wasteful of power, on account of the indirect way in which the original source of the force, namely, the pressure of the steam, had to reach its point of application by giving the blow to the hammer. Not only did the necessary mechanism for communicating the force in this roundabout manner interfere with the space necessary for the proper handling of the article to be forged, but the range of the fall of the hammer being only about 18 in., caused a very rapid decrease in the energy of the blow when only a very moderate-sized piece of iron was introduced. For example, a piece of iron 9 in. in dia- meter reduced the fall of the mass forming the hammer to one-half, and the work it could accomplish was diminished in like proportion. Besides, as the hammer was attached to a lever working on a centre, the striking face of the hammer was parallel to the anvil only at one particular point of its fall ; and again, as the hammer was always necessarily raised to the same height at each stroke, there was absolutely no means of controlling 22 THE STEAM HAMMER. the force of the blow. When we reflect on the fact that the rectilinear motion of the piston in the cylinder of the engine had first to be converted into a rotary one, by beams, connecting-rod, crank, &c., and then this rotary movement transformed into a lifting one by the intervention of wheels, shafts, cams, &c., while all that is required in the hammer is a straight up and-down movement, the wonder is that such an indirect and cumbersome application of power should have for so many years been contentedly used. But in November, 1839, Mr. Nasmyth, an eminent engineer of Manchester, received a letter from a correspondent, informing him of the difficulty he had found in carrying out an order received for the forging of a shaft for the paddle-wheels of a steamer, which shaft was required to be 3 ft. in diameter. There was in all England no forge ham- mer capable of executing such a piece of work. This caused Mr. Nasmyth to reflect on the construction of forge hammers, and in a few minutes he had formed the conception of the steam hammer. He immediately sketched the design, and soon afterward the steam hammer was & fait accompli, for Mr. Nasmyth had one at once executed and erected at his works, where he invited all concerned to come and witness its performances. Will it be believed that four years elapsed before this admirable application of steam power found employment outside the walls of Mr. Nasmyth's work- shops ? After a time he succeeded in making those best able to profit by such an invention aware of the new power for such it has practically proved itself, having done more to revolutionize the manufacture of iron than any other inventions that can be named, except, perhaps, those of Cort and Bessemer. The usual prejudice attending the introduction of any new machine, however obvious its advantages are afterward admitted to be, at length cleared away, and the steam hammer is from henceforth an absolute necessity in every engineering workshop, and scarcely less so for some of the early stages of the process of manufacturing crude wrought iron. Whether blows of enormous energy or gentle taps are required, or strokes of every gradation and in any order, the steam hammer is ready to supply them. A steam hammer of the smaller kind is represented in Fig. 13. The general mode of action will easily be understood. The steam is admitted below the piston, which is thus raised to any required height within the limits of the stroke. When the communication with the boiler is shut off and the steam below the piston is allowed to escape, the piston, with the mass of iron forming the hammer attached to the piston-rod, falls by its own weight. This weight, in the large steam hammers, amounts to several tons ; and the force of the blow will depend jointly upon the weight of the hammer, and upon the height from which it is allowed to fall. The steam is admitted and allowed to escape by valves, moved by a lever under the control of a workman. By allowing the hammer to be raised to a greater or less height, and by regulating the escape of the steam from beneath the piston, the operator has it in his power to vary the force of the blow. Men who are accustomed to work the valves can do this with great nicety. They sometimes exhibit their perfect control over the machine by cracking a nut on the anvil of a huge hammer ; or a watch having been placed face upwards upon the anvil, and a moistened wafer laid on the glass, a. practised operator will bring down the ponderous mass with such exactitude and delicacy that it will pick up the wafer, and the watch-glass will not even be cracked. The steam hammer has recently been improved in several ways, and its power has been more than doubled, by causing the steam, THE STEAM HAMMER. during the descent, to enter above the piston and add its pressure to the force of gravity. Probably one of the most powerful steam hammers ever constructed is that recently erected at the Royal Gun Factory at Wool- wich, for the purpose of forging great guns for the British Navy. It has been made by Nasmyth & Co., and is in shape similar to their other steam hammers. Its height is upwards of 50 ft., and it is surrounded with fur- naces and powerful cranes, carrying the huge iron tongs that are to grasp the glowing masses. The hammer descends not merely with its own weight FlG. 13. Nasmyttis Steam Hammer. of 30 tons ; steam is injected behind the falling piston, which is thus driven down with vastly enhanced rapidity and impulse. Of the lower portion of this stupendous forge, nothing is visible but a flat table of iron the anvil level with the floor of the foundry. But more wonderful, perhaps, than anything seen aboveground, is the extraordinarily solid foundation beneath. " Huge tablets of foot-thick castings alternate with concrete and enor- mous baulks of timber, and, lower down, beds of concrete, and piles driven deep into the solid earth, form a support for the uppermost plate, upon which the giant delivers his terrible stroke. Less than this would render it unsafe to work the hammer to its full power. As the monster works THE STEAM HAMMER. soberly and obediently though he does it the solid soil trembles, and everything movable shivers, far and near, as, with a scream of the steam, our 'hammer of Thor' came thundering down, mashing the hot iron into shape as easily as if it were crimson dough, squirting jets of scarlet and yellow yeast. The head of the hammer, which of course works vertically, is detachable, so that if the monster breaks his steel fist upon coil or anvil, another can be quickly supplied. These huge heads alone are as big as a sugar-hogshead, and come down upon the hot iron with an energy of more than a thousand foot-tons."* By the courteous permission of Major E. Maitland, Superintendent of the Royal Gun Factories, we are enabled to present our readers with the view of the monster hammer which forms the Plate I. Mr. Condie, in his form of steam hammer, utilizes the mass of the cylinder itself to serve as the hammer. The piston-rod is hollow, and forms a pipe, through which the steam is admitted and discharged, and the piston is sta- tionary, the cylinder moving instead between vertical guides. A hammer face is attached to the bottom of the cylinder by a kind of dovetail socket, so that if the strikirg surface becomes injured in any way, another can easily be substituted. The massive framework which supports the moving parts of Condie's hammer has its supports placed very far apart, so as to leave ample space for the handling of large forgings. FIG. nMerryweather's Steam Fire-Engine. " Daily Telegraph.* FIG. 1 5. A Foundry. IRON. ' T RON and coal," it has been well said, "are kings of the earth." No substance can indeed be named possessing so many useful qualities and capable of such a variety of applications as iron. The employment of this metal has been so vastly extended by modern improvements in the modes of manufacturing it, and by increased knowledge of its properties and behaviour, that we should be passing over some of the most important inventions of the age were we to omit a description of Bessemer's process and certain other operations, which facilitate the production of unlimited quantities of excellent iron and steel from the dull stony ore. The nature and value of these recent inventions will, however, be quite unintelligible to a reader not possessing some knowledge of the ordinary mode of extracting iron, and of the modifications in the properties of the metal produced by the presence in it of small quantities of certain other substances. We shall, therefore, begin by a few words on the smelting of iron, noticing some of the more remarkable improvements. Iron is the most common of the metals, in more than the ordinary sense. It is so widely diffused that almost every mineral contains some iron, and it is a chief constituent of a great many. It is not absent from the organic kingdoms, and in the blood of all vertebrate animals occurs in notable pro- portions. It is not a little remarkable that while this commonest of metals 25 26 IRON. is never found in the metallic state amongst terrestrial minerals, the aero- lites or meteoric stones, which fall upon our planet from out of the realms of immeasurable space, consist usually of little else than metallic iron, alloyed with a little nickel. These meteorites are sometimes of a considerable size ; for example, one found in South America is calculated to weigh fourteen tons ; but they fall too rarely to be of any value as a source of iron, although the natives of India and other places have sometimes forged swords from FIG. 1 6. Aerolite in the British Museum. the sky-descended masses. The minerals which yield iron are plentiful and easily obtained, but in them not a particle of iron exists in a metallic state, the metal being combined with oxygen and other bodies, so that all the properties of the simple substance are entirely disguised. The richest and most valuable ores of iron are oxides, such as the magnetic iron ore of Sweden, and haematite, which is abundant in certain parts of England and France. The chief source, however, of the large quantity of iron produced in Great Britain is the clay ironstone of South Wales and Staffordshire. This contains an impure carbonate of iron, mixed with clay, lime, oxide of manganese, and other minerals. It has the great advantage of being IRON. 27 FIG. i-]. Blast Furnace. associated with strata yielding coal and lime, and this circumstance more than compensates for the small per-centage of iron it contains as compared with the oxide ores. It is not definitely settled how far iron was used in ancient times, but it is certain that if it were known at all it must have been obtained directly from the oxides by the simple process of heating the ore with charcoal in a small furnace or on a hearth, the necessary temperature being attained by urging the fire with some rude blowing apparatus. In certain localities, where the nature of the ore is suitable and fuel abun- dant, malleable iron is still obtained from the ore by a single process. This is practised in the Pyrenees, and on a larger scale in the bloomery forges of North America. But in Great Britain the enormous quantity of iron produced from the clay ironstone is first obtained in the state of cast iron by the process of smelting. In this process the clay ironstone is roasted after having been broken up 28 IRON. into lumps. This operation is now almost invariably performed in large calcining kilns, which are charged with ore intermixed with a small pro- portion of the cheapest kind of coal. This preliminary treatment removes all the volatile matters, expelling the whole of the carbonic acid, and entirely driving off, in great clouds of steam, the water which is contained in iron- stone in notable quantity. The calcined ore is then ready for the blast furnace, represented in section in Fig. 18. This is a structure about 70 to 100 ft. high, and from 20 to 30 ft. in in- ternal diameter at the widest part. At E is the crucible, the bottom of which is termed the hearth, and is usually formed of a certain kind of very FIG. 1 8. Section of Blast Furnace. infusible sandstone ; A is called the tympstone, and above it is an opening through which the slag overflows. At O is one of the openings for the pipes or tuyeres, which are connected with blowing machines supplying a constant blast of air at a pressure of about 3 Ibs. per square inch. The arrangement of the tuyeres is shown in the plan, Fig. 19. At the lowest point of the furnace is the tap-hole, which is completely closed by sand and clay, except when it is opened at certain intervals to allow the melted metal to be drawn off. Surrounding the chimney or throat of the furnace is a gallery, and at C is an opening through which the charges are introduced. Access is obtained to this gallery by the gangway of which a portion is shown on the left-hand side of the figure. IRON. 29 The interior of the furnace is lined with the very best fire-bricks, and the exterior is formed of a casing of solid masonry strengthened with iron hoops. Great care and expense are bestowed in the construction of these furnaces, and when the fire has been kindled it is never allowed to go out until the furnace needs repairs ; so that such a furnace will continue in activity without intermission for a period of ten years, being regularly sup- plied at proper intervals with alternate charges of coal and a mixture of roasted ore with limestone in fragments. The limestone is added in order to render fusible the clay, and other earthy matter, which are associated with the carbonate of iron ; these matters melt and form the glassy-looking slags that in a molten state are continually flowing over the tympstone. FIG. 19. Plan of Blast Furnace. The air which issues from the tuyeres is soon deprived of its oxygen by the excess of carbon, producing carbonic oxide, and this gas ascending, penetrates the heated ore, rendered porous by the previous roasting. Carbonic oxide is a combustible gas, and at the high temperature of the furnace it takes away oxygen from the iron, which then combines with a small proportion of the carbon in the coal, fuses, and, together with the melted slag, drops into the crucible of the furnace, where it sinks to the lowest part ; while the slag floats on its surface, and flows over the tymp- stone as it accumulates at the same time covering the surface of the melted iron, which otherwise would be oxidized by the blast, and would, in fact, burn by again uniting with oxygen. The iron which thus prepared always contains from 2 to 5 per cent, of carbon is drawn off at intervals of 12 or 24 hours by the removing with an iron rod the plug of clay stopping the tap-hole. The liquefied metal is allowed to run into a series of shallow channels formed in sand, where it solidifies, and when cold, is broken into suitable lengths, and then presents itself in the form of the bars commonly known as pig iron. This is the crude cast iron; and here it will be desirable to refer to the fact that cast iron is not simply iron, but a chemical compound of iron and carbon. As it runs from the smelting furnace, however, it contains many other sub- stances, such as silicon, uncombined carbon, manganese, and other metals, phosphorus, sulphur, &c. The properties of the metal are greatly modified by these bodies, even when present in very small proportions. In the' variety of cast iron known as white, perhaps the carbon is chemically com- bined with the metal, while in the grey and mottled varieties some of the carbon is separated from the iron, and is merely mechanically diffused IRON. throughout the mass in the form of small crystals, similar to those of graphite or plumbago. The following table shows the per-centage composition of some samples of cast iron : White. White. Mottled. Grey. Iron 88-81 89-304 QV2Q 00-376 Combined carbon 4.-Q4. 2'4-t;? 278 I'O2I Graphite, or uncombined carbon. 0-871 I'QQ 2 '64. 1 Silicon O'7C I 'I2A O'7I vo6i Sulphur ... . trace 2-1:16 trace I'l 3Q Phosphorus O'I2 O'QIT I '2~l O'Q28 Manganese rs8 2-8I5 trace O'8^4. These examples will suffice to show that the composition of cast iron varies greatly ; and there are also wide differences in the mechanical pro- perties of each variety. Grey iron, that which is commonly produced from the furnace, is not so hard as the other kinds of cast iron ; it can be filed, or drilled, or turned in a lathe. The mottled variety is very tough, and on that account is preferred for casting guns. White cast iron has, when broken across, a white lustre, and the fracture presents a lamellar crystal- line texture. It is more fusible than the others, but it is very brittle, and so hard that it cannot be worked with steel tools. Iron possesses a pro- perty which is by no means common to all the metals a property upon which chiefly depends its admirable suitability for receiving any required form by casting. At the moment that a mass of the fluid metal solidifies, its bulk increases, so that this metal is capable of receiving even delicate markings from the moulds in which it is cast. No greater improvement was ever effected in any art by simple means than that introduced into iron smelting about forty-five years ago by Neilson's substitution of hot air for cold in the blast furnace. So great was the saving of fuel, especially in Scotland, where the hot blast first came into use, that the trade of iron smelting was completely revolutionized. Thus, at the Clyde Ironworks, in 1829, the production of each ton of iron required 153 cwts. of coal, and 10^ cwts. of limestone ; whereas, in 1833, after the hot blast had been adopted, 59 cwts. of raw coal, and 7 cwts. of limestone, sufficed for the same work. In carrying his invention into practice Neilson had to encounter many difficulties. The iron ovens in which he at first heated the air were very rapidly oxidized ; and when thick cast iron pipes were substituted, it was found that they were very liable to leak at the joints on account of the expansions and contractions caused by the alterations of temperature. Like other new inventions, this also had to contend with established prejudices and misconceptions. The iron- masters having observed that the quantity of ore reduced was greater in winter than in summer, attributed this to the coldness of air blown in overlooking the fact that with equal volumes of cold air and warm air, the quantity or weight of oxygen must be greater in the former. The quantity of air blown into a blast furnace is enormous, its weight being greater than that of all the coke, ore, and limestone put together. It is calculated that the weight of the air passing through a smelting fur- nace amounts, in some cases, to between 2,000 and -3,000 tons in a week. IRON. 31 At the ironworks at Dowlais an engine of 650 horse-power is appro- priated to the blowing apparatus, in which a piston moving in a cylinder 12 ft. in diameter, and the same in length, is constantly forcing air at a pressure of 3^ Ibs. per square inch through the immense mains or discharge- pipes, 5 ft. in diameter, by which it is conducted to the furnaces. Various theories have been advanced to explain why i cwt. of fuel ex- pended in heating the blast should do the work of perhaps 4 cwt. burnt in the furnace itself. Recently, however, furnaces have been built of larger dimensions, some having as much as six times the capacity of those formerly in use ; and experience has shown that this increase of size, of itself, pro- duces results similar to those which were obtained when the hot blast was applied to smaller furnaces. Nevertheless, the use of the hot blast has not been abandoned ; for as the air can thus be heated by cheap coal at a less cost than by the fuel within the furnace, Neilson's invention still affords very important advantages. There is, however, much difference of opinion among practical metallurgists as to the temperature which should be given to the blast, and as to its influence on the quality of the iron produced. The common practice has been to pass the air through a series of pipes or chambers heated by a separate furnace, so that it might enter the smelting furnace at a temperature sufficiently high to melt a strip of lead held in a current of it, that is at about 620 F. Mr. Siemens has recently introduced a mode of still further economizing fuel by causing the hot gases which escape from the smelting furnace to pass through a chamber filled with fire-bricks, so arranged that the gases circulate freely round them ; and when the bricks are by this means heated to redness, the current of gases is shut off from the chamber, and conducted through another precisely similar ; while the blast of cold air is made to circulate through the heated chamber, and thus acquires a temperature of 1,200 or 1,300 F. Two of these chambers are used alternately, so that while one is employed in heating the blast, the other is acquiring from the furnace gases the heat necessary for it to perform the same duty. In this manner a great part of the heat of the waste gases is utilized. Other modes of employing the gases escaping from the chimneys of the smelting furnaces have also been adopted. It will be remembered that the most active agent in the reduction of the metal is the carbonic oxide gas produced in the furnace by the incomplete combustion of the carbon of the coal. A large excess of this gas is pro- duced, and escapes from the chimney of the furnace. The unconsumed carbonic oxide so escaping, together with the hydrogen and other combus- tible gases, forms more than a third of the gases issuing from the furnace. It is the combustion of these inflammable gases which produces the lam- bent flames often seen at the mouths of smelting furnaces, and imparting its most striking features to the night view of an iron-smelting region. Now, until recently, all the heat produced by these flames was absolutely wasted, but by various expedients a portion or the whole of these gases is now drawn off and allowed to burn in such a manner that the heat is available for raising steam or for heating the air-blast. It is obvious that if the gases are withdrawn from the furnace without checking the upward current, the processes could not suffer in any way. One very effective manner of doing this is shown in Fig. 20, which is a section through the upper part of a smelting furnace, with the " cup and cone " arrangement. The mouth of the furnace is covered by a shallow iron cone, a, open at the bottom, into which fits another cone, b, attached to a chain, c, sustained by an arm of the lever, d, which is firmly held in the required position by 32 IRON. the chain e, and is also provided with a counterpoise, f. In this position the mouth of the furnace is closed, and the gases find an exit by the opening g, seen behind the cones, and leading into a passage through which the gases are conveyed to the place where they are required to be burnt. The charge for the furnace is filled into the hopper, a, and at the proper time the chain, e, is slackened, when the weight of the materials resting in the suspended cone overcomes that of the counterpoise, and the charge slides down over the surface of the cone, b, which is immediately drawn up again by the counterpoise, so that the opening is at once closed. FlG. 20. Cup and Cone. By such contrivances as we have described, and by minor improvements, the consumption of coal in iron smelting has been so much economized in recent years that Mr. I. L. Bell-, in his presidential address to the members of the Iron and Steel Institute in 1873, mentions that in the quantity of coal consumed for iron smelting an annual saving of three and a half mil- lions of tons had been effected over that consumed fifteen years before, although the quantity of iron produced had very greatly increased. Cast iron, it is well known, cannot be wrought with the hammer ; it can- not be rolled into plates, nor welded on an anvil ; it is not readily filed or worked with steel tools ; and it possesses no ductility, and little tenacity, or power to resist a pulling force. On the other hand, wrought iron pos- sesses all these properties in the highest degree, but can be fused only by an extremely intense heat. In chemical composition the latter is essentially pure iron, whereas the former is essentially a compound of iron and carbon. Cast iron is converted into wrought iron by the processes of refining and puddling. The object of these operations is, as may be imagined, the removal of the carbon and other substances. The pig iron is re-melted on the hearth of a furnace in such a manner that it is exposed to the action of the air for about two hours. The silicon, most of the carbon, and some of the iron itself are oxidized in this process. At one stage in the process the metal appears to boil, owing to the escape of jets of carbonic acid gas, formed by the union of oxygen and carbon in the midst of the mass. The oxidizing action is carried on through the whole, for the puddler stirs it up, so that every part is brought into contact with the oxygen, or with the oxide of iron formed on the surface, and carried by the stirring into the heart o IRON. 33 FIG. 21. Rolling Mill. fromthentT i is sometimes mixed with oxide of iron from the forge. As the iron loses its carbon it becomes less fusible, and at dTm^l H m Th >f C meS a maSS 1 lo Sely adherin S Drains, like so much damp sand. The heat is now raised, and the grains begin to fuse and be- come more adherent, so that the puddler is able to collect all the metal at the end of an iron rod into a spongy mass. He forms this into balls or blooms, weighing about 70 Ibs. each. The blooms consist of particles of nearly pure iron, partly fused together, but retaining in the interstices between them a portion of the fused slags, oxides, and other extraneou >,T' Ji Y a !" e n w dellvered to the shingUr, by whom they are sub- Bitted whilst still intensely hot, to the action of a steam hammer or other heavy hammer, or subjected to strong compression in a shingling press or comnrl tr? n i^ aSS !T e iro * rollere - Th ^ design of this treatment is to compress the half-fused particles of iron into a compact mass, and to tho- F Ut thC ^ and fused Oxide ' In a ^ case the ^ration ^ Y PaS ?u g the metal between ^ ooved rollers > b y wh 'ch it is roffi rS ; / T ^ afterwards cut into lengths/re-heated, and 5 rolling repeated, and so on several times, when the icon is required to possess great tenacity. The rolling of the blooms into bars i^s riki- 34 IRON. spectacle. The furnace door is opened, the workman pulls out a white-hot ball, he throws it on the ground, whence it is instantly snatched up by an attendant Vulcan, armed with a pair of tongs, and applied to the largest- sized groove in a pair of massive rollers. The lump of iron is shot through the rolls, and is doubled in length by their pressure. No sooner has ii come out than it is again seized and tossed back over the rollers, to be again passed through a smaller groove ; and it thus passes backwards and forwards several times, increasing its length and improving in quality with each passage through the grooves. The whole operation is effected in an incredibly short time, and before the spectator has been able fully to comprehend the process the result is before him a long, straight, finished bar of glowing iron is stretched on the sand. Scarcely has the bar been laid out to cool than another fiery mass is following the same course through the rolls. The view of a rolling mill given in Fig. 21 will enable the reader to understand the nature of the operation we have just described. If the spectator's gaze is not entirely fascinated by the glowing metal, he may perceive a massive fly-wheel, perhaps 20 ft. in diameter and as many tons in weight, spinning through the air at the rate of sixty revolu- tions per minute, and communicating by certain toothed wheels the rotary motion to the powerful rolls. At Dowlais each pair of rolls has an iron fly-wheel, 20 ft. in diameter, with a rim 12 in. square, and the engine driving these mills has itself a fly-wheel 25 ft. in diameter and 2 ft. wide. The rolls themselves are 1 1 in. in diameter in the grooves, and elsewhere double that diameter. The reader, from these details, and from the fact that such massive fly-wheels are made to revolve at the rate of sixty or a hundred revolutions per minute, may form some idea of the enormous momentum possessed by them. The following, from the " Times " of November 22, 1859, may enable him to realize the amount of force stored up in a heavy body in rapid motion : " About eleven o'clock on the night of Friday last a most destructive accident occurred at the extensive ironworks of Messrs. Gibbs Brothers, Deepfield, near Wolverhampton, by which a large iron- rolling mill was levelled with the ground, one man killed, and several others severely injured. A number of the workmen had fortunately gone to supper, and others were disengaged while some alteration was being made in the rollers, when sud- denly the large driving-wheel, some tons in weight, broke into fragments, which were propelled on all sides with great force. Several of the iron pillars that supported the roof, the principal iron beam, and several of the lesser ones were broken, and the entire roof shortly afterwards came down. Of the men who were in the mill at the time, one, named John Taylor, was dug out of the debris insensible, and expired shortly afterwards, three others sustained severe fractures and other injuries, and two or three escaped unhurt in a manner little short of miraculous. The whole place looks as if it had been blown up with gunpowder, and the damage is esti- mated at about ,3,000." Wrought iron is never pure, but always contains a small proportion of carbon, varying from one-fifth to one-half per cent. The presence of this small quantity of carbon enhances rather than impairs its useful qualities ; but some of the other ingredients of cast iron, if not entirely removed in the refining and puddling processes, act most injuriously upon the qualities of the wrought iron. For example, the presence of a small proportion of phosphorus renders wrought iron brittle and rotten when cold, although at a red heat it may be forged. Iron having this defect is technically said to IRON. 35 be cold short, while the term red short is applied to metal which when hot breaks and crumbles under the hammer, a condition occasioned by the presence of a small quantity of sulphur or of silicon. Wrought iron is quite unlike cast iron in texture, for while the latter when broken across shows a distinctly crystalline structure, which we may compare to that of loaf- sugar, the former exhibits a fibrous grain, not unlike that of a piece of wood. This fibrous structure depends upon the mechanical treatment the iron has received, and in bars the fibres always arrange themselves parallel to the length of the bar. Fig. 22 shows the fibrous structure in a piece of iron where a portion has been wrenched off. Like wood, wrought iron has much greater tenacity along the fibres than across them ; that is, a much less force is required to tear the fibres asunder than to break them across. Consequently, to obtain the greatest advantage from the strength of wrought FlG, 22. Fibrous Fracture of Wrought Iron. iron, the metal must be so applied that the chief force may act upon it in the direction of the fibres. Steel is a nearly pure compound of iron with carbon, the proportion of the latter element being much less than in cast iron, and always less than one-fiftieth. Good steel contains from 07 to 17 per cent, of carbon, and some excellent qualities have been found to have from 1*3 to 1*5 per cent, of carbon, together with o'i per cent of silicon. Steel is thus, as regards carbon, intermediate in composition between wrought and cast iron, and in a great measure possesses the most valuable properties of both. It may be wrought on the anvil at a white heat, and at a higher temperature it may be melted and cast in a mould. It is distinguished from cast iron by its fine, close grain, and still more by its remarkable tenacity, in which pro- perty it excels wrought iron, for while a bar of the best wrought iron, i in. square, breaks with a load of thirty tons, a bar of good steel of the same size will sustain seventy-five tons, and if the steel has been subjected to certain processes, its tenacity may even reach a hundred and twenty tons. Still more remarkable is the wonderful property possessed by steel by which it may at will be made soft or hard, tough or elastic. If suddenly quenched in water when it is red hot, it acquires a hardness rivalling that 3 2 36 IRON. of the diamond, and is also rendered very brittle and extremely elastic. The hardness and brittleness can be reduced by tempering 'to any required extent. Steel may be made either by stopping the processes by which carbon is removed from cast iron at such a stage as to leave the requisite quantity . of carbon in combination with the iron, or by reversing the decarbonizing process and re-combining wrought iron with carbon. In the latter plan bars of the finest wrought iron are heated for a week in a closed receptacle, lightly packed with powdered charcoal. But all the older plans for producing steel by the partial decarbonization of cast iron have been eclipsed in interest and importance by Bessemer's discovery that the carbon and silicon may be readily removed by forcing currents of cold air through a mass of the fused metal. This discovery was first announced to the world in a paper read before the British Association by Mr. Bessemer at the meeting in 1856. This paper bore the startling title : " On the Manufacture of Iron and Steel without Fuel." In this pro- cess the whole mass of melted iron is exposed to the action of atmo- spheric air without the intervention of the puddler with his stirring-rod, the stirring being performed by the air itself, while the heat produced by the combustion of the silicon and carbon, and of a considerable proportion of the iron itself, maintains the metal in a perfectly fluid state, even when all the extraneous matters have been burnt off. Thus Mr. Bessemer showed the world the unprecedented spectacle of several tons of pure or malleable iron in a molten state. Mr. Bessemer's first patent, dated iyth October, 1855, was for " forcing currents of air, or of steam, or of air and steam, into and among the particles of molten crude iron, or of re-melted pig or refined iron, until the metal so treated is thereby rendered malleable, and has acquired other properties common to cast steel, and still retaining the fluid state of such metal, and pouring or running the same into suitable moulds." It was found that the steam had an injurious effect, but when air alone was forced through the metal, the temperature rose from a red to a white heat. Mr. Bessemer afterwards took out other patents for improvements in his process and in the apparatus, using air alone. The experiments re- quisite for the perfection of the invention were carried out at Baxter House, where Mr. Bessemer then resided, and Dr. Percy describes the process, as he there witnessed it, as the most startling and impressive metallur- gical operation he had ever seen. " After the blast was turned on," he says, " all proceeded quietly for a time, when a volcano-like eruption of flame and sparks suddenly occurred, and bright red-hot scoriae or cinders were forcibly ejected, which would have inflicted serious injury on any unhappy by- standers whom they might perchance have struck. After a lew minutes all was again tranquil, and the molten malleable iron was tapped off. At first I doubted whether the metal which I saw flowing was actually malleable iron ; and after the analysis in my laboratory of a portion of this identical iron, and the detection in it of phosphorus somewhat exceeding i per cent, my scepticism was rather confirmed than otherwise. However, I soon became convinced that Mr. Bessemer was correct in asserting that he had succeeded in producing a temperature higher than ever before attained in metallurgical operations ; sufficient, indeed, to render malleable iron as liquid as water." The apparatus used in these experiments was a cylin- drical vessel, like a large crucible, in the bottom of which were a number of clay nozzles, or tuyeres, through which the blast from a blowing machine could be forced (see Fig. 23.) The first experiments were not undertaken with a view to the production of steel, but to ascertain the possibility of IRON. 37 obtaining malleable iron without submitting the metal to the ordinary re- fining and puddling operations. Into the cylindrical vessels Mr. Bessemer caused crude iron from a blast furnace to be poured until the liquid metal filled the vessel to about the depth of two feet. But the blast of air was turned on before the metal was allowed to enter the vessel, and thus the tuyeres were kept clear, for the pressure of air in the blast sent through them supported the weight of the liquid column of metal. The air rushing FIG. 23. Experiments at Baxter House. through the liquid kept it in constant agitation, and the union of the oxygen with the carbon not only maintained the temperature of the mass, but even caused it to increase greatly. In about fifteen or twenty minutes, all the carbon diffused through the iron having been burnt up, the metal itself began to combine with the oxygen, producing oxide, which, being well mixed with the rest by the violent ebullition going on, was brought in con- tact with any silica or earthy matter which might be present, and thus rendered them fusible, and caused them to appear as frothy slags. At the end of from thirty to thirty-five minutes the process was complete, and the fluid decarbonized iron was run out at a tap-hole. It cannot properly be said that the operation is conducted without fuel ; for not only the carbon burnt off forms the fuel, but a large quantity of the iron itself burns into oxide, which helps to form the slag, and this quantity, most of which must be accounted as loss, amounts to upwards of twenty per cent. In the manner just described, the iron may be completely decarbonized, and it is plain that if the process were stopped at precisely the right point, which 38 IRON. would leave the proper proportion of carbon in combination with the iron, the result should be good cast steel. There is, however, great difficulty in ascertaining the exact point of decarbonization at which the operation is to be stopped, and although valuable indications are afforded by the colour of the flame which issues from the vessel, especially when this is examined by the spectroscope, it is found better in practice to obtain the required composition in another manner. The Bessemer process was on its first announcement welcomed with great enthusiasm, but the earlier results obtained by it disappointed the expectations of every one, for both the iron and steel made by it were of bad quality. Thus, Dr. Percy, in the foregoing account of the process, mentions the large per-centage of phosphorus he found in the product, and we have already referred to bad effects produced by the presence of this element in iron. In fact, though by this process silicon and carbon are com- pletely removed from the iron, it appears that sulphur and phosphorus are not eliminated. Hence it is necessary to employ cast iron free from these impurities, and metal containing even so small a quantity of phosphorus as seven parts in ten thousand must be rejected. But this is not all, for even when very pure pig iron, containing but mere traces of substances other than carbon, has been employed, the product has been found valueless for practical purposes. At one time it seemed that the new process would prove a failure in practice, when the happy thought occurred to Mr. Mushet that the defects of the method as regards steel might be remedied by the addition of manganese to the contents of the converter. The presence of manganese in the iron best adapted for making into steel had long before been pointed out. It having also been found that in the production of steel it was very difficult to determine with any certainty the proper mo- ment for arresting the decarbonization, the Bessemer process was modified in the manner described above, by adding to the completely decarbonized metal such a quantity of good cast iron (spiegeleisen}, containing about ten per cent, of manganese, as would convert it into steel of the finest quality. Spiegeleisen (literally mirror-iron}, a white hard cast iron, has had that name given to it by the Germans from the fact that when it is broken across, the fractured surfaces present numerous little mirror-like faces of crystals,, having a pure silvery lustre. This iron is procured from ores which con- tain manganese, and it also retains a very large proportion of carbon. The composition of an average specimen of spiegeleisen may be stated as showing 4 to 4! per cent, of carbon, 10 per cent, of manganese, and I per cent, of silicon. The precise composition of the spiegeleisen about to be used in the Bessemer operation is always first determined with great accu- racy ; and, according to its richness in carbon, a quantity is added which will convert the iron contained in the Bessemer vessel into good steel. The quantity thus added to the contents may form a proportion varyjng from, 5 to 10 per cent. Manganese is by no means the only metal the pre- sence of which in small quantities has been found to improve the quality of steel, for silver, rhodium, and chromium have also been observed to produce a beneficial effect, and Mr. Mushet obtained a patent for the addition of titaniferous iron to steel. A short account of the manner in which the Bessemer operation is now conducted will, with the annexed figures, serve to render the process easily intelligible. The vessel in which the operation is conducted is termed a converter, and its construction will be understood by reference to Fig. 24. It is an egg-shaped vessel, about 3! ft. in diameter, made of wrought iron in two I It OAT. 39 parts, and lined in the inside with a thick infusible coating made from ground fire-bricks and a certain kind of sandstone. The two portions are united by flanges strongly bolted together, and the converter swings on trunnions, one of which is hollow and admits the air-blast by the pipe b to the base of the vessel. This pipe, which of course turns on the trunnion with die converter, conducts the air to a kind of chamber, d, from which it passes into the vessel through about fifty holes of ^ in. diameter. To the other FlG. 24. Bessemer Converter. A, Front view, showing the mouth, c ; B, Section. trunnion a toothed wheel is attached, which engages the teeth of a rack receiving motion from hydraulic pressure in a cylinder. The iron for the operation is melted in a furnace having its hearth above the level of the converter, which is turned so that its axis is horizontal and its mouth up- wards. In this position it is ready to receive the molten iron, which is con- veyed to it by a trough, lined with sand, when the furnace is tapped. The metal is allowed to pour in until its surface is nearly at the level of the lowest holes through which the air enters. Usually about five tons of iron are thus operated on. The blast having first been turned on at a pressure of 1 5 Ibs. to the square inch, the hydraulic power is set to work and the con- verter is slowly brought back to an upright position. The pressure of the cur- rent of air prevents any of the fluid metal from entering the blow-holes, and the blast of cold air is continued for a period varying from twelve to twenty minutes until, in fact, all the silicon and carbon have been entirely con- sumed. The converter is then slowly turned back into the horizontal posi- tion, and the blast is shut off, while a certain weight of melted cast iron of a particular composition is run in ; the blowing is resumed, the vessel brought to the upright position, and the blast continued for about five minutes, in order thoroughly to incorporate the ingredients. At the end of this time the vessel is again lowered, the blast is shut off, and the contents of the converter are run off into a vessel of wrought iron, lined with sand and provided with an iron plug, coated with sand, fitting into a socket, so that, when required, the plug may be raised, and the molten steel allowed to flow out into the moulds, which it does in a stream about an inch in dia- 4 o IRON. IRON. 41 meter. In this process it will be observed that the steel is not produced by stopping the decarbonization at a certain stage, but by adding to it cast iron containing such a proportion of carbon as, when added to the pure iron in the converter, will produce good steel. In a paper read before the Institute of Civil Engineers the effects when the blast of cold air is put on are described : " The process is then in an instant brought into full activity, and small, though powerful, jets of air spring upwards through the fluid mass. The air, expanding in volume, divides itself into globules, or bursts violently upwards, carrying with it some hundredweights of fluid metal, which again falls into the boiling mass below. Every part of the apparatus trembles under the violent agitation thus produced. A roaring flame rushes from the mouth of the vessel, and as the process advances, it changes its violet colour to orange, and finally to a voluminous pure white flame. The sparks, which at first were large, like those of ordinary foundry iron, change to small hissing points, and these gradually give way to small floating specks of bluish light as the state of malleable iron is approached. There is no eruption of cinder, as in the early experiments, although it is formed during the process ; the improved shape of the converter causes it to be retained, and it not only acts beneficially on the metal, but it helps to confine the heat, which during the process has risen from the comparatively low temperature of melted pig iron to one vastly greater than the highest known welding heat, by which malleable iron only becomes sufficiently soft to be shaped by the blows of the hammer. But here it becomes perfectly fluid, and even rises so much above the melting-point as to admit of its being poured from .the converter into a foundry ladle, and from thence transferred to several suc- cessive moulds." The whole series of operations connected with the Bessemer process may be easily followed by the help of Fig. 25, which is taken from a beau- tiful model in the Museum of Practical Geology. This model, which was presented to the museum by Mr. Bessemer himself, represents every part of the machinery and appliances of the true relative sizes. C is the trough, lined with infusible clay, by which the liquid pig iron is conveyed to the converters, A. The hydraulic apparatus by which the vessels are turned over is here below the pavement, but the rack which turns the pinion on the axis of the converter is shown at B. The vessel into which the molten steel is poured from the converter is marked E, and this vessel is swung round on a crane, D, so as to bring it exactly over the moulds, placed in a circle, ready to receive the liquid steel, which on cooling is turned out in the form of solid ingots. The valves which control the blast, and those which regulate the movements of the converter through the hydraulic apparatus, are worked by the handles seen at H. The crane, or revolving table, D, is also under perfect control, so that the crude pig iron is converted into steel, and the moulds are filled with a rapidity and ease that are posi- tively marvellous to a spectator. Although the results of the process are not perfect, yet the method has already caused a new development of the applications of steel. Not only was the production of five tons of malleable metal from pig iron, in one operation, in the short space of half an hour, a surprise for metallurgists ; but there was the unprecedented circumstance of this mass of metal being in a state of perfect fusion. By the modified method 400,000 tons of steel, or steely iron, are now produced annually in this country ; and a substance which has hitherto been so costly as to be employed only for knives, springs, 42 IRON. and other small articles, is now produced on a scale which admits of its being used in the construction of bridges, railways, and buildings, or, in fact, applied to any purpose where great tenacity, hardness, elasticity, or durability would be desirable. The production of large castings in steel was long a matter of impossibility, for steel cannot be fused, except in com- paratively small crucibles ; but by Bessemer's process large castings may be made, for quantities of pig iron, about ten tons in weight, are now sometimes operated on in the converter. The great difficulty, however, in steel castings is to obtain them perfectly uniform in texture throughout. Sir. J. Whitworth applies to the steel, after it has been run into the mould, a very powerful pressure, by which process excellent results have been ob- tained in cases where uniformity of texture is of the highest importance, as, for example, in castings intended for the construction of cannons. But everything that has previously been done in the way of casting steel of uniform texture has been surpassed by the scale on which the German firm of Krupp and Co. carry on their operations. The largest mass of steel ever made in one piece was exhibited by this firm at the London Inter- national Exhibition of 1862. Its weight was 21 tons, and it had been pur- posely broken across, to show that its texture was sound throughout ; yet this enormous piece of metal had been formed in separate portions of perhaps not more than 70 Ibs. each, melted in separate crucibles, and poured so continuously and regularly into the mould as to form one uniform cast- ing without flaw or defect. When we consider how many crucibles con- taining 70 Ibs. of metal each would be required to make up such a casting, we see that a large number of men must have been employed, and that there must have been a perfect organization and harmony in their working. Krupp is largely engaged in making steel cannons for the German and other govern- ments, and his works are the largest of the kind in the world, no fewer than 10,000 men being employed in them. FIG. 26. Cupola Furnace. FIG. 27. Sir Joseph Whitworth. TOOLS OF the immense variety of tools and mechanical contrivances employed in modern times, by far the greatest number are designed to impart to certain materials some definite shape. The brickmaker's mould, the joiner's plane, the stonemason's chisel, the potter's wheel, are examples of simple tools. More elaborate are the coining press, the machine for planing iron, the drilling machine, the turning lathe, the rolling mill, the Jacquard loom. But all such tools and machines have one principle in common a principle which casual observers may easily overlook, but one which is of the highest importance, as its application constitutes the very essence of the modern process of manufacture as distinguished from the slow and laborious mode of making things by hand. The principle will be easily understood by a single example. Let it be required to draw straight lines across a sheet of paper. Few persons can take a pen or pencil, and do this with even an approach to accuracy, and at best they can do it but slowly and imperfectly. But with the aid of a ruler any number of straight lines may be drawn rapidly and surely. The former case is an instance of making by hand, the latter represents manufacturing, the ruler being the tool or machine. Let it be observed that the ruler has in itself the kind of form required that is to say, straightness and that in using it we copy or transfer this straightness to the mark made on the paper. This is. a 43 44 70OLS. simple example of the copying principle, which is so widely applied in machines for manufacturing ; for, in all of these, materials are shaped 01 moulded by various contrivances, so as to reproduce certain definite forms, which are in some way contained within the machine itself. This will be distinctly seen in the tools which are about to be described. Probably no one mechanical contrivance is so much and so variously applied as the Screw. The common screw-nail, which is so often used by carpenters for fastening pieces of metal on wood, or one piece of wood to another, is a specimen of the screw with which everybody is familiar. The projection which winds spirally round the nail is termed the thread of the screw, and the distance that the thread advances parallel to the axis in one turn is called the pitch. It is obvious that for each turn the screw makes it is advanced into the wood a depth equal to the pitch, and that there is formed in the wood a hollow screw with corresponding grooves FIG. T&.WhitwortKs Screw Dies and Tap. and projections. Screws are formed on the ends of the bolts, by which various parts are fastened together, and the hollow screws which turn on the ends of the bolts are termed nuts. The screws on bolts and nuts, and other parts of machines, were formerly made with so many different pitches that, when a machine constructed by one maker had to be repaired by another, great inconvenience was found, on account of the want of uni- formity in the shape and pitch of the threads. A uniform system was many years ago proposed by Sir Joseph Whitworth, and adopted by the majority of mechanical engineers, who agreed to use only a certain defined series of pitches. The same engineer also contrived a hand tool for cutting screws with greater accuracy than had formerly been attained in that process, A mechanic often finds it necessary to form a screw-thread on a bolt, and al3o to produce in metal a hollow screw. The reader may have observed gasfitters and other workmen performing the first operation by an instru- ment having the same general appearance as Fig. 28. This contains hard steel dies, which are made to press on the bolt or pipe, so that when the guide-stock is turned by the handles, the required grooves are cut out. TOOLS, 45 The arrangement of these dies in Sir Joseph Whitworth's instrument is shown in Fig. 28, which represents the central part of the guide-stock; A, B, c are the steel dies retained in their places, when the instrument is in wse, by a plate which can be removed when it is necessary to replace one set of dies by another, according to the pitch of thread required. The figure also shows the set of dies, A, B, c, removed from the guide-stock. D is the work, pressed up against the fixed die, A, by B and c, the pressure being applied to these last as required by turning the nut, thus drawing up the key, E, so that the inclined planes, /, g, press against similar surfaces forming the ends of the dies. For producing the hollow screws, taps are provided, which are merely well-formed screws, made of hard steel and having the threads cut into detached pieces by several longitudinal grooves, as repre- sented in the lower part of Fig. 28. The method of forming screws by dies and taps is, however, applicable only to those of small dimensions, and even for these it is not employed FIG. 29. Screw-cutting Lathe. where great accuracy is required. Perfect screws can only be cut with a lathe, such as that represented in Fig. 29. In this we must first call the reader's attention to the portion of the apparatus marked A, which receives the name of the slide-rest. The invention of this contrivance by Maudsley had the effect of almost revolutionizing mechanical art, for by its aid it became possible to produce true surfaces in the lathe. Before the slide-rest was introduced, the instrument which cut the wood or metal was held in the workman's hand, and whatever might be his skill and strength, the steadiness and precision thus obtainable were far inferior to those which could be reached by the grip of an iron hand, guided by unswerving bars. The slide-rest was contrived by Maudsley in the first instance for cut- ting screws, but its principle has been applied for other purposes. This principle consists in attaching the cutting tool to a slide which is incapable of any motion, except in the one direction required. Thus the slide, A, represented in Fig. 29, moves along the bed of the lathe, B, carrying the cutter with perfect steadiness in a straight line parallel to the axis of the lathe. There are also two other slides for adjusting the position of the cutter ; the handle, a, turns a screw, which imparts a transverse motion to the piece, , and the tool receives another longitudinal movement from the 46 TOOLS. handle, c. The pieces are so arranged that these movements take place in straight lines in precisely the required direction, and without permitting the tool to be unsteady, or capable of any rocking motion. In Whitworth's lathe, between the two sides of the bed, and therefore not visible in the figure, is a shaft placed perfectly parallel to the axis of the lathe. One end of this shaft is seen carrying the wheel, C, which is connected with a train of wheels, D, and is thus made to revolve at a speed which can be made to bear any required proportion to that of the mandril, E, of the lathe, by properly arranging the numbers of the teeth in the wheels.; and the machine is provided with several sets of wheels, which can be substituted for each other. The greater part of the length of this shaft is formed with great care into an exceedingly accurate screw, which works in a nut forming part of the slide-rest. The effect, therefore, of the rotation of the screw is to cause the slide-rest to travel along the bed of the lathe, advancing with each revolution of the screw through a space equal to its pitch dis- tance. There is an arrangement for releasing the nut from the guiding- screw, by moving a lever, and then by turning the winch the slide-rest is moved along by a wheel engaging the teeth of a rack at the back of the lathe. Now, if the train of wheels, C D, be so arranged that the screw makes one revolution for each turn of the mandril, it follows that the cut- ting tool will move longitudinally a distance equal to the pitch of the guiding-screw while the bar placed in the lathe makes one turn. Thus the point of the cutter will form on the bar a screw having the same pitch as the guiding-screw of the lathe. Here we have a striking illustration of the copying principle, for the lathe thus produces an exact copy of the screw which it contains. The screw-thread is traced out on the cylindrical bar, which is operated upon by the combination of the circular motion of the mandril with the longitudinal movement of the slide-rest. By modifying the relative amounts of these movements, screw-threads of any desired pitch can be made, and it is for this purpose that the change wheels are provided. If the thread of the guiding-screw makes two turns in one inch, one revolution of the wheel C will advance the cutter half an inch along the length of the bar. If the numbers of teeth in the wheels be such that the wheel D makes ten revo- lutions while C is making one, then in the length of half an inch the thread of the screw produced by the cutter will go round the core ten times, or, in technical language, the screw will be of $ inch pitch. Since a screw turning in a nut advances only its pitch distance at each revolution, a finely-cut screw furnishes an instrument well adapted to im- part a slow motion, or to measure minute spaces. Suppose a screw is cut so as to have fifty threads in an inch, then each turn will advance it -^ in.; half a turn y^ in. ; a quarter of a turn, ^o, and so on. It is quite easy to attach a graduated circle to the head of the screw, so that, by a fixed pointer at the circumference, any required fraction of a revolution may be read off. Thus if the circle had two hundred equal parts, we could, by turning the screw so that one division parsed the index, cause the screw to advance through ^j of -^ inch, or 10 ^ oQ part of an inch. This is the method adopted for moving the cross-wires of the instruments for measuring very small spaces under the microscope. Sir Joseph Whitworth, who has done so many great things in mechanical art, was the first mechanician to perceive the importance of extreme accuracy of workmanship, and he invented many beautiful instruments and processes by which this accuracy might be attained. Fig. 30 represents one of his measuring machines, TOOLS. 47 intended for practical use in the workshop, to test the dimensions of pieces of metal where great precision is required. The base of the machine is constructed of a rigid cast iron bed bearing a fixed headstock, A, and a movable one, B, the latter sliding along the bed, c, with a slow movement, when the handle, D, is turned. This slow motion is produced by a screw on the axis, a, working in the lower part of the headstock, just as the slide- rest is moved along the bed of the lathe. The movable headstock, when it has been moved into the position required, is firmly clamped by a thumb- screw. The face of the bed is graduated into inches and their subdivisions. Here it should be explained that the machine is not intended to be used for ascertaining the absolute dimensions of objects, but for showing by what FlG. 30. Whitworttts Measuring Machine. fraction of an inch the size of the work measured differs from a certain standard piece. Each headstock carries a screw of ^ inch pitch, made with the greatest possible care and accuracy. To the head of the screw in the movable headstock is attached the wheel, b, having its circumference divided into 250 equal parts, and a fixed index, c, from which its graduations may be counted. An exactly similar arrangement is presented in connection with the screw turning in the fixed headstock, but the wheel is much larger, and its circumference is divided in 500 equal parts. It follows, therefore, that if the large wheel be turned so that one division passes the index, the bar moves in a straight line -^ of the ^ of an inch, that is, lQ Soo f an mcn - The ends of the bars, d and e, are formed with perfectly plane and parallel surfaces, and an ingenious method is adopted of securing equality of pressure when comparisons are made. A plate of steel, with perfectly parallel faces, called a gravity-piece, or feeler, is placed between the flat end of the bar and the standard-piece, and the pressure when the screw-reading is taken must be just sufficient to prevent this piece of steel from slipping down, and that is the case when the steel remains suspended and can nevertheless be easily made to slide about by a touch of the finger. Thus any piece which, 48 TOOLS. with the same screw- readings, sustains the gravity-piece in the same manner as the standard, will be of exactly the same length ; or the number of divi- sions through which the large wheel must be turned to enable it to do so tells the difference of the dimensions in ten-thousandth parts of an inch. By this instrument, therefore, gauges, patterns, &c., can be verified with the greatest precision, and pieces can be reproduced perfectly agreeing in their dimensions with a standard piece. Thus, for example, the diameters of shafting can be brought with the greatest precision to the exact size required to best fit their bearings. In another measuring machine on the same principle the delicacy of the measurement has been carried still farther, by substituting for the large divided wheel one having 200 teeth, which engage an endless screw or worm. This will easily be understood by reference to Fig. 3 1 , where a similar arrangement is applied to another purpose. Imagine that a wheel like P, Fig. 31, but with 200 teeth, has taken the place of E in Fig. 30, and that the wheel, T, on the axis of the endless screw is shaped like E, Fig. 30. One turn of the axis carrying the endless screw, therefore, turns the wheel through T^ of a revolution, and as this axis bears a graduated head, having 250 divisions, the screw having 20 threads to the inch, is, when one divi- sion passes the index, advanced through a space equal to ^Q X ^o X ch. This TOOOQOO f an mcn J that i s > the one-millionth part of an inch. This is an interval so small that ten times its length would hardly be appreciated with the highest powers of the microscope, and the machine is far too delicate for any practical requirements of the present day. It will indicate the expansion caused by heat in an iron bar which has merely been touched with the finger for an instant, and even the difference of length produced by the heat radiated from the person using it. A movement of TQOOOOO f an inch is shown by the gravity-piece remaining suspended instead of fall- ing, and the piece falls again when the tangent-screw is turned back through -^(j of a revolution, a difference of reading representing a possible movement of the measuring surface through only TOO 000 f an inch. This proves the marvellous perfection of the workmanship, for it shows that the amount of play in the bearings of the screws does not exceed one-millionth of an inch. A good example of a machine-tool is the Drilling Machine, which is used for drilling holes in metal. Such a machine is represented in Fig. 31, where A is the strong framing, which is cast in a single piece, in order to render it as rigid as possible. The power is applied by means of a strap round the speed pulley, B, by which a regulated speed is communicated to the bevel wheel, C, which drives D, and thus causes the rotation of the hollow shaft, E. In the lower part of the latter is the spindle which carries the drilling tool, F, and upon this spindle is a longitudinal groove, into which fits a projection on the inside of E. The spindle is thus forced to rotate, and is at the same time capable of moving up and down. The top of the spindle is attached to the lower end of the rack, G, by a joint which allows the spindle to rotate freely without being followed in its rotation by the rack, although the latter communicates all its vertical movements to the spindle, as if the two formed one piece. The teeth of the rack are engaged by a pinion, which carries on its axis the wheel H, turned by an endless screw on the shaft, I, which derives its motion by means of another wheel and endless screw from the shaft, K. The latter is driven by a strap passing over the speed pulleys, L and M, and thus the speed of the shaft K can be modified as reauired by passing the strap from one pair of pulleys to TOOLS. 49 another. The result is that the rack is depressed by a slow movement, which advances the drill in the work, or, as it is technically termed, gives \htfeed to the drill. By a simple piece of mechanism at N the connection of the shafts K and I can be broken, and the handle O made to communi- cate a more rapid movement to I, so as to raise up the drill in a position to FIG. 31. WHUwortKs Drilling Machine. begin its work again, or to bring it quickly down to the work, and then the arrangement for the self-acting feed is again brought into play. By turning the wheel, P, the table, Q, on which the work is fastened, is capable of being raised or lowered, by means of a rack within the piece R, acted on by a pinion carried on the axle, P. The table also admits of a horizontal motion by the slide s, and may besides be swung round when required. The visitor to an engineer's workshop cannot fail to be struck with the 50 TOOLS. operation of the powerful Lathes and Planing Machines, by which long thick flakes or shavings of iron are removed from pieces of metal with the same apparent ease as if the machine were paring cheese. The figure on the opposite page represents one of the larger forms of the planing machine, as constructed by Sir J. Whitworth. The piece of work to be planed is firmly bolted down to the table, A, which moves upon the V-shaped surfaces, running its whole length, and accurately fitting into corresponding grooves in a massive cast iron bed. The bevel wheel, of which a portion is seen at B, is keyed on a screw, which extends longitudinally from end to end of the bed. This screw works in nuts forming part of the table, and as it turns in sockets at the ends of the bed, it does not itself move forward, but imparts a progressive movement to the table, and therefore to the piece of metal to be planed. As this table must move backwards and forwards, there must be some contrivance for reversing the direction of the screw's rotation, and this is accomplished in a beautifully simple manner by an arrangement which a little consideration will enable any one to understand. It will be observed that there are three drum-pulleys at C. Let the reader confine, for the present, his attention to the nearest one, and picture to him- self that the shaft to which it is attached is placed in the same horizontal plane as the axis of the screw and at right angles to it, passing in front of bevel wheel B. A small bevel wheel turning with this shaft, and engaging the teeth of the wheel B, may, it is plain, communicate motion to the screw. Now let the reader consider what will be the effect on the direction of the rotation of B of applying the bevelled pinion to the nearer or to the farther part of its circumference, supposing the direction of the rotation of this pinion to be always the same. He will perceive that the direction in one case will be the reverse of that in the other. The shaft to which the nearest pulley is attached carries a pinion engaging the wheel at its farther edge, and therefore the rotation of this pulley in the same direction as the hands of a watch causes the wheel B to rotate so that its upper part moves towards the spectator. The farthest pulley, fei- N . ^^"-7*- _. -~V FIG. 42. Rails and Cramp-gauge. When it has been decided to construct a railway between two places, the laying-out of the line is a subject requiring great consideration and the highest engineering skill for the matter is, on account of the great cost, much more important than the setting-out of a common road. The idea of a perfect railroad is that of a straight and level line from one terminus to another ; but there are many circumstances which prevent such an idea from being ever carried into practice. First, it is desirable that the line should pass through important towns situated near the route ; and then the cost of making the roadway straight and level, in spite of natural ob- stacles, would be often so great, that to avoid it detours and inclines must be submitted to the inconvenience and the increased length of road being balanced by the saving in the cost of construction. It is the business of the engineer who lays out the line to take all these circumstances into con- sideration, after he has made a careful survey of the country through which the line is to pass. The cost of making railways varies, of course, very much according to the number and extent of the tunnels, cuttings, em- bankments, or other works required. The average cost of each mile of railway in Great Britain may be stated as about ,35,000. The road itself when the rails are laid down is called the permanent way, perhaps origin- ally in distinction to the temporary tramways laid down by the contractors during the progress of the works. The permanent way is formed first of RAILWAYS. o.i ballast^ which is a layer of gravel, stone, or other carefully chosen material, about 2 ft. deep, spread over the roadway. Above the ballast and partly em- bedded in it are placed the sleepers, which is the name given to the pieces of timber on which the rails rest. These timbers are usually placed trans- versely that is, across the direction of the rails, in the manner shown in Fig. 42. This figure also represents the form of rails most commonly adopted, and exhibits the mode in which they are fastened down to the sleepers by means of the iron chairs, b c, the rail being firmly held in its place by an oak wedge, d. These wedges are driven in while the rails are maintained at precisely the required distance apart by the implement, e f, called a ! B tramp guage, the chairs having previously been securely attached to the sleepers by bolts or nails. The double T form of rail has several important advantages, such as its capability of being reversed when the upper surface is worn out, and the readiness with which the ends of the rails can be joined by means si fish-plates. These are shown in Fig. 43, where in A we are supposed to be looking down on the rails, and in B to be looking at them sideways. In Fig. 44 we have the rail and fish-plates in section. The holes in the rails through which the bolts pass are not round but oval, so that a certain amount of play is permitted to the ends of the rails. It may easily be seen on looking at a line of rails that they are not laid with the ends quite touching each other, or, at least, they are not usually in contact. The reason of this is that space must be allowed for the ex- pansion which takes place when a rise in the temperature occurs. If the rails are laid down when at the greatest temperature they are likely to be subject to, they may then be placed in actual contact ; but in cold weather a space will be left by their contraction. For this reason it is usual when rails are laid to allow a certain interval ; thus rails 20 ft. long laid when the temperature is 70, are placed with their ends ^th f an mcn apart, at 30 y^th of an inch apart, and so on. The neglect of this precaution has some- times led to damage and accidents. A certain railway was opened in June, and after an excursion train had in the morning passed over it, the mid- 6 4 KAIL WAYS. day heat so expanded the iron, that the rails became in some places ele- vated 2 ft. above the level, and the sleepers were torn up ; so that, in order to admit of the return of the train, the rails had to be hastily relaid in a kind of zigzag. In June, 1856, a train was thrown off the metals of the North-Eastern Railway, in consequence of the rails rising up through expansion. The distance between the rails in Great Britain is 4 ft. 8^ in., that width having been adopted by George Stephenson in the construction of the earlier lines. Brunei, the engineer of the Great Western, adopted, however, in the construction of that railway, a gauge of 7 ft., with a view of obtain- ing greater speed and power in the engines, steadiness in the carriages, FIG. 44. Section of Rails and Fish-plates. and increased size of carriages for bulky goods. The proposal to adopt this gauge gave rise to a memorable dispute among engineers, often called " The Battle of the Gauges." It was stated that any advantages of the broad gauge were more than compensated by its disadvantages. The want of uniformity in the gauges was soon felt to be an inconvenience to the public, and a Parliamentary Committee was appointed to consider the subject. They reported that either gauge supplied all public requirements, but that the broad gauge involved a great additional outlay in its construc- tion without any compensating advantages of economy in working ; and, as at that time 2,000 miles of railway had been constructed on the narrow gauge, whereas only 270 miles were in existence on the broad gauge, they recommended that future railways should be made the prevailing width of 56| in. The Great Western line has engines, bridges, tunnels, viaducts, c., on a larger scale than any other railway in Britain. The difference of gauge has, however, been felt to involve so much inconvenience that lines which adopted the 7-ft. gauge have since relaid the tracks at the more common width. In all probability the Great Western line will soon be completely reconstructed on the narrow gauge system, in order that trains may run without interruption in connection with other lines. The Avheels of railway carriages and engines differ from those of ordinary AIL WAYS. carriages in being fastened in pairs upon the axles, with which they revolve (see Fig. 45). The tire of the wheel is conical, the slope being about i in 20 ; that is, in a wheel 5 in. broad the radius of the outer edge is ^ in. less than that of the inner ; and the rails are placed sloping a little inwards. The effect of this conical figure is to counteract any tendency to roll off the rails ; for if a pair of wheels were shifted a little to one side, the parts of the tires rolling upon the rails being then of unequal circum- -. ference,wouldcausethe J wheels to roll towards the other side. The conical shape produces "l IN 20 Lk. FIG. ^.Conical Wheels. this kind of adjustment so well that the flanges do not in general touch the rails. They act, however, as safeguards in passing over curves and junc- tions. In curves the outer line of rails is laid higher than the inner, so that in passing over them the train leans slightly inwards, in order to counteract what is called the centrifugal force, to which any body moving in a curve is subject. This so- called force is merely the result of that tendency which every moving body has to continue its motion in a straight line. A very good example of the effect of this may be seen when a circus horse is going rapidly round the ring. The inclination inwards is still more perceptible when a rider is standing on the horse's back, as shown in Fig. 46. The earth's attraction of gravity is pulling the performer straight down, and the centrifugal force would of itself throw her outwards hori- zontally. The resultant or com- bined effect of both acts is seen in the exact direction in which she is leaning, and it presses her feet on the horse's back, the animal itself being under similar FIG. 46. Centrifugal Force. conditions. It is obvious that the amount of centrifugal force, and therefore of inward slope, will increase with the speed and sharpness of the curve, and on the railways the raite 5 66 RAIL WA YS. are placed so that the slope counteracts the centrifugal force when the train travels at about the rate of twenty miles per hour. A very important part of the mechanism of a railway is the mode of passing trains from one line of rails to another. Engines and single car- riages are sometimes transferred by means of turn-tables, but the more general plan is by switches, which are commonly constructed as shown in Fig. 47. There are two rails, A and B, tapering to a point and fixed at the other end, so that they have sufficient freedom to turn horizontally. A train passing in the direction shown by the arrow would continue on the main line, if the points are placed as represented ; but if they be moved SHORT TONGUE FIG. 47 Points. so that the long tongue is brought into contact with the rail of the main line, then the train would run on to the side rails. These points are worked by means of a lever attached to the rod, C, the lever being either placed near the rails, or in a signal-box, where a man is stationed, whose sole duty it is to attend to the points and to the signals. The interior of a signal- box near an important junction or station is shown in Fig. 48, and we see here the numerous levers for working the points and the signals, each of these having a connection, by rods or wires, with the corresponding point or signal-post. The electric telegraph is now an important agent in railway signalling, and in a signal-box we may see the bells and instruments which inform the pointsman whether a certain section of the line is " blocked " or " clear." The signals now generally used on British railways are made by the semaphore, which is simply a post from which an arm can be made to project. When the driver of the train sees the arm projecting from the left-hand side of the post, it is an intimation to him that he must stop his train ; when the arm is dropped half-way, so as to project 45 from the post, it is meant that he must proceed cautiously ; when the arm is down the line is clear. These signals, of course, are not capable of being seen at night, when their place is supplied with lamps, provided with coloured glasses red and green and also with an uncoloured glass. The lamp may have the different glasses on three different sides, and be turned round so as to present the required colour ; or it may be made to do so without turning, if provided with a frame having red and green glasses, which can be moved RAILWAYS. FIG. 48. Signal-box on the North London Railway like spectacles in front of it. The meanings of the various coloured lights and the corresponding semaphore signals are these : I 1 1 White Green Red . All right Caution . Danger . Go on. Proceed slowly. Stop. A very clear account of the mode of working railway signals on what is now called the block system, together with a graphic description of a signal- box, was given in a paper which appeared some years ago in " The Popular Science Review," from the pen of Mr. Charles V. Walker, F.R.S., the telegraph engineer to the South-Eastern Railway Company, who was the first to organize an efficient system of electric signalling for railways. We may remark that the signalling instruments on the South-Eastern line, and indeed on all the lines at the present day, address themselves both to the ear and to the eye, for they consist of first, bells, on which one, two, or more blows are struck, each series of blows having its own particular meaning ; and, second, of a kind of miniature signal-post, with arms capable of being moved by electric currents into positions similar to those of the arm of an actual signal-post, so that the position of the arms is made always to indicate the state of the line. One arm of the little signal-post the left is red, and it has reference to receding trains ; the other viz., the right arm is white, and relates to approaching trains. Mr. Walker thus de- scribes the signalling 68 RAILWAYS. "The ordinary position of the arms of the electro-magnetic telegraph semaphores will be down ; that is to say, when the line is clear of all trains, and business begins, say in early morning, all the arms will be down, indi- cating that no train is moving. When the first train is ready to start, say from Charing Cross, the signalman will give the proper bell-signal to Belvi- dere two, three, or four blows, according as the train is for Greenwich, jor North Kent, or Mid- Kent, or for the main line; and the Belvidere man will acknowledge this by one blow on the bell in reply, and without raising the Charing Cross red or left arm. This is the signal that the train may go on ; and when the train has passed, so that the Charing Cross man can see the tail lights, he gives the out signal a second time, which the Belvidere man acknowledges, at the same time raising the red arm at Charing Cross, behind the train, and so protecting it until it has passed him at Belvidere, when he signals to that effect to Charing Cross, at the same time putting down the red arm there, as an indication that the line is again clear. While these operations are going on for down trains, others precisely similar, but in the reverse direction, are going on for up trains. . . . One and the same pressure on the key sends a bell signal and raises or depresses the semaphore arm as the case may require, a single telegraph wire only being required for the combined system, as for the more simple bell system." In one of the signal-boxes on the South-Eastern line, Mr. Walker states, on a certain day 650 trains or engines were signalled and all particulars accurately entered in a, book, the entries requiring the writing down of nearly 8,000 figures : an illustration of the amount of work quietly carried on in a signal-box for the advantage and security of the travelling public. Mr. Walker also gives us a peep into the inside of one of the signal-boxes, thus : " The interior of a large signal-box exhibits a very animated scene, in which there are but two actors, a man and a boy, both as busy as bees, but with no hurry or bustle. The ruling genius of the place is the strong, active, intelligent signalman, standing at one end of the apartment, the monarch for the time being of all he surveys. Immediately before him in one long line, extending from side to side, is a goodly array of levers, bright and clean from constant use and careful tending, each one labelled for its respective duty. Before him to the right and left are the various electro- magnetic semaphores, each one in full view and adjusted in position to the pair of roads to which it is appropriated, and all furnished with porcelain labels. Directly in front of him is a screen, along which are arranged the various semaphore keys ; and on brackets, discreetly distributed, are the bells and gongs, the twin companions each of its own semaphore. Before the screen are the writing-desk and books, and here stands the youngster, the ministering spirit, all on the alert to take or to send electric signals and to record them, his time and attention being devoted alternately to his semaphore keys and to his books, being immediately under the eye and control of the signalman. This is no place for visitors, and the scenes enacted here have little chance of meeting the public gaze ; indeed, the officers whose duties take them hither occasionally are only too glad to look on, and say as little as may be, and not interrupt the active pair, be- tween whom there is evidently a good understanding in the discharge of duties upon the accurate performance of which so much depends. Looking on, the man will be seen in command of his rank and file : signals come, are heard and seen by both man and boy ; levers are drawn and .with- drawn, one, two, three, or more ; the arms and the lamps on the gigantic masts outside, of which there are three, well laden, are displayed as re- RAIL WA VS. C 9 quired ; distant signals are moved, points are shifted and roads made ready; telegraph signals are acknowledged ; and on looking out for the box is glazed throughout trains are seen moving in accordance with the signals made ; and on the signal-posts at the boxes, right and left for here they are within easy reach of each other arms are seen up and down in sym- pathy with those on the spot, and with the telegraph signals that have been interchanged. There is no cessation to this work, and there is no confu- sion in it ; one head and hand directs the whole, so that there are no con- flicting interests and no misunderstandings ; all is done in perfect tran- quillity, and the great secret is that one thing is done at a time. All this, which is so simple and so full of meaning to the expert, is to the uninitiated intricate and vague ; and though he cannot at first even follow the descrip- tion of the several processes, so rapidly are they begun and ended, yet, as the cloud becomes thin, and his ideas become clearer, he cannot fail to be gratified, and to be filled with admiration at the great results that are brought about by means so simple." Post Office Railway Van. Most of the carriages used on railways are so familiar to every one^that it is unnecessary to give any description of them. We give, however, Illus- trations of two forms which have special features of interest. The first of these is the Travelling Post Office, Fig. 49. In such vans as that here represented letters are sorted during the journey, and for this purpose the interior is provided with a counter and with pigeon-holes from end to end. When the train stops bags may, of course, be removed from or received into the van in the ordinary manner ; but by a simple mechanism bags may be delivered at a station and others taken up while the train continues its journey at full speed. A bar can be made to project from the side of the carriage, and on this the bag is hung by hooks, which are so contrived that they release the bag when a rod, projecting from the receiving appa- ratus, strikes a certain catch on the van. The bag then drops into a netting, which is spread for its reception ; and in order to receive the bags taken up, a similar netting is stretched on an iron frame attached to the van. 70 RAILWAYS. This frame is made to fold up against the side of the carriage when not in use. When the train is approaching the station where the bag is to be taken up, this frame is let down, and a projecting portion detaches the bags, so that they drop into the net, from which they are removed into the interior of the vehicle. These travelling post offices are lighted with gas, and are padded at the ends, so that the clerks may not be liable to injury by concussions of the carriages. The other illustration shows the interior of one of the Pullman's Cars, so much used in the United States, and in one of its forms lately introduced into England on the Midland Railway. Some of these vehicles on the American railways are luxurious hotels upon wheels ; they contain accom- modation for forty persons, having a kitchen, hot and cold water, wine, china and linen closets, and more than a hundred different articles of food, besides an ample supply of tablecloths, table napkins, towels, sheets, pillow- cases, &c. Then there are other Pullman inventions, such as the " palace " and the " sleeping " cars, in which the traveller who is performing a long journey makes himself at home for days, or perhaps for a week, as, for in- stance, while he is being carried across the American continent from ocean to ocean at the easy rate of twenty miles an hour on the Pacific and other connecting lines. Mr. C. Nordhoff, an American writer, giving an account of his journey to the Western States, writes thus : " Having unpacked your books and unstrapped your wraps in your Pullman or Central Pacific palace car, you may pursue all the sedentary avocations and amusements of a parlour at home ; and as your housekeeping is done and admirably done for you by alert and experienced servants ; as you may lie down at full length, or sit up, sleep, or wake at your choice ; as your dinner is sure to be abundant, very tolerably cooked, and not hurried ; as you are pretty certain to make acquaintances in the car ; and as the country through which you pass is strange and abounds in curious and interesting sights, and the air is fresh and exhilarating you soon fall into the ways of the voyage ; and if you are a tired business man or a wearied housekeeper, your careless ease will be such a rest as certainly most busy and overworked Americans know how to enjoy. You write comfortably at a table in a little room called a ' drawing-room,' entirely closed off, if you wish it, from the re- mainder of the car, which room contains two large and comfortable arm- chairs and a sofa, two broad clean plate-glass windows on each side (which may be doubled if the weather is cold), hooks in abundance for shawls, hats, &c., and mirrors at every corner. Books and photographs lie on the table. Your wife sits at the window sewing and looking out on long ranges of snow-clad mountains or on boundless ocean-like plains. Children play on the floor or watch at the windows for the comical prairie dogs sit- ting near their holes, and turning laughable somersaults as the car sweeps by. The porter calls you at any hour you appoint in the morning ; he gives half an hour's notice of breakfast, dinner, or supper ; and while you are at breakfast, your beds are made up and your room or your section aired. About eight o'clock in the evening for, as at sea, you keep good hours the porter, in a clean grey uniform, comes in to make up the beds. The two easy-chairs are turned into a berth ; the sofa undergoes a similar transformation ; the table, having its legs pulled together, disappears in a corner, and two shelves being let down furnish two other berths. The freshest and whitest of linen and brightly-coloured blankets complete the outfit ; and you undress and go to bed as you would at home, and, unless you have eaten too heartily of antelope or elk, will sleep as soundly." RAILWAYS. An important general truth may find a familiar illustration in the subject now under notice. The truth in question may be expressed by saying that, in all human affairs, as well as in the operations of nature, the state of things at any one time is the result, by a sort of growth, of a preceding state of things. And in this way it is certainly true of inventions, that they never make their appearance suddenly in a complete and finished state like Minerva, who is fabled to have sprung from the brain of Jupiter fully grown and completely armed ; but rather their history resembles the slow and progressive process by which ordinary mortals attain to their full stature. We have already seen that railways had their origin in the tram- ways of collieries ; and, in like manner, the railway carriage grew out of the colliery truck and the stage coach ; for when railway carriages to convey passengers were first made, it did not occur to their designers that anything better could be done than to place coach bodies on the frame of the truck ; and accordingly the early railway carriages were formed by mounting the body of a stage coach, or two or three such bodies side by side, on the timber framework which was supported by the flanged wheels. The cut, Fig. 56, is from a painting in the possession of the Connecticut Historical Society, and it represents the first railway train in America on its trial trip (1831), in which sixteen persons took part, who were then thought not a little courageous. Here we see that the carriages were regular stage coaches, and the same was the case in England. But it is very significant that, to this day, the stage coach bodies are traceable in many of the car- riages now running on English lines, especially in the first-class carriages, where, in the curved lines of the mouldings which are supposed to orna- ment the outside, one may easily recognize the forms of the curved bodies of the stage coaches, although there is nothing whatever, in the real framing of the timbers of the railway carriage, which has the most distant relation to these curves. Then again, almost universally on English lines, the old stage coach door-handles are still retained on the first-class carriages, in the awkward flat oval plates of brass which fold down with a hinge. Many other points might be named which would show the persistence of the stage coach type on the English railways. The cut, Fig. 56, proves that the Americans set out with the same style of carriages; but North America, as compared with the Old World, is par excellence the country of rapid developments, and there carriages, or cars, as our Transatlantic cousins call them, have for a long time been made with numerous improve- ments, and in forms more in harmony with the railway system, than the conservatism of English ideas, still cleaving to the stage coach type, per- mitted to be attempted in this country. The perfection of comfort in railway travelling appears to be attained in the famous Pullman Cars, of which travellers in the States have long enjoyed the benefit, while, in the birthplace of railways, no attempt has been made to effect any material improvements in the general plan of the carriages. Quite recently, how- ever, the Midland Railway Company, to their credit be it said, have entered into an agreement with the Pullman Car Company, by which, for the first time, these luxurious vehicles are placed within reach of the English tra- veller. In Plate III. we give a representation of the interior of one of these cars as now fitted up for the Midland line, and known as the Parlour Car. The elegance and comfort of the arrangements are almost too obvious to require description. We see the luxuriously padded chairs, which, by turning on swivels, permit the traveller to adjust his position according to his individual wishes, so that he can, with ease, place his seat either to gaze 72 RAILWAYS. directly sn the passing landscape, or turn his face towards his fellow- travellers opposite or on either side. The chairs are also provided with an arrangement for placing the backs at any required inclination, and the light and refined character of the decorations of the carriage should not escape the reader's notice. Pullman Cars of another kind, providing sleeping accommodation for night journeys, are also in use on the Midland line, and they are fitted up with the same thoughtful regard to comfort as the Parlour Ca/ represented in our plate. The great engineering feats which have been accomplished in the con- struction of railways are numerous enough to fill volumes. We give, there- fore, only a short notice of one or two recently constructed lines which have features of special interest, passing over, for want of space, even such remarkable constructions as the railway by which the traveller may now go up the Righi> and the railway which is to ascend Mount Vesuvius. JHE METROPOLITAN RAILWAYS. 'XIT'HEN the traffic in the streets of London became so great that the * * ordinary thoroughfares were unable to meet public requirements, the bold project was conceived of making a railway under the streets. The con- struction of a line of railway beneath the streets of a populous city, amidst a labyrinth of gas-pipes, water-mains, sewers, &c., is obviously an undertaking presenting features so remarkable that the London Underground Railway cannot here be passed over without a short notice. Its construction occu- pied about three years, and it was opened for traffic in 1863. The line commences at Paddington, and passing beneath Edgware Road at right angles, it reaches Marylebone Road, under the centre of which it proceeds, and passing beneath the houses at one end of Park Crescent, Portland Place, it follows the centre of Euston Road to King's Cross, where a junc- tion with the Great Northern and Midland system is effected. Here the line bends sharply southwards, and proceeds to Farringdon Street Station, near Smithfield Market, from which point it again takes an easterly direc- tion and reaches Moorgate Street, the present terminus. The crown of the arch which covers the line is in some places only a few inches beneath the level of the streets ; in other places it is several feet below the surface, and, in fact, beneath the foundations of the houses and other buildings. The steepest gradient on the line is I in 100, and the sharpest curve has a radius of 200 yards. The line is nearly all curved, there not being in all its length three-quarters of a mile of straight rails. The difficulties besetting an undertaking of this kind would be tedious to describe, but may readily be imagined. The line traverses every kind of soil clay, gravel, sand, rubbish, all loosened by previous excavations for drains, pipes, foundations, c. ; and the arrangements of these drains, water and gas-pipes, had to be reconciled with the progress of the railway works, without their uses being interfered with even for a time. Of the stations the majority have roofs of the ordinary kind, open to the sky ; but two of them, namely, Baker Street and Gower Street, are completely underground stations, and their roofs are formed by the arches of brickwork immediately below the streets. The arrangements at these stations show great boldness and JRAJLWAYS. 73 FIG. 50. Cower Street Station, Metropolitan Railway. inventiveness of design. The booking offices for the up line are on one side of the road, and those for the down line on the other. Fig. 50 repre- sents the interior of the Gower Street Station, and the other is very similar. In each the platforms are 325 ft. long and 10 ft. broad, and the stations are lighted by lateral openings through the springing of the arch which forms the roof. This arch is a portion of a circle of 32 ft radius, with a span of 45 ft. and a rise of 9 ft. at the crown. The lateral openings are arched at the top and bottom, but the sides are flat. The width of each is 4 ft. 9 in., and the height outside 6 ft., increasing to 10 ft. at the ends opening on the platform. The openings are entirely lined with white glazed tiles, and the outward ends open into an area, the back of which is inclined at an angle of 45, and the whole also lined with white glazed tiles, and covered with glass, except where some iron gratings are provided for ven- tilation. The tiles reflect the daylight so powerfully that but little gas is required for the illumination of the station in the day-time. The arched roofs of these stations are supported by piers of brickwork, 10 ft. apart, 5 ft. 6 in. deep, and 3 ft. 9 in. wide. In the spaces between the piers vertical arches, like parts of the brick lining of a well, are wedged in, to resist the thrust of the earth, and a straight wall is built inside of this between the piers, to form the platform wall of the station. The tops of the piers are connected by arches, and are thus made to bear the weight of the arched roof, which has 2 ft. 3 in. thickness of brickwork at the crown, and a much greater thickness towards the haunches. The benefit derived by the public from the completion of the Metropolitan Railway was greatly increased by the subsequent construction of another railway " The Metropolitan District," which, joining the Metropolitan at Paddington, makes a circuit about the west-end of Hyde Park, and passing close to the Victoria Terminus of the London, Chatham, and Dover and the Brighton and South Coast Railways, reaches Westminster Bridge, and then follows the Thames Embankment to Blackfriars Bridge, when it 74 RAILWAYS. leaves the bank of the river to reach its terminus at the Mansion House Station. This line, taken in conjunction with the Metropolitan, forms the " inner circle" of the railway communication in London. But the circuit is not complete, being broken at the east by the want of connection between the Mansion House Station and that at Moorgate Street, although these stations are but little more than half a mile apart. A line connecting these two points will probably soon be constructed, and then the public will possess a complete circle of communication. The number of trains each day entering and leaving some of the stations on the Metropolitan system is very great. Moorgate Street Station a terminus into which several companies run may have about 800 trains arriving or departing in the course of a day. THE PACIFIC RAILWAY. HP HE remarkable development of railways which has taken place in the * United States has its most striking illustration in the great system of lines by which the whole continent can be traversed from shore to shore. The distance by rail from New York to San Francisco is 3,215 miles, and the journey occupies about a week, the trains travelling night and day. The traveller proceeding from the Eastern States to the far west has the choice of many routes, but these all converge to Omaha. From this point the Pacific Railroad will convey him towards the land of the setting sun. The map, Fig. 51, shows the course of this railway, which is the longest in the world. It traverses broader plains and crosses higher mountains than any other. Engineering skill of the most admirable kind has been displayed in the laying-out and in the construction of the line, with its innumerable cuttings, bridges, tunnels, and snow-sheds. The road/rom Omaha to Ogden, near the Great Salt Lake a distance of 1,032 miles is owned by the Union Pacific Company, and the Central Pacific joins the former at Ogden and completes the communication to San Francisco, a further length of 889 miles the whole distance from Omaha to San Francisco being 1,911 miles. The Union Pacific was com- menced in November, 1865, and completed in May, 1869. There are at Omaha extensive workshops provided with all the appliances for construc- ting and repairing locomotives and carriages, and these works cover 30 acres of ground, and give employment to several thousand men. The population of Omaha rose during the making of the railway from under 3,000 in 1864 to more than 16,000 in 1870, and it is now a flourishing town. A little dis- tance from Omaha the line approaches the Platte River, and the valley of this river and one of its tributaries is ascended to Cheyenne, 516 miles from Omaha, the line being nowhere very far from the river's course. Cheyenne is 5,075 ft. higher above the sea than Omaha, the elevation of which is 966 ft. The Platte River is a broad but very shallow stream, with a channel continually shifting, owing to the vast quantity of sand which its muddy waters carry down. This portion of the line passing through a dis- trict where leagues upon leagues of fertile land await the hand of the tiller, has opened up vast tracts of land hedgeless, gateless green fields, free to all, and capable of receiving and supporting millions of human inhabitants. RAIL WA YS. 75 Cheyenne, a town of 3,000 inhabitants, is entirely the creation of the railways, for southward from Cheyenne a railway passes to Denver, a distance of 106 miles, through rich farming and grazing districts. Seven miles beyond Cheyenne the line begins to ascend the Black Hills by steep gradients, and at Granite Can- yon, for example, the rise in five miles is 574 ft. , or about 1 2 1 ft. per mile. Many lime-kilns have been erected in this neighbourhood, where limestone is very abundant. A little beyond this point the road is in many places protected by snow-sheds, fences of timber, and rude stonework. At Sherman, 549 miles from Omaha, the line attains the sum- mit of its track over the Black Hills, and the highest point on any railway in the world, being 8,242 ft. above the level of the sea. Wild and desolate scenery characterizes the district round Sherman, and the hills, in places co- vered with a dense growth of wood, will furnish an immense supply of timber for years to come. The timber-sheds erected over the line, and the fences beside it are not so much on account of the depth of snow that falls, but to pre- vent it from blocking the line by being drifted into the cuts by the high wind. A few miles beyond Dale Creek at Sher- man is the largest bridge on the line. It is a trestle bridge, 650 ft. long and 126 ft. high, and has a very light ap- pearance indeed, to an English eye unaccustomed to these impromptu timber structures, it looks unpleasantly light. From Sherman the line descends to Laramie, which is 7,123 ft. above the sea level and 24 miles from Sherman, and here the railway has a workshop, for good coal is found within a few miles. A fine tract of grazing land, 60 miles long and 20 miles broad, stretches around this station, and it is said that nowhere in the whole North American continent can cattle be reared and fattened more cheaply. The line, now descending the Black Hills, crosses for many miles a long stretch of rolling prairie, covered in great part with sage- bush j and forming a tableland lying be- 7 6 RAILWAYS. FIG. 52. -Trestle Bridge. tween the western base of the Black Hills and the eastern base of the snowy range of the Rocky Mountains, which latter reach an elevation of from 10,000 to 17,000 ft. above the sea level and are perpetually covered with snow. Such tablelands are termed in America " parks." Before the line reaches the summit of the pass by which it crosses the range of the snowy mountains, it traverses some rough country among the spurs of the hills through deep cuts and under snow-sheds, across ravines and rivers, and through tunnels. At Percy, 669 miles, is a station named after Colonel Percy, who was killed here by the Indians when surveying for the line. He was surprised by a party of the red men, and retreated to a cabin, where he withstood the attack of his assailants for three days, killing several of them ; but at length they set fire to the cabin, and the unfortunate Colonel rushing out, fell a victim to their ferocity. Near Creston, 737 miles from Omaha, the highest point of the chief range is reached, though at an elevation lower by 1,212 ft. than the summit of the pass where the line crosses the Black Hills, which are the advanced guard of the Rocky Mountains. Here is the water-shed of the continent, for all streams rising to the east of this flow ultimately into the Atlantic, while these, having their sources in the west, fall into the Pacific. Before reaching Ogden the line passes through some grand gorges, which open a way for the iron RAIL WA VS. 77 horse through the very hearts of the mountains, as if Nature had foreseen railways and providently formed gigantic cuttings such as the Echo and Weber Canyons, which enable the line to traverse the Wahsatch Moun- tains. Echo Canyon is a ravine 7 miles long, about half a mile broad, flanked by precipitous cliffs, from 300 to 800 ft. high, and presenting a succession of wild and grand scenery. In Weber Canyon the river foams and rushes along between the mountains, which rise in massive grandeur on either side, plunging and eddying among the huge masses of rock fallen from the cliffs above. Along a part of the chasm the railway is cut in the side of the steep mountain, descending directly to the bed of the stream. FIG. 53. American Canyon. Where the road could not be carried round or over the spurs of the moun- tains it passes through tunnels, often cut through solid stone. A few miles farther the line reaches the city of Ogden, in the state of Utah, the territory of the Mormons. This territory contains upwards of 65,000 square miles, and though the land is not naturally productive, it has, by irrigation, been brought into a high state of cultivation, and it abounds in valuable minerals, so that it now supports a population of 80,000 persons. We have now arrived at Ogden, where the western portion of the great railway line connecting the two oceans unites to the Union Pacific we have just described. This western portion is known as the Central Pacific Rail- road, and it stretches from Ogden to San Francisco, a distance of 882 miles. The portion of the line which unites Sacramento to Ogden, 743 miles, was commenced in 1863 and finished in 1869, but nearly half of the entire length was constructed in 1868, and about 50 miles west of Ogden, the re- markable engineering feat of laying 10 miles of railway in one day was performed. It was thus accomplished : when the waggon loaded with the 78 RAILWAYS. rails arrived at the end of the track, the two outer rails were seized, hauled forward off the car, and laid upon the sleepers by four men, who attended to this duty only. The waggon was pushed forwards over these rails, and the process of putting down the rails was repeated, while behind the waggon came a little army of men, who drove in the spikes and screwed on the fish- plates, and, lastly, a large number of Chinese workmen, with pickaxes and spades, who ballasted the line. The average rate at which these opera- tions proceeded was about 240 ft. of track in 77^ seconds, and in these 10 miles of railway there were 2,585,000 cross-ties, 3,520 iron rails, 55,000 spikes, 7,040 fish-plates, and 14,080 bolts with screws, the whole weighing 4,362,000 Ibs. ! Four thousand men and hundreds of waggons were re- quired, but in the 10 miles all the rails were laid by the same eight men, each of whom is said to have that day walked 10 miles and lifted 1,000 tons of iron rails. Nothing but the practice acquired during the four previous years and the most excellent arrangement and discipline could have made the performance of such a feat possible as the laying of eight miles of the track in six hours, which was the victory achieved by these stalwart navvies before dinner. The line crosses the great American desert, distinguished for its desolate aspect and barren soil, and so thickly strewn with alkaline dust that it appears almost like a snow-covered plain. The alkali is caustic, and where it abounds no vegetation can exist, most of the surface of this waste being fine, hard grey sand, mixed with the fragments of marine shells and beds of alkali. The third great mountain range of the North American continent is crossed by this line, at an elevation of 7,043 ft. above the sea level. The Sierra Nevada, as the name implies, is a range of rugged wild broken mountain-tops, always covered with snow. The more exposed portions of the road are covered with snow-sheds, solidly constructed of pine wood posts, 16 in. or 20 in. across : the total length of snow-sheds on the Sierra Nevada may equal 50 miles. These sheds sometimes take fire ; but the company have a locomotive at the Summit Station, ready to start at a moment's notice with cars carrying tanks of water. The snow falls there sometimes, to a depth of 20 ft. in one winter; and in spring, when it falls into the valleys in avalanches, sweeping down the mountain-sides, they pass harmlessly over the sloping roofs of the snow-sheds. Where the line passes along the steep flank of a mountain, the roofs of these snow-sheds abut against the mountain-side, so that the masses of snow, gliding down from its heights, continue their slide without injury to line, or sheds, or trains. Where, however, the line lies on level ground, or in a ridge, the snow-sheds are built with a strong roof of double slope, in order to support or throw off the snow. From Summit (7,017 ft.) the line descends continuously to Sacramento, which is only 30 ft. above the sea level, and 104 miles from Summit. About 36 miles from Summit, the great American Canyon, one of the wildest gorges in the Sierra Nevada range, is passed. Here the American River is confined for a length of two miles between precipitous walls of rock, 2,000 ft. in height, and so steep that no human foot has ever yet followed the stream through this tremendous gorge (Fig. 53). A few miles beyond this the line is carried, by a daring feat of engineering, along the side of a mountain, overhanging a stream 2,500 ft. below. This moun- tain is known as " Cape Horn," and is a place to try the nerves of timid people. When this portion of the line was commenced, the workmen were lowered and held by ropes, until they had hewn out a standing-place on RAIL WA YS. 79 FlG. 54. Cape Horn. the shelving sides of the precipice, along whose dizzy height, where even the agile Indian was unable to plant his foot, the science of the white man thus made for his iron horse a secure and direct road. (Fig. 54.) These lines of railway, connecting Omaha with Sacramento, are remark- able evidences of the energy and spirit which characterize the Anglo-Saxon race in America. The men who conceived the design of the Central Pacific Railroad, and actually carried it into effect, were not persons experienced in railway construction; but five middle-aged traders of Sacramento, two of whom where drapers, one a wholesale grocer, and the others ironmongers, believing that such a railway should be made, and finding no one ready to undertake it, united together, projected the railway, got it completed, and now manage it. These gentlemen were associated with an engineer RAIL WA YS. FIG. 55. Snow Plough. named Judah, who was a sanguine advocate of the scheme, and made the preliminary surveys, if he did not plan the line. The line is considered one of the best appointed and best managed in the States ; yet the project was at first ridiculed and pronounced impracticable by engineers of high repute, opposed by capitalists, and denounced by politicians. An eminent banker, who personally regarded the scheme with hopefulness, would not venture, however, to take any stock, lest the credit of his bank should be shaken, were he known to 'be connected with so wild a scheme. And, indeed, the difficulties appeared great. Except wood, all the materials required, the iron rails, the pickaxes and spades, the waggons, the loco- motives, and the machinery had to be sent by sea from New York, round Cape Horn, a long and perilous voyage of nine months duration, and transhipped at San Francisco for another voyage of 120 miles before they could reach Sacramento. Add to this that workmen were so scarce in California, and wages so high, that to carry on the work it was necessary to obtain men from New York; and during its progress 10,000 Chinamen were brought across the Pacific, to work as labourers. Subscriptions came in very slowly, and before 30 miles of the line had been constructed, the price of iron rose in a very short time to nearly three times its former amount. At this critical juncture, the five merchants decided to defray, out of their own private fortunes, the cost of keeping 800 men at work on the line for a whole year. We cannot but admire the unswerving confidence in their enterprise displayed by these nVe country merchants, unskilled in RAILWAYS. 8 1 railway making, unaided by public support, and even discouraged in their project by their own friends. The financial and legal obstacles they suc- cessfully surmounted were not the only difficulties to be overcome. They had the engineering difficulties of carrying their line over the steep Sierra, a work of four years ; long tunnels had to be bored ; one spring when snow 60 ft. in depth covered the track, it had to be removed by the shovel for 7 miles along the road ; saw-mills had to be erected in the mountains, to pre- pare the sleepers and other timber work ; wood and water had to be carried 40 miles across alkali plains, and locomotives and rails dragged over the mountains by teams of oxen. The chief engineer, who organized the force of labourers, laid out the road, designed the necessary structures, and suc- cessfully grappled with the novel problem of running trains over such a line in all seasons, was Mr. S. S. Montague. The requirements of the traffic necessitate not only solidly constructed iron-covered snow-sheds, but massive snow-ploughs to throw off the track the deep snow which could in no other way be prevented from interrupting the working of the line. These snow-ploughs are sometimes urged forward with the united power of eight heavy locomotives. Fig. 55 represents one of these ploughs cleaning the line, by throwing off the snow on to the sides of the track. The cutting apparatus varies in its arrangements, some forms being designed to push the snow off on one side, some on the other, and to fling it down the precipices ; and others, like the one represented, are intended merely to throw it off the track. FIG. 56. The first Steam Railroad Train in America. Sacramento is 1,775 miles from Omaha, and is connected with San Fran- cisco by a line 139 miles long. At San Francisco, or rather at Oakland, 1,911 miles from Omaha and 3,212 miles from New York, is the terminus of the great system of lines connecting the opposite shores of the vast North American continent. San Francisco, situated on the western shore of a bay, is connected with Oakland by a ferry ; but the railway company have recently constructed a pier, which carries the trains out into the bav for 2.\ miles. This pier is strongly built, and is provided with a double set of rails and a carriage-road, and with slips at which ships land and em- bark passengers, so that ships trading to China, Japan, and Australia can load and unload directly into the trains, which may pass without change from the shores of the Pacific to those of the Atlantic Ocean. San Fran- cisco is a marvellous example of rapid increase, for the population now numbers 170,000, yet a quarter of a century ago 500 white settlers could not be found in as many miles around its site. The first house was erected in 1846, and in 1847 not a ship visited the bay, but now forty large steam- ships ply regularly, carrying mails to China, Japan, Panama, South America, Australia, &c., and there are, of course, hundreds of other steamers and ships. 6 RAIL WA YS. The descriptions we have given of only two lines of railway may suffice to show that the modern engineer is deterred by no obstacles, but boldly drives his lines through places apparently the most impracticable. He shrinks from no operations however difficult, nor hesitates to undertake works the mere magnitude of which would have made our forefathers stand aghast. Not in England or America alone, but in almost every part of the world, the railways have extended with wonderful rapidity ; the continent of Europe is embraced by a network of lines ; the distant colonies of Australia and New Zealand have thousands of miles of lines laid down, and many more in progress ; the map of India shows that peninsula traversed in all direc- tions by the iron roads ; and in the far distant East we hear of Japan having several lines in successful operation, and the design of laying down more. In connection with such works, at home and abroad, many constructions of great size and daring have been designed and erected ; many navigable rivers have been bridged, and not seldom has an arm of the sea itself been spanned ; hundreds of miles of embankments and viaducts have been raised ; hills have been pierced with innumerable cuttings and tunnels, and all these great works have been accomplished within the experience of a single generation of men, and have sprung from one single successful achievement of Stephenson's the Liverpool and Manchester Railway, completed not half a century ago. We in England should also have pride in remembering that the growth of the railways here is due to the enter- prise, industry, and energy of private persons ; for the State has furnished no funds, but individuals, by combining their own resources, have executed the works, and manage the lines for their common interest and the public good. It is said that the amount of money which has been spent on railways in Great Britain is not far short of 500 millions of pounds sterling. The greatest railway company in the United Kingdom is the London and North- Western, which draws in annual receipts about seven millions of pounds ; and the total receipts of all the railway companies would nearly equal half the revenue of the State. FIG. 57. Railway Embankment near Bath. I FIG. 58. The Great Eastern at Anchor. STEAM NAVIGATION. THE first practically successful steamboat was constructed by Symington, and used on the Forth and Clyde Canal in 1802. A few years after- wards Fulton established steam navigation in American waters, where a[ number of steamboats plied regularly for some years before the invention! had received a corresponding development in England, for it was not until 1814 that a steam-packet ran for hire in the Thames. From that time, how- ever, the principle was quickly and extensively applied, and steamers made their appearance on the chief rivers of Great Britain, and soon began also to make regular passages from one sea-port to another, until at length, in . 1819, a steamer made the voyage from New York to Liverpool. It does not appear, however, that such ocean steam voyages became at once common, d for we read that in 1825 the captain of the first steam-ship which made the voyage to India was rewarded by a large sum of money. It was not until 1838 that regular steam communication with America was commenced by the dispatch of the Great Western from Bristol. Other large steamers were soon built expressly for the passage of the Atlantic, and a new era in steam navigation was reached when, in 1845, the Great Britain made her first voyage to New York in fourteen days. This ship was of immense size, compared with her predecessors, her length being 320 ft., and she was 83 6 2 84 STEAM NA VIGA TION. } moreover made of iron, while instead of paddles, she was provided with < a screw-propeller, both circumstances at that time novelties in passenger ships. Fulton appears to have made trial in America of various forms of mechanism for propelling ships, through the water. Among other plans he tried the screw, but finally decided in favour of paddle-wheels, and for a long time these were universally adopted. Many ships of war were built with paddle-wheels, but the advantages of the screw-propeller were at length perceived. The paddle-wheels could easily be disabled by an enemy's shot, and the large paddle-boxes encumbered the decks and obstructed the operations of naval warfare. Another circumstance perhaps had a greater share in the general adoption of the screw, which, had long before been proposed as a means of applying steam power to the propulsion of vessels. This was the introduction of a new method of placing the screw, so that its powers were used to greater advantage. Mr. J. P. Smith obtained a patent in 1836 for placing the propeller in that part of the vessel techni- cally called the dead-wood, which is above the keel and immediately in front of the rudder. When the means of propulsion in a ship of war is so placed, this vital part is secure from injury by hostile projectiles, and the decks are clear for training guns and other operations. Thus placed, the screw has been proved to possess many advantages over paddle-wheels, so that at the present time it has largely superseded paddle-wheels in vessels of every class, except perhaps in those intended to ply on rivers and lakes. Many fine paddle-wheel vessels are still afloat, but sea-going steamers are nearly always now built with screw-propellers. In the application of the steam engine to navigation the machine has received many modifications in the form and arrangement of the parts, but in principle the marine engine is identical with the condensing engine already described. The engines in steam-ships are often remarkable for the great diameter given to the cylinders, which may be 8 ft. or 9 ft. or more. Of course other parts of the machinery are of corresponding dimensions. Such large cylinders re- quire the exercise of great skill in their construction, for they must be cast in one piece and without flaws. The engraving, Fig. 59, depicts the scene presented at the works of Messrs. Penn during the casting of one of these large cylinders, the weight of which may amount to perhaps 30 tons. Only the top of the mould is visible, and the molten iron is being poured in from huge ladles, moved by powerful cranes. In paddle vessels the great wrought iron shaft which carries the paddle-wheels crosses the vessel from side to side. This shaft has two cranks, placed at right angles to each, and each one is turned by an engine, which is very commonly of the kind known as the side-lever engine. In this engine, instead of a beam being placed above flie cylinder, two beams are used, one being set on each side of the cylinder, as low down as possible. The top of the piston-rod is attached to a cross- head, from each end of which hangs a great rod, which is hinged to the end of the side-beam. The other ends of the two beams are united by a cross-bar, to which is attached the connecting-rod that gives motion to the crank. Another favourite form of engine for steam-ships is that with oscil- lating cylinders. The paddle-wheels are constructed with an iron frame- work, to which flat boards, or floats, are attached, placed usually in a radial direction. But when thus fixed, each float enters the water obliquely, and in fact its surface is perpendicular to the direction of the vessel's course only at the instant the float is vertically under the axis of the wheel. In order to avoid the loss of power consequent upon this oblique movement of the floats, they are sometimes hung upon centres, and are so moved by suitable STEAM NA VIGA TION. FIG. 59. Casting Cylinder of a Marine Steam Engine. mechanism that they are always in a nearly vertical position when passing through the water. Paddle-wheels constructed in this manner are termed feathering wheels. They do not appear, however, to possess any great advantage over those of the ordinary construction, except when the paddles are deeply immersed in the water, and this result may be better understood when we reflect that the actual path of the floats through the water is not circular, as it would be if the vessel itself did not move ; for all points of the wheel describe peculiar curves called cycloids, which result from the combination of the circular with the onward movement. The next figure, 60, exhibits a very common form of the screw propeller, and shows the position which it occupies in the ship. The reader may not at once understand how a comparatively small two-armed wheel revolving in a plane perpendicular to the direction of the vessel's motion is able to propel the vessel forward. In order to understand the action of the propeller, he should recall to mind the manner in which a screw-nail in a piece of wood advances by a distance equal to its pitch at every turn. If he will conceive a gigantic screw-nail to be attached to the vessel extending along the keel, and suppose for a moment that the water surrounding this screw is not able to flow away from it, but that the screw works through the water as the nail does in the wood, he will have no difficulty in understanding that, under such circumstances, if the screw were made to revolve, it would ad- vance and carry the vessel with it. The reader may now form an accurate notion of the actual propeller by supposing the imaginary screw-nail to have the thread so deeply cut that but little solid core is left in the centre, 86 STEAM NA VI G A TION. and supposing also that only a very short piece of the screw is used say the length of one revolution and that this is placed in the dead-wood. Such was the construction of the earlier screw-propellers, but now a still shorter portion of the screw is used ; for instead of a complete turn of the thread, less than one-sixth is now the common construction. Such a strip or segment of the screw-thread forms a blade, and two, three, four, or more blades are attached radially to one common axis. The blades spring when there are two from opposite points in the axis, and in other cases from points on the same circle. The blades of the propeller are cut and carved into every variety of shape according to the ideas of the designer, but the fundamen- tal principle is the same in all the forms. It need hardly be said that the particles of the water are by no means fixed like those of the wood in which a screw advances. But as the water is not put in motion by the screw without offering some resistance by reason of its inertia, this re- sistance reacting on the screw operates in the same manner, but not to the same extent, as the wood in the other case. When we know the pitch of the screw, we can calculate what distance the screw would be moved for- ward in a given number of revolutions if it were working through a solid. This distance is usually greater than the actual distance the ship is pro- pelled, but in some cases the vessel is urged through the water with a greater velocity than if the screw were working in a solid nut. The shaft which carries the screw extends from the stem to the centre of the ship where the engines are placed, and it passes outward through a bearing lined with wood, of which lignum vita is found to be the best kind, the lubricant for this bearing being not oil but water. The screw would not have met with the success it has attained but for this simple contrivance ; for it was found that with brass bearings a violent thumping action was soon produced by the rapid rotation of the screw. The wearing action between the wood and the iron is very slight, whereas brass bearings in this position quickly wear and their adjustments become impaired. The screw-shaft is very massive and is made in several lengths, which are supported in appro- priate bearings ; there is also a special arrangement for receiving the thrust of the shaft, for it is by this thrust received from the screw that the vessel is propelled, and the strain must be distributed to some strong part of the ship's frame. There is usually also an arrangement by which the screw- shaft can, when required, be disconnected from the engine, in order to allow the screw to turn freely by the action of the water when the vessel is under sail alone. A screw-propeller has one important advantage over paddle-wheels in the following particular : whereas the paddle-wheels act with the best effect when the wheel is immersed in the water to the depth of the lowest FIG. 60. Screw- Propeller. STEAM NA VIGA TION. 87 float, the efficiency of the screw when properly placed is not practically altered by the depth of immersion. As the coals with which a steamer starts for a long voyage are consumed, the immersion is decreased hence the paddle-wheels of such a steamer can never be immersed to the proper extent throughout the voyage ; they will be acting at a disadvantage during the greater part of the voyage. Again, even when the immersion of the vessel is such as to give the best advantage to the paddle-wheels, that ad- vantage is lost whenever a side-wind inclines the ship to one side, or when- ever by the action of the waves the immersion of the paddles is changed by excess or defect. From all such causes of inefficiency arising from the position of the vessel the screw-propeller is free. The reader will now understand why paddle-wheel steamers are at the present day constructed for inland waters only. A great impulse was given to steam navigation, by the substitution of iron for wood in the construction of ships. The weight of an iron ship is only two-thirds that of a wooden ship of the same size. It must be remembered that, though iron is many times heavier than wood, bulk for bulk, the re- quired strength is obtained by a much less quantity of the former. A young reader might, perhaps, think that a wooden ship must float better than an iron one ; but the law of floating bodies is, that the part of the floating body which is below the level of the water, takes up the space of exactly so much water as would have the same weight as the floating body, or in fewer words, a floating body displaces its own weight of water. Thus we see that an iron ship, being lighter than a wooden one, must have more buoyancy. The use of iron in ship-building was strenuously advocated by the late SirW. Fairbairn, and his practical knowledge of the material gave .great authority to his opinion. He pointed out that the strains to which ships are exposed are of such a nature, that vessels should be made on much the same prin- ciples as the built-up iron beams or girders of railway bridges. How successfully these principles have been applied will be noticed in the case of the Great Eastern. This ship, by far the largest vessel ever built, was designed by Mr. Brunei, and was intended to carry mails and passengers to India by the long sea route. The expectations of the promoters were disappointed in regard to the speed of the vessel, which did not exceed 1 5 miles an hour ; and no sooner had she gone to sea than she met with a series of accidents, which appear, for a time, to have destroyed public con- fidence in the vessel as a sea-going passenger ship. Some damage and much consternation were produced on board by the explosion of a steam jacket a few days after the launch. Then the huge ship encountered a strong gale in Holyhead Harbour, and afterwards was disabled by a hurri- cane in the Atlantic, in which her rudder and paddles were so damaged, that she rolled about for several days at the mercy of the waves. At New York she ran upon a rock, and the outer iron plates were stripped off the bottom of the ship for a length of 80 ft. She was repaired and came home safely ; but the companies which owned her found themselves in financial difficulties, and the big ship, which had cost half a million sterling, was sold for only ^25,000, or only about one-third of her value as old materials. The misfortunes of the Great Eastern, and its failure as a commercial speculation in the hands of its first proprietors, has been quoted as an illustration of the ill luck, if it might be so called, which seems to have attended several of the great works designed by the Bmnels for the Thames Tunnel was, commercially, a failure ; the Great Western Railway, with its magnificent embankments, cuttings, and tunnels, is reverting to 88 STEAM NA VIGATION. the narrow gauge, and therefore the extra expenses of the large scale has been financially thrown away ; the Box Tunnel, a more timid engineer would have avoided ; and then there is the Great Eastern. It is, however, equally remarkable that all these have been glorious and successful achieve- ments as engineering works, and the scientific merit of their designers remains unimpaired by the merely accidental circumstance of their not bringing large dividends to their shareholders. Nor is their value to the world diminished by this circumstance, for the Brunels showed mankind the way to accomplish designs which, perhaps, less gifted engineers woulw never have had the boldness to propose. The Box Tunnel led the way to other longer and longer tunnels, culminating in that of Mont Cenis ; but for the Thames Tunnel once ranked as the eighth wonder of the world we should probably not have heard of the English Channel Tunnel a scheme which appears less audacious now than the other did then ; if no FlG. 61. Section of Great Eastern Amidships. Great Eastern had existed, we should not now have had an Atlantic Tele- graph. Possibly this huge ship is but the precursor of other still larger, and it is undoubtedly true that since its construction the ideas of naval architects have been greatly enlarged, and the tendency is towards increased size and speed in our steam-ships, whether for peace or war. The accidents which had happened to the ship had not, however, materi- ally damaged either the hull or the machinery ; and the Great Eastern was refitted, and afterwards employed in a service for which she had not been designed, but which no other vessel could have attempted. This was the work of carrying and laying the whole length of the Atlantic Telegraph Cable of 1865, of which 2,600 miles were shipped on board in enormous tanks, that with the contents weighed upwards of 5,000 tons. The ship has since been constantly engaged in similar operations. The Great Eastern is six times the size of our largest line-of-battle ships, and about seven times as large as the splendid steamers of the Cunard line, which run between Liverpool and New York. She has three times the steam power of the largest of these Atlantic steamers, and could carry twenty times as STEAM NA VIGATION. 89 FIG. 62. The Great Eastern in course of Construction. many passengers, with coal for forty days' consumption instead of fifteen. Her length is 692 ft. ; width, 83 ft. ; depth, 60 ft. ; tonnage, 24,000 tons ; draught of water when unloaded, 20 ft. ; when loaded, 30 ft. ; and a pro- menade round her decks would be a walk of more than a quarter of a mile. The vessel is built on the cellular plan to 3 ft. above the water-line ; that is, there is an inner and an outer hull, each of iron plates f in. thick, placed 2 ft. 10 in. apart, with ribs every 6 ft., and united by transverse plates, so that in place of the ribs of wooden ships, the hull is, as it were, built up of curved cellular beams of wrought iron. The ship is divided longitudinally by two vertical partitions or bulkheads of wrought iron, \ in. thick. These are 350 ft. long and 60 ft. high, and are crossed at intervals by transverse bulk- heads, in such a manner that the ship is divided into nineteen compart- ments, of which twelve are completely water-tight, and the rest nearly so. The diagram (Fig. 61) represents a transverse section, and shows the cellular construction below the water-line. The strength and safety of the vessel are thus amply provided for. The latter quality was proved in the accident to the ship at New York ; and the former was shown at the launch, for when the vessel stuck, and for two months could not be moved, it was found that, although one-quarter of the ship's length was unsupported, it exhibited no deflection, or rather the amount of deflection was imperceptible. Fig. 62 is from a photograph taken during the building of the ship, and Fig. 63 shows the hull when completed and nearly ready for launching, while the vignette at the head of the chapter exhibits the big ship at anchor when completely equipped. The paddle-wheels are 56 ft in diameter, and are 9 STEAM NA VIGAT10N. FIG. 63. The Great Eastern ready for Launching. turned by four steam engines, each having a cylinder 6 ft. 2 in. in diameter, and 14 ft. in length. The vessel is also provided with a four-bladed screw- propeller of -24 ft. diameter, driven by another engine having four cylinders, six boilers, and seventy-two furnaces. The total actual power of the engines is more than that of 8,000 horses, and the vessel could carry coals enough to take her round the world a capability which was the object of her enormous size. The vessel as originally constructed contained accommo- dation for 800 first-class passengers, 2,000 second class, and 1,200 third class that is, for 4,000 passengers in all. The principal saloon was 100 ft. long, 36 ft. wide, and 13 ft. high. Each of her ten boilers weighs 50 tons, and when all are in action, 1 2 tons of coal are burnt every hour, and the total displacement of the vessel laden with coal is more than 20,000 tons. The use of steam power in navigation has increased at an amazing rate. Between 1850 and 1860 the tonnage of the steam shipping entering the port of London increased threefold, and every reader knows that there many fleets of fine steamers plying to ports of the United Kingdom. There are, for example, the splendid Atlantic steamers, some of which almost daily enter or leave Liverpool, and the well-appointed ships belong- ing to the Peninsular and Oriental Company. The steamers on the Holy- head and Kingston line may be taken as good examples of first-class pas- senger ships. These are paddle-wheel boats, and are constructed entirely of iron, with the exception of the deck and cabin fittings. Taking one of these as a type of the rest, we may note the following particulars : the vessel is 334 ft. long, the diameter of the paddle-wheels is 31 ft, and each has fourteen floats, which are 12 ft. long and 4 ft. 4 in. wide. The cylinders of the engines are 8 ft 2 in. in diameter, and 6 ft 6 in. long. The ship cost STEAM NA VIGA TION. about ,75,000. The average passage between the two ports a distance of 65! miles occupies 3 hours 52 minutes, and at the measured mile the vessel attained the speed of 2O'8i i miles per hour. As an example of the magnificent vessels owned by the Cunard Company, we shall give now a few figures relating to one of their largest steam-ships, the Persia, launched in 1858, and built by Mr. N. Napier, of Glasgow, for the company, to carry mails and passengers between Liverpool and New York. Her length is 389 ft., and her breadth 45 ft. She is a paddle-wheel steamer, with engines of 850 horse-power, having cylinders 100 in. in diameter with a stroke of i oft. The paddle-wheels are 38 ft. 6 in. in diameter, and each has twenty- eight floats, 10 ft. 8 in. long and 2 ft. wide. The Persia carries 1,200 tons of coal, and displaces about 5,400 tons of water. 1858 FIG. 64. Comparative Sizes of Steamships. 1838, Great Western; 1844, Great Britain ; 1856, Persia; 1858, Great Eastern. A, Section amidships of Great Eastern ; B, The same of Great Western. Both on the same scale, but on a larger one than their profiles. A velocity of twenty-one miles per hour appears to be about the highest i ever attained by a steamer. This is probably near the limit beyond whicriV the speed cannot be increased to any useful purpose. The resistance offered ' by water to a vessel moving through it increases more rapidly than the velocity. Thus, if a vessel were made to move through the water by being pulled with a rope, there would be a certain strain upon the rope when the vessel was dragged, say, at the rate of five miles an hour. If we desired the vessel to move at double the speed, the strain on the rope must be increased fourfold. To increase the velocity to fifteen miles per hour, we should have to pull the vessel with nine times the original force. This is expressed by saying that the resistance varies as the square of the velocity. Hence, to double the speed, the impelling force must be quadrupled, and as that force is exerted through twice the distance in the same time, an engine would be required of eight times the power or, in other words, the power STEAM NA VIGA TION. of the engine must be increased in proportion to the cube of the velocity ; so that to propel a boat at the rate of 1 5 miles an hour would require engines twenty-seven times more powerful than those which would suffice to propel it at the rate of five miles an hour. The actual speed attained by steam-ships with engines of a given power and a given section amidships will depend greatly upon the shape of the vessel. When the bow is sharp, the water displaced is more gradually and slowly moved aside, and therefore does not offer nearly so much resistance as in the opposite case ; but the greater part of the power required to urge the vessel forward is employed in overcoming a resistance which in some degree resembles friction between the bottom of the vessel and the water. The wonderful progress which has, in a comparatively short time, taken place in the power and size of steam-vessels, cannot be better brought home to the reader than by a glance at Fig. 64, which gives the profiles of four steamships, drawn on one and the same scale, thus showing the re- lative lengths and depths of those vessels, each of which was the largest ship afloat at the date which is marked below it, and the whole period in- cludes only the brief space of twenty years ! for this, surely, is a brief space in the history of such an art as navigation. All these ships have been named in the course of this article, but in the following table a few particulars concerning each are brought together for the sake of comparing the figures : Date. Name. Propulsion. Length. Breadth. 1838 Great \Vestern ... Paddles 236 ft. 36ft. 1$M4 Great Britain . . . 322 CI i8t;6 Persia . . . . Paddles ^QO 4.C .. 1858 Great Eastern ... Screw and paddles 690 TO 83 FIG. 65. The Great Eastern at Ni%ht. STEAM NA VIGATION. 93 FlG. 66. Bessemer Steamer. THE BESSEMER CHANNEL STEAMER. T^HE latest, most novel, and most ingenious invention connected with -* steam navigation is certainly the steamer which Mr. Bessemer has built at Mull. This invention will entirely abolish all the unpleasant sensa- tions which landsmen are apt to experience in a sea voyage. The problem of effectually removing the cause of the distressing malde mer now appears likely to be successfully solved. Mr. Bessemer's ship is built for plyi between the shores of France and England, and the method in which he purposes to carry passengers over the restless sea which separates us from our Gallic neighbours is bold and ingenious in the highest degree. He will carry the passengers in a spacious saloon, which, instead of partaking of the rolling and tossing of the ship, will be maintained in an absolutely level position. The saloon will be suspended on pivots, much in the same way as a mariner's compass is suspended ; and, by an application of hydraulic power, forces will be applied to counteract the motion of the ship and maintain the swinging saloon perfectly horizontal. It was origin- ally intended that the movements should be regulated by a man stationed for that purpose, where he may work the levers that bring the machinery into action, so as to preserve the saloon in the required position. This plan has, however, been improved upon, and the adjustments will be auto- matic. It may be well to mention that it is a mistake to suppose that any- thing freely suspended, like a pendulum, on board a ship rolling with the waves, will hang vertically. If, however, we cause a heavy disc to spin very rapidly, say in a horizontal plane, the disc cannot be moved out of the 94 STEAM NA VIGA TION. horizontal plane without the application of some force. A very well-made disc may be made to rotate for hours, and would, by preserving its original plane of rotation, even show the effect of the earth's diurnal motion. Mr. Bessemer makes use of such a gyroscope to move the valves of his hydraulic apparatus, and so to keep his swinging saloon as persistently horizontal as the gyroscope itself. The following interesting details regarding Mr. y/Bessemer's latest invention are extracted from the " Daily Telegraph " of /* the 23rd September, 1874: " At the water-line Mr. Bessemer's ship is 350 ft. long, and each end, for a distance of 48 ft., will be only about 4 ft. from the line of floating. In rough weather the water will wash over the ends, which have been rounded, so as to throw it off as easily as possible. These cigar-shaped extremities are fitted with capstans, worked by Messrs. Brown's (Edinburgh) hydraulic apparatus, and the vessel is provided with two of Martin's patent anchors. When at sea there will seldom be any necessity for any one to go upon the ends, but they will be used chiefly when the vessel is entering a harbour or port. Above the low ends a breastwork is raised, about 8 ft. high, 254 ft. long, and extending the entire length of the vessel. In the centre, and occupying the space of 90 ft., is the swinging saloon, which is intended for first-class passengers. At either end of this apartment are the engines and boilers; on the breastwork deck are deck-houses, for private families or parties, smoke-rooms, refreshment bars, &c. Although both extremities of the vessel have the same appearance, each being fitted with a rudder sheeted by Messrs. Brown's hydraulic machinery, they may for convenience be named the fore and after ends. As the latter will be the part first to take the water at the launching, temporary bulwarks are being erected, so that as little wash as possible may be caused. At the after-part of the vessel, and entirely independent of the swinging saloon, is the accommo- dation for second-class passengers. On the first floor is a spacious cabin for ladies, and two other cabins, which can be used by families or other persons requiring privacy. On the next floor is the second-class saloon, 5 1 ft. in length. The entrances to the engine-rooms are of a convenient character, and there is every probability that in fine weather this part of the vessel will be frequently resorted to by passengers. Running in all direc- tions in the engine-room are huge pipes, conveying steam to different engines, and water to the numerous pumps situated in various parts of the vessel. " The engines are oscillating and expansive, working up to 4,600 horse- power, which, should it be required, can be increased to as much as 5,000; but it is expected that the former will be quite sufficient to drive the ship at the required speed of twenty miles per hour. There are two pairs of engines, one set at either end of the ship, and each having two cylinders of 80 in. in diameter, and a stroke of 5 ft, working with steam of 30 Ibs. pressure per square inch, supplied from four box-shaped boilers, each boiler having four large furnaces. The paddle-wheels, of which there are a pair on either side of the vessel, are 27 ft. 10 in. in diameter outside the outer ring, and each wheel has twelve feathering floats. It is expected the leading pair of wheels, when working at full speed, will make thirty-two revolutions per minute. The following pair of wheels will of course move faster, as they will receive some of the wash from the leading ones. Every care has been taken to so secure the engines in their places that when developing their enormous power they shall not strain or injure the ship. For starting, stopping, and reversing the engines, Brown's hydraulic starting-gear has STEAM NA VIGA TION. 95 been adopted. All orders to the engineers will be conveyed from the bridges to the engine-room by telegraph. The whole of the levers for working an engine are brought within a space of three feet, so that the engineer, with- out moving from the spot, can reach any one of them. Great care has been taken to make the gear as simple as possible, so that one engineer only will be required to actually work the engines, and either can be started and reversed in a few seconds. The levers, in fact, are so easily worked that a child could set the engines in motion or stop them. There will be no necessity for the stokers ever to appear on deck. Those engaged in stoking for the after engine can walk along a passage which brings them to the fore engine, near which their bunks are situated. This passage will also enable the engineers to get at once from one engine-room to the other, in case anything should be the matter with the machinery. " Entrance to the Bessemer saloon is gained by two broad staircases lead- ing to one landing, and a flexible passage from this point to the saloon will be laid. The saloon rests on four steel gudgeons, one at each end, and two close together near the middle. These are not only employed to support the saloon, but are also utilized for conveying the water to the hydraulic engines, by which the saloon is kept steady. For this purpose the after one has been made hollow, and is connected with the water mains from two pairs of powerful engines made by Messrs. Galloway and Co., Man- chester, and also with a supply-pipe leading to a central valve-box, by means of which the two hydraulic cylinders on either side are supplied with water. Between the two middle gudgeons is placed a gyroscope, worked by a small turbine, filled with water from one of the gudgeons. The introduction of this gyroscope has enabled Mr. Bessemer to dispense with the services of a man, and has thus completed his scheme of a steady saloon, by making the machinery completely automatic ; but should at any time the gyroscope fail to act, it is so arranged that a man can at once take charge of the controlling machinery, and thus prevent the cabin becoming useless for its first object. The saloon is 70 ft. long, 35 ft. wide, and 20 ft. high. On the left-hand side, just within the apartment, is a spacious stair- case leading to the top, and thus enabling passengers to enjoy the sea breeze, and at the same time be free from feeling the motion of the ship, and this without having first to traverse any portion of the vessel proper. On the right-hand side of the engine will be a retiring-room, so that if, even in the saloon, any person should find the * sea air ' too much for him, he can with- draw from the public gaze. Ranged round the saloon will be a row of seats, and it is intended to ornament the sides in a very tasteful manner with panels. There will be carved oak shields bearing the monogram ' B. S. C.' (Bessemer Steamship Company), and above these will be handsome oil paintings. At the far end of the saloon is a retiring-room to be used by ladies, and over that another. It may be mentioned that the different names and uses of the various auxiliary cabins have not yet been definitely decided upon, and they will remain in abeyance until the requirements of the traffic have made themselves manifest " One great desideratum in a ship's saloon is good ventilation, and this has been attended to with the greatest care. Two fans or blowers are employed, and are worked by the small auxiliary engine. One fan forces fresh air into long tubes passing under the seats and throughout the whole length and breadth of the saloon, and the pipes are punctured with small holes, so as not to create a draught in any particular part. The air thus supplied will be kept at a convenient temperature by passing through a 96 STEAM NA VIGA TION. heating apparatus, resembling very much an ordinary surface condenser. Thus, in cold weather, by regulating the supply of steam to the apparatus, the temperature may be comfortably warmed, and, by always maintaining a supply of fresh air, it will never become in any degree vitiated. The heat is obtained from the exhausted steam from one of Galloway's engines. The other fan draws the foul air from the saloon, and discharges it overboard. The tubes conveying the air from the blowers are connected with those in the saloon by means of an intermediate pipe fitted with flexible joints. It is confidently expected that the arrangement of the saloon, together with the steadiness of the vessel, will entirely prevent sea-sickness. The recent addition to the original plan has been the placing of two light masts, one at either end : sails attached to these will tend to materially steady the steamer ; but another great consideration was to give the vessel some as- sistance in turning, which, owing to her great length, would probably have been a somewhat slow process, as she would have to be moved in a wide circle. Entrance-ports or gangways are situated at the outer side of the paddle-box, so as to insure a safe and easy landing-place. The steering apparatus will be under the control of the officer on the bridge, which is fitted between each pair of paddle-boxes. The kitchens are on the spon- sons at the fore end, and are capable of serving up a hot dinner. In the fore part is the accommodation for the crew, and a large space is appro- priated to the stowage of luggage. For the latter purpose a crane is being fitted on board, and such arrangements are made as to prevent the boxes and other property of the passengers from being tumbled about, as is some- times the case. The vessel will be provided with two large life-rafts, on the principle patented by Mr. Christie, and four other boats will be ranged along the side of the vessel."* THE CASTALIA. A NOTHER very remarkable ship has recently been constructed for ** carrying passengers across the English Channel without the unplea- sant rolling which is experienced in the ordinary steamboats. The vessel which has received the above name has been designed by Captain Dicey, who formerly held an official position at the Port of Calcutta. His Indian experience furnished him with the first suggestion of the new ship in the device which is adopted there for steadying boats in the heavy surf. The plan is to attach a log of timber to the ends of two outriggers, which pro- ject some distance from the side of the vessel- ; or sometimes two canoes, a certain distance apart, are connected together. Some of these Indian boats will ride steadily in a swell that will cause large steamers to roll heavily. Improving on this hint, Captain Dicey has built a vessel with two hulls, each of which acts as an outrigger to the other. Or, perhaps, the Castalia may be described as a flat-bottomed vessel, with the middle part of the bottom raised out of the water throughout the entire length, so that * Since the above account was written, the Bessemer has been tried at sea. Her speed has by no means realized the expectations of her designers. The automatic regulator will not be adopted at present ; and the swinging saloon has not yet (Aug. 1875) been brought into operation. STEAM NAVIGATION. 97 FIG. 67. The Castalia in Dover Harbour. the section amidships has a form like this . . . The two hulls are connected by what we may term " girders," which extend completely across their sec- tions, forming transverse partitions or bulkheads, and these girders are strongly framed together, so as to form rigid triangles. These unite the two hulls so completely, that there is not the least danger of the vessel being strained in a sea-way. The decks are also formed of iron although covered with wood, so that the whole vessel really forms a box girder of enormous section. The reason why the steamers which have hitherto run between Dover and Calais, Folkestone and Boulogne, and other Channel ports, are so small, is because the harbours on either side could not receive vessels with such a draught as the fine steamers, for example, which run on the Holy- head and Kingston line. Now, the Castalia draws only 6 ft. of water, or i ft. 6 in. less than the present small Channel steamers, and she will there- fore be able to enter the French ports at all states of the tide. Yet the extent of the deck space is equalled in few passenger ships afloat, except the Great Eastern and some of the Atlantic steamers. The vessel is 290 ft. in length, with an extreme breadth of 60 ft. The four spacious and ele- gantly-fitted saloons two of which are 60 ft. by 36 ft., and two 28 ft. by 26 ft., and the roomy cabins, retiring-rooms, and lavatories, offer the greatest possible contrast to the " cribbed, cabined, and confined " accom- modation of the ordinary Channel steamers. There are also a kitchen and all requisites for supplying dinners, luncheons, &c., on board. But besides the above-named saloons and cabins, there is a grand saloon, which is 9 8 STEAM NAVIGATION. FIG. 68. The Castalia in Dover Harbour End View. 1 60 ft. long and 60 ft. wide ; and the roof of this forms a magnificent pro- menade 14 ft. above the level of the sea. There is comfortable accommo- dation in the vessel for more than 1,000 passengers. The inner .sides of the hulls are not curved like the outside, but are straight. The space between them is 35 ft. wide, and the hulls are each 20 ft. in breadth, and somewhat more in depth. There are two paddle- wheels, placed abreast of each other in the water-way between the two hulls, and each of these contains boilers and powerful engines. The de- signers of this vessel calculated that she would attain a speed of I4f knots per hour, but this result has not been realized. Probably there were no data for the effect of paddles working in a confined water-space. The posi- tion of the paddles is otherwise an advantage, as it leaves the sides of the vessel free and unobstructed. The ship is of the same form at each end, so that it can move equally well in either direction. There are rudders at both ends, and the steering qualities of the ship are admirable. Although the speed of the Castalia is below that intended, the vessel is quite a success as regards steadiness, for the rolling and pitching have been vry greatly reduced, and the miseries and inconveniences of the Channel passage have, it is hoped, been at length obviated. The Castalia is represented in Figs. 67 and 68. She was constructed by the Thames Iron Shipbuilding Co., and launched in June, 1874, but after she had been tried at sea, it was found necessary to fit her with improved boilers, and this has caused a delay in placing the vessel on her station. FIG. (x)H.M.S. Devastation in Queenstown Harbour. SHIPS OF WAR. '"T * AKE it all in all, a ship of the line is the most honourable thing that man, as a gregarious animal, has ever produced. By himself, un- helped, he can do better things than ships of the line ; he can make poems, and pictures, and other such concentrations of what is best in him. But as a being living in flocks, and hammering out with alternate strokes and mutual agreement, what is necessary for him in those flocks to get or produce, the ship of the line is his first work. Into that he has put as much of his human patience, common sense, forethought, experimental philosophy, self- control, habits of order and obedience, thoroughly wrought hand-work, defiance of brute elements, careless courage, careful patriotism, and calm expectation of the judgment of God, as can well be put into a space of 300 ft. long by 80 ft. broad. And I am thankful to have lived in an age when I could see this thing so done." So wrote Mr. Ruskin nearly twenty years ago, referring, of course, to the old wooden line-of-battle ships. It may be doubted whether he would have written thus enthusiastically about so unpicturesque an object as the Glutton, just as it may be doubted whether the armour-plated steamers will attain the same celebrity in romance and in. TOO SHIPS OF WAR. verse as the old frigates with their " wooden walls." Certain it is that the patience, forethought, experimental philosophy, thoroughly wrought hand- work, careful patriotism, and other good qualities which Mr. Ruskin saw in the wooden frigates, are not the less displayed in the new ironclads. Floating batteries, plated with iron, were employed in the Crimean War at the instigation of the French Emperor. About the same time the ques- tion of protecting ships of war by some kind of defensive armour was forced upon the attention of maritime powers, by the great strides with which the improvements in artillery were advancing ; for the new guns could hurl projectiles capable of penetrating, with the greatest ease, any wooden ship afloat. The French Government took the initiative by con- structing La Gloire, a timber-framed ship, covered with an armour of rolled iron plates, 4| in. thick. The British Admiralty quickly followed with the Warrior, a frigate similar in shape to the wooden frigates, but built on an iron frame, with armour composed of plates 4! in. thick, backed by 18 in. of solid teak-wood, and provided with an inner skin of iron. The Warrior is 380 ft. long, but only 213 ft. of this length is armoured. The defensive armour carried by the Warrior, and the ironclads constructed immediately afterwards, was quite capable of resisting the impact of the 68 Ib. shot, which was at that time the heaviest projectile that could be thrown by naval guns. But to the increasing power of the new artillery it soon became necessary to oppose increased thickness of iron plates. The earlier iron- clads carried a considerable number of guns, which could, however, deliver only a broadside fire, that is, the shots could, for the most part, be sent only in a direction at right angles to the ship's length, or nearly so. But in the more recently built ironclads there are very few guns, which are, however, six times the weight of the old sixty-eight pounders, and are cap- able of hurling projectiles of enormous weight. The ships built after the Warrior were completely protected by iron plates, and the thickness of the plates has been increased from time to time, with a view of resisting the increased power which has been progressively gi^en to naval guns. A contest, not yet terminated, has been going on between the artillerist and the ship-builder ; the one endeavouring to make his guns capable of pene- trating with their shot the strongest defensive armour of the ships, the other adding inch after inch to the thickness of his plates, in order, if pos- sible, to render his ship invulnerable. One of the finest of the large ironclads is the Hercules, of which a section amidships is presented on the opposite page. This ship is 325 ft. in length, and 59 ft. in breadth, and is fitted with very powerful engines which will work up to 8,529 indicated horse-power. The tonnage is 5,226 ; weight of hull, 4,022 tons ; weight of the armour and its backing, 1,690 tons ; weight of engines, boilers, and coals, 1,826 tons; weight of equipment and arma- ment, 8,676 tons. Although the Hercules carries this enormous weight of armour and armament, her speed is very great, excelling, in fact, that of any merchant steamer afloat, for she can steam at the rate of nearly 17 miles an hour. She also possesses, in a remarkable degree, the property which naval men call handinessj that is, she can be quickly turned round in a comparatively small space. The handiness of a steamer is tested by causing her to steam at full speed with the helm hard over, when the vessel will describe a circle. The smaller the diameter of that circle, and the shorter the time required to complete it, the better will the vessel execute the movements required in naval tactics. Comparing the performances of the Warrior and the Hercules,, we find that the smallest circle the former SfffPS OF WAR. 101 FIG. 70. -Section ofH.M.S. Hercules, 102 SffJPS OF WAR. can describe is 1,050 yards in diameter, and requires nine minutes for its completion, whereas the latter can steam round a circle of only 560 yards diameter in four minutes. The section (Fig. 70) shows that, like the Great Eastern, the Hercules is constructed with a double hull, so that she would be safe, even in the event of such an accident as actually occurred to the Great Eastern, when a hole was made by the stripping off of her bottom plates, 80 ft. long and 5 ft. wide. The defensive armour of the Hercules is, it will be observed, greatly strengthened near the water-line, where damage to the ship's side would be most fatal. The outer iron plates are here 9 in. thick, while in other parts the thickness is 8 in., and in the less important positions 6 in. The whole of the hull is, however, completely protected above the water-line, and the iron plates are backed up by solid teak-wood for a thickness of from 10 in. to 12 in. The teak is placed between girders, which are attached to another iron plating \\ in. thick, supported by girders 2 ft. apart. The spaces between these girders are also filled with teak, and the whole is lined with an inner skin of iron plating, f in. thick. The belt along the water-line has thus altogether I \\ in. of iron, of which 9 in. are in one thickness, and this part is, moreover, backed by additional layers of teak, as shown in the section ; so that, besides the 1 1^ in. of iron, the ship's side has here 3 ft. 8 in. total thickness of solid teak-wood. The deck is also covered with iron plates, to protect the vessel from vertical fire. The Hercules carries eight 1 8-ton guns as her central battery, and two 1 2-ton guns in her bow and stern : these guns are rifled, and each of the larger ones is capable of throwing a shot weighing 400 Ibs. The guns can be trained so as to fire within 1 5 of the direction of the keel ; for near the ends of the central battery the ports are indented, and the guns are mounted on Scott's carriages, in such a manner that any gun-slide can be run on to a small turn-table, and shunted to another port, just as a railway-carriage is shunted from one line to another. Targets for artillery practice were built so as to represent the construction of the side of the Hercules, and it was found, as the result of many experiments, that the vessel could not be penetrated by the 600 Ib. shot from an Armstrong gun, fired at a distance of 700 yds. The production of such iron plates, and those of even greater thickness which have since been used, forms a striking example of the skill with which iron is worked. These plates are made by rolling, and it will be understood that the machinery used in their formation must be of the most powerful kind, when it is stated that plates from 9 in. to 1 5 in. thick are formed with a length of 16 ft. and a breadth of 4 ft. The plates are bent, while red hot, by enormous hydraulic pressure, applied to certain blocks, upon which the plates are laid, the block having a height adjusted according to the curve required. The operation requires great care, as it must be accomplished without straining the parts in a manner injurious to the strength of the plate. Fig. 7 1 on the next page is the section of another ship of war, the Incon- stant, which has not, like the Hercules, been designed to withstand the impact of heavy projectiles, but has been built mainly with a view to speed. The Inconstant has only a thin covering of iron plating, except in that portion of the side which is above water, where there is a certain thickness of iron diminishing from the water-line upwards, but not enough to entitle the Inconstant to be classed as an armoured vessel. This ship, however, may be a truly formidable antagonist, for she carries a considerable number of heavy guns, which her speed would enable her to use with great effect against an adversary incapable of manoeuvring so rapidly. She could give SHIPS OF WAR. 103 FIG. 71. Section of H.M.S. Inconstant. chase, or could run in and deliver her fire, escaping by her speed from hostile pursuit in cases where the slower movements of a ponderous iron- clad would be much less effective. The Inconstant carries ten 1 2-ton guns of 9 in. calibre, and six 6-ton 7 in. guns, all rifled muzzle-loaders, mounted on improved iron carriages, which give great facilities for handling them- IO4 SHIPS OF WAR. The ship is a frigate 338 ft. long and 50 ft. broad, with a depth in the hold of 17 ft. 6 in. She is divided by bulkheads into eleven water-tight com- partments. The engines are of 6,500 indicated horse-power, and the vessel attains an average speed of more than 1 8| miles per hour. A new system of mounting very heavy naval guns was proposed by FIG. 72. Section, Elevation, and Plan of Turret of H. M.S. Captain. Captain Coles about 1861. This plan consists in carrying one or two very heavy guns in a low circular tower or turret, which can be made to revolve horizontally by proper machinery. The turret itself is heavily armoured, so as to be proof against all sho.t, and is carried on the deck of the ship, which is so arranged that the guns in the turret can be fired at small angles with the keel. The British Admiralty having approved of Captain SHIPS OF WAR. io6 SHIPS OF WAR. Coles' plans, two first-class vessels were ordered to be built on the turret system. These were the Monarch and the Captain the latter of which we select for description on account of the melancholy interest which at- taches to her. On page 105 a diagram is given representing the profile of the Captain, in which some of the peculiarities of the ship are indicated the turrets with the muzzles of two guns projecting from each being easily recognized. The Captain was 320 ft. long and 53 ft. wide. She was covered with armour plates down to 5 ft. below the water-line, as repre- sented by the dark shading in the diagram. The outer plating was 8 in. thick opposite the turrets, and 7 in. thick in other parts. It was backed up by 12 in. of teak ; there were two inner skins of iron each f in. thick, then a framework with longitudinal girders 10 in. deep. The deck was plated in the spaces opposite the turrets with iron i^ in. thick. The Captain was fitted with twin screws that is, instead of having a single screw, one was placed on each side, their shafts being, of course, parallel with the vessel's length. The object of having two screws was not greater power, for it is probable that a single screw would be more effectual in propelling the ship ; but this arrangement was adopted because it was considered that, had only one screw been fixed, the ship might easily be disabled by the breaking of a blade or shaft ; whereas in the case of such an accident to one of the twin screws, the other would still be available. The twin screws could also be used for steering, and the vessel could be controlled without the rudder, as the engines were quite independent of each other, each screw having a separate pair. The diameter of the screws was 17 ft. The erections which are shown on the deck between the turrets afforded spacious quarters for the officers and men. These structures were about half the width of the deck, and tapered off to a point towards the turrets, so as leave an unimpeded space for training the guns, which could be fired at so small an angle as 6 with the length of the vessel. Above these erections, and quite over the turrets, was another deck, 26 ft. wide, called the "hurricane deck." The ship was fully rigged and carried a large spread of canvas. But the special features are the revolving turrets, and one of these is represented in detail in Fig. 72, which gives a section, part elevation, and plan. Of the construction of the turret, and of the mode in which it was made to revolve, these drawings convey an idea sufficiently clear to obviate the necessity of a minute description. Each turret had an outside diameter of 27 ft, but inside the diameter was only 22 ft. 6 in., the walls being, therefore, 2 ft. 3 in. thick nearly half this thickness consisting of iron plating. Separate engines were provided for turning the turrets, and they could also be turned by men working at the handles shown in the figures. Each turret carried two 25-ton Armstrong guns, capable of receiving a charge of 70 Ibs. of gunpowder, and of throwing a 600 Ib. shot. After some preliminary trials the Captain was sent to sea, and behaved so well, that Captain Coles and Messrs. Laird, her designer and contrac- tors, were perfectly satisfied with her qualities as a sea-going ship. She was then sent in the autumn of 1870 on a cruise with the fleet, and all went well until a little after midnight between the 6th and 7th September. 1870, when she suddenly foundered at sea off Cape Finisterre. The news of this disaster created a profound sensation throughout Great Britain, for, with the exception of nineteen persons, the whole crew of five hundred persons went down with the ship. Captain Coles, the inventor of the turrets, was in the ill-fated vessel and perished with the rest, as did also Captain Burgoyne, the gallant commander, and the many other distinguished naval SHIPS OF WAR. 107 officers who had been appointed to the ship ; among the rest was a son of Mr. Childers, then First Lord of the Admiralty. Although the night on which this unfortunate ship went down was squally, with rain, and a heavy sea running, the case was not that of an ordinary shipwreck in which a vessel is overwhelmed by a raging storm. It might be said, indeed, of the loss of the Captain as of that of the Royal George : "It was not in the battle ; No tempe>t gave the shock ; She sprang no fatal leak ; She ran upon no rock." One of the survivors, Mr. James May, a gunner, related that, shortly after midnight he was roused from his sleep by a noise, and feeling the ship uneasy, he dressed, took a light, and went into the after turret, to see if the guns were all right. He found everything secure in the turret, but that moment he felt the ship heel steadily over, and a heavy sea having struck her on the weather side, the water flowed into the turret, and he got out through the hole in the top of the turret by which the guns were pointed, only to find himself in the water. He swam to the steam-pinnace, which he saw floating bottom upwards, and there he was joined by Captain Burgoyne and a few others. He saw the ship turn bottom up, and sink stern first, the whole time from her turning over to sinking not being more than a few minutes. Seeing the launch drifting within a few yards, he called out, " Jump, men ! it is your last chance." He jumped, and with three others reached a launch, in which were fifteen persons, all belonging to the watch on deck, who had found means of getting into this boat. One of these had got a footing on the hull of the ship as she was turning over, and he actually walked over the bottom of the vessel, but was washed off by a wave and rescued by those who in the meantime had got into the launch. It appears that Captain Burgoyne either remained on the pinnace or failed to reach the launch. Those who were in that boat, finding the captain had not reached them, made an effort to turn their boat back to pick him up, but the boat was nearly swamped by the heavy seas, and they were obliged to let her drift. One man was at this time washed out of the boat and lost, after having but the moment before exclaimed, " Now, lads, I think we are all right." After twelve hours' hard rowing, without food or water, the survivors, numbering sixteen men and petty officers and three boys, reached Cape Finisterre, where they received help and attention. On their arrival in England, a court-martial was, according to the rules of the service, formally held on the survivors, but in reality it was occupied in investigating the cause of the catastrophe. The reader may probably be able to understand what the cause was by giving his attention to some general considerations, which apply to all ships whatever, and by a careful examination of the diagrams, Figs. 74 and 75, which are copied from dia- grams that were placed in the hands of the members of the court-martial. The letters B and G and the arrows are, however, added, to serve in illus- tration of a part of the explanation. The vessel is represented as heeled over in smooth water, and the gradations on the semicircle in Fig. 74 will enable the reader to understand how the heel is measured by angles. If the ship were upright, the centre line would coincide with the upright line, marked o on the semicircle, and drawn from its centre. Suppose a level line drawn through the centre of the semicircle, and let the circumference between the point where the last line cuts it and the point o be divided into ninety equal parts, and let these parts be numbered, and straight lines io8 SNIPS OF WAR. drawn from the centre to each point of division. In the figure the lines are drawn at every fifth division, and the centre line of the ship coincides with that drawn through the forty-fifth division. In this case the vessel is said to be inclined, or heeled, at an angle of forty-five degrees, which is usually written 45. In a position half-way between this and the upright the angle of heel would be 22^, and so on. The reader no doubt perceives that a ship, like any other body, must be supported, and he is probably aware that the support is afforded by the upward pressure of the water. He may also be familiar with the fact that the weight of every body acts upon it as if the whole weight were concentrated at one certain point, and that this point is called the centre of gravity of the body. Whatever may be the position of the body itself, its centre of gravity remains always at FIG. 74. the same point with reference to the body. When the centre of gravity happens to be within the solid substance of a body, there is no difficulty in thinking of the force of gravitation acting as a downward pull applied at .the centre of gravity. But this point is by no means always within the substance of bodies : as often as not it is in the air outside of the body. Thus the centre of gravity of a uniform ring or hoop is in the centre, where, of course, it has no material connection with the hoop ; Lut in whatever position the hoop may be placed, the earth's attraction pulls it as z/"this cen- tral point were rigidly connected with the hoop, and a string were attached to the point and constantly pulled downwards. This explanation of the meaning of centre of gravity may not be altogether superfluous, for, when rJie causes of the loss of the Captain were discussed in the newspapers, it became evident that such terms as " centre of gravity " convey to the minds of many but very vague notions. One writer in a newspaper enjoying a SHIPS OF WAR. 109 large circulation seriously attributed the disaster to the circumstance of the ship having lost her centre of gravity ! The upward pressure of water which supports a ship is the same upward pressure which supported the water before the ship was there that is, supported the mass of water which the ship displaces, and which was in size and shape the exact counterpart of the immersed part of the ship. Now, this mass of water, considered as a whole, had itself a centre of gravity through which its weight acted down- wards, and through which it is obvious that an equal upward pressure also acted. This centre of gravity of the displaced water is usually termed the " centre of buoyancy," and, unlike the centre of gravity, it changes its posi- tion with regard to the ship when the latter is inclined, because then the immersed part becomes of a shape different for each inclination of the ship. Now, recalling for an instant the fundamental law of floating bodies namely, that the weight of the water displaced is equal to the weight of the FIG. 75- floating body we perceive that in the case of a ship there are two equal forces acting vertically, viz., the weight of the ship or downward pull of gravitation acting at G, Fig. 74, the centre of gravity of the shipj and an equal upward push acting through B, the centre of buoyancy. It is obvious that the action of these forces concur to turn a ship placed as in Fig. 74 into the upright position. It is by no means necessary for this effect that the centre of gravity should be below the centre of buoyancy. All that is requisite for the stability of a ship is, that when the ship is placed out of the upright position, these forces should act to bring her back, which condition is secured so long as the centre of buoyancy is nearer to the side towards which the vessel is inclined than the centre of gravity is. When there is no other force acting on a ship or other floating body, these two points are always in the same vertical line. The two equal forces thus applied in parallel directions constitute what is called in mechanics a "couple," and the effect of this in turning the ship back into the upright position is the same as if a force equal to its weight were applied at the end of a lever equal in length to the horizontal distance between the lines through B and no SHIPS OF WAR. G. The righting force, then, increases in proportion to the horizontal dis- tance between the two points, and it is measured by multiplying the weight of the ship in tons by the number of feet between the verticals through G and B, the product being expressed in statical foot-tons, and representing the weight in tons which would have to be applied to the end of a lever i ft. long, in order to produce the same turning effect. When a ship is kept steadily heeled over by a side wind, the presence of the wind and the re- sistance of the water through which the vessel moves constitutes another couple exactly balancing the righting couple. The moment of the righting couple, or the righting force, or statical stability as it is also called, are determined by calculation and experiment from the design of the ship, and from her behaviour when a known weight is placed in her at a known dis- tance from the centre. Such calculations and experiments were made in the case of the Captain, but do not appear to have been conducted with sufficient care and completeness to exhibit her deficiency in stability. After the loss of the ship, however, elaborate computations on these points were made from the plans and other data. The following table gives some of the results, with the corresponding particulars concerning the Monarch for the sake of comparison : I. Angle at which the edge of the deck is immersed II. Statical righting force in foot-tons at the angle at which the deck is immersed III. Angle of greatest stability IV. Greatest righting force in foot-tons V. Angle at which the righting force ceases VI. Reserve of dynamical stability at an angle of 14 in dynamical foot-tons Monarch. \ Captain. 28' 12,542 ^9 15 6,500 54 410 From No. V. in the above table we learn that if the Captain had been heeled to 54, the centre of gravity would have overtaken the centre of buoyancy that is, the two would have been in one vertical line. Any further heeling would have brought the points into the position shown in Fig. 75, where it is obvious that the action of the forces is now to turn the vessel still more on its side, and the result is an upsetting couple instead of a righting couple. These figures and considerations refer to the case of the vessel floating in smooth water, but the case of a vessel floating on a wave is not different in principle. The reader may picture to himself the diagrams inclined so that the water-line may represent a portion of the wave's surface ; then he must remember that the very action which heaves up the water in a slop- ing surface is so compounded with gravity, that the forces acting through G and B retain nearly the same position relatively to the surface as before. No. VI. in the foregoing table requires some explanation. To heel a ship over to a certain angle a certain amount of work must be done, and in the scientific sense work is done only when something is moved through a space against a resistance. When the weight of a ton is raised I ft. high, one foot ton of work is said to be done; if 2 tons were raised i ft., or I tori were raised 2 ft., then two foot-tons of work would be done, and so on. The SHIPS OF WAR. in same would be the case if a pressure equal to those weights were applied so as to move a thing in any direction through the same distances. It should be carefully noticed that the foot-ton is quite a different unit in this case from what it is as the moment of a couple. If we heel a ship over by applying a pressure on the masts, it is plain that the pressure must act through a certain space, and the same heel could be caused either by means of a smaller pressure or a greater, according as we apply it higher up j>r lower down ; but the space through which it must act would vary, so that the product of the pressure and space would, however, be always the same. No. VI. shows the amount of work that would have to be done in order completely to upset each of the vessels when already steadily heeled over to 14. The amounts in the two cases are so different that we can easily understand how a squall which would not endanger the Monarch might throw the Captain over. A squall suddenly springing up would do more than heel a vessel over to the angle at which it is able to maintain it : it would swing it beyond that position by reason of the work done on the sails as they are moving over with the vessel, and the latter would come to a steady angle of heel only after a series of oscillations. Squalls, again, which, although suddenly springing up in this manner, could not heel the ship over beyond the angle where the stability vanishes, might yet do so if they were intermittent and should happen to coincide in time with the oscillations of the ship just as a series of very small impulses, coinciding with the time of the vibrations of a heavy pendulum, may accumulate so as to increase the range of vibration to any extent. It is believed that in the case of the Captain the pressure of the wind on the under-side of the hur- ricane assisted in upsetting the vessel. This, however, could only have exerted a very small effect compared to that produced by the sails. The instability of the Captain does not appear to have been discovered by such calculations as were made before the vessel went to sea. It was observed, however, that the ship when afloat was I ft. 6 in. deeper in the water than she should have been in other words, the freeboard, or side of the ship out of the water, instead of being 8 ft. high as intended, was only 6 ft. 6 in., and such a difference would have a great effect on the stability. The turret system has been applied to other ships on quite a different plan. Of these the Glatton is one of the most remarkable. Her appear- ance is very singular, and totally unlike that which we look for in a ship as may be seen by an inspection of Fig. 76, page 112. The Glatton, which was launched in 1871, is of the Monitor class, and was designed by Mr. E, J. Reed, who has sought to give the ship the most complete protection With this view the hull is covered with iron plates below the water-line, and the deck also is cased with 3 in. iron plates, to resist shot or shell fall ing vertically. The base of the turret is shielded by a massive breastwork, which is a peculiarity of this ship. The large quantity of iron required for all these extra defences has, of course, the effect of increasing the immer- sion of the vessel, and therefore of diminishing her speed. The freeboard when the ship is in ordinary trim is only 3 ft. high, and means are provided for admitting water to the lowest compartment, so as to increase the im- mersion by i ft., thus reducing the freeboard to only 2 ft. when the vessel is in fighting trim, leaving only that small portion of the hull above water as a mark for the enemy. The water ballast can be pumped out when no longer needed. The Glatton is 245 ft. long and 54 ft. broad, and she draws 19 ft of water with the freeboard of 3 ft., displacing 4,865 tons of water, while, with the 2 ft. freeboard, the displacement is 5,179 tons. This ship 112 SHIPS OF WAR. cost ,210,000. Mr. Reed wished to construct a vessel of much larger size on the same plan a proposal to which, however, the Ad- miralty did not then con- sent. The Glatton is, never- theless, one of the most powerful ships of war ever built, and may be considered as an impregnable floating fortress. Above the water- line the hull is covered with armour plates 12 in. thick, supported by 20 in. of teak backing, and an inner layer of iron I in. thick. Below the water-line the iron is 8 in. thick, and the teak 10 in. The revolving turret carries two 25-ton guns, firing each a 600 Ib. shot, and is covered by a massive plating of iron 14 in. in thickness. Besides this the base of the turret is protected by a breastwork rising 6 ft. above the hull. This breastwork is formed of plates 12 in. thick, fast- ened on 1 8 in. of teak. The turret rises 7 ft. above the breastwork, and therefore the latter in no way impedes the working of the guns. The Glattoru has a great advantage over all the other turret ships in having a per- fectly unimpeded fore range for her guns, for there is no mast or other object to pre- vent the guns being fired directly over the bow. There are no sails, the mast being intended only for flying sig- nals and hoisting up boats, &c. The hull is divided by vertical partitions into nine water-tight compartments, and also into three horizon- tal flats the lowest being air-tight, and having ar- rangements for the admis- sion and removal of water, SHIPS OF WAR. IT3 as already mention- ed. The stem of the ship is protruded forwards below the water for about 8 ft., thus forming a huge ram which would it- self render the Glat- ton a truly formid- able antagonist at close quarters even if her guns were not used. The engines are capable of being worked up to 3,000 horse-power, giving the ship a speed of 9^ knots per hour, and means are pro- vided for turning the turret by steam power. The turret can be rotated by manual labour, re- quiring about three minutes for its com- plete revolution, but by steam power the operation can be effected in half a mi- nute. The comman- der communicates his orders from the pilot-house on the hurricane deck to the engine-room, steering-house, and turret, by means of speaking-tubes and electric telegraphs. The Glatton was not designed to be ocean-going, but is intended for coast defence. The British navy contains two pow- erful turret -ships constructed on the same general plan as the Glatton, but larger, and capable of steaming at a 8 n 4 SHIPS OF WAR. greater speed, and of carrying coal for a long voyage. These sister ships are named the Devastation, Fig. 69, and the Thunderer ; Fig. 77. The Thun- derer has two turrets and a freeboard of 4 ft. 6 in. Space is provided for a store of 1,800 tons of coal, of which the Glatton can carry only 500 tons. The vessel is fitted with twin screws, turned by two pairs of independent engines, capable of working up to 5,600 horse-power, and she can steam at the rate of 12 knots, or nearly 14 miles, an hour. With the large supply of coal she can carry, the Thunderer could make a voyage of 3,000 miles without re-coaling. Though the freeboard of the heavily-plated hull is only 4 ft. 6 in., a lighter iron superstructure, indicated in the figure by the light shading, rises from the deck to the height of 7 ft, making the real freeboard nearly 12 ft. This gives the ship much greater stability, and prevents her from rolling heavily when at sea. The length is 285 ft. and the width 58 ft., and the draught 26 ft. The hull is double, the distance between the outer and inner skins of the bottom being 4 ft. 6 in. The framing is very strong and on the longitudinal principle, and the keel is formed of Bessemer steel. Each turret is 24 ft. 3 in. in internal diameter, and is built with five layers of teak and iron. Beginning at the inside, there is a lining of 2f in. iron plates ; then 6 in. of teak in iron frames, arranged horizontally ; 6 in. of armour plates ; 9 in. of teak, placed verti- cally ; outside of all, 8 in. armour plates. Each turret carries two Fraser 35-ton guns, rifled muzzle-loaders. The turrets revolve by hand or by steam-power. There are no sails, and thus a clear range for the guns is afforded fore and aft. The bases of the turrets are protected by the armoured breastwork, of which a portion is seen in the figure in advance of the fore turret. Another very powerful ship of war, which possesses some special features, is represented in the diagram on the opposite page, Fig. 78. This vessel, named the Konig Wilhelm, was built at Blackwall for the Prussian Govern- ment by the Thames Ironworks and Steam Shipbuilding Company, from designs by Mr. Reed. Her length is 365 ft, width 60 ft. ; burthen, 6,000 tons ; displacement, 8,500 tons. She is framed longitudinally, that is, girders pass from end to end, about 7 ft. apart, and the stem projects into a pointed ram. In this case also the hull is double ; there is, in fact, one hull within another, with a space of 4! ft. between them. The armour plates are 8 in. in thickness, with 10 in. of teak backing; but on the less important parts the thickness of the iron is reduced to 6 in., and in some places to 4 in. This ship has a broadside battery, and there are no turrets, but on the deck there are, fore and aft, two semicircular shields, formed of iron plates and teak, pierced with port-holes for cannon, and also with loop-holes for muskets. From these a fore-and-aft fire may be kept up. The ship is fully rigged, and has also steam engines of 7,000 horse-power, by Maudslay and Co. Her armament consists of four three-hundred- pounders, capable of delivering fore-and-aft as well as broadside fire, and twenty-three other guns of the same size between decks. These guns are all Krupp's steel breech-loaders. The great contest of armour plates versus guns has already been alluded to, and to the remarks then made it may be added that, while on the one hand guns 80 tons in weight are in course of construction, ships are already designed with 18 in. and even 20 in. of iron armour plates. It would be very difficult to predict which side will sooner reach the limit beyond which increase of size and power cannot go. The gradual increase of thickness of plating, attended by increased weight of guns, projectiles, and charges SHIPS OF WAR u6 SHIPS OF WAR. of powder, may be illustrated by stating in a condensed form some details of ships already named, as regards the thickness of armour, and its resisting power, which is nearly in proportion to the square of its thickness ; and some particulars respecting the guns carried by those ships are also given. Warrior. Hercules. Glatton. Thunderer. Thickness of iron pla- tin r in inches 4.1 Q 12 Relativeresisting power of plating 2O 8 1 M4 1 06 Guns carried Cast iron, Wrought iron, Wrought iron, Wrought iron, Weight of guns in tons Charge of powder in Ibs. Weight of projectiles in Ibs smooth bore. 4l 16 68 nned. 18 60 4.00 nned. 25 70 6OO rifled. 35 1 20 700 Destructive power of projectiles at 1,000 yards range, in foot- tons 4.C2 7,863 s,i6s 8.4O4. In official trials the power of iron plates to resist projectiles was tested not only by firing at targets built to resemble the sides of the vessels, but real floating batteries, like that represented below, were made the sub- ject of experiment ; and although the sides of the battery here depicted were formed of 4^ in. slabs of iron, backed by 22 in. of teak, Sir J. Whit- worth's flat-headed projectiles completely penetrated them, punching a clean six-sided hole in the iron plates. The bright flash of light emitted at the instant of the impact is shown in the cut, and the effect to a spectator was much the same as if a gun had been fired from the battery in reply. FlG. 79. AV////V Works, at Essen, Prussia, FIRE-ARMS. THE invention of gunpowder or rather its use in war appears at first sight a device little calculated to promote the general progress of mankind. But it has been pointed out by some historians that the intro- duction of gunpowder into Europe brought about the downfall of the feudal system with its attendant evils. In those days every man was practically a soldier : the bow or the sword he inherited from his father made him ready for the fray. But when cannons, muskets, and mines began to be used, the art of war became more difficult. The simple possession of arms did not render men soldiers, but a long special training was required. The greater cost of the new arms also contributed to change the arrangements of society. Standing armies were established, and war became the calling of only a small part of the inhabitants of a country, while the majority were left free to devote themselves to civil employments. Then the useful arts of life received more attention, inventions were multiplied, commerce began to be considered as honourable an avocation as war, letters were cultivated, and other foundations laid for modern science. If such have really been the indirect results of the invention of gunpowder, we shall hardly share the regret of the fine gentleman in " Henry IV." : " That it was great pity, so it was, That villanous saltpetre should be digged Out of the bowels of the harmless earth, Which many a good tall fellow had destroyed So cowardly.'' We often hear people regretting that so much attention and ingenuity as are shown by the weapons of the present day should have been expended 117 n8 FIRE-ARMS. upon implements of destruction. It would not perhaps be difficult to show that if we must have wars, the more effective the implements of destruction, the shorter and more decisive will be the struggles, and the less the total loss of life, though occurring in a shorter time. Then, again, the exaspe- rated and savage feelings evoked by the hand-to-hand fighting under the old system have less opportunity for their exercise in modern warfare, which more resembles a game of skill. But the wise and the good have in all ages looked forward to a time when sword and spear shall be every- where finally superseded by the ploughshare and the reaping-hook, and the whole human race shall dwell together in amity. Until that happy time arrives " Till the war-drum throbs no longer, and the battle flags are furl'd In the Parliament of man, the Federation of the world When the common sense of most shall hold a fretful realm in awe, And the kindly earth shall slumber, lapt in universal law," we may consider that the more costly and ingenious and complicated the implements of war become, the more certain will be the extension and the permanence of civilization. The great cost of such appliances as those we are about to describe, the ingenuity needed for their contrivance, the elabo- rate machinery required for their production, and the skill implied in their use, are such that these weapons can never be the arms of other than wealthy and intelligent nations. We know that in ancient times opulent and civilized communities could hardly defend themselves against poor and barbarous races. But the world cannot again witness such a spectacle as Rome presented when the savage hordes of Alaric swarmed through her gates, and the mighty civilization of centuries fell under the assaults of the northern barbarians. In our day it is the poor and barbarous tribes who are everywhere at the mercy of the wealthy and cultivated nations. The present age has been so remarkably fertile in warlike inventions, that it may truthfully be said that the progress made in fire-arms and war-ships within the last few years surpasses that of the three previous centuries. Englishmen have good reason to be proud of the position taken by their country, and may feel assured that her armaments will enable her to hold her own among the most advanced nations of the world. We shall bring before the reader a description of some of the improvements in artillery, beginning with RIFLED CANNON. n^HE large guns which are now constructed at Woolwich, though often -* called Armstrong guns, are not the production of one inventive mind only, but may be described more fully as wrought iron guns built on Sir W. Armstrong's principles, improved by Mr. Anderson's method of hooping the coils,^-with solid-ended steel tubes toughened in oil, rifled on the French system, modified as recommended by the Ordnance Committee to suit projectiles studded according to Major Palliser's plan. Sir W. Arm- strong's principle consists in disposing of the fibre of the iron so as best to resist the strains in the several parts of the gun. Wrought iron being fibrous in its texture has, like wood, much more strength in the direction of the grain than across it. The direction of the fibre in a bar of wrought iron is parallel to its length, and in that direction the iron is nearly twice as strong as it is transversely. A gun may give way either by the bursting of the barrel or FIRE-ARMS. 119 by the blowing out of the breech. The force which tends to produce the first effect acts transversely to the axis of the gun ; hence the best way to resist it is to wrap the iron round the barrel, so that the fibres of the metal encircle it like the hoops of a cask. The force which tends to blow out the breech is best resisted by disposing the fibres of the iron so as to be parallel to the axis of the gun ; hence Sir W. Armstrong makes the breech-piece from a solid forging with the fibre in the required direction. Until the year 1867 the guns made at Woolwich were constructed accord- ing to the original plan proposed by Sir W. Armstrong, and on this system, one of the large guns consisted of as many as thirteen separate pieces. These guns, though unexceptionable as to strength and efficiency, were necessarily so very costly that it became a question whether anything could be done to lessen the expense by a simpler mode of construction or by greater economy in the material. The problem was solved in the most satisfactory manner by Mr. Fraser, of the Royal Gun Factory, who pro- posed an important modification of the original plan, and the adoption of a kind of iron cheaper than had been previously employed, yet perfectly BREECH'COIL FIG. 80. Section of 9 in. Fraser Gun. suited for the purpose. Mr. Fraser's modification consisted in building up the guns from only a few coils, instead of several, the coils being longer than Sir W. Armstrong's, and the iron coiled upon itself two or even three times : a plan which enabled him to supersede the breech-piece, formerly made in one large forging, by a piece formed of coils. In order to perceive the increased simplicity of construction introduced by Mr. Fraser, we need but glance at the section of a 9 in. gun constructed according to his system, Fig. 80, and remember that a piece of the same size made after the original plan had ten distinct parts, whereas the Fraser is seen to have but four. Com- pare also Figs. 8 1 and 94. We shall now describe the process of making the Fraser 9 in. gun. The parts of the gun as shown in the section, Fig. 80, are : i, the steel barrel ; 2, the B tube ; 3, the breech-coil ; 4, the cascable screw. The inner steel barrel is made from a solid cylinder of steel, which is supplied by Messrs. Firth, of Sheffield. This steel is forged from a cast block, the casting being necessary in order to obtain a uniform mass, while the subsequent forging imparts to it greater solidity and elasticity. After the cylinder has been examined, and the suitable character of the steel tested by trials with portions cut from it, the block is roughly turned and bored, and is then ready for the toughening process. This consists in heat- ing the tube several hours to a certain temperature in an upright furnace, and then suddenly plunging it into oil, in which it is allowed to remain for a day. By this treatment the tenacity of the metal is marvellously increased. 120 FIRE-ARMS. A bar of the steel I in. square previous to this process, if subjected to a pull equal to the weight of 13 tons, begins to stretch and will not again recover its original form when the tension is removed, and when a force of 31 tons is applied it breaks. But the forces required to affect the tough- ened steel in a similar manner are 31 tons and 50 tons respectively. The process, unfortunately, is not without some disadvantages, for the barrel is liable to become slightly distorted and even superficially cracked. Such cracks are removed by again turning and boring ; the hardness the steel acquires by the toughening process being shown by the fact that in the first boring 8| in. diameter of solid steel is cut out in 56 hours, yet for this slight boring, in which merely a thin layer is peeled off, 25 hours are required ; and lest there should be any fissures in the metal, which, though not visible to the eye, might make the barrel unsound, it is filled with water, which is subjected to a pressure of 8,000 Ibs. per square inch. If under this enormous pressure no water is forced outwards, the barrel is considered safe. It is now ready to have the B tube shrunk on it. The B tube, like certain other portions of these guns, is constructed from coiled iron bars, and this constitutes one great peculiarity of Sir W. Arm- strong's system. It has the immense advantage of disposing the metal so that its fibres encircle the piece, thus applying the strength of the iron in the most effective way. The bars from which the coils are prepared are made from " scrap " iron, such as old nails, horse-shoes, &c. A pile of such fragments, built up on a wooden framework, is placed in a furnace and intensely heated. When withdrawn the scraps have by semi-fusion become coherent, and under the steam hammer are soon welded into a compact mass of wrought iron, roughly shaped as a square prism. The glowing mass is now introduced into the rolling-mill, and in a few minutes it is rolled out, as if it were so much dough, into a long bar of iron. In order to form this into a coil it is placed in a very long furnace, where it can be heated its entire length. When sufficiently heated, one end of the bar is seized and attached to an iron core of the required diameter, and the core being made to revolve by a steam engine, the bar is drawn out of the fur- nace, winding round the core in a close spiral, so that the turns are in con- tact. The coil is again intensely heated, and in this condition a few strokes of the steam hammer in the direction of its axis suffice to combine the spires of the coil into one mass, thus forming a hollow cylinder. The B tube for the 9 in. gun is formed of two double coils. When the two portions have been completely welded together under the steam hammer, the tube, after cooling, is roughly turned and bored. It is again fine bored to the required diameter, and a register of the diameter every few inches down the bore is made. These measurements are taken for the purpose of adapting most accurately the dimensions of the steel barrel to the bore of the B tube, as it is found that perfect exactness is more easily obtained in turning than in boring. The steel barrel is therefore again turned to a size slightly larger than the bore of the B tube, and is then placed muzzle end upwards, and so arranged that a stream of water, to keep it cool, shall pass into it and out again at the muzzle, by means of a syphon, while the B tube, which has been heated until it is sufficiently expanded, is passed over it and gradually cooled. Jf now the B tube were allowed to cool spontaneously, its ends would, by cooling more rapidly than the central part, contract upon the steel barrel and grip it firmly at points which the subsequent cooling would tend to draw nearer together longitudinally, and thus the barrel would be subjected FIRE-ARMS. 121 to injurious strains. In order to prevent this, the B tube is made to cool progressively from the breech end, by means of jets of water made to fall upon it, and gradually raised towards the muzzle end, which has in the meanwhile been prevented from shrinking by having circles of gas-flames playing upon it. The breech-coil, or jacket, is formed of three pieces welded together. First, there is a triple coil made of bars 4 in. square, the middle one being coiled in the reverse direction to the other two. After having been intensely heated in a furnace for ten hours, a few blows on its end from a powerful steam hammer welds its coils perpendicularly, and when a solid core has been introduced, and the mass has been well hammered on the sides, it becomes a compact cylinder of wrought iron, with the fibres all running round it. When cold it is placed in the lathe, and the muzzle end is turned down, leaving a shoulder to receive the trunnion-ring. The C coil is double, welded in a similar manner to the B coil, and it has a portion turned off, go that it may be enclosed by the trunnion-ring. The trunnion-ring is made by punching a hole in a slab of heated iron first by a small conical mandrel, and then enlarging by repeating the pro- cess with larger and larger mandrels. The iron is heated for each operation, and the trunnions are at the same time hammered on and roughly shaped or, rather, only one has to be hammered on for a portion of the bar which serves to hold the mass from the other. The trunnion-ring is then bored out, and after having been heated to redness, is dropped on to the triple breech-coil which is placed muzzle end up, and the turned end of the C coil (of course, not heated) is then immediately placed within the upper part of the trunnion-ring. The latter in cooling contracts so forcibly as to bind the ends of the coils together, and the whole can thus be placed in a furnace and heated to a high temperature, so that when removed and put under the steam. hammer, its parts are readily wielded into one mass. The breech-coil in this state weighs about 16 tons ; but so much metal is re- moved by the subsequent turnings and borings, that it is reduced to nearly half that weight in the gun. It is then turned in a lathe of the most mas- sive construction, which weighs more than 100 tons. Fig. 34, page 53, is from a drawing taken at Woolwich, and shows one of the large guns in the lathe. No one who witnesses this operation can fail to be struck with the apparent ease with which this powerful tool removes thick flakes of metal as if it were so much cheese. The projections of the trunnions prevent the part in which they are situated from being finished in this lathe, and the gun has to be placed in another machine, where the superfluous metal of the trunnion-ring is pared off by a tool moving parallel to the axis of the piece. Another machine accomplishes the turning of the trunnions, the "jacket" being made to revolve about their axis. The jacket is then ac- curately bored out with an enlargement or socket to receive the end of the B tube,' and a hollow screw is cut at the breech end for the cascable. The portion of the gun, consisting of the steel barrel with the B tube shrunk on it, having been placed upright with the muzzle downwards, the breech-piece, strongly heated, is brought over it by a travelling crane, and slips over the steel barrel, while the recess in it receives the end of the B tube. Cold water is forced up into the inside of the barrel in order to keep it cool. As the breech cools, which it is allowed to do spontaneously, it contracts and grips the barrel and B tube with great force. The cascable requires to be very carefully fitted. It is this piece which plays so important a part in resisting the force tending to blow out the end of the barrel. The 122 FIRE-ARMS. cascable is a solid screw formed of the very best iron, and its inner end is wrought by scraping and filing, so that when screwed in there may be per- fect contact between its face and the end of the steel barrel. A small annular space is left at the circumference of the inner end, communicating- through a small opening with the outside. The object of this is, that in case of rupture of the steel barrel, the gases escaping through it may give timely warning of the state of the piece. Besides minor operations, there remain the important processes of finish- ing the boring, and of rifling. The boring is effected in two operations, and after that the interior is gauged in every part, and " lapping " is resorted to where required, in order to obtain the perfect form. Lapping consists in wearing down the steel by friction against fine emery powder and oil, spread on a leaden surface. The piece is then ready for rifling. The machinery by which the rifling is performed cannot be surpassed for its admirable ingenuity and simplicity. In this operation the gun is fixed horizontally, its axis coinciding with that of the bar, which carries the grooving tools. This bar is capable of two independent movements, one backwards and forwards in a straight line in the direction of the length of the bar, and the other a rotation round its axis. The former is communicated by a screw parallel to the bar, and working in a nut attached to the end of it. For the rotatory movement the bar carries a pinion, which is engaged by a rack placed horizontally and per- pendicularly to the bar, and partaking of its backward and forward move- ment, but arranged so that its end must move along another bar placed at an angle with the former. It is this angle which determines the pitch of the rifling, and by substituting a curved guide-bar for the straight one, an increasing twist may be obtained in the grooves. The projectile used with these guns is of a cylindrical form, but pointed at the head, and the moulds in which these shots are cast are so arranged that the head of the shot is moulded in iron, while the body is surrounded with sand. The rapid cooling induced by the contact of the cold metal causes the head of the shot to solidify very quickly, so that the carbon in the iron is not separated as in ordinary casting. In consequence of this treatment, the head of the shot possesses the hardness of steel, and is there- fore well adapted for penetrating iron plates or other structures. The pro- jectiles are turned in a lathe to the exact size, and then shallow circular cavities are bored in them, and into these cavities brass studs, which are simply short cylinders of a diameter slightly larger than the cavities, are forced by pressure. The projecting studs are then turned so as accurately to fit the spiral grooves of the guns. Thus the projectile in traversing the bore of the piece is forced to make a revolution, or part of a revolution, about its axis, and the rapid rotation thus imparted has the effect of keep- ing the axis of the missile always parallel to its original direction. Thus vastly increased accuracy of firing is obtained. There is a very curious deviation to which the projectiles from rifled cannon are liable, a deviation which is not caused by the wind, but is con- stant in amount for the same gun with the same projectile and charge of powder. A shot deviates to the right or left according to the twist of the rifling of the piece, and according to the form of the head of the shot. This is found to depend upon certain laws which govern all rotating bodies. To explain these laws fully would be to exceed our limits, but the reader who is interested in this subject may see them in operation in the gyroscope, a simple form of which is now a cheap and common toy. When the axis FIRE-ARMS. 123 of the disc is placed in an inclined position, the unsupported extremity would, if the disc were not rotating quickly, be depressed by gravity ; but when the disc is in rapid rotation, it descends very slowly, and the axis at the same time slowly moves round horizontally about the supported point. It is easy to see that the projectile from the rifled cannon is under similar conditions, for the reaction on its head of the air through which it moves tends to depress or to elevate the anterior end of the axis, according as the projectile is flat-headed or pointed. The constant deflection of shot here mentioned is allowed for in pointing the piece, its direction and amount having once been determined by experiment. FIG. 81. They^-ton Fraser Gun. Shells are also used with the Woolwich rifled guns. The shells are of the same shape as the solid shots, from which they differ in being east hollow, and having their interior filled with gunpowder. Such shells when used against iron structures require no fuse : they explode on coming into collision with their object. In other cases, however, the shells are provided with fuses, which cause the explosion when the shot strikes. The cut above, Fig. 81, represents one of the 35-ton guns, made on the plan introduced by Mr. Fraser. This piece of ordnance is 16 ft. long, 4 ft. 8 in. in diameter at the breech, and i ft. 9 in. at the muzzle. The bore is about I ft. Each gun can throw a shot or bolt 700 Ibs. in weight, with a charge of 120 Ibs. of powder. It is stated that the shot, if fired at a short range, would pene- trate a plate of iron 14 in. thick, and that at a distance of 2,000 yards it would retain sufficient energy to go through a plate 12 in. thick. The effect of these ponderous missiles upon thick iron plates is very remarkable. Targets or shields have been constructed with plates and timber backing, graders, &c., put together in the strongest possible manner, in order to test the resisting power of the armour plating and other constructions of our iron-clad ships. The next two cuts, Figs. 82 and 83, are representations of the appearance of the front and back of a very strong shield of this de- scription, after having been struck with a few 600 Ib. shots fired from the 25-ton gun. The shots with chilled heads, already referred to, sometimes were found to penetrate completely through the 8 in. front plate, and the 6 in. -of solid teak, and the 6 in. of plating at the back. Other plans of constructing guns quite different from that adopted at 124 FIRE-ARMS. Woolwich have been proposed. Sir Joseph Whitworth makes his guns of steel, and they are striking examples of beautiful and accurate workman- ship. His system of rifling consists in forming the bore of the gun so that its section is a regular polygon, and the projectile is an elongated bolt with FlG. 82. Millwall Shield after being battered with Heavy Shot. Front View. sides exactly fitting the barrel of the gun : the projectile is, in fact, a twisted prism, usually hexagonal. Sir Joseph's guns are breech-loaders, and they are remarkable for their long range and accuracy of fire. One of these FIG. 83. Rear View of the Millwall Shield. guns with a charge of 50 Ibs. of gunpowder threw a 250 Ib. shot a distance of nearly six miles, and on another occasion a 310 Ib. shell was hurled through the air, and first touched the ground at a distance of more than six and a quarter miles from the gun. These are the greatest distances to which shot or shell have ever been thrown. FIRE-ARMS. 125 As the material of which these guns are constructed is costly, and very- perfect workmanship is required in the formation of the barrel and the shots, the expense attending their manufacture and use is much greater than that incurred in the case of the Armstrong guns. Sir W. Armstrong's estimate for a 35-ton gun was .3,500, and Sir J. Whitworth's 6,000. The gun, as constructed at Woolwich on Mr. Eraser's plan, is estimated to cost ,2,500. The first cost of a gun is a matter for consideration, since each piece, even the strongest, is able only to fire a limited number of rounds before it becomes unsafe or useless. It appears that no cannon has yet been constructed capable of withstanding without alteration the tremendous shocks given by the explosion of the gunpowder, and these alterations, however small at any one discharge, are summed up and ultimately bring to an end what may be termed the " life of the piece." By means of .1 kind of pressure-gauge, in which the compression of a piece of metal produced by the force of the gases in the powder-chamber is compared with the amount of compression produced in the same metal by a known FlG. 84. Comparative sizes of '35 and'&i-ton Guns. A. 35-ton. B. 8i-ton. pressure, it was estimated that in the 35-ton gun a pressure of more than 47 tons must have been applied to every square inch of the internal surface of the powder-chamber. When we remember that these enormous pres- sures are applied so suddenly that they partake rather of the nature of a blow than of a pressure gradually applied, we cannot be surprised at the ultimate failure of even the strongest piece of ordnance. The great 35-ton guns, hitherto the most powerful of cannon, have been humorously called " Woolwich Infants ; " but if the reader will refer to Fig. 84, he will see that even these impressive pieces of ordnance are, indeed, " infants " in almost a literal sense, compared with the monsters which are now in course of con- struction. The figure represents the 35-ton and the 8i-ton gun on the same scale. The new gun will weigh 8 1 tons, the length will be 27 ft., with a length of bore equal to about 24ft., and the diameter of the bore will be 14 in. in the first instance ; but if after experiments it be found desirable, the bore may be enlarged, first to 15 and afterwards to 16 in. The trunnions for this gun will be 16 in. in diameter. The shot for the 14 in. bore will weigh about 1,000 Ibs, and with the larger bores 1,200 Ibs. The energy of the 1,000 Ib. projectile when it leaves the muzzle will, it is calculated, be equal to 11,700 foot-tons, and that of 1,200 Ib. shot will exceed 14,000 foot-tons, or nearly double that possessed by the shot from the 35-ton gun. Such projectiles will completely penetrate plates of iron 20 in. in thickness. The 126 FIRE-ARMS. Si-ton gun will be built up in the same manner as the 35-ton, but the coils will necessarily be much longer, and the chase is formed in three parts instead of two. It was for the manufacture of these enormous guns that the great steam hammer (represented in the frontispiece) was erected at Woolwich. An extreme contrast to the 8i-ton gun is presented in the little gun which forms the subject of Fig. 85. This piece weighs only 148 Ibs., and with a charge of 8 oz. of gunpowder has been found to hit with shot or shell a target with great precision at a distance of 1,000 yards. It has been found admirably adapted for mountain warfare and in situations where the ab- sence of good roads would make heavier pieces an impediment to an army on the march. In the wars in Abyssinia and in Ashantee these little weapons jocosely called " steel pens "-were found of great service. In the battle which preceded the fall of Magdala and the death of King Theodore, the shells thrown by these little guns at a distance of several hun- dred yards cut up and threw into con- fusion the great bodies of men which the Abyssinian King had collected. The Armstrong and the Whitworth guns are not the only ordnance which claim attention on account of their power , ~ and accuracy of fire. There are tKe The founder Rifled Steel Gun. sted guns of the famous Krupp? whose works at Essen in Prussia are of enor- mous extent. The head-piece to this chapter gives but a very faint idea of the 500 acres of ground covered by this huge gun factory, where 310 steam engines are at work, and more than 100 furnaces are burning night FIG. 85. FlG. 86. The \\v-pojinder Breech-loading Krupp's Gun, with Breech- piece, or Slide, open and ready to load. Rnd day. The forges, the lathes, and the steam hammers may be reck- oned by hundreds, and to crown all, more than 10,000 men are busily engaged in feeding and controlling the innumerable machines. These works originally were not devoted exclusively to the manufacture of guns, but now the production of the cast steel breech-loading ordnance absorbs much attention at these gigantic steel works. The peculiarity of FIRE-ARMS. 127 these steel guns is the perfectly uniform character of the steel and its free- dom from flaws. Krupp has exhibited solid cylinders of steel, 2 ft. in diameter, broken across in order to show that the steel was perfectly sound and uniform throughout the mass. The details of the processes by which these castings are made are kept secret. FlG. 87. The \\Q-po :>r.dcr Breech-loading Krupp Siege Gun, loaded, the Slide pusiied in and fixed, and ready for firing. The hundred-and-ten-pounder breech-loading gun, as constructed by Krupp, is represented in Figs. 86 and 87. In the former the sliding breech- piece is drawn out, and the piece is ready for loading ; in the latter the slide is pushed in, and fixed ready for firing. The reader will, of course, under- stand that the gun is represented in these figures without its carriage and FlG. 88. The yz-pounder Prussian Krupp Siege GUJI, a Breech-loader, with Breech-piece open for admission of the Shell and Gunpowder. other adjuncts. These guns, it lias been stated, will throw a shot a distance of five miles, although at the siege of Paris, where they were used by the Prussians, the utmost range of the shells was about four and a half miles, or more usually three and a half miles. A much lighter Krupp gun, a thirty- two-pounder, is represented in Fig. 88, mounted on a traversing platform and well-constructed gun-carriage, with an ingenious contrivance for re- ceiving the force of the recoil. Krupp also makes guns throwing heavy projectiles, 600 Ibs. and upwards. These large guns were at one time consi- FIRE-ARMS. FIG. 89. Appearance of the Deck of a Ship after the bursting of a large Gun, dered by British artillerists as very unreliable weapons, because steel is a material subject to certain unobserved flaws, that occasionally cause the guns to burst in the most capricious manner. As a matter of fact, a few FIG. 90. Another view of the same disaster, at which, strange to say, no one was killed. Krupp guns have burst, and in one case with disastrous consequences. The reader may picture to himself the probable results to the gunners of an accident of this kind. He will doubtless understand that every part of the interior of the powder- chamber receives at the moment of the discharge FIRE-ARMS. 129 FIG. 91. The Citadel of btrasburg after the Prussian Bombardment. a sudden strain or blow of the same power as that which urges on the shot. It is v the effect of this strain acting on the end of the barrel which pro- duces the recoil of the gun in ordinary cases. But if the breech of the gun is blown in pieces, these pieces are hurled about with as much force as would have otherwise been expended on the shot. The cuts, Figs. 89 and 90, are from photographs taken on board ship immediately after the bursting of a large gun. The reader cannot fail to notice the manner in which a rnast has been completely cut away by a fragment of metal. The Krupp guns, as may be imagined from the scale of their production are, however, finding great favour with continental nations. Austria, for example, has resolved upon completely changing her armament, by intro- ducing these weapons, in power and accuracy of fire they having been found on investigation to surpass the Austrian artillery in the proportion of five to one. The power of the Krupp guns may be illustrated by the reproduc- tion of a photograph, given in Fig. 91, of a portion of the fortifications of Strasburg after the Prussian bombardment of that fortress. This section may fitly close with the two drawings Fig. 92 and 93, and a few words of description of the truly scientific and ingenious invention of Captain Moncrieff, by which he utilizes the force of the recoil as a power for raising the gun into a firing position, after it has been loaded under cover. The mode in which Captain MoncriefFs gun-carriage acts will be understood by a simple inspection of the figures. Fig. 93 represents the gun raised above the parapet and ready for firing. When thfc discharge takes place the gun would, if free, move backwards with a speed which would be greatest at the first. The arrangements of the carriage are such I 3 o FIRE-ARMS. FIG. 92. Moncrieff's Gun-carriage j gun lowered j or loading. _.- ./ FIG. 93. The Moncrieff Gim raised and ready for firing. that this initial velocity does not receive a sudden check, but the force is made to expend itself in raising a heavy counterpoise ; at the same time the gun is permitted to descend, retaining a direction parallel to its first position. At the end of the descent of the gun (which, it must be under- stood, is not caused by its weight, but by the force of the recoil, for the FIRE-ARMS. 13 i counterpoise more than balances the gun) it is retained in that position until it has been re-loaded, and when released the counterpoise descending brings the gun up again into the firing position. The great advantage of this invention is that the artillerymen are not exposed to any danger while loading; and even when pointing the gun expedients may be adopted whereby the whole danger is reduced to that arising from " vertical fire," by which little harm is likely to be done. The plan of using a counter- poise in the manner just described would manifestly be inapplicable with naval guns, but Captain Moncrieff has matured a most elegant application of the elastic force of compressed air. The force of the recoil is made to urge a piston, so as to compress the air, and the reaction is used, like the descent of the counterpoise, to bring the gun back into the firing position. FlG. 94. foo-pounder Muzzle-loading Armstrong Gun. BREECH-LOADING RIFLES. "VX/TIEN in 1864 a committee which had been appointed to investigate * the question of proper arms for our infantry, recommended that that branch of the service should be supplied with breech-loaders, our Government, considering that no form of breech-loader had up to that time been invented which would unequivocally meet all the requirements of the case, wisely determined that, pending the selection of a suitable arm, the service muzzle-loaders should meanwhile be converted into breech- loaders. The problem of how this was to be done was solved by the gun- maker Snider, and in the "Converted Enfield" or "Snider" the British army was provided for a time with an arm satisfying the requirements of that period. This change of weapon was effected at a comparatively small outlay, for the conversion cost less than twenty shillings an arm. The arrangement was confessedly only temporary, and the Snider-Enfield has been passed on to the volunteers and reserve, while the line are now pro- vided with a still more effective weapon in the Martini-Henry rifle. The 9 2 132 FIRE-ARMS. authorities having, in 1866, offered gunmakers and others handsome prizes for the production of rifles best fulfilling certain conditions, nine weapons were selected out of 104 as worthy to compete. No first prize was awarded, but the second was given to Mr. Henry, while Mr. Martini was seventh on the list. In order to obtain a weapon fulfilling all the requirements, a vast number of experiments were made by the committee appointed for that purpose, as to best construction of barrel, size of bore, system of rifling, kind of cartridge, and other particulars, and assistance was rendered by several eminent gunsmiths and engineers. FIG. 95. Section of M artinir Henry Lock. After a severe competition it appeared that the best weapon would be produced by combining Henry's system of rifling with Martini's mechanism for breech-loading. The parts constituting the lock and the mechanism for working the breech, shown in Fig. 95, are contained in a metal case, to which is attached the woodwork of the stock, now constructed in two parts. To this case is attached the butt of the rifle by a strong metal bolt 6 in. in length, A, which is inserted through a hole in the heel-plate. The part that closes the breech termed the " block " is marked B. It turns loosely on a pin, c, passing through its rear end and fixed into the case at a levei some- what higher than the axis of the barrel. The end of the block is rounded off so as to form with the rear end of the case, D, which is hollowed out to receive it in a perfect knuckle joint. Let it be observed that this rounded surface, which is the width of the block, receives the whole force of the recoil, no strain being put on the pin, C, on which the block turns. In the experiments a leaden pin was substituted, and the action of the mechanism was not in the least impaired. This arrangement serves greatly to diminish the wear and the possibility of damage from the recoil. As the pin on which the block turns is slightly above the axis at the barrel, it follows that the block, when not supported, immediately drops down below the barrel. Behind the trigger-guard is a lever, E, working on a pin, F, fitted into the lower part of the case. To this lever is attached a much shorter piece called the " tumbler," which projects into the case, G. It is this tumbler which acts as a support for the block, and raises it into its firing position FIRE-ARMS. 133 or lowers it according as the lever, E, is drawn toward a firer or pushed forward. How this is accomplished will be readily understood by observ- ing the form of the notch, H, in which the upper end of the tumbler moves. It will be noticed that the piece being in the position for firing, if the lever be pushed back, G slides away from the shallower part of the notch into the deeper, and the block accordingly falls into the position shown in the figure ; and if again the lever is drawn backward, G acting on H will raise the block to its former position. The block or breech-piece is hollowed out on its upper surface, I, so as to permit the cartridge to be readily in- serted into the exploding-chamber, J. The centre of the block is bored out, and contains within the vital mechanism for exploding the cartridge, namely, a spiral spring, of which the little marks at K are the coils in section. These coils pass round a piece of metal called the " striker," which is armed with a point, capable of passing through a hole in the front face of the block exactly behind the percussion cap of the cartridge when the block is in the firing position. When the lever handle is moved for- ward, it causes the tumbler, which works on the same pin, to revolve, and one of its arms draws back the striker, compressing the spring in so doing, so that as the block drops down the point of the striker is drawn inwards. In this position the piece receives the cartridge into the chamber. The lever, E, being now drawn backward, the piece is forced into the notch, H, and the block is kept firmly in its place ; besides this, there is a further compression of the spring by the tumbler, and in this position the spring is retained by the rest-piece, L, which is pushed into abend in the tumbler. By pulling the trigger this piece is released, so that the tumbler can revolve freely, and relieve the pent-up spring, whose elasticity impels the striker forward, so that this enters the cartridge directly. A very important and ingenious part of this arrangement is the contrivance for extracting the case of the exploded cartridge. The extractor turns on the pin, M, and has two arms pointing upwards, N, which are pressed by the rim of the cartridge pushed home into two grooves cut in the sides of the barrel. It has another arm, o, bent only slightly upwards and pointing towards the centre of the case, and forming an angle of about 80 with the above- mentioned upright arm ; when, by pushing forward the lever, its short arm drops into the recess, the block, no longer supported, falls, and hits the point of the bent arm of extractor, so causing the two upright arms to extract the cartridge-case a little way. The barrel is of steel ; the calibre is 0*45 1 in. It is rifled on Mr. Henry's patent system. The section of the bore may be generally described as a heptagon with re-entering angles at the junctions of the planes, so that there are fourteen points of contact for the bullet, viz., one in the middle of each plane, and one at each of the re-entering angles. The twist of rifling is one turn in 22 in. The charge consists of 85 grains of powder, and a bullet weighing 480 grains, of a form designed by Mr. Henry. The cartridge is of the same general construction as the " Boxer " cartridge, used in the Snider rifle, but it is bottle-shaped, the diameter being en- larged from a short distance in rear of the bullet, in order to admit of its being made shorter, and consequently stronger, than would be otherwise possible. A wad of bees'-wax is placed between the bullet and powder, by which the barrel is lubricated at each discharge. The sword-bayonet to be used with this rifle is of a pattern proposed by Lord Elcho. It is a short sword, broad towards the point, and furnished on a portion of the back with a double row of teeth, so as to form a stout saw. It is so balanced 134 FIRE-ARMS. as to form a powerful chopping implement, so that, in addition to its pri- mary use as a bayonet, it will be useful for cutting and sawing brushwood, small trees, &c. FIG. 96. The Martini-Henry Rifle. A, ready for loading ; B, loaded and reaay for nriiig. The following are the principal particulars of weight, dimensions, &c,,of the Martini-Henry rifle : C Without bayonet . 4 ft. I in. < With bayonet fixed I Of barrel Length of rifle Calibre . Rifling Weight . Bayonet . Charge of powder ?) w 9-22 0-451 Grooves 7 Twist . i turn in 22 in. Without bayonet. 8 Ibs. 7 oz. With bayonet Jo 4 Length . 2 ft. i% in. Weight without 7 scabbard J I lb. 8 oz. 85 grains. Weight of bullet . ' . . . . 480 The rifle is sighted to 1,400 yards. As an evidence of the accuracy of fire in this rifle, it may be stated that of twenty shots fired at 1,200 yards, the mean absolute deflection of the hits from the centre of the group was 2'28 ft. The highest point in the trajectory at 500 yards is rather over 8 ft, so that the bullet would not pass over a cavalry soldier's head within that distance. The trajectory of the Snider at the same range rises to nearly 12 ft. The bullet will pass through from thirteen to seventeen ^ in. elm planks placed I in. apart at 20 yards distance ; the number pierced by the Snider under similar circumstances being from seven to nine. As regards rapidity of fire, twenty rounds have been fired in 53 seconds ; and one arm which had been exposed to rain and water artificially applied for seven days and nights, and had during that time fired 400 rounds, was then fired, without cleaning, twenty rounds in I minute 3 seconds. A competent military authority, after instituting a careful comparison between the breech-loading small arms of Prussia, France, and England, FIRE-ARMS. '35 concludes that in the Martini-Henry we have a weapon "giving great accuracy at long ranges, combined with a very flat trajectory, and therefore fearfully destructive at shorter ranges, with terrible smashing power and great penetration, not liable to fouling, easily cleaned, and not injured by being left uncleaned, with grooves which do not lead or wear out ; a breech mechanism easily constructed by machinery, easily worked, not liable to get out of order, utterly unaffected by rust or dirt, wearing well, quickly loaded, light, handy, strong, and durable." FIG. wThe Chassepot Rifle. Section of theHYeech. "^ Another breech-loading military rifle of note is the Chassepot, the weapon of the French infantry, of which we heard so much before and during the disastrous campaign against the Germans. This arm has a rifled barrel, with a breech mechanism of great simplicity, which is represented in section in Fig. 97. The piece marked B corresponds to what is called the "hammer" in the old lock used with percussion-caps, and the first operation in charging the rifle consists in drawing out B, as shown in the cut, until, by the spring, C, connected with the trigger, A, falling into a notch, the hammer, if we may so term it, is retained in that position. The effect of this movement xs to draw out also a small rod attached to the hammer, and terminated in front by a needle, about \ in. long, at the same time that a spiral spring surrounding the rod is compressed, the spring being fastened to the front end of the rod, and abutting against a screw-plug, which closes the hinder end of F, and permits only the rod to pass through it. The piece F, which is also movable, has projecting from its front end a little hollow cylinder, through the centre of which the needle passes, and this little cylinder has a collar, serving to retain its position, an india-rubber ring surrounding a portion of the cylinder, and forming a plug to effectually close the rear end of the barrel. It will be noticed that the cylinder is continued by a smaller projection, which forms a sheath for the point of the needle. The movable breech-piece, F, is provided with a short lever, E, by which it is worked. The second movement performed by the person who is charging the piece is to turn this lever from a horizontal to a vertical position, which thus causes the piece F to turn 90 about its axis, and then by drawing the lever towards him he removes the piece F from the end of the barrel, which, thus i 3 6 FIRE- ARMS. exposed, is ready to receive the cartridge. The cartridge contains the powder and the bullet in one case, the posterior portion containing also a charge of fulminate in the centre, and it is by the needle penetrating the case of the cartridge and detonating this fulminate that the charge is exploded. When the cartridge has been placed in the barrel, the piece F is pushed forward, the metallic collar and india-rubber ring stop up the rear of the barrel, and on turning the levef, E, into a horizontal position, the breech is entirely closed. If now the trigger be drawn, the hammer is released, and the spring carries it forward, at the same time impelling the needle through the base of the cartridge-case, where it immediately causes the explosion of the fulminate. The bullet is conical, and its base having a slight enlargement, the latter moulds itself to the grooves with which the barrel is rifled. When the piece has not to be fired immediately, the lever is not placed horizontally, but in an inclined position, in which the hammer cannot move forward, even if the trigger be drawn. The Chassepot has an effective range of 1,093 yards, and the projectile leaves the piece with a velocity of 1,345 ft. per second, the trajectory being such that at 230 yards the bullet is only 18 in. above the straight line. The piece can be charged and fired by the soldier in any position, and it was found that it could be discharged from seven to ten times per minute, even when aim was taken through the sights with which it is furnished, and fourteen or fifteen times per minute without sighting. The ordinary rifled musket, which this arm superseded, could only be fired twice in a minute, and could only be loaded when the soldier was standing up. MITRAILLEURS, OR MACHINE GUNS. '"PHE idea 01 combining a number of musket-barrels into one weapon, so - 1 - that these barrels may be discharged simultaneously or in rapid suc- cession, is not new. Attempts were made two hundred years ago to con- struct such weapons ; but they failed, from the want of good mechanical adjustments of their parts. Nor would the machine gun have become the effective weapon it is, but for the recent invention of the rigid metallic- cased cartridge. Several forms of machine guns have recently attracted much attention. There is the Mitrailleur (or Mitrailleuse), of which we heard so much at the commencement of the Franco- German War, and of whose deadly powers the French managed to circulate terrible and mys- terious reports, while the weapon itself was kept concealed. Whether this arose from the great expectations really entertained of the destructive effects of the mitrailleur, or whether the reports were circulated merely to inspire the French troops with confidence, would be difficult to determine. Our own policy in regard to new implements of war is not to attempt to conceal their construction. Experience has shown that no secret of the least value can long be preserved within the walls of an arsenal, although the French certainly apparently succeeded in surrounding their invention with mystery for awhile. The machine gun, or " battery," invented by Mr. Catling, an American, is said by English artillerists to be free from many defects of the French mitrailleur. In 1870 a committee of English military men was appointed to examine the powers of several forms of mitrailleur, with a view to reporting upon the advisability or otherwise of introducing FIRE-ARMS. 137 this arm into the British service. They recommend for certain purposes the Catling Battery Gun. In the Catling the barrels, ten in number, are distinct and separate, being screwed into a solid revolving piece towards the breech end, and passing near their muzzles through a plate, by which they are kept parallel to each other. The whole revolves with a shaft, turning in bearings placed front and rear in an oblong fixed frame, and carrying two other pieces, which rotate with it. These are the " carrier " and the lock cylinder. Fig. 98 gives a rear view, and Fig. 99 a side view, of the Catling battery gun. The weapon is made of three sizes, the largest one firing bullets i in. in dia- meter, weighing \ lb., the smallest discharging bullets of -45 in. diameter. FIG. gS.Ttie Gatling Battery Gun. Rear View. The small Gatling is said to be effective at a range of more than a mile and a quarter, and can discharge 400 bullets or more in one minute. Mr. Gatling thus describes his invention : " The gun consists of a series of barrels in combination with a grooved carrier and lock cylinder. All these several parts are rigidly secured upon a main shaft. There are as many grooves in the carrier, and as many holes in the lock cylinder, as there are barrels. Each barrel is furnished with one lock, so that a gun with ten barrels has ten locks. The locks work in the holes formed in the lock cylinder on a line with the axis of the barrels. The lock cylinder, which contains the lock, is surrounded by a casing, which is fastened to a frame, to which trimmers are attached. There is a partition in the casing, through which there is an opening, and irrto which the main shaft, which carries the lock cylinder, carrier, and barrels, is journaled. The main shaft is also at its front end journaled in the front part of the frame. In front of the partition in the casing is placed a cam, provided with spiral surfaces or inclined planes. i 3 8 FIRE-ARMS. " This cam is rigidly fastened to the casing, and is used to impart a reciprocating motion to the locks when the gun is rotated. There is also in the front part of the casing a cocking ring which surrounds the lock cylinder, is attached to the casing, and has on its rear surface an inclined plane with an abrupt shoulder. This ring and its projection are used for cocking and firing the gun. This ring, the spiral cam, and the locks make up the loading and firing mechanism. " On the rear end of the main shaft, in rear of the partition in the casing, is located a gear-wheel, which works to a pinion on the crank-shaft. The rear of the casing is closed by the cascable plate. There is hinged to the frame in front of the breech- casing a curved plate, covering partially the grooved carrier, into which is formed a hopper or opening, through which the cartridges are fed to the gun from feed-cases. The frame which sup- ports the gun is mounted upon the carriage used for the transportation of the gun. " The operation of the gun is very simple. One man places a feed-case filled with cartridges into the hopper; another man turns the crank, which, by the agency of the gearing, revolves the main shaft, carrying with it the lock cylinder, carrier, barrels, and locks. As the gun is rotated, the car- tridges, one by one, drop into the grooves of the carrier from the feed- cases, and instantly the lock, by its impingement on the spiral cam surfaces, moves forward to load the cartridge, and when the butt-end of the lock gets on the highest projection of the cam, the charge is fired, through the agency of the cocking device, which at this point liberates the lock, spring, and hammer, and explodes the cartridge. As soon as the charge is fired, the lock, as the gun is revolved, is drawn back by the agency of the spiral surface in the cam acting on a lug of the lock, bringing with it the shell of the cartridge after it has been fired, which is dropped on the ground. Thus, it will be seen, when the gun is rotated, the locks in rapid succession move forward to load and fire, and return to extract the cartridge-shells. In other words, the whole operation of loading, closing the breech, discharging, and expelling the empty cartridge-shells is conducted while the barrels are kept in continuous revolving movement. It must be borne in mind that while the locks revolve with the barrels, they have also, in their line of travel, a spiral reciprocating movement ; that is, each lock revolves once and moves forward and back at each revolution of the gun. " The gun is so novel in its construction and operation that it is almost impossible to describe it minutely without the aid of drawings. Its main features may be summed up thus : ist. Each barrel in the gun is provided with its own independent lock or firing mechanism. 2nd. All the locks revolve simultaneously with the barrels, carrier, and inner breech, when the gun is in operation. The locks also have, as stated, a reciprocating motion when the gun is rotated. The gun cannot be fired when either the barrels or locks are at rest. "There is a beautiful mechanical principle developed in the gun, viz., that while the gun itself is under uniform constant rotary motion, the locks rotate with the barrels and breech, and at the same time have a longi- tudinal reciprocating motion, performing the consecutive operations of loading, cocking, and firing without any pause whatever in the several and continuous operations." The small Catling is supplied with another improvement called the " drum feed." This case is divided into sixteen sections, each of which contains twenty-five cartridges, and is placed on a vertical axis on the top FIRE-ARMS. FlG. 99. The Catling Battery Gun. Front View. of the gun. As fast as one section is discharged, it rotates, and brings another section over the feed aperture, until the whole 400 charges are expended. The committee of military officers before referred to caused many experi- ments to be performed, with a view of testing the power of the new weapon. The nature of some of the experiments will be understood from the follow- ing table : TRIAL OF MONTIGNY AND SMALL CATLING MITRAILLEURS v. 12-pouNDER BREECH-LOADER AND 9-POUNDER MUZZLE- LOADING RIFLED GUNS, At 800 yards, against three rows of 45 ft. X 9 ft. targets, 15 yards apart, representing columns of infantry and cavalry. (Tffftfj two minutes.) 1 Range. Nature of Ordnance. Projectile. Number of rounds fired. Total number of hits on three screens. Total Cavalry dis- abled. Total Infantry dis- abled. Remarks. ab nit yards. c Numerous frag- 800 i2-pr. breech- loader, rifled 1 Seg- j raent 6 496 62 93 1 ments, yth round in the gun when V. time was up. Two rounds prema- 9-pr. muzzle- loader, rifled ) Shrap- ) nel 7 254 45 59 ] ture and one burst over. Total effec- (^ tive rounds, 4. Mitrailleur n 81 34 21 { 1 2th plate in gun. Fired in volleys. ( Gun worked stiff- Small Catling *- 82 191 Si H ly, and the tire checked twice by t cartridge jamming. i 4 o FIRE-ARMS. After a careful comparison of the effects of field artillery firing shrapnel, the committee concluded that the Catling would be more destructive in the open at distances up to 1,200 yards, but that it is not comparable to artillery in effect at greater distances, or where the ground is covered by trees, brushwood, earthworks, &c. The mitrailleur, however, would soon be knocked over by artillery if exposed, and therefore will probably only be employed in situations under shelter from such fire. An English officer, who witnessed the effects of mitrailleur fire at the battle of Beaugency, looks upon the mitrailleur as representing a certain number of infantry, for whom there is not room on the ground, suddenly placed forward at the proper moment at a decisive point to bring a crushing fire upon the enemy. Many other eye-witnesses have spoken of the fearfully deadly effect of the mitrailleur in certain actions during the Franco-German War. Mr. Catling contends that, shot for shot, his machine is more accurate than infantry, and certainly the absence of nerves will insure steadiness ; while so few men (four) are necessary to work the gun that the exposure of life is less. No re-sighting and re-laying are necessary between each dis- charge. When the gun is once sighted its carriage does not move, except at the will of the operator ; and the gun can be moved laterally when firing is going on, so as to sweep the section of a circle of 12 or more without moving the trail or changing the wheels of the carriage. The smoke of battle, therefore, does not interfere with its precision. The small Catling is supplied with another improvement called the " drum feed." This case is divided into sixteen sections, each of which contains twenty-five cartridges, and is placed on a vertical axis on the top of the gun. As fast as one section is discharged, it rotates, and brings another section over the feed aperture, until the whole 400 charges are expended. Whatever may be the part this new weapon is destined to play in the wars of the future, we know that every European Power has now provided itself with some machine guns. The Germans have those they took from the French, who adhere to their old pattern. The Russians have made numbers of Catlings, each of which can send out, it is said, i ,000 shots per minute, and improvements have been effected, so as to obtain a lateral sweep for the' fire. A competitor to the Catling presents itself in the Belgian mitrailleur, the Montigny, Fig. 100. This gun, like the Catling, is made of several diffe- rent sizes, the smallest containing nineteen barrels and the largest thirty- seven. The barrels are all fitted into a wrought iron tube, which thus con- stitutes the compound barrel of the weapon. At the breech end of this barrel is the movable portion and the mechanism by which it is worked. The movable portion consists mainly of a short metallic cylinder of about the same diameter as the compound barrel, and this is pierced with a number of holes which correspond exactly with the position of the gun- barrels, of which they would form so many prolongations. In each of the holes or tubes a steel piston works freely ; and when its front end is made even with the front surface of the short cylinder, a spiral spring, which is also contained in each of the tubes, is compressed. The short cylinder moves as a whole backwards and forwards in the direction of the axis of the piece, the movement being given by a lever to the shorter arm of which the movable piece is attached. When the gun is to be loaded this piece is drawn backwards by raising the lever, when the spiral springs are relieved from compression, and the heads of the pistons press lightly against a flat steel plate in front of them. The withdrawal of the breech-block gives space for a steel plate, bored with holes corresponding to the barrels, to be slid FIRE-ARMS. 141 FIG. ioo. The Montigny Mitrailleur. down vertically ; and this plate holds in each hole a cartridge, the head of each cartridge being, when the plate has dropped into its position, exactly opposite to the barrel, into which it is thrust, when the movable breech- block is made to advance. The anterior face of this breech-block is formed of a plate containing a number of holes again corresponding to the barrels, and in each hole is a little short rod of metal, which has in front a project- ing point that can be made to protrude through a small aperture in the front of the plate, the said small apertures exactly agreeing in position with the centres of the barrels, and being the only perforations in the front of .the plate. The back of the plate has also openings through which the heads of the pistons can pass, and by hitting the little pieces, or strikers, cause their points to pass out through the apertures in front of the plate, and enter the base of the cartridges, vfhzre fulminate is placed. The plate filled with cartridges has a bevelled edge, and the points of the strikers are pushed back by it as it descends. The heads of the pistons are separated until the moment of discharge from the recesses containing the strikers by the flat steel plate or shutter already mentioned. The effect, therefore, of pushing the breech-block forward is to ram the cartridges into the barrels, and at the same time the spiral springs are compressed, and the heads of the pistons press against the steel shutter which separates them from the strikers, so that the whole of the breech mechanism is thus closed up. When the piece is to be fired a handle is turned, which draws down the steel shutter and permits the pistons to leap forward one by one, and hit the strikers, so that the points of the latter enter the cartridges and inflame the fulminate. The shutter is cut at its upper edge into steps, so that no two adjoining barrels are fired at once. The whole of the thirty-seven barrels can be fired by one and a quarter turns of the handle, which may, of course, be given almost instantly, or, by a slower movement, the barrels can be discharged at any required rate. The barrels of the machine guns we have described do not, as is generally supposed, radiate ; on the contrary, they are arranged in a perfectly parallel direction. It is found that at the proper ranges the bullets spread suffi- ciently, for there are many causes which prevent them from pursuing a perfectly parallel course. 142 FIRE-ARMS. FIG. loi. Mallefs Mortar, SHELLS AND EXPLOSIVE BULLETS. PHESE missiles were formerly called "bombs," from the Latin bombus, , ,. 1T } allu sion to the noise they make when exploded. The bomb is a hollow iron globe cast pretty thick, and having a round aperture by which it can be filled and lighted. After being carefully examined, to ascer- tain if there are any flaws, it is nearly filled with gunpowder, and a fuse inven m. Bombs are now called "shells," whether discharged from FIRE-ARMS. 143 mortars or guns. The largest formerly used in the British service had a diameter of 13 in. ; was rather more than 2 in. thick ; it contained about 10 Ibs. of gunpowder ; and weighed I cwt. 3 qrs. 2 Ibs. The shells now in use vary from a few pounds to many hundreds of pounds : the largest naval guns throwing a shell weighing 700 Ibs. the Mallet, 36 in. Mortar, remains unsurpassed in respect of the weight of metal which it is capable of throwing, viz., 2,986 Ibs.! This enormous weight was thrown one mile and a half at the Plumstead practice range. The shells were embedded to a great depth ; a few were dug up, but most of them were buried fully 30 feet ; and as it costs about 21 to recover FIG. 102. The Shrapnel and Segment Shells. them, they are allowed to remain. Two mortars were made at a cost of ^5,000 each ; only one has been fired, and they weigh 35 tons being the precise weight of " England's Thunderer," the 7oo-pounder already alluded to. Mallet's mortar gave way with charges of from 60 to 70 Ibs. of gun- powder. The 7oo-pounder fires 120 Ibs. of powder. The most destructive shells now used are those called " The Shrapnel " and " Segment Shells." The shrapnel consists of an iron case containing a number of bullets. The bursting charge of gunpowder is behind them, and when exploded by the fuse, which is ignited by the flash of the gun, they travel forward with a greater velocity than the shell, and, spreading out like the sticks of a lady's fan, carry death and destruction before them. The segment shell is made up of forty-nine pieces of iron, some of which are shown between the two shells in Fig. 102. The pieces of cast iron are held together with lead ; the bursting charge is placed in the centre, and is ignited by a percussion fuse, which explodes when the shell strikes any object. i 4 4 FIRE-ARMS. A military authority writing on this subject says : "A shrapnel shell may be said to be a short cannon containing its charge of powder in a thick chamber at the breech end ; the sides of the fore part of the shell are thinner than those of the chamber, and may be said to form the barrel of the cannon. This cannon is loaded up to the muzzle with round balls, which vary with the shell in size. An iron disc between the powder and the bullets represents the wad used in ordinary fowling-pieces. A false conical head is attached to the shell, so that its outward appearance is very similar to that of an ordinary cylindro-conoidal shell : that is to say. it looks like a very large long Enfield bullet. When fired, the flash of the gun lights a time-fuse in the head of the shell, which at the proper moment communicates with the powder charge in the chamber through a pipe which lies in the axis of the shell. The spinning motion which had been communicated to the shell by the rifling of the gun from which it had been fired, causes the barrel filled with bullets to point in the direction of the object at which the gun had been aimed. Consequently, when the shrapnel shell is burst, or rather fired off, the bullets which it contained are streamed forward with actually greater velocity than that at which the shell had been moving ; and the effect produced is similar to firing grape and canister from a smooth-bore cannon at a short range. " The segment shell consists of a thin casing like a huge conical-headed thimble, with a false bottom attached to it. It is filled with small pieces of iron called ' segments/ cast into shapes which enable them to be built up inside the outer casing into two or more concentric circular walls. The internal surface of the inmost wall forms the cavity of the compound or segment shell, and contains the bursting charge. The segment shell is fitted with a percussion fuse, which causes it to explode when it strikes. In the shrapnel shell, the powder charge is situated in rear of the bullets, and consequently produces the chief effect in a forward direction. In the segment shell, the powder is contained inside the segments, and therefore produces the chief effect in a lateral direction. When the shrapnel shell is burst at the right moment, its effect is greatly superior to that of the segment shell-; on the other hand, the segment shell when employed at unknown or varying distances is far more likely to explode at the proper time. " Shrapnel and segment shells can be used with field artillery, i.e^ 9-pounders, 12-pounders, i6-pounders ; and also with heavy rifled guns in fortresses, viz., 4o-pounders, 64-pounders, 7-in. and Q-in. guns. But the conditions of their service are very different in each case. With regard to field artillery, the distance of the enemy is rarely known and is constantly changing, and hence it is very difficult to cut the time-fuse of the shrapnel shell to the proper length. Moreover, the men who have to adjust the fuses would probably be exposed to the fire of the enemy's artillery, and conse- quently could not be expected to prepare the fuses with the great care and nicety which are absolutely necessary to give due effect to the shells. There are, however, some occasions when the above objections would not hold good as for instance, when field artillery occupy a position in which they wait the attack of an enemy advancing over ground in which the distances are known. " Segment shells require no adjustment of their percussion fuse. They enable the artillerymen to hit off the proper range very quickly, since the smoke of the shell which bursts on striking tells them at once whether they are aiming too high or too low. This authority, therefore, coincides with FIRE-ARMS. that of the Dartmoor Committee, who decided that both shrapnel and segment shells should be served out in certain proportions to field batteries. " With regard, however, to the service of heavy rifled guns in fortresses, the conditions are quite different. In the first place, the distance of all objects in sight would be well known beforehand ; and in the second place, the fuses of the shells would be carefully cut to the required length in the bomb-proofs, where the men would be completely sheltered. The 7-in. shrapnel contains 227 bullets, and a 9-in. shrapnel would contain 500 bullets of the same size, and these shells could be burst with extraordinary accuracy upon objects 5,000, 6,000, or 7,000 yards off." This admirable digest was written in answer to the statement in the " Times," that shrapnel shells were rarely used with very heavy guns, and the author goes on to say " that there is nothing to prevent strategical fortresses in England from being armed with 7-in. and 9-in. rifled shell guns, and thus it would be easy to estimate the chances of success with which any invading army could attack a properly constructed fortress armed with rifled guns firing 7-in. and 9-in. shrapnel shells." Captain C. Orde-Browne, Royal Artillery Laboratory Instructor at the Royal Arsenal, Woolwich, in a recent lecture given by him in speaking of the changes which had taken place in guns and projectiles remarked that the bullets from a shrapnel shell might kill and wound a great number of men, while the fragments of a common shell in bursting could only blow one or two into atoms. In the late experiments at Dartmoor against rows of targets representing columns of troops, it was shown that a single field battery advantageously placed, and firing the improved shell, could kill or disable 20,000 men in one hour. FIG. 103. Norton's Explosive Bullets or Rifle Shells. The Prussians used the ogivo-cylindrical form of percussion shell. These projectiles burst by means of fuses called "percutent." At the moment when the shell strikes the ground, a small metallic rod or stalk, terminat- ^ g iJ n a ? mt ' acts upon a fu } minatin g capsule screwed on the head of the shell, and the powder is ignited. The shell bursts, and the fragments to the number of twenty or more of different sizes are violently scattered, mostly in front, to a distance of several hundred yards. The shell has a leaden covering which is separated from the shell, and adds to the de- structive effect produced by the cast-iron fragments. This article will hardly be complete without some reference to Explosive Bullets, which have been well described, and their proper use indicated, by Major Fosbery. Of course, such bullets are in fact shells in miniature, and it has been mercifully urged that it is quite sufficient to maim or otherwise render useless an enemy, without blowing him into a shapeless 10 146 FIRE-ARMS. mass, or causing increased torture by making the bullet that penetrates the body explode instantly. Explosive bullets or rifle shells have been, and are now, regularly used in shooting elephants, lions, tigers, and other larger animals ; and the advocates for their use declare it is more merciful to kill an animal with one blow and flash, than to condemn it to a slow and lingering death by a number of single bullet wounds. The first rifle shell was prepared by Captain Norton in the year 1826, and is shown in Fig. 103. In one case, a, the bullet is exploded by some FIG. 104. General John Jacobs' Explosive Bullet. percussion powder, fired by a wooden plug ; in the other, <:, the bullet contained a small tin tube fitted into the hollow; this was filled with gunpowder, and a percussion cap placed on the end of the tube, which explodes on contact ; b represents the external shape of Norton's explosive bullet. This was succeeded by another invented by General John Jacobs. FIG. 105. Major Fosbery's Explosive Bullets. The next were the Metford and Boxer Shells, succeeded by Major Fosbery's invention. This rifle shell was carefully tried in India in 1863, and used for the purpose of blowing up artillery tumbrils. It was subsequently employed by Major Fosbery, not for destroying human life, but for judging distances in the hill country of India ; and with the greatest success at the Umbeyla Pass, where, by watching the smoke and detonation on the rocks, they were enabled to judge the distance and aim the mountain guns with just as much precision, as they might have done on level ground. FIG. 106. Harvey's Torpedo. Working the Brakes. TORPEDOES. 'T^HE notion of destroying ships or other structures by explosions of gun- * powder, contained in vessels made to float on the surface of the water, or submerged beneath it, is not of very modern origin. Two hundred and fifty years ago the English tried "floating petards " at the siege of Rochelle. During the American War of Independence similar contrivances were used against the British, and from time to time since then " torpedoes," as they were first termed by Fulton, have been employed in warfare in various forms ; but up to quite a recent period the use of torpedoes does not appear to have been attended with any decided success, and it is probable that but \ for the deplorable Civil War in the United States we should have heard / little of this invention. During that bitter fratricidal struggle, however, when so much ingenuity was displayed in the contrivance of subsidiary means of attack and defence, the torpedo came prominently into notice, having been employed by the Confederates with the most marked effects. It is said that thirty-nine Federal ships were blown up by Confederate torpedoes, and the official reports own to twenty-five having been so destroyed. This caused the American Government to turn their attention to the torpedo, and they became so convinced of the importance of this class of war engine that they built boats expressly for torpedo warfare, and equipped six Monitors for the same purpose. It has been well remarked that the torpedo plays the same part in naval warfare as does the mine in operations by land. This exactly describes the 117 10 2 148 TORPEDOES. purpose of the torpedo where it is used defensively, but the comparison fails to suggest its capabilities as a weapon of offence. There are few occa- sions where a mine is made the means of attack, while the torpedo readily admits of such an employment, and, used in this way, it may become a conspicuous feature of future naval engagements. Many forms of this war engine have been invented, but all may be classified, in the first place, under two heads : viz., stationary torpedoes, and mobile or offensive torpedoes ; while independent distinctions may be made according to the manner of firing the charge ; or, again, according to the mode of determining the instant of the explosion. The stationary torpedo may be fixed to a pile or a raft, or attached to a weight ; the offensive torpedo may be either allowed to float or drift against the hostile ships, or it may be propelled by machinery, or attached to a spar of an ironclad or other vessel. The charge may be fired by a match, by percussion, by friction, by electricity, or by some con- trivance for bringing chemicals into contact which act strongly upon each other, and thus generate sufficient heat to ignite the charge. The instant of explosion may be determined by the contact of the torpedo with the hostile structure (in which case it is said to be " self-acting "), or by clock- work, or at the will of persons directing the operations. In some cases lines attached to triggers are employed; in others electric currents are made use of. FlG. 107. Submerged lurpeao. In the American Civil War the stationary torpedoes at first laid down were self-acting, that is, they were so arranged as to explode when touched by a passing vessel. Such arrangements present the great disadvantages I being as dangerous to friendly as to hostile ships. The operation of placing them is a perilous one, and when once sunk, they can only be removed at great risk. Besides this, they cannot be relied on for certain TORPEDOES. 149 action in time of need, as the self-acting apparatus is liable to get out of order. The superiority of the method of firing them from the shore when the proper instant arrived, became so obvious that the self-acting torpedo was soon to a great extent superseded by one so arranged that an observer could fire it at will, by means of a trigger-line or an electric current. Similar plans had often been previously employed or suggested. For example, during the war between Austria and Italy the Austrian engineers at Venice had very large electric torpedoes sunk in the channels which form the ap- proaches to the city. They consisted of large wooden cases capable of con- taining 400 Ibs. of gun-cotton, moored by chains to a wooden framework, to which weights were lashed that sufficed to sink the whole apparatus, Fig. 107. A cable containing insulated wires connected the torpedo with an electrical arrangement on shore, and the explosion could take place only by the operator sending a current through these wires. The torpedo was wholly submerged, so that there was nothing visible to distinguish its posi- tion. There was no need of a buoy or other mark, as in the case of self- acting torpedoes, to warn friendly vessels off the dangerous spot, and there- fore nothing appeared to excite an enemy's suspicions. But it is, however, absolutely necessary that the defenders should know the precise position of each of their submarine mines, so that they might explode it at the moment the enemy's ship came within the range of its destructive action. This was accomplished at Venice in a highly ingenious manner, by erecting a camera obscura in such a position that a complete picture of the protected channels was projected on a fixed white table. While the torpedoes were being placed in their positions an observer was stationed at the table, who marked with a pencil the exact spot at which each torpedo was sunk into the water. Further, those engaged in placing the torpedoes caused a small boat to be rowed round the spot where the torpedo had been placed, so as to describe a circle the radius of which corresponded to the limit of the effective action of the torpedo. The course of the boat was traced on the picture in the camera, so that a very accurate representation of the posi- tions of the submarine mines in the channels was obtained. Each circle traced on the table was marked by a number, and the wire in connection with the corresponding torpedo was led into the camera, and marked with the same number, so that the observer stationed in the camera could, when he saw the image of an enemy's ship enter one of the circles, close the electric circuit of the corresponding wire, and thus instantly explode the proper torpedo. The events of the war did not afford an opportunity of testing practically the efficiency of these preparations. Another mode of exploding torpedoes from the shore has been devised by Abel and Maury. It has the advantage of being applicable by night as well as by day. The principle will be easily understood with the assist- ance of the diagram, Fig. 108, in which, for the sake of simplicity, the positions of only three torpedoes, I, 2, 3, are represented. In this arrangement two observers are required at different stations on the shore. At each station which should not, of course, be in any con- spicuous position is a telescope, provided with a cross-wire, and capable of turning horizontally about an upright axis. The telescope carries round with it, over a circular table of non-conducting substance, a metallic pointer which presses against narrow slips of metal let into the circumference of the table. To each slip of metal a wire passing to a torpedo is attached, and another wire is connected with the axis of the pointer, so as to be put into electric contact with each of the others when the pointer touches the TORPEDOES. C l-l A N N E. L KEY KEY FIG. 108. Mode of Firing Torpedo. corresponding piece of metal on the rim of the table. The mode in which these wires are connected with the torpedoes, the telescopes, and the elec- tric apparatus is shown by the lines in the diagram. At each station is a key, which interrupts the electric circuit except when it is pressed down by the operator. There are thus four different points at which contacts must be simultaneously made before the circuit can be complete or a torpedo ex- plode. In the diagram three of these are represented as closed, and in such a condition of affairs it only remains for the observer to depress the handle of the key at station B to effect the explosion of torpedo No. 2. The observer at station A is supposed to see the approaching vessel in the line of torpedo No. 2, and recognizing this as an enemy's ship, he depresses the key at his station. The operator at B, by following the course of the vessel with his telescope, will have brought the pointer into contact with the wire leading to No. 2 torpedo, and he then causes the explosion to take place by com- pleting the circuit by depressing his key. A modification of this plan is proposed by which the position of the torpedoes is indicated by placing marks, such as differently-coloured flags, or by night lamps with coloured glasses, throwing their light only towards the telescopes. These marks are placed in the line of direction of each torpedo from the telescope as at c \-> C
  • , containing 67 Ibs. of Gun-Cotton. The employment of torpedoes develops, as a matter of course, a system of defence against them. Nets spread across a channel will catch drifting torpedoes, and stationary ones may be caused to explode harmlessly by nets attached to spars pushed a great distance forward from the advancing ship. Another very interesting form of torpedo, which is adapted for offensive operations, has been approved by the English Government after satisfac- tory results had been obtained with it in various official trials. It is the invention of Commander Harvey, and is worthy of a detailed description for the ingenuity of its construction. The shape of Harvey's torpedo, as may be noticed on reference to Fig. 1 1 8, is not symmetrical, but it has some remote resemblance to a boat, though constructed with flat surfaces throughout. The outside case is formed of wood well bound with iron, all the joints being made thoroughly water-tight. The length is 5 ft. and the depth if ft, while the breadth is only 6 in. Within this wooden case is another water-tight case made of TORPEDOES. thick sheet copper, from the top of which two very short wide tubes pass upwards to what we may term the deck of the wooden case. These are the apertures through which the charge of gunpowder or other explosive material is introduced ; and when the tubes have been securely stopped with corks, brass caps are screwed on. The centre of the internal case is FIG. 113. Explosion of^lbs. ofGun-Cotton in 37 feet of Water. occupied by a copper tube, g, Fig. 115, which passes the entire depth, and is soldered to the top and bottom of the copper case, so that the interior of the tube has no communication with the body of the torpedo, the principal charge merely surrounding it. Thus the tube forms a small and quite in- FIG. 114. Explosion 0/432 Ids. of Gun-Cotton in 27 feet of Water. dependent chamber in the midst of the large one. which latter is capable of containing 80 Ibs. of gunpowder. The copper tube or priming-case con- tains also a charge, a, which when exploded bursts the tube, and thus fires the torpedo in its centre. The priming charge is put in from the lower end of the tube, which is afterwards closed by a cork and brass cap, h ; TORPEDOES. for the centre of the priming-case is occupied by a brass tube, b, closed at the bottom, but having within a pointed steel pin projecting upwards. In this tube works the exploding bolt c d, which requires a pressure of 30 or 40 Ibs. to force it down upon the steel pin. This pressure is communicated to the bolt by the straight lever working in the slot at its head, d, and itself acted on at its extremity by the curved lever to which it is attached. Thus from the mechanical advantage at which the levers act a moderate downward pressure suffices to force the exploding bolt to the bottom of the brass tube. The lower end of this bolt has a cavity containing an exploding composition sufficient in itself to fire the torpedo, even independently of the priming charge contained in the copper tube. This composition is safely retained in the end of the bolt by a metallic cap- sule,*:, which, when the bolt is forced down, is pierced through by the steel pin at the bottom of the brass tube, and then the ex- plosion takes place. The bolts are not liable to explosion by concussion or expo- sure to moderate heat, and they can be kept for an indefinite period without de- terioration. The mode of producing the explosion is not stated : it consists probably of an arrangement for bringing chemicals into contact. Besides the two levers already mentioned, a shorter curved lever working horizontally will be noticed. The object of this is to make a lateral pressure also T, c, j- r> effective in forcing down the bolt a result FIG. ^.-Section of Priming- accomplished . b y attaching to the short arm of the lever a greased cord, which, after passing horizontally through a fair- leader, runs through an eye (see Fig. 117) in the straight lever, and has its extremity fastened so that a horizontal movement of the short lever draws the other down. A very important part of the apparatus is the safety key,/, Fig. 1 1 5, a wedge which passes through a slot in the exploding bolt, and resting on the brasswork of the priming-case, retains the muzzle i in. above the pin. Through the eye of the safety key and round the bolts passes a piece of packthread, *?, which being knotted is strong enough to keep the key securely in its place, but weak enough to yield when the strain is put on the line, d', used for withdrawing the safety key at the proper moment. This line is attached to the eye of the key, and passes through one of the handles forming the termination of the iron straps. As repre- sented in Fig. 117, it forms the centre one of the three coils of rope. The bottom of the torpedo is ballasted with an iron plate, to which several thicknesses of sheet lead can be screwed on as occasion requires. Fig. 117 shows the arrangement of the slings by which the torpedo is attached to the tow-rope, and it will be seen that another rope passes backwards through an eye in the stern to the spindle-shaped object behind the tor- pedo. This is a buoy, of which two at least are always used, although only Case and Exploding Bolt. TORPEDOES. 157 FIG. 1 1 6. Harvey's Torpedo. one is represented in the figure. Each buoy, in length 4^ ft., is made of solid layers of cork built up on an iron tube running through it lengthways, so that the buoys admit of being strung upon the rope. Having thus described the construction of the torpedo, we proceed to explain how it is used. It must be understood that if the torpedo and its attached buoys are left stationary in the water, the tow-rope being quite slack, the torpedo will, from its own weight, sink several feet below the surface. But when they are towed, the strain upon the tow-line brings the torpedo to the surface, to dip below it again as often as the tow-line is slackened. There is another peculiarity in the behaviour of the torpedo, and that is that, when towed, it does not follow in the wake of the vessel, but diverges from the ship's track to the extent of 45. Its shape and the mode in which it is attached to the tow-line are designed so as to obtain this divergence. But, according as the torpedo is required to diverge to the right or to the left, there must be the corresponding shape and arrangement of tow-line and levers ; hence two forms of torpedo are required, the star- board and the port. The figures represent the port torpedo, or that which is launched from the left side of the torpedo-ship, and diverges to the left of its course. The efficiency of the torpedo depends upon the readiness and certainty with which it can be brought into contact with the hostile ship, and this is accomplished by duly arranging the course of the torpedo vessel, and by skilfully regulating the tow-line so as to obtain the requisite amount of divergence, and to cause the torpedo to strike at the proper depth. The tow-rope is wound on a reel, furnished with a powerful brake, the action of which will be readily understood by inspoction of Fig. 116, i 5 8 TORPEDOES. which represents also a similar smaller reel for the line attached to the safety key. Leather straps, sprinkled with rosin to increase the friction, encircle the drums of the reels, and can be made to embrace them tightly by means of levers, so that the running out of the lines can be checked as quickly as may be desired. Handles are attached to the straps, so that they can be lifted off the drum when the line is being drawn in by working FIG. 117. Harvey's Torpedo. the handles. When the torpedo is ready for action and has been launched, a suitable length of tow-line, which is marked with knots every ten fathoms, is allowed to run off its reel, while the safety key-line is at the same time run off the small reel, care being taken to avoid fouling or such strains on the line as would prematurely withdraw the key. Fig. 106 will make clear the mode of controlling the lines, but it is not intended to represent the actual disposition in practice, where the men and the brakes would be placed under cover. On the left of the figure a starboard torpedo is about to be launched ; on the right a port torpedo has been drawn under the iron- clad and is in the act of exploding, the safety key having been withdrawn by winding in its line when the torpedo came into proximity to the attacked vessel. When the torpedo has been launched over the vessel's side, the latter being in motion, the -torpedo immediately diverges clear of the ship ; and when the buoys have also reached the water, the men working the reels pay out the line steadily, occasionally checking the torpedo to keep it near the surface, but avoiding a sudden strain upon the slacked tow-rope, which would cause the torpedo to dive, and in shallow water this might lead to the injury or loss of the torpedo. The torpedo can be gradually veered out to the distance required, at the same time that the safety-key is so managed that sufficient strain may be put upon it to prevent it from forming a long TORPEDOES. 159 FIG. 1 1 8. Harvey's Torpedo. bight astern of the torpedo, but avoiding such a strain as would break the yarn holding the safety-key in its place. The distance to which the tow- line* may be paid will depend upon the circumstances of the attack. More FIG. 119. Official Trial of" Harvey's Sea Torpedo? February, 1870. than 50 fathoms is, however, a disadvantage, as the long bight of tow- lines makes the torpedo drag astern. The torpedo can always be made to dive several feet below the surface by suddenly letting out two or three fathoms of tow-line. The torpedo vessel should, of course, be a steamer of i6o TORPEDOES. considerable speed able to outstrip when necessary all her antagonists, and, as a rule, it is found best to make the attack at night. Let us imagine two ships of war at anchor, and parallel to each other at perhaps a distance of 60 fathoms ; and suppose that, under cover of darkness, a hostile tor- pedo vessel boldly steams up between them, having launched both its starboard and port torpedoes. In such a case neither ship could fire at the torpedo vessel for fear of injuring the other, while the torpedo vessel would in all probability succeed in bringing its floating mines into contact with both its enemies. FIG. 1 20. Model of Submarine Guns. Another device for submarine attacks upon vessels on which much inge- nuity has been expended is the submarine gun. It has been sought to propel missiles beneath the surface of the water, these missiles being usually provided with a charge which, on contact with the vessel's side, would ex- plode, and by making a hole below the water-line, cause the certain destruc- tion of the ship. It is obvious that such a mode of attack would reach the only vulnerable parts of a thickly-plated ironclad, and therefore the project has been recently revived in several forms. Fig. 120 is taken from the photo- graph of a model of an invention of this kind. The guns which are to propel the submarine projectiles, have port-holes formed by valves in such a manner that the gun when loaded can be run out without allowing water to enter ; it can then be fired while the muzzle is below the surface, and again drawn in without the port being at any time so opened that water can pour into the vessel. All contrivances of this kind have hitherto been failures ; indeed, it does not appear possible that they could succeed, except at very close quarters, for the resistance offered by water to a body moving rapidly in it is extremely great, and, as we have already had occasion to state, the resist- ance increases as the square of the velocity, and probably in even a higher degree for very great velocities. Any one who will remember the effort it TORPEDOES. 161 requires to move one's hand quickly backwards and forwards through water will easily understand that the resistance it presents would, in a com- paratively short space, check the speed of a projectile, however great that speed might be at first. Then the currents in the water will have a great effect on the path of the projectile, so that taking an aim would under such circumstances be practically impossible. For these reasons and others, submarine guns could, therefore, be used only at very close quarters, where probably other modes of attack, such as torpedoes attached to long spars, would be more effective. The real power of the offensive torpedo in actual warfare remains yet to be proved, for the few cases recorded during the Civil War in America have by no means settled the point ; and many inventions which appear quite successful when tested merely in deliberate experiments, where everything is previously arranged to insure the most favourable conditions, completely fail when applied in actual warfare. Many years ago Mr. Warner pro- duced a great sensation by an invention which appears to have been essen- tially a floating torpedo. The cut below, Fig. 121, represents the result of an experiment publicly made by him off Brighton, in 1844, upon a barque, which was towed out by a steamer to a distance of a mile and a half from the shore. Mr. Warner was on board the steamer, and the barque was 300 yards astern. Five minutes after a signal had been made from the shore, the torpedo was caused to explode, striking the barque amidships, throwing up a large column of water and debris, shooting the mainmast clean out of the vessel, the mizen going by the board, and dividing the hull into two parts, so that she sank immediately. Yet this invention, though apparently so successful, does not seem to have ever been put in practice. Fig. 121. The Warner Experiment off Brighton. 11 Portrait of M. Lesseps. THE SUEZ CANAL. THE reader who wishes to understand the exact position of the great engineering work called the Suez Canal must take down his atlas, and look for the Eastern Hemisphere, when no difficulty will be experienced in discovering the position of the vast continent of Africa, of which many parts still remain unexplored. Africa is washed on the north by the Medi- terranean Sea, on the west by the Atlantic, on the south by the Southern Ocean, and by the Indian Ocean and Red Sea on the east and north-east. The traveller who went to India in the famous sailing ships called East Indiamen was obliged formerly to sail round the " cape of storms," the Cape of Good Hope, and would pass from the Southern to the Indian Ocean. If the waterway round Africa is now traced out, we come to the Red Sea, and find that the only obstacle which would have prevented a ship making the circuit of 15,000 miles is a narrow neck of land, called the Isthmus of Suez. It has been well said that had there been a strait like that of Gibraltar, or that of Messina, at Suez, instead of a sandy isthmus, the achievements of Bartholomew Diaz, Vasco da Gama, and Columbus might have lost 162 THE SUEZ CANAL. 163 much of their significance ; but the progress of the human race would have been infinitely more rapid, and the advantages to the world's economy would have been incalculable. If the names of these hardy mariners would have come less prominently forward, it is fair to suppose that we should not have heard of another great man, viz., Monsieur Ferdinand Lesseps, whose portrait graces the head of this chapter, and who has been the persevering and successful pioneer and engineer to achieve the completion of the work. It would appear that there are many claimants for the honour of having first sug- gested the feasibility of the plan for cutting through the Isthmus of Suez. According to some recent discoveries in the chief archives of Venice, it was so early as the end of the fifteenth century, when Vasco da Gama had discovered the Cape of Good Hope, and the Portuguese took that new route to India, hitherto the exclusive property of the Venetian and Genoese merchants, it was already at that period that a re-cutting of the Isthmus of Suez was thought of. Plans were compiled and embassies sent to Egypt, for paving the way for the accomplishment of this great enterprise, which, it is said, was only foiled by the persistent opposition of some patricians (probably bribed by foreign gold) which prevented the execution of the plan. The ancient Egyptians did not cut through the entire isthmus, although they formed therein certain canals, the remains of which are visible to this day. One of the early English poets, Christopher Marlowe, born in the reign of King Edward VI., educated at Cambridge, and who appeared on the stage in the reign of Elizabeth, and was unfortunately assassinated with his own sword by a footman, appears to have anticipated M. de Lesseps, in the following lines : " Thence marched I into Egypt and Arabia, And here, not far from Alexandria, Whereat the Terrene and the Red Sea meet, Being distant less than full a hundred leagues, I meant to cut a channel to them both, That men might quickly sail to India." But the idea of carrying out the project, it is stated by Lord Houghton, was due to Pere Enfantin, of the St. Simonians, who, in the year 1833, interested M. Ferdinand Lesseps, the French vice-consul, and Mehemet Ali, the Pasha of Egypt, in taking practical measures towards its accom- plishment. Surveys were made, but, owing to the breaking out of a plague and to other causes, not much more was heard of the scheme till 1845. In 1846 La Societe d* Etude du Canal de Suez was formed, and among the names of those who then applied their attention to the subject was to be found that of Robert Stephenson. But he instituted inquiries, the result of which was wholly unfavourable to the enterprise. He recommended the con- struction of a railway through Egypt, and a line was made between Alex- andria and Suez. But, notwithstanding the opinion of Mr. Stephenson, M. Lesseps persevered with wonderful energy, believing, on the report of other engineers, that the scheme could be successfully carried out. Lord Houghton very properly said that M. Lesseps had not invented the Suez Canal. In fact, who did invent anything ? An invention is the result oi the information and the deductions of thousands of other men. M. Lesseps undertaking was the complement of what had already been done by other men ; but no less honour was due to him because it was so. 112 1 64 THE SUEZ CANAL. It is right to state that Mr. Stephenson did not say it was impossible to complete the Suez Canal ; he merely gave it as his opinion that the cost of making the canal, and keeping it in a proper state for navigation, would be so great that the scheme would not pay. If a great English engineer appeared to oppose the project, another eminent one, Mr. Hawkshaw, certainly helped it on at a moment when the Viceroy of Egypt was losing confidence, and had his opinion been adverse to the project reported upon, the Viceroy would certainly not have taken upon himself additional liability in connection with the undertaking, and the money expended up to that date would have been represented only by some huge mounds of sand and many shiploads of artificial stone thrown into the bottom of the sea, to make the harbour of Port Said. The canal is, however, an accomplished fact, and the magnitude of the enterprise may be appreciated when it is remembered that fourteen years have elapsed in carrying it out, and that in round numbers the Suez Canal is 100 miles long, 100 yards wide, having a minimum depth of 26 ft. The Suez Canal Company, it is said, has expended twelve millions of money in what was considered to be chiefly shifting sands. And that M. Lesseps appreciated the good offices of Mr. Hawkshaw is shown from the fact that when he introduced that engineer to various distinguished persons, on the occasion of the opening of the canal, he said, " This is the gentleman to whom I owe the canal." It cannot, therefore, be said of the English nation that they were jealous of the peaceful work of their French neighbours, or opposed it in any other sense but as a " non-paying " and apparently unprofitable scheme. No opposition, however influential, can now hinder the Suez Canal route from being the sea highway between Europe and India. The iron ship trade is at present yielding immense profits, and numbers of iron vessels are still being built for this particular route. It does not require much com- prehension to understand that the distance from London or Liverpool to Bombay by the Suez Canal as compared with that by the Cape is some 5,000 less out of 10,000 miles, and that in due time the whole course of trade between East and West will be changed by the piercing of the Isthmus of Suez. The traveller who wishes to see the canal should go to France, and embarking at the port of Marseilles, cross the Mediterranean Sea, and steam to Port Said, which is about 150 miles east of the port of Alexandria, where the isthmus is crossed by the railroad, and is used by travellers to India, being known by the cognomen of the " overland route." And this railway conveys the mail to and from India, thus saving the great sea voyage round Africa and the Cape of Good Hope. Nevertheless, it involves two transhipments from the steamer to the rail at Alexandria, and from the railway to the steamer at Suez. Port Said is the little town at the northern or Mediterranean entrance of the canal. It is chiefly built of wood, with straight wide streets and houses, and although now containing a population of six thousand inhabi- tants, it would have been difficult to have got together one hundred people at that spot twenty years ago. It is said to have a very flourishing and bright appearance, as seen from the canal, and, like all the other towns on the Suez route, has a striking resemblance to the newly settled cities of America, and being composed of very combustible materials, would be burnt down in a very short space of time. Port Said is provided with docks, basins, quays, and warehouses, and THE SUEZ CANAL. 165 FIG. 1 22. Port Said, the Mediterranean entrance to the Suez Canal. has a harbour stretching out a couple of miles or so into the sea, which is enclosed by two piers, or rather breakwaters. One of the most serious obstacles to the advancement of Port Said is the absence of fresh water, which is supplied from the "fresh-water canal" at Ismailia, where two engines of 25 -horse power are at work night and day pumping the water through a distance of 40 miles. If the canal which carries the beautiful Nile- water from Cairo to Ismailia was prolonged to Port Said, this diffi- culty would no longer prevail, but it could only be carried out by a great expenditure of time and money. The harbour (Fig. 1 24)however, is now completed ; two converging break- waters have been built out into the Mediterranean from the coast, where a lighthouse is erected, the larger and more westerly one being one mile and a half long, the shorter about a mile and a quarter, and the distance between the two lighthouses erected on the extremities of the breakwater being half a mile. The piers are made of concrete and sand, cast into blocks weighing 10 tons each. This composition has of late years been greatly approved by engineers, where stone cannot be procured. The sea-face of the great canal now being completed in Holland is composed of a similar artificial stone, and it is found to bear the wear and tear of the waves almost, if not quite, as well as ordinary stone. It is stated that 25,000 blocks, each weighing 10 tons, were used. They are not laid with the regularity of ordinary masonry, but have been dropped from large barges, so that they present a very rugged and uneven appearance. (Fig. 1 23.) Experienced naval officers say that Port Sa'id is superior to Alexandria as a port, and the object of throwing out these great bulwarks is for the purpose of preventing the sand brought down by the Nile silting in and closing up the Suez Canal. Along the western pier there is a constant settlement of sand from this cause, which, they say, is partially washed through the interstices left be- tween the blocks of artificial stone, and may in time give some trouble by i66 THE SUEZ CANAL. FIG. 123. One of tke Breakwaters at Port Said. forming sand-banks in the harbour ; but it is quite possible to conceive that this will be easily prevented by the introduction of smaller stones, which could be readily inserted by workmen from boats, going out at the low tide. FIG. 124. Bird's-eye View of Port Said. " Rob Roy" describes Port Said as " a little town at the north entrance of the canal. It is built of wood, with wide straight streets, and houses that look like brown paper, and that would burn from end to end in ten THE SUEZ CANAL. 167 FIG. i2$.-Map of the Suez Canal. minutes. Hotels, cafe's, shops, and bazaars are crowded by six thousand people of every nation, but with the Greek and Levantine element largely preponderating." At Port Said are seen vessels of con- siderable tonnage waiting their turn to enter the canal, which is indicated by two obelisks (Fig. 122), being part of the festive arrangements prepared for the reception of Her Majesty the Empress of the French. They consist of a frame- work of wood, covered with painted canvas, and will no doubt be some day replaced by granite. Having now fairly entered the canal, it is quite as well to consult the map (Fig. 125), in order to know the precise localities of the places passed by the traveller in going from Port Said to Suez and the Red Sea. The arrow points in the direction of the compass, and shows that the canal runs very nearly from north to south. Beginning with the Mediterranean Sea and Port Sa'id, there is a run of 28 miles through Lake Menzaleh to Kantara. Although called a lake, it is in truth nothing but a shallow lagoon or swamp, and here it was found better to enclose the canal with high embankments of sand, in order to keep out the waters of the lake, which were of no practical service in the feeding of the canal, but were very detrimental to its completion. Of all portions of the undertaking, this one, M. Lesseps states, was the most arduous and difficult, though at the time it attracted the least attention. A trough had to be dredged out of the bed of the shallow lagoon, and on either side of this hollowed-out space high sand-banks had to be erected, and the difficulty of making a solid foundation for these sand-banks was found to be extreme. The difficulty, however, was sur- mounted, and such is the excellence and stability of the work that the water of the canal neither leaks out, nor does any of the brackish water of the lagoon infiltrate and undermine the great em- bankments. A passenger on board a steamer going through this part of the 168 THE SUEZ CANAL. FIG. 126. A Group of Egyptian Fellahs ', and their Wives. canal states that " the only curiosity worthy of notice is the constant appear- ance of the curious effect of the ' mirage/ which brought up on the horizon the illusory effect of trees and houses, that could not have 'existed in the position where they were seen. The waters of the lake were covered with wild ducks, and white pelicans stalked about in the marshy places." At Kantara the canal crosses the track of the highway between Cairo and Syria ; a floating bridge carries the caravans across. And near this spot is stationed an Egyptian man-of-war, which supplies the police for the proper watch and ward of the canal. There are two hotels here, which condescend to supply the traveller at exorbitant prices. From Kantara to El Fendane is a distance of 15 miles, that is to say, to the southern extremity of Lake Ballah, where the canal still passes through sand embankments raised within a mere. This lake is, however, almost dried up, and therefore the difficulties which had to be surmounted at Lake Menzaleh were not felt here. According to the original programme, the canal was to have been con- structed by forced labour, supplied by the Viceroy. The unhappy peasantry of the country, called "fellahs" (Fig. 126) were to be induced, by a liberal supply of stick, to give their labour for a miserable pittance of rice. No doubt, in ancient times, the corve, or forced labour, was in use, when every peasant might cheerfully work, because it was for the general benefit to bring sweet water from the Nile to other dry and thirsty places in Egypt ; but to be obliged to work at a waterway of salt water, which was only to be of use to foreigners who passed through their country, could not be expected of frail human beings, and thus the cruelties of the work fre- quently equalled the horrors of other slave countries. This was one of the reasons why the late Lord Palmerston opposed the canal scheme ; for the kind-hearted statesman bore in mind the loss of health and life occasioned to poor Egyptians by this mode of labour, and the more so because it had been originally proposed that one of the conditions on which the French THE SUEZ CANAL. 169 company was to take up the project should be the execution of the work by free labour. In consequence, no doubt, of representations from free countries, the Porte was induced to put a veto on the employment of forced labour, and every one thought that this would be the death-blow to the completion of the canal ; but M. Lesseps did not give way to despair, and has since stated that if he had depended only on the labour of the fellahs, the gigantic difficulties of the work never could have been surmounted, and, in fact, that he owed the success of his canal to his not having had labourers of that sort, because he had turned his attention to the mechanical con- trivances used for dredging on the Thames and Clyde, and the result was that the work had been done in half the time and at half the expense that would have been necessary if the dredging had been done by the manual labour of the poor fellahs. Mr. Fowler, the eminent engineer, has stated that the dredges used in the construction of the canal were of a new description. They were won- derful mechanical contrivances, and but for them the canal would not have been finished. They were not the contrivance of M. Lesseps, but of one of the contractors, a distinguished engineer, who received his technical education in France, but his practical experience in England. The use of the dredging machines was prepared for by digging out a rough trough by spade-work, and as soon as it had been dug to the depth of from 6 ft. to 12 ft. the water was let in. After the water had been let in, the steam- dredges were floated down the stream, moored along the bank, and set to work. These dredges are of two kinds. The great " couloirs " * consist of a long, broad, flat-bottomed barge, on which there stands a huge frame- work of wood, supporting an endless chain of heavy iron buckets. The chain is turned by steam, and the height of the axle is shifted from time to time, so that the empty buckets, as they revolve round and round, shall always strike the bottom of the canal at a fixed angle. As they are dragged over the soil they scoop up a quantity of mud and sand and water; and as each bucket reaches its highest point in the round, it discharges its contents into a long open iron pipe, which runs out at right angles to the oarge. The further extremity of this pipe stretches for some yards beyond the bank of the canal ; and, therefore, when the dredging is going on there is a constant stream of liquid mud pouring from the pipe's mouth upon the shore, and thus raising the height of the embankment. When the hollow scooped out by the buckets has reached the required depth, the dredge is moved to another place, and the same process is repeated over and over again. These stationary dredges, however, though very effective, require much time in moving, and the lighter work of the canal has been chiefly effected by movable dredges of a smaller size. These machines are of the same construction as those described ; the only difference is, that the mud raised by their agency is not poured directly on shore by pipes attached to the dredges, but is emptied in the first instance into large barges, divided into partitions, and moored alongside the dredge. Each partition contains a railway truck, and when the barge is filled it steers away to the bank, where an elevator is fixed. The trucks filled with mud are raised by a crane worked by steam power, and placed upon inclined rails, attached to the elevator, which slope upwards at an angle of 45 towards the bank. They are then drawn up the rails by an endless rope, and as each truck ' " Couloir " signifies a long open pipe, in allusion to the pipe attached to the dredging machine. i7o THE SUEZ CANAL. FIG. 127 '.Dredges and Elevators at Work. reaches the end of the rails, its side falls open, the mud is shot out upon the bank, and the empty truck returns by another set of rails to the plat- form on which the elevator is placed, and is thence lowered into the barge to which it belongs. As the elevator can unload and re-load a barge much faster than the dredges can fill it with mud, each elevator is fed by half a dozen dredges, and thus the mud raised from the canal by several dredges is carted away without difficulty at one and the same time. As these floating dredges are much easier to shift than those encumbered by the long couloir pipes, the work of excavating the bed goes on much more rapidly. But in places where there is any great mass of earth or sand to be removed, the large couloirs can scoop out a given volume in a shorter time. The chief contractor, M. Lavalle, calculated that the expense of keeping the canal clear from silting up might be estimated at about ,40,000 per annum ; but it does not appear that this is the case, and happily so, from the following authentic statement in the "Times," made I7th June, 1871, by Sir David A.' Lange, English director of the Suez Canal Company: " The following are the measures adopted for maintaining the Suez Canal at its present minimum depth of 26 English feet of water. " Two dredgers will be assigned for dredging the channel of the outer harbour at Port Said, and removing any silt which may have accumulated during last year. " Two other dredgers are to be employed in excavating the new circular basin to be formed on the Asiatic shore of the canal, and to continue dredging operations in the Cherif basin. *' A fifth dredger is destined to maintain the canal along the entire line of section terminating at kilometre 60*500. " The second section, commencing at kilometre 60-500, will only have two dredgers at work one for widening the bend north of El-Guisr, and the other to complete the siding at the Bitter Lakes. "All along the Suez section a single dredger has been deemed sufficient to maintain the canal." The marvellous nature of the operations required to push on the canal to completion appears to have struck many independent observers, and amongst them none are more entertaining than " Rob Roy," who says : " The sensation of wonder at the prodigious scale of the operations in THE SUEZ CANAL. 171 progress increases day by day as one moves along what seems to be a wide river, with villages on the banks, and smoky funnels and white sails on the surface. The hydraulic machines, which groan and snort and rattle their chains as they work, are of enormous size; and though each of them seems to be pouring forth a volume of mud, yet the mind finds it hard to believe that all of these together can lift up and throw over the banks enough to make any appreciable progress between yesterday and to-day. The sand dredged from below is either carried out to sea in barges, or (farther inland) is delivered in a stream from a lofty iron tube, 220 ft. long, with its mouth over one bank, or it is hoisted up an iron inclined plane and cast upon the shore, until the heap on each side of the water is 50 ft. high. The engines for this purpose are forty in number, and each of them cost ^40,000. The expenses at present amount to ^200 coo every month, and the work has already absorbed eight millions sterling.** From El Fendane, at the end of Lake Ballah, to Lake Timsah, a distance of about five miles, the canal was dug out by the " fellahs ; " nearly 60,000 men were engaged upon it, organized in separate great gangs, 35,000 work- ing by day and 25,000 by night. A tramway was laid down, and the sand was taken in ballust-trucks and shot into the desert. The traveller is now supposed to have arrived at Lake Timsah, where no doubt, in the days of the Pharaohs, a lake existed. When taken in hand by M. Lesseps, it was a barren sandy hollow, containing a few shallow pools, through which a man could easily wade, but now it is filled with the waters of the Mediterranean Sea. It is a pretty inland salt-water lake about three miles in width. On the northern shore stands the town, or rather small settlement, of Ismaiiia, which is, in fact, the " half-way house," where most of the officials of the Suez Canal Company reside, as they can get to either end of the canal with greater facility, or to Cairo, by the rail- road, which comes to this point and continues with the canal to Suez. Before the station and railroad were constructed the desert was perfectly solitary, varied only by the tents of half-naked and ragged Arabs ; now it is a scene of bustle and prosperity. The Viceroy of Egypt is building a stone palace ; here and there are a few villas and an hotel. The chief reason why Ismaiiia is likely to become a flourishing place is because the 172 THE SUEZ CANAL. FIG. 129. Lake 'limsah and Ismailia. settlement is well supplied with water, which has been brought from Cairo by canal, and called very properly " The Sweet-Water Canal." " Rob Roy " speaks most enthusiastically of it. " The Sweet- Water Canal is already a blessing to Egypt. It is from 30 to 40 ft. wide, and boats with all sorts of cargoes are towed through it by men on foot, or sail along gaily if there is a breeze to fill their snowy wings. My canoe excited the greatest delight among all this river population, both when she skimmed over the water with her blue sails, or rested by the bank with her cabin rigged up, and my dinner cooked, and my little reading lamp and mosquito curtains arranged for the night. I managed to sleep thus in the canoe very comfortably, though the nights were cold ; and on Lake Timsah a jackal paid me a visit at a very unfashionable hour by moonlight. During one day a violent gale swept across the canal. To look at the desert was to see a vast yellow picture of men and camels dimly floating in a sea of sand without any horizon. The quantity of sand whisked from the plain, and cast into the canal-water by a wind like this, will be a serious matter to deal with. An ounce of sand per square yard amounts to 500 tons on the whole canal, and the wind sometimes blows in this way for a month together." The palace of the Viceroy is situated on the Fresh-Water Canal, and M. Lesseps has built a pretty villa at this spot, and within eight days of the opening of the canal, took to himself, in the little church of Ismailia, a young and handsome wife. Travellers speak highly of the beautiful gardens on the banks of this canal, and indeed vegetation has sprung up on all sides ; orange trees and acacias, tall enough to shelter you from the noontide sun, are to be found in the garden of M. Lesseps. Lake Timsah is the head-quarters of the land works, and, in fact, the depot of the company, where vessels going up or down the Suez Canal may cross each other safely. The course is marked out with buoys. It commences at the north-east corner, and after sweeping round in a wide curve, passes out at the south-east. From Lake Timsah to the Bitter Lakes the canal again passes for eight miles or so through the desert, where by partial excavations by hand labour, and subsequent flooding to admit the dredges, it was considered that a THE SUEZ CANAL. 173 sufficiently deep channel could be made. The couloirs were set to work, when suddenly "a lion arose in their path" in the shape of a great rock, about 80 ft. in length, and lying 12 ft. only below the surface, and right in the middle of the main channel. If anything could show the indomitable energy of M. Lesseps, it would be his courage in dealing with this diffi- culty, and at a time when a few months only could elapse before the adver- tised day of the opening. He attacked the sunken rock with gunpowder. A large raft, or floor, supported on barges, was moored over the sunken rock, and from this men, armed with long poles shod with steel, drilled numerous holes, into which charges of gunpowder were placed, and fired in the usual manner by the electric battery. This temporary obstruction occurred opposite to the landing-place at Serapeum, which is an interesting spot, where the remains of the ancient canal of the Pharaohs exist, in reference to which Lord Houghton observes : " Looking at the vast works that had been executed in Egypt ages ago, he could not but think that if the canal had been regarded as a necessary undertaking in former days, it would have been accomplished in those days. He believed that inventions were carried out when they were wanted, and that consequently the Suez Canal was the result of a want of our own time. " When he looked at the works of irrigation that had been constructed in Egypt from the days of the Pharaohs to the days of the Ptolemies, he felt convinced that if the canal had been wanted it would have been made before the nineteenth century. The truth was no one had wanted it. In former times, the Egyptians would have regarded the intersection cf the isthmus by a canal as a contrivance which would lead to the unnecessary intrusion of foreigners, and to the destruction of the nationality of Egypt,. In the times to which he referred, the Egyptians were anxious for free communication between the Nile and the Red Sea, but not for free com- munication between the Red Sea and the Mediterranean? Passing by Se'rapeum, the traveller arrives at a vast expanse of water, called the " Bitter Lakes," because the dry sandy hollow formerly contained a marsh, or mere, of very brackish water. Here it was that theory pro- claimed and bitterly contested the impossibility of keeping such an enor- mous area filled with sea-water. The opponents of the canal said it would sink into the sand, or be evaporated by the intense heat of the sun ; but none of these prognostications have been verified, and it is now a great inland sea, far surpassing Lake Timsah, being 25 miles long and from 6 to 7 miles wide. The only difficulty in filling this enormous natural basin arose from the rapidity and force with which the waters flowed in, which carried away certain great barriers, erected to prevent accident. This was done when the water at Suez was at the low tide, and then subsequently the Red Sea was allowed to flow in. The last embankment which separated the two oceans was cut through by the Viceroy in person, and at last the two seas were united. Since the two seas have joined their waters, a strong current has set in from south to north, but there is no eddy or fall at the place where the waters meet. The tide runs up the canal with great force, and there is a difference of 6 or 7 ft. between high and low water; but the tide does not extend beyond the Bitter Lakes, where it is gradually diffused and lost. The colour of the current of water from Suez is said to be green, whilst that portion fed by the Mediterranean is blue. Since the Bitter Lakes have 174 THE SUEZ CANAL. FIG. iy>.The Viceroy of Egypt cutting the last embankment of the Reservoir of the Plain of Suez, to unite the two seas the Mediterranean and the Red Sea. been filled the mean temperature of the districts on the banks has fallen 5 Centigrade. It is also stated that, although the canal literally swarms with sea-fish, they keep to their respective ends of the canal, as if the Mediterranean fish would not consort with those of the Red Sea, or rather make themselves at home in strange waters. There is also, perhaps, another cause, and that is the very bitter nature of the water at the northern end of the Bitter Lakes, which acts as a natural barrier, through which the fish may decline to pass. The bed of the Bitter Lakes is the only portion of the canal's course in which it has not been necessary to make a cutting. Buoys are laid down to mark the best channel, but such is the width and depth of the water that vessels need not keep exactly within them. On quitting the Bitter Lakes the canal proper is again entered, and the work is reported to be very complete for ten miles from this point, passing Chalouf, where " Rob Roy " makes the following note : " At Chalouf I found 14,000 men at work. They labour very hard indeed, running up the hill with baskets of sand on their heads. About 1,000 donkeys walk in long lines with neat mat baskets on their backs. In curious and close contrast to these simple carriers the mighty power of steam toils and puffs as it hurls up huge bulks of heavy clay ; and it is, perhaps, only in Egypt one could see human and animal power exerted in such competi- tion with steam power. The labourers are sent from all parts of Egypt. They must come, but they are highly paid from 2 francs to 3 francs a day. Prices both of labour and of food have risen very much since the canal has been begun, but the supply of fish has rapidly increased." In order to reach the vast docks which the Suez Canal Company has erected on the western coast of the Red Sea, the canal is now quitted, and the vessel crosses the neck of the Red Sea. The Cairo and Alexandria Railway has been extended for a distance of THE SUEZ CANAL. 175 two miles, and is carried through the sea on an embankment, which lands the train close to the docks and quays of the canal, so that passengers by the overland route are able to embark from the train on board the steamer, and thus escape the troublesome transhipment of themselves and luggage. At the opening of the canal M. Lesseps showed his cleverness and courage by permitting a flotilla of fifty vessels to pass through, and on the 2 ist November, 1869, the following statement was made by the " Official Journal" of Paris, announcing the arrival of the Empress at Suez : " The canal has been traversed from end to end without hindrance, and' the Imperial yacht Aigle, after a splendid passage, now lies at her moorings in the Red Sea. " Thus are realized the hopes which were entertained of this great under- taking the joining of two seas. " The Government of the Emperor cannot but look with satisfaction upon the success of an enterprise which it has never ceased to encourage. " A work like this, successfully accomplished in the face of so many obstacles, does honour to the energetic initiative of the French mind, and is a testimony to the progress of modern science." An Imperial decree was then issued, dated the iQth November, appoint- ing M. de Lesseps to the rank of Grand Cross of the Legion of Honour, in consideration of his services in piercing the Isthmus of Suez. The report of Captain Richards, Hydrographer to the Admiralty, con- taining information from Captain S. S. Nares, of H.M. surveying vessel Newport, is so very comprehensive and important, as further elucidating the important question of the pilotage and future prospects of the canal, that it is given here in extenso. " The coast in the neighbourhood of Port Sa'id is unusually low, being out of sight at three miles distance. The lighthouse, town, and shipping are the only objects seen from the offing. At present there are two tall obelisks, one on each side of the canal entrances, but as they are merely built of boarding they can only be temporary. At six miles to the west the coast is marked by a Gemileti tower, a low square building, standing by itself on a low sandy coast ; but to the eastward of the port there is nothing to mark the low shore. The current off the coast is very uncertain. It generally runs with the wind, from to \\ knots an hour. The general set is to the eastward. Owing to the current and low shore, more than usual caution is necessary in approaching the harbour. The harbour is formed by two concrete breakwaters running off from the sandy shore. Inside the piers the harbour is at present constantly silting up, in consequence of the current, heavily laden with sand, running through numerous openings in the piers, and depositing the sand in the quieter water inside. A good straight channel of 26 ft. of water has been dredged, leading into the inner basins, about 100 yards inside, and parallel to the west pier. It is marked by black buoys on the east side, and red ones on the west side ; and it may be presumed that the authorities will be careful to keep it clear. The basins inside the harbour have a depth of 26 ft. water : they are sufficiently large for the trade which may be expected ; if not, there is ample space for en- larging them. On the outer end of each breakwater there is a low light red on the west pier, and green on the east one. The Port Sa'id lighthouse is a tall white stone tower, 180 ft. high, standing close to the inshore end of the west breakwater. It shows a flashing white lime-light, visible 18 miles. The pilot-boats carry a Blue Peter flag. The best anchorage in 6 fathoms is with the low red light on with the high lighthouse ; or J7 6 THE SUEZ CANAL. the west pier-head a little open of the lighthouse on either side. The bottom is mud and very good holding ground. A bank with 12 ft. water has been formed to the eastward of the harbour. The east pier-head light (green) on with the high lighthouse leads over the west edge of the bank ; therefore, these marks must be kept well open. In approaching, allowance must be made for a bank which is forming outside the west pier end. In November, 1869, there was 6 fathoms at half a mile from the pier end, with the anchorage marks in one. The entrance to the canal is conveniently situated at the inner end of the basins. The usual depth of water is from 26 to 29 ft. ; immediately south of the Campement de Cap is a short bank of 24 ft. ; and a mile north of Kantara, opposite the 43rd kilometre mark, is a bank of 23 ft. The whole of this distance, 24^ miles, with the excep- tion of one-sixth of a mile at the Campement, which is higher, the canal runs through a wet, flat sandy plain scarcely higher than the level of the water on the east side, and a little below it on the west side, which, with a 'high Nile' is completely overflooded, and the sand rendered firm by the deposit of mud from the river. In this part of the canal there is no sand- drift, and it may be considered as completed. The debris thrown upon the banks is firm, black, sandy mud, protecting the canal from the water in Lake Menzaleh, without any opening in the whole distance. The canal passes through sand-hills from 20 to 30 ft. high, and has a depth of from 26 to 28 ft. This part of the canal is completed, but it is subject to a very severe sand-drift in high winds. The canal here passes through a lagoon, with a depth varying from 19 to 24 ft. ; but the dredges are still at work. There is constant trouble in this part of the canal, in consequence of the banks on each side, which are composed of fine sand- debris, not being firm enough to resist the constant ebb and flow of the water between the lake and the canal, which, carrying large quantities of sand with it, is constantly altering the depth of water. In this cutting the sand-hills are about 40 ft. high. The depth in the canal varies from 22 to 24 ft. ; but there is work still going on in the shallow parts. All this part is subject to heavy sand- drift. " For about four miles in the neighbourhood of El Guisr the canal is cut through a stratum of soft lime or sandstone. The sharp turns between El Guisr and Lake Timsah are probably owing to the engineers having fol- lowed the softest part of the rock. Ships can pass round the curves without trouble. The central station in the canal is well situated for a stopping- place. There is at present only 22 ft. in the middle of the lake ; but the dredges will soon give deeper water. The depth varies from 22 ft. to 27 ft., except one bank of 20 ft., in the lagoon. The debris banks here, of pure sand, like those in Lake Ballah, are not adhesive enough to form a barrier between the canal and the lagoons, to keep the silt from run- ning into the channel, but the canal is sufficiently wide to allow dredges to work without stopping the traffic. In this cutting the canal is carried through a stratum of sandstone, with depths from 22 ft. to 24 ft., except in one place a mile south of Serapeum, where for about 30 yards there is a narrow ridge, with only 18 ft. water over hard rock. A strong party of men are at work, and the obstruction will soon be reduced. At the south end of the cutting the deep channel is narrow and incomplete. This cut- ting is subject to a very heavy sand-drift. From the debris on the bank it would appear that the narrow ridge of stone running across the canal had only lately been discovered. The margin of the deep water in the lake i \ miles from the entrance is marked on the east side by a red iron pillar THE SUEZ CANAL. 177 lighthouse, 40 ft. high, showing a fixed white light, visible 10 or 12 miles. The excavated channel leading into the deep water has a depth of from 24 ft. to 29 ft. It is conspicuously marked on each side by iron beacons^ 15 ft. high, with a black ball 3 ft. in diameter on the top. As we passed, each beacon was lighted; but whether the lamps are to remain could not be ascertained. The margin of the deep water at the south end of the lake is, conspicuously marked on the east side by a lighthouse similar to the north one, and by a buoy on the west side. A straight run may be made between the lighthouses (a distance of 8 miles) with not less than 22 ft. depth of water ; 26 ft. may be obtained by passing nearer to the west shore of the lake. The water in this part of the lake being shallower, a cutting has been made, giving from 26 ft. to 27 ft. depth. The channel is well marked by numerous iron beacons on each side (from four to six to a mile) similar to those at the north end of the lake. This part is quite complete, with hard banks and depths of water from 26 ft. to 30 ft. at low water. It is subject to sand-drifts. At Chalouf the cutting is carried through sand- stone; the debris is hard and lumpy. South of latitude 30 6' N. the canal passes through sand-hills, it increases in width, and the debris on the bank is more than usually large. At Madama the banks are of firm marl or soft clay. This part of the canal is incomplete : the debris banks are sand. The soundings were irregular, the depth varying from 21 ft. to 26 ft. at low water. A large number of men are still at work here. At the entrance a good stone wall is built on the west bank, but it requires to be raised and extended. Another is much wanted on the east side, where the curve already shows the usual signs of scouring out on the outer, and depositing on the inner side. The south end of the canal may be said to extend if miles beyond the two red lights, passing the Suez creek and the new dock and harbour works, into the Gulf of Suez, with not less than 27 ft. at low water. With a flood tide a great quantity of silt pours into the canal from the sand-bank on the east side of the entrance ; but doubtless means will be taken to prevent it. A breakwater has already been carried across the sea-face of the bank. The mouth of the canal is marked by a red light on the west side at the extreme end of the new harbour works, and by a green light on the opposite side on the nearest end of the breakwater. Both lights are at present only hoisted on temporary poles. Outside these marks the channel is further shown by a line of buoys, white on the east side and red on the west side. The dry dock is 430 ft. long, 83 ft. broad, and can dock a ship drawing 23 ft. when the channel outside is completed. " The current depends on any variation in the height of water in the Mediterranean. The banks show that the canal here is subject to a rise and fall of one foot, the current and height lessening as the distance from the entrance increases. There is no tide or current in Lake Timsah or the upper Bitter Lake. The tidal influence extends from Suez to four miles north of the southern end of the Bitter Lakes. The stream commences to flow from two or three hours after low water at Suez. A spring tide rises 6 ft. at Suez, 2 ft. at Madama, i ft. at Chalouf, and \ ft. at the south entrance of the Bitter Lakes. At Kabiet there is no rise and fall. The immense reservoir of water in the Bitter Lakes with an ebb tide, and in the Gulf of Suez with the flood, will prevent the tide ever having a greater range. With a strong southerly wind in the Gulf of Suez the water rises to from 8 ft. to 9 ft. at the head of the Gulf, and may affect the water in the canal to some small extent. From two to three hours before high water at Suez the flood spring tide was running i\ knots at Chalouf, increasing to 2 or 2\ knots at 178 THE SUEZ CANAL. Madama, with the water very much discoloured. By starting from Suez an hour before low water a vessel will arrive in the Bitter Lake before the flood tide overtakes her, and having nearly slack water all the way. Every five or six miles a short widening in the canal (a gare) gives room for a vessel to haul in and allow another to pass her with ease. Vessels can pass each other at any part by using warps ; but they cannot do so without stopping, except at a great risk of running on shore, and delaying the whole traffic of the canal. A single ship could pass through in from fourteen to sixteen hours ; and two small ships, entering one at each end, could pass each other with- out slackening speed. But it is impossible to carry a train of large ships through in one day. Lake Timsah and the town of Ismailia are conveniently situated and sufficiently large for a stopping-place ; and, doubtless, arrange- ments will be made for ships to start from each end on one day, for all to meet and anchor for the night at Lake Timsah, and to start for their re- spective ends the following morning. This, allowing eight hours for passing through each end of the canal, and twelve hours for remaining at Ismailia, will give twenty-eight hours for the transit. With a full moon, a handy ship, by entering the canal in the evening and arriving at Ismailia in the morning early enough to join the train of vessels, might perform the voyage in from sixteen to twenty hours. With a train of only two or three ships, and no delay at nights, the transit would occupy about eighteen hours. There is no doubt that every vessel will cause more or less damage to the banks on passing, but screw ships only going five or six knots will hurt the canal very slightly, except in the lagoons, where the banks are formed of very fine sand. The Pera, a large paddle-wheel steamer, on passing with great speed (8 knots), and displacing the water in the whole breadth of the canal, did considerable damage, the wave she made swamping several boats. Large vessels should be made to reduce speed more than small ones. Should a vessel touch the ground in any part of the canal, except in the tidal part at the Suez end, she will sustain no damage, merely being thrown out of her turn in the line. A good coating of sand has formed at the bottom of the canal in the sandstone cuttings. In the tidal part near Suez, if a vessel is passing through with a following tide, and the bow touches either bank, there will be great danger of her swinging across the canal, with a two- knot current running against her broadside. With a wind blowing across the canal, vessels touching the lee side will be blown at once against the bank, but without any damage. The present pilots will rapidly gain expe- rience ; with trained leadsmen and a lead going on each side of the ship, there is no difficulty whatever in navigating the canal and keeping in mid channel." SAND. HTHE foregoing account of the Suez Canal is by Mr. J. H. Pepper, being, - in fact, the substance of a lecture delivered nightly by him at the Polytechnic Institution, in Regent Street, at the time when the canal was the latest novelty of the day. The lecture was illustrated by some experi- ments, designed to exhibit certain properties of sand which had reference to the construction of the canal ; and though the properties in question are by no means to be classed among recent discoveries, yet the experiments were novel in form, and well calculated to interest a popular audience. The want of cohesion among the sand-particles, which is the cause of the lateral pressure, although it is also the great feature of liquids, is far from conferring upon sand all the characteristic properties of liquids. When the Suez Canal was projected, many prophesied evil to the undertaking, from the sand of the Desert being drifted by the wind into the canal, and others were apprehensive that where the canal was cut through the sand, the Bottom would be pushed up by the pressure of the banks. They imagined that the sand would behave exactly like the ooze of a soft peat-bog, through which, when a trench has been cut, the bottom of the trench soon rises, for the soft matter has virtually the properties of a liquid : it acts, in fact, exactly like very thick treacle. Sand, however, is not possessed of liquid properties ; it has a definite angle of repose, which is not the case with thin bog, with which material the experiments described in the following pages could not possibly be performed. The behaviour of sand, which became a matter of interest to the public in connection with the Suez Canal, was, however, popularly confounded with that of peat-bog and similar materials; but it need not be said that all the apprehensions as to the safety of the canal have proved unfounded. The following account of the experiments shown at Mr. Pepper's lecture is given in his own words. The first experiment may be made by filling a large corked funnel (sup- ported by a proper stand) with dry sand ; and attention must be paid to this point, viz., dryness, as the property of cohesion is conferred on the separate particles of sand if they are at all wet, or even damp ; and thus fine white sand, from which all salt has been carefully washed, is the best kind to use in these experiments ; and if any salt remains, the sand becomes humid in damp weather, and the particles will not roll properly one upon the other, but stick or cling together. The corked funnel containing the sand being arranged and standing on a tray, the oxy-hydrogen light is thrown on and the cork removed. The sand flows out with great regularity, and forms a heap below, which takes the form of a cone, having a certain angle, which may be determined by a quadrant, and is found to be 30. By the sea-side, in the summer months, the sand is frequently very dry, and by burying a child's hoop half-way in the sand, and then allowing some loose sand to fall, a cone is obtained, and if the half of the hoop has been roughly graduated, the angle at which the particles of sand will remain in a posi- 179 122 i8o SAND. FIG. 131. Apparatus for showing Sand Experiments. A, funnel from which the sand is flowing ; B, scale and wedge to show angle at which sand rolls ; c, model of section of cylinder, with chalk-marks to show direction of the pressure ; r>, E, F, tin vessels with tubes ; G, press to contain eggs and sand ; H, tube and piston ; i, j, K, K, American pails, containing sand for the experiments, and prepared with tissue-paper ; L, framework and sides of tissue-paper to represent a pail ; N, oxy-hydrogen light to show experiments. tion of stability is readily determined. It follows, therefore, from this cir- cumstance that the pressure of sand is not perpendicular, but lateral, and it is well shown by taking a wooden section of a tube, having a sliding piece of wood, the sides of which have been cut to an angle of 30. The piece of wood is first brought to the lower part of the model, and the out- line marked with a piece of chalk. It may then be moved higher up and chalked again, and by repeating this three or four times, certain lines are marked on the model, which are intended to show that, after allowing for the weight of the cone of sand having its sides sloping at an angle oi 30, and which covers the bottom of the tube, the remainder heaped upon the first cone does not press upon the bottom, but almost entirely on the sides of the containing vessel (Fig. 132). The chalk-marks will remind the spectator of the chevrons, or good- conduct badges, on the arms of soldiers. The direction of the pressure is shown by putting some sand in a strong cylindrical iron vessel, and packing therein some eggs (Fig. 133), the whole being well bedded and covered with sand. On the upper part a thick piece of wood is now laid, and the operator may take a 14 Ib. weight and bring it down with force upon the piece of wood, and if the table upon which the arrangement is standing is strong enough, several smart blows may be given with a sledge-hammer. This being done, the 14 Ib. weight and the piece of wood are removed, and the sand poured out into the tray, when the eggs will be found to be intact ; and thus it is shown that when certain animals deposit their eggs in the sand, they cannot be broken by any human being walking on the surface, SAND. 181 and not even if the bulky hippopotamus should chance to be taking a pro- menade and tread over the exact spot where the eggs are deposited. FlG. 132. The Model marked with Chalk in several places, like Che- vrons. FIG. 133. Iron Cylinder, Sand, and Eggs. The principle of lateral pressure may now be strikingly illustrated by taking an American wooden pail, and having previously cut a large circular hole in the bottom, this is now covered with fine tissue-paper, which should be carefully pasted on, to prevent the particles of sand flowing through the small openings between the paper and the wood. The pail with its paper bottom may be held before the oxy-hydrogen light, to show the thinness of the tissue-paper, and being placed upright and rapidly filled with sand, it may be carried about by the handle without the slightest fear of the weight of the sand breaking through the thin medium. On ejecting the sand from the pail, it may be held as before in front of the light, and if the hand is thrust through the tissue-paper, more evidence is given of the peculiar manner in which sand presses, and that the wooden sides of the pail receive the chief pressure, and not the bottom. With large tin cylindrical cans, into which tubes of at least an inch in diameter are inserted in various positions, other and very interesting expe- riments may be performed. If the vessel is furnished with a tube standing out at right angles, as shown at E, Fig. 134, and then filled with sand, on removing the cork from the end of the tube, no sand will flow out, because it forms a cone, one side of which crosses the tube at an angle of 30, and effectually stops any further movement of the sand. The same result is obtained by inserting a tube into the containing vessel, like the spout, or rather in the same direction as the spout, of a tea-pot (D, Fig. 134). It 182 SAND. matters not how high the sand may be piled above the level of the top of the spout, it cannot flow out ; whereas, if it was filled with water, of course :;hat fluid would flow out until it had found its level. FlG. 1 34. The three Cylindrical Vessels and Tubes. The third arrangement (F,Fig. 134) in which the tube is inserted at triQ proper angle of 30, is the only one out of which the sand will move, and even here it may be noted that the particles of sand require time for their movement, as there is, first, the inertia of the particles to be overcome, and, secondly, the friction is great. The sand flows out of vessel r(Fig. 134) FlG. 135. Framework and sides of FlG. 136. Tube (one end closed -with Tissue-paper to represent a Pail. Tissue-paper), Sand, and Sledge- hammer. and a very instructive result is obtained, because it stops when the sand forms a cone inside the vessel, the sides of which correspond with the tube ; and in flowing out on a sheet of paper it forms a cone of the same angle. Probably one of the most convincing experiments is that which may be performed with a cylindrical tube (Fig. 135)1 8 in. long and 2 in. wide, open at SAND. 183 FIG. 137. The Hour-Glass on the Screen at the Polytechnic. both ends. A piece of tissue paper is carefully pasted on one end, so that when dried no cracks or interstices are left ; the tube is filled with dry sand to a height say of 12 in. ; in the upper part is inserted a solid plug of wood 12 in. long, and of the same or very nearly the same diameter as the inside of the tube, so that it will move freely up and down like the piston of an air- pump. The tube, sand, and piston, being arranged as described, may now be held by an assistant, and the demonstrator, taking a sledge-hammer, may proceed to strike steadily on the end of the piston, and although the paper will bulge out a little, the force of the blow will not break it. If the assistant holding the tube allows it to jerk or rebound after each blow of the hammer, the paper may break, because air and sand are driven down by the succeeding blow, and therefore it must be held steadily, so that the piston beds fairly on the sand each time. A still more conclusive and striking experiment (Fig. 136) may be shown with a framework of metal constructed to represent a pail, the sides of which are closed up by pasting sheets of tissue-paper inside and over the lower part. As before demonstrated, when a quantity of sand is poured into the pail, the tissue-paper casing the bottom does not break, but if a suffi- cient quantity is used, the sides formed of tissue-paper bulge out, and usually give way, in consequence of the lateral pressure exerted by the particles of sand exerted in the direction shown by the chalk-marks in the model of the section of the cylinder, shown at Fig. 132. One of the symbols of time is the hour-glass, a simple contrivance con- sisting of a glass vessel contracted in the centre so as to leave a narrow tube in communication with both sides. Fine dry sand is placed into one 1 84 SAND. side, and the quantity is adjusted, so that it will run for a few minutes or for the period of one hour. By fitting a flattened hour-glass into a slide which was placed in a powerful lantern, with achromatic lenses, and lighted with the oxy-hydrogen light, a large picture of the " hour-glass " was projected on to the screen at the Polytechnic. It was the more amusing because, the object being inverted, the sand appeared to flow upwards instead of downwards, and whilst flowing, it was noticed that as each particle of sand struck a tiny blow on the top of the fallen cone of sand, the force passed down from the top of the cone to the bottom in a wave-like figure, by the communicating particles of sand, the angle being also well defined. The experiment was satisfactory, as explaining the principle of the hour-glass, and why the sand flows always at the same rate (see Fig. 137). It is thus clearly shown that as sand rolls down, it will always maintain a conical surface, whose inclination at the base is about 30, consequently, if the banks of the Suez Canal are formed of dry sand, this angle, as already demonstrated, must not be exceeded, as any attempt to construct them at sharper angles must be labour in vain, because the sand would roll off and fill up the hollow or cutting between. THE SAND-BLAST. THE properties of sand, which have been so fully illustrated by these experiments of Mr. Pepper's, are perhaps not more interesting than a recent application of sand to industrial purposes, almost entitling it to be called a new mechanical agent. Persons who. have dwelt near the sea- shore have often remarked how soon, by exposure to winds which drift the sand, the glass of windows loses its polish. Engineers have noticed that solid particles carried over mechanically with the steam from the boiler of an engine soon erode the pipes, especially at places where there is a deflection. Many instances might be given where a stream of small rapidly-moving particles produces such effects that their wearing power has to be taken into account and guarded against. It is remarkable that such effects have been so long observed, and that no attempt appears to have been made to turn this cutting power of moving particles to good account, until a paper, communicated to the Franklin Institute, announced that Mr. Tilghman had found that sand, impelled by a jet of steam escaping under high pressure, constituted a very efficient means of grinding the surface of glass, an operation often required for ornamental purposes. Mr. Tilghman at first made use of high-pressure steam, into the midst of a current of which he introduced a stream of sand. He found, however, that a very high velocity was not necessary in order to obtain useful results. For re- moving the polish from glass, a blast of air, giving a pressure equal only to four inches of water, sufficed to impart enough impetus to the sand particles. He then adopted an ordinary rotatory fan, making about fifteen hundred revolutions per minute, and sending the current of air downwards through an upright tube, 5 ft. in height, at the top of which the sand is supplied, which, falling into the current of air, acquires additional velocity, and is thus impelled against the plates of glass, which were slowly moved SAND. 185 across the end of the tube at a little distance from it. The tube was of a narrow oblong section, 2 ft. by I in., and sheets of glass moved slowly forward at the rate of about five inches per minute were completely ground by the sand-blast of i in. across, as ten or fifteen seconds' time was found enough to enable the sand completely to depolish common glass. Arrangements were made for an automatic replacement of the spent sand at the top of the tube, and the dust produced was carried off by the air- blast and re-conveyed into the fan, again to mix with the shower of sand upon the glass. Glass ground in this manner shows under the microscope a number of little indentations or cavities, produced by the impact of the particles of sand, and the surface is much more uniform in its texture than any that could be formed by rubbing. By covering portions of the surface of the glass with paint, or by protecting it in part with lace, paper, collo- dion, or other material preventing the access of the sand, patterns of any kind may be very quickly engraved. Beautiful effects are produced in the ordinary processes of glass-cutting by operating upon colourless glass, which has been covered " flashed," as it is called by a thin layer of co- loured glass. On removing the coloured layer to a greater or less depth, varied tints or clear-cut patterns may be obtained. The sand-blast may be employed to produce such effects in great perfection, by protecting the portion in which it is desired to retain the colour by some tough or elastic substance applied to the glass. The flashing of ordinary coloured glass is cut through by the sand-blast in from four to twenty minutes. If a current of air of less velocity be used (for example, one equivalent in pressure to about an inch of water), it is found possible to depolish glass with the sand long before a fern-leaf, or other delicate material, will be pierced, and thus patterns having the outlines of leaves, &c., are readily and directly transferred to the glass. By continuing the blast, the softer parts of leaves may be cut through, while the glass is still protected by the thicker veins and stems, and thus something more than a mere outline can be obtained directly from natural leaves. A piece of common window- glass, covered by a piece of wire gauze, exposed to the sand-blast, was com- pletely cut through in the meshes of the gauze, thus producing from a solid piece of glass a glass sieve, with meshes ^ in. wide, separated by portions of glass only ^ in. thick. A photograph taken, on glass covered with a film of gelatine and bichromate of potash, may be subjected to the sand-blast, so that the parts not protected by the gelatine are acted on by the sand. Beautifully engraved pictures produced by this elegant process were exhi- bited by Mr. Tilghman, at a meeting of the Photographic Society of Phila- delphia, in 1871. The process of engraving these occupied only from three to ten minutes. The most delicate results were obtained by using finely sifted sand and a blast of about one inch pressure of water, with a longer time of exposure. That a material so hard as glass should be ground away in the manner described, while a substance so delicate as a piece of lace or a film of gelatine appears to resist the action, is explicable on the sup- position that the soft materials yield to the impact of the grains of sand, which may become for a moment half embedded on the surface, and then be thrown off, while in the case of the glass the sharp angles of the sand particles cut out a minute portion of the more unyielding body. An instructive result was obtained in a series of experiments to elucidate these singular facts. It was found that by imparting sufficient velocity to even comparatively soft bodies, they may be made to wear the surfaces of substances much harder than themselves. Thus a stream of fine leaden i86 SAND. shot, carried in a current of steam escaping from a pressure of 50 Ibs. to trie square inch, worked out a cavity in a piece of hard quartz, and the shot were found on examination to be but very slightly flattened by the blow. Sharp hard sand has been made to cut a hole i^ in. diameter and i^ in. deep through a piece of corundum, a substance much harder than sand itself. This was accomplished by employing a steam-jet of very high pres- sure, namely, 300 Ibs. to the square inch. But 100 Ibs. of steam sufficed to urge the sand with a velocity that enabled it to perforate a file \ in. thick in ten minutes. In using steam in this way, the sand is introduced in the centre of the jet of steam, through a tube about in. in diameter. The steam escapes from a larger pipe completely surrounding the sand-pipe, and projecting 6 in. beyond its orifice. Thus the escaping steam, acting as a piston, tends to produce a vacuum behind it, into which the rush of air draws down the sand-particles from above, and they are impelled upon the material to be operated upon, which is placed about one inch from the end of the tube. This invention is also applied to cutting stones, and for this purpose the steam-jet is always used as the propelling agent. It has been found that steam at a pressure of 125 Ibs. will enable the sand to cut away i^ cubic inches of granite, 3 cubic inches of marble, or 10 cubic inches of sand- stone in one minute. By making use of stencil-plates, patterns of any kind may thus be quickly cut, even in the hardest stone. It is singular that stencil-plates of metal soon curl up under the sand-blast, while a soft sub- stance, like paper, is not liable to this defect. Hence metallic templates, if used, have to be protected from the impact of the sand by some soft material. When the sand-blast is working on stone, a red light is perceptible. As this does not appear to be due to heat, it may possibly be connected with the luminous effects that are observed when certain crystals are split. It has been suggested that the ancient Egyptian carvings, which are so neatly executed in some of the hardest stone, may have been produced by a similar process.- It is, perhaps, a great descent from Egyptian sculpture to mention an application of the sand-blast so humble as that of cleaning out the interior of cast iron pans. But to this purpose, however, it has been advantageously applied, supplanting the operation of turning in a lathe, which formerly was requisite, in order to obtain a clean surface for tinning. FIG. 138. Britannia Bridge, Menai Straits. IRON BRIDGES. THE credit of having invented the arch is almost universally assigned to the ancient Romans, though the period of its introduction and the date of its first application to bridge building are unknown. That some centuries before the Christian era, the timber bridges of Rome had not been superseded by those of more permanent construction is implied in the legend of the defence of the gate by Horatius Codes a tale which has stirred the heart of many a schoolboy, and is known to everybody by Macaulay's spirited verses, in which " Still is the story told, How well Horatius kept the bridge, In the brave days of old." Some of the arched bridges built by the Romans remain in use to this day to attest the skill of their architects. The Ponte Molo at Rome, for example, was erected 100 B.C. ; and at various places in Italy and Spain many of the ancient arches still exist, as at Narni, where an arch of 150 ft 187 i88 IRON BRIDGES. span yet remains entire. Until the close of the last century the stone or brick arch was the only mode of constructing substantial and permanent bridges. And in the present century many fine bridges have been built with stone arches. The London and Waterloo Bridges across the Thames are well-known instances, each having several arches of wide span, attain- ing in the respective cases 1 52 ft. and 120 ft. The widest arch in England, and one probably unsurpassed anywhere in its magnificent stride of 200 ft., is the bridge across the Dee at Chester, built by Harrisson in 1820. At the end of last century cast iron began to be used for the construction of bridges, a notable example being the bridge over the Wear at Sunderland, of which the span is 240 ft. But with the subsequent introduction of wrought iron into bridge building a new era commenced, and some of the great results obtained by the use of this material will be described in the present article. In order that the reader may understand how the pro- perties of wrought iron have been taken advantage of in the construction of bridges, a few words of explanation will be necessary regarding the strains to which the materials of such structures are exposed. Such strains may be first mentioned as act most directly on the materials of any structure or machine, and these are two in number, namely, exten- sion and compression. When a rope is used to suspend a weight, the force exerted by the latter tends to stretch the rope, and if the weight be made sufficiently great, the rope will break by being pulled asunder. The weight which just suffices to do this is the measure of the tenacity of the rope. Again, when a brick supports a weight laid upon it, the force tends to com- press the parts of the brick or to push them closer together, and if the force were great enough, the brick would yield to it by being crushed. Now, a brick offers so great a resistance to a crushing pressure, that a single ordinary red brick may be capable of supporting a weight of 18 tons, or 40,320 Ibs. that is, about 1,000 Ibs. on each square inch of its surface. Thus the bricks at the base of a tall factory chimney are in no danger of being crushed by the superincumbent weight, although that is often very great. The tenacity of the brick, however, presents the greatest possible contrast to its strength in resisting pressure, for it would give way to a pull of only a few pounds. Cast iron resembles a brick to a certain extent in opposing great resistance to being crushed compared to that which it offers to being pulled asunder, while wrought iron far excels the cast metal in tenacity, but is inferior to it in resistance to compression. The following table expresses the forces in tons which must be applied for each square inch in the section of the metals, in order that they may be torn apart or crushed : Tenacity per square inch, in tons. Crushing pres- sure per square inch, in tons. Cast iron 8 CO Wrought iron -3Q 17 Iron wire 4 Besides the direct strains which tend to simply elongate or compress the materials of a structure or of a machine, there are modes of applying forces which give rise to transverse strains, tending to twist or wrench the pieces. IRON BRIDGES. 189 or to bend them, or rupture them by causing one part of a solid to slide away from the rest. Strains of this kind no doubt come into play in cer- tain subordinate parts of bridges of any kind; but if we divide bridges according to the nature of the strains to which the essential parts of the structure are subject, we may place in a class where the materials are exposed to crushing forces only, all bridges formed with stone and brick arches ; and in a second class, where the material is subjected to extension only, we can range all suspension bridges ; while the third class is made up of bridges in which the material has to resist both compression and ex- tension. This last includes all the various forms of girder bridges, whether trussed, lattice, or tubular. The only remark that need be here made on arched bridges is, that when cast iron was applied to the construction of bridges, the chief strength of the material lying in its resistance to pres- sure, the principle of construction adopted was mainly the same as that which governs the formation of the arch ; but as cast iron has also some tenacity, this permitted certain modifications in the adjustment of the equilibrium, which are quite out of the question in structures of brick and stone. FIG. 139. The general principle of the construction of girder bridges is easily explained by considering a simple case, which is almost within every- body's experience. Let us suppose we have a plank supported as in Fig. 1 39. The plank will by its own weight sink down in the centre, becoming curved in the manner shown ; or if the curvature be not sufficiently obvious, it may always be increased by placing weights on the centre, as at g. If the length of the plank had been accurately measured when it was extended flat upon the ground, it would have been found that the upper 01 concave surface, a b, had become shorter, and the lower or convex surface, c d, longer when the plank is supported only at the ends a result sufficiently obvious from the figure it assumes. It is plain, then, that the parts of the wood near the upper surface are squeezed together, while near the lower surface the wood is stretched out. Thus, the portions in the vicinity of the upper and lower surfaces are in opposite conditions of strain ; for in the one the tenacity of the material comes into play, and in the other its power of resisting compression. There is an intermediate layer of wood, how- ever, which, being neither extended or compressed, receives no strain. The position of this is indicated by the line 697 The mileage of the carriers sent was much greater in the lead than in the iron pipes, although the total lengths of each kind were respectively 5,974 yards and 6,826 yards. The result is remarkable, as showing the effect of apparently slight differences when their operation is summed up by numerous repetitions. The circuit at Charing Cross having been divided on account of the difficulty mentioned above, the tubes act as separate pipes one for " up " traffic (i.e., to Central Telegraph Station), the other for " down" (i.e., from the Central Station). The air, however, still accomplishes a circuit, being exhausted at one end and compressed at the other. A very noticeable and curious difference is found between the times required by the carriers to perform the " up " and the " down" journeys : 16 242 PNE UMA TIC DISPA TCH. An " up " carrier requires 6*5 minutes A " down " carrier requires I2'5 Together 19-0 When two pipes were separated at Charing Cross so that the air no longer circulated from one to the other, but both were left open to the atmosphere, while the " up " pipe was worked by a vacuum only and the " down " pipe by pressure only, the times wefe for An"up" carrier 8*5 minutes A " down " carrier 11*3 Together 19-8 The time, therefore, for the whole circuit was practically the same whether the tubes were worked by a continuous current of air or separated, and one worked by the' vacuum and the other by pressure. It was also seen that when the tubes were connected so that the air current was con- tinuous, and the pump producing a vacuum at one end and a compression at the other, the neutral point where the pressure was equal to that of the atmosphere was not found midway between the two extremities that is, at Charing Cross Station but much nearer the vacuum end. When the tubes were disconnected, it appeared, as already shown by the figures given above, that there was a gain of speed on the down journey, and a loss of speed on the up journey ; and as the requirements of the traffic happened to require greater dispatch for the down journeys, the tubes have been worked in this manner. It has been proposed to convey letters by pneumatic dispatch between the General and Suburban Post Offices, and the Post Office authorities have even consulted engineers on the practicability of sending the Irish mails from London to Holyhead by this system. It was calculated, however, that although the scheme could be carried out, the proportion of expense for great speeds and long distances would be enormously increased. A speed of 130 miles per hour was considered attainable, but the wear and tear of the carriers would be extremely great at this high velocity, and it was considered doubtful whether this circumstance might not operate seriously against the practical carrying out of the plan. The prime cost would be very great, for the steam power alone which would be requisite would amount to 390 horse-power for every four miles. We thus see that very high velocities would introduce a new order of difficulties in the practical working. The case as regards the velocity with which electric signals can be sent round the world is very different. An amusing hoax appears to have been perpetrated by some waggish telegraph clerk on an American gentleman at Glasgow, with regard to the pneumatic system of sending messages; for the gentleman sent to the " Boston Transcript" a letter, in which he relates that having sent a tele- graphic message from Glasgow to London, he received in a few minutes a reply which indicated a mistake somewhere, and then he went to the Glasgow telegraph office, and asked to see his message. " The clerk said, ' We can't show it to you, as we have sent it to London/ ' But,' I replied, ' you must have my original paper here. I wish to see that/ He again said, ' No, we have not got it : it is in the post office at London/ - What do you rnean ?' I asked. ' Pray, let me see the paper I left here half PNEUMATIC DISPATCH. 243 an hour ago.' ' Well, 1 said he, ' if you must see it, we will get it back in a few minutes, but it is now in London.' He rang a bell, and in five minutes or so produced my message, rolled up in pasteboard. ... I inquired if I might see a message sent. ' Oh, yes ; come round here.' He slipped a number of messages into the pasteboard scroll, popped it into the tube, and made a signal. I put my ear to the tube and heard a slight rumbling noise for seventeen seconds, when a bell rang beside me, indicating that the scroll had arrived at the General Post Office, 400 miles off. It almost took my breath away to think of it." . In the journal called " Engineering," into which this curious letter was copied, it is pointed out that to travel from London to Glasgow, a distance of 405 miles, in seventeen seconds, the carrier must have moved at the rate of 24 miles per second, or 5 miles a second faster than the earth moves in its orbit, and the carrier would have in such a case become red hot by its friction against the tube before it had travelled a single second. A plan of conveying, not telegraph messages, but parcels, was proposed and carried into effect some time ago, and more recently has been applied to lines of tubes in connection with the General Post Office. These tubes pass from Euston Station down Drummond Street, Hampstead Road, Totten- ham Court Road, to Broad Street, St. Giles's, whence, with a sharp bend, they proceed to the Engine Station at Holborn, and then to the Post Office. The tube is formed chiefly of cast iron pipes of a Q-shaped section, 4 ft. 6 in. wide and 4 ft. high, in 9 ft. lengths. There are curves with radii of 70 ft. and upwards, and at these parts the tube is made of brickwork and not of iron. The carriages run on four wheels, and are so constructed that the ends fit the tubes nearly, and the interval left is partly closed by a project* ing sheet of india-rubber all round. The carriages are usually sent through the tube in trains of two or three, and the trains are drawn forward by an exhausting apparatus formed by a fan, 22 ft. in diameter, worked by two horizontal steam engines having cylinders 24 in. in diameter and a stroke of 20 in. The air rushes by centrifugal force from the circumference of the fan, and is drawn in at the centre, where the exhaust effect is produced. The tubes which convey the air from the main tube open into the latter at some distance from its extremities, which are closed by doors, so that after the carriage passes the entrance of the suction tube, its momentum is checked by the air included between it and the doors, which air is, of course, compressed by the forward movement of the carriage. At the proper moment the doors are opened by a self-acting arrangement, and the carriage emerges from the tube. There are two lines of tube an " up " and a " down " line and means are provided for rapidly transferring the carriages from one to the other at the termini. The time occupied in the transit is about 12 minutes. Some of the inclines have as much slope as I in 14, yet loads of 10 or 12 tons weight are drawn up these gradients with- out difficulty. The mails are sent between Euston Station and the Post Office by means of these tubes. Passengers have also made the journey as an experiment by lying down in the carriages. Fig. 174 shows one of the carriages and the entrance to the tubes. Great expectations have been formed by some persons of the applica- tions of pneumatic force. Some have suggested its use for moving the trains in the proposed tunnel between England and France. But calcula- lations show that for long distances and large areas such modes of impart- ing motion are enormously wasteful of power. Thus, in the tunnel alluded to it must be remembered that not only the train, but the whole mass of 16 2 244 PNE UMA TIC DISPA TCH. air in the tunnel would have to be be drawn or pushed forward. The drawing of a train through by exhausting the air would be very similar to drawing it through by a rope ; in fact, the mass of air may be regarded as a very elastic rope, but by no means a very light one, or one that could be drawn through without some opposing force which has a certain resem- blance to friction coming into operation. Indeed, it has been calculated that in the case named, only five per cent, of the total power exerted by the engines in exhausting the air could possityy produce a useful effect in moving the train. Air has also been made the medium for conveying intelligence in another manner than by shooting written messages through tubes, for its property of transmitting pressure has been applied to produce at a distance signals like those made use of in the electric telegraph system. A few years ago, an apparatus for this object was contrived by Signer Guattari, whose in- vention is known as the " Guattari Atmospheric Telegraph." In this there is a vessel charged with compressed air by a compression-pump, and the pressure is maintained by the same means, while the reservoir is being drawn upon. A valve is' so arranged that the manipulator can readily admit the compressed air to a tube extending to the station where the signals are received, at which the pressure is made to move a piston as often as the sender opens the valve. This movement is made to convey intelligence when a duly regulated succession of impulses is sent into the tube the receiving apparatus being arranged either to give visible or audible signals, or to print them on slips of paper, according to any of the methods in use with the electric telegraph. Certain advantages over the electric system are claimed for this pneumatic telegraph as, for example, greater simplicity and less liability to derangement. The tubes, which are merely leaden piping of small .bore, are also exempt from the inconvenient interruptions which electric communication sometimes suffers from elec- trical disturbances in the atmosphere. The pneumatic system is easily arranged, and from its great simplicity any person can in a few hours learn to use the whole apparatus, while it is calculated that the expense of con- struction and working would not be above half of that incurred for the electric system. For telegraphs in houses, ships, warehouses, and short lines, this invention will doubtless prove very serviceable ; but for long lines a much greater force of compression would be required, and the time needed for the production of an impulse at the distant ends of the tubes would be considerably increased. FIG. 178. The Sommeiller Boring Machines. ROCK BORING. A LLUSION has already been made to one great characteristic of our *T*- age, namely, the replacement, in every department of industry, of manual labour by machines. A brief notice of even the main features of the various contrivances which have been made to take the place of men's hands would more than occupy this volume. Accordingly, we must omit all reference to many branches of manufacture, although the products may be of very great utility, and the processes of very high interest ; and in taking one example here and. another there, we must be guided mainly by the extent and depth of the influence which the new invention appears destined to exert. This consideration has, with scarcely an exception, decided the selection of the topics already discussed, and it has also de- termined the introduction of the present article, which relates to machines of no less general importance than the rest, although at first sight it might seem to enter upon the details of merely a special branch of industry. But so general are the interests connected with the subject we are about to lay before our readers, that we are not sure it would not have been more logical to have placed the present article before all the rest. For whence comes the iron of which our steam engines, tools, rails, ships, cannon, bridges, and printing presses are made? whence 'comes the fuel which supplies force to the engines? whence come, in fine, the substances which form the materiel of every art ? Plainly from the earth the nurse 245 246 ROCK BORING. and the mother of all, and in most cases from the bowels of the earth, for her treasures are hidden far below the surface the coal, and the ores of iron and other metals, are not ready to our hand, exposed to the light of day. The railways also, and the canals, can be made only on condition that we cut roads through the solid rocks, and pierce with tunnels the towering mountains. Hence the tools which enable us to penetrate into the sub- stance of the earth present the highest general interest from a practical point of view, and this interest is enhanced by the knowledge of the struc- ture and past history of our planet acquired in such operations. The operations by which solid rocks are penetrated in the sinking of shafts for mines, or in the driving of tunnels, drifts, headings, galleries, or cuttings for railways, mines, or other works, are easily understood. In the first place a number of holes perhaps 3 ft. or 4 ft. deep and 2 in. or 3 in. in diameter are formed in the rock. The holes are then charged with gun- powder or other explosive materials, a slow-burning match is adjusted, the miners retire to a safe distance, the explosion takes place detaching, shattering, and loosening masses of the rock more or less considerable ; and then gangs of workmen clear away the stones and debris which have been detached by the explosion, and the same series of operations is renewed. The holes for the blasting charges are formed by giving repeated blows on the rock with a kind of chisel called a jumper the end of which is formed of very hard steel, so that the rock is in reality chipped away. The dtbris resulting from this operation is cleared away from time to time by a kind of auger or some similar contrivance. But for many purposes it is necessary to drill holes in rocks to great depths, hundreds of feet perhaps, as for example, in order to ascertain the nature of underlying strata, or to verify the presence of coal or other minerals before the expense of sinking a shaft is incurred. These bore-holes were commonly formed in exactly the same manner as the blast-holes already mentioned, by repeated blows of a chisel or jumper, which was attached to the end of a rod ; and as the hole deepened, additional lengths of rod were joined on, and the rods were withdrawn from time to time to admit of the removal of the debris by augers, or by cylinders having a valve at the bottom. The reciprocating movement is given to the chisels and rods either by hand or by steam or water power. When the length of the rods becomes considerable, of course the difficulty of giving the requisite blows in rapid succession is greatly increased, for the whole length of rods has to be lifted each time, and if allowed to fall with too much violence, the breaking of the chisel or the rods is the inevitable result The time requisite for drawing out the rods, removing the fragments chipped out, and again attaching the rods and lowering, also increases very much as the bore gets deeper. Messrs. Mather and Platt, the Manchester engineers, have, in order to obviate these difficulties, constructed machines in which the chipping or cutting is done by the fall of a tool suspended from a rope, the great advantage re- sulting from the arrangement being the facility and rapidity with which the tools used for the cutting and for the removal of the debris are lowered to their work and drawn up. It is necessary in using the juniper, whether in cutting blast-holes or bore-holes, to give the tool a slight turn after eac& blow, in order that the rock may be chipped off all round, and the action of the tool equalized. Many attempts have been made to drill rocks after the fashion in which iron is drilled that is, by drilling properly so called, in which the tool has a rapid rotary motion. But even in comparatively soft rock, it is found that no steel can sufficiently withstand the abrading action ROCK BORING. 247 of the rock, for the tool becomes quickly worn, and makes extremely slow progress. We shall have presently to return to the subject of bore-holes ; but now let us turn our attention to an example which will illustrate the nature and advantages of the machinery which has in recent times been applied to work the jumpers by which the holes for blasting are formed. THE MONT CENIS TUNNEL. / "P HE successful construction, by the direction of Napoleon, of a broad and * easy highway from Switzerland into Italy, crossing the lofty Alps amid the snows and glaciers of the Simplon, has justly been considered a feat of skill redounding to the glory of its designers. But we have recently wit- nessed a greater feat of engineering skill, for we have seen the Alps con- quered by the stupendous work known as the Mont Cenis Tunnel. This tunnel is 7\ English miles in length ; but it is not the mere length which has made the undertaking remarkable. The mountain which is pierced by the tunnel is formed entirely of hard rock, and what added still more to the apparently impracticable character of the proposal when first announced was the circumstance that it was quite impossible to sink vertical shafts, so that the work could not, as in the usual process, be carried on at several points simultaneously, but must necessarily be continued from the two extremities only, a restriction which would occasion a vast loss of time and much expense, to say nothing of the difficulties of ventilating galleries of more than three miles in length. The reader must bear in mind that the importance of this question of ventilation depends not simply on the re- newing of the air contaminated by the respiration of the workmen, but on the quick removal of the noxious gases produced in the explosions of the blasting charges. A work surrounded by such difficulties would probably have never been attempted had not Messrs. Sommeiller and Co. invited the attention of engineers to an engine of their invention, worked by com- pressed air, and capable of automatically working "jumpers" which could penetrate the hardest rock. These rock-boring machines, having been ex- amined by competent authorities in the year 1857, were pronounced so efficient that the execution of the long-spoken-of Alpine tunnel was at once resolved upon, and before the close of that year the work had actually been commenced, after a skilful and accurate survey of the proposed locality had been made, and the direction of the tunnel set out. The tunnel does not pass through Mont Cenis, although the post road from St. Michel to Susa passes over part of Mont Cenis, which gives its name to the pass. The mountain really pierced by the tunnel is known as the Grand Vallon, and the tunnel passes almost exactly below its summit, but at a depth the perpendicular distance of which is as nearly as possible one mile. The northern end of the tunnel is near a village named Fourneaux. Pending the construction of the Sommeiller machines, and other machi- nery which was to supply the motive force, the work of excavation was commenced at both ends, in 1857, in the ordinary manner, that is, by hand labour, and in 1858 surveys of the greatest possible accuracy were mean- while made, in order that the two tunnels might be directed so that they would meet each other in the heart of the mountain. The reader will at once perceive that the smallest error in fixing on the direction of the two straight lines which ought to meet each other would entail very serious 248 SOCK BORING. consequences. The difficulties of doing this may be conceived when we remember that the stations were nearly 8 miles apart, separated by rugged mountains, in a region of snows, mists, clouds, and winds, over which the levels had to be taken, and a very precise triangulation effected. So suc- cessfully were these difficulties overcome, and so accurately were the mea- surements and calculations made, that the junction of the centre lines of the completed tunnel failed by only a few inches^ a length utterly insigni- cant under the conditions. The work was carried on by manual labour only, until the beginning of 1 86 1, for it was found, on practically testing the machinery, that many important modifications had to be made before it could be successfully em- ployed in the great work for which it was designed. After the machinery had been set to work, at the Bardonneche end, breakages and imperfec- tions of various parts of the apparatus, or the contrivances for driving it, caused delay and trouble, so that during the whole of 1861 the machines were in actual operation for only 209 days, and the progress made averaged only 1 8 in. per day, an advance much less than could have been effected by manual labour. The engineers, not disheartened or deterred by these difficulties and disappointments, encountered them by making improve- ment after improvement in the machinery as experience accumulated, so that a wonderful difference in the rate of progress showed itself in 1862, when the working days numbered 325, and the average rate of advance was three feet nine inches per day. At the Fourneaux extremity more time was required for the preparation of the air-compressing machinery, and the machines had been at work in the other extremity, with more or less interruption, for nearly two years before the preparations at Fourneaux were completed. The illustration at the head of this article, Fig. 178, represents the Som- meiller machines at work, the motive power being compressed air, conveyed by tubes from receivers, into which it is forced until the pressure becomes equal to that of six atmospheres, or 90 Ibs. per square inch. The com- pression was effected by taking advantage of the natural heads of water, which were made to act directly in compressing the air ; the pressure due to a column of water 160 ft. high being made to act upwards, to compress air, and force it through valves into the receivers ; then the supply of water was cut off, and that which had risen up into the vessel previously con- taining air was allowed to flow out, drawing in after it through another valve a fresh supply of air ; and then the operations were repeated by the water being again permitted to compress the air, and so on, the whole of the movements being performed by the machinery itself. The compressed air, after doing its work in the cylinders of the boring tools, escaped into the atmosphere, and in its outrush became greatly cooled, a circumstance of the greatest possible advantage to the workmen, for otherwise, from the internal warmth of the earth, and that produced by the burning of lights, explosions of gunpowder, and respiration, the heat would have been into- lerable. At the same time, the escaping air afforded a perfect ventilation of the workings while the machines were in action. At other times, as after the explosion of the charges, it was found desirable to allow a jet of air to stream out, in order that the smoke and carbonic acid gas should be quickly cleared away. Even had the work been done by manual labour alone, a plentiful supply of compressed air would have been required merely for ventilation, so that there was manifest advantage in utilizing it as the motive power of the machines. ROCK BORING. 249 FIG. 179. Transit by Diligence over Mont Cenis. The experience gained in the progress of the work suggested from time to time many improvements in the machinery and appliances, which finally proved so effectual that the progress was accelerated beyond expectation. At the end of 1864, when the machines had been in work about four years, it was calculated that the opening of the tunnel might be looked for in the course of the year 1875. But in point of fact it happened that on the 2$th December, 1870, perforator No. 45 bored a hole from Italy into France, by piercing the wall of rock, about 4 yards thick, which then separated the workings from each other. The centre lines of the two workings, as set out from the different sides of the mountain, failed to coincide by only a foot, that set out on the Fourneaux side being this much higher than the other, but their horizontal directions exactly agreeing. The actual length of the tunnel was found to be some 1 5 yards longer than the calculated length, the calculation having given 7*5932 miles for the length, whereas by actual measurement it was found to be 7*6017 miles. The heights above the sea-level of the principal points are these : Feet. Fourneaux, or northern entrance 3>8oi Bardonneche, or southern entrance 4> 2 36 Summit of tunnel 4> 2 4-6 Highest point of mountain vertically over the tunnel 95 2 ? 250 ROCK BORING. The tunnel is lined with excellent brick and stone arching, and it is con- nected with the railways on either side by inclined lines, which are in part tunnelled out of the mountain, so that the extremities of the tunnel referred to above are not really entered by the trains at all ; but these lateral tunnels join the other and increase the total distance traversed underground to very nearly 8 miles, or more accurately, 7 '9^806 miles. The time required by a train to pass from one side to the other is about 25 minutes. What a contrast is this to the old transit over the Mont Cenis pass by "diligence" ! We have the scene depicted in Fig. 179, where we perceive, sliding down or toiling up the steep zigzag ascents, a series of curious vehicles drawn by horses with perpetually jingling bells. The cost of the Mont Cenis Tunnel was about .3,000,000 sterling, or upwards of ,200 per yard ; but as a result of the experience gained in this gigantic work, engineers consider that a similar undertaking could now be carried out for half this cost. It is supposed that the profit to the contrac- tors for the Mont Cenis Tunnel was not much less than 100 per yard. The greatest number of men directly employed on the tunnel at one time was 4,000, and the total horse-power of the machinery amounted to 860. From 1857 to 1860, by hand labour alone, 1,646 metres were excavated; from 1 86 1 to 1870 the remaining 10,587 metres were completed by the machines. The most rapid progress made was in May, 1865, in which month the tunnel was driven forward at one end the length of 400 feet. When the workings were being carried through quartz, a very hard rock, the speed was greatly reduced as, for example, during the month of April, 1866, when the machines could not accomplish more than 35 ft. The perforators used in the Mont Cenis Tunnel were worked by com- pressed air, conveyed to a small cylinder, in which it works a piston, to the rod of which the jumper is directly attached. The air, being admitted be- hind the piston, impels the jumper against the rock, and the tool is then immediately brought back by the opening of a valve, which admits com- pressed air in front of the piston, at the same time that the air which has driven it forward is allowed to escape, communication with the reservoir of compressed air having previously been closed behind it. The whole of these movements are automatic, and they are effected in the most rapid manner, four or five blows being struck in every second, or. between two and three hundred in one minute. Water was constantly forced into the holes, so as to remove the debris as quickly as it was formed. A number of these machines were mounted on one frame, supported on wheels, run- ning on the tramway which was laid along the gallery. The perforators had no connection with each other, for each one had its own tube for the con- veyance of compressed air, and its own tube to carry the water used for clearing out the hole, and the cylinders were so fixed on the frames that the jumpers could be directed in any desired manner against any selected portion of the rock. They were driven to an average depth of about -2\ ft., and the process occupied from forty to fifty minutes. When a set of holes had thus been formed, the cylinders were shifted and another series com- menced, until about eighty holes had been bored, the formation of the whole number occupying about six or seven hours, and the holes being so arranged that the next operation would detach the rock to the required extent. The flexible tubes, which conveyed the air and water to the ma- chines from the entrances, were then removed from the machines and stowed away, the frame bearing the perforators was drawn back along the tramway, 'workmen advanced whose duty it was to wipe out the holes, ROCK BORING. 251 charge them with powder, and fire the fuses ready for the explosion. When the slow-burning match was ignited, all retired behind strong wooden bar- ricades, at a safe distance, until the explosion had taken place ; and after the compressed air had been allowed to stream into the working, so as to clear away all the smoke and gas generated by the explosion, the workmen ran up on a special tramway the waggons which were to carry away all the detached stones ; and when this had been done, the floor was levelled, the tramways were lengthened, and the frame bearing the drilling machines was brought up to begin a fresh series of operations, which were usually repeated about twice in the course of every twenty-four hours. A great part of the rock consists of very hard calcareous schist, interspersed with veins of quartz, one of the hardest of all rocks, which severely tries the temper of the steel tools, for a few blows on quartz will not unfrequently cause the point of a jumper to snap off. ROCK-DRILLING MACHINES. O EVERAL forms of rock-drills, or perforators, have been constructed on *-2 the same principle as that used in the Mont Cenis Tunnel, and a de- scription of one of them will give a good notion of the general principle of all. We select a form devised by Mr. C. Burleigh, and much used in America, where it has been very successfully employed in driving the Hoosac Tunnel, effecting a saving in the cost of the drilling amounting to one-third of the expense of that operation, and effecting also a still greater saving of time, for the tunnel, which is 5 miles in length, is to be completed in four years, instead of twelve, as the machines make an advance of 1 50 ft. per month, whereas the rate by hand labour was only 49 ft. per month. These ma- chines are known as the " Burleigh Rock Drills," and have been patented in England for certain improvements by Mr. T. Brown, who has kindly supplied us with the following particulars : The Burleigh perforator acts by repeated blows, like Bartlett and Som- meiller's, but its construction is more simple, and the machine is lighter and not half the size, while its action is even superior in rapidity and force. The Burleigh machines are composed of a single cylinder, the compressed air or steam acting directly on the piston, without the necessity of fly- wheel, gearing, or shafting. The regular rotation of the drills is obtained by means of a remarkably simple mechanical contrivance. This consists of two grooves, one rectilinear, the other in the form of a spiral cut into the piston-rod. In each of these channels, or grooves, is a pin, which works freely in their interior : these pins are respectively fixed to a concentric ring on the piston-rod. A ratchet wheel holds the ring, and the pin slides into the curve, causing it to turn always in the same direction, without being able to go back. By this eminently simple piece of mechanism, the regular rotation of the drill-holder is secured. The slide-valve is put into motion by the action of a projection, or ball-headed piston-rod, on a double curved momentum-piece, or trigger, which is attached to the slide-rod or spindle by a fork, thus opening and shutting the valve in the ascent and descent of the piston. Fig. 180 represents one of the machines attached in this instance by a clamp to the frame of a tripod. The principal parts 252 ROCK BORING. of the machine are the cylinder, with its piston, and the cradle with guide- ways, in which the cylinder travels. The action of the piston is similar to that of the ordinary steam hammer, with this difference, that, in addition FlG. 1 80. Bur high Rock Drill on Tripod. to the reciprocating, it has also a rotary, motion. The drill-point is held in a slip-socket, or clamp, at the end of the piston-rod, by means of bolts and nuts. The drill-point rotates regularly at each stroke of the piston, making a complete revolution in every eighteen strokes. For hard rocks it is gene- rally made with four cutting edges, in the form of a St. Andrew's cross, ROCK BORING. 253 thus striking the rock in seventy-two places in one revolution, each cut- ting edge chipping off a little of the stone at each stroke in advance of the one preceding. The jumper makes, on an average, 300 blows per minute, and such is the construction of the machine, that the blows are of an elastic, and not of a rigid, nature, thus preventing the drill-point from being soon blunted. It has been found in practice, that a drill-point used in the Burleigh machine can bore on an average 20 ft. of Aberdeen granite with- out re-sharpening. As the drill pierces the rock, the machine is fed down the guide-ways of the cradle by means of the feed-screw (see Fig. 180), according to the nature of the rock and the progress made. When the cylinder has been fed down the entire length of the feed-screw, and if a greater depth of hole is required, the cylinder is run back, and a longer drill is inserted in the socket at the end of the piston-rod. The universal clamp may be attached to any form of tripod, carriage, or frame, according to the requirements of the work to be done ; it enables the machines to work vertically, horizontally, or at any angle. The following advantages are claimed for this machine : Any labourer can work it ; it combines strength, lightness, and compactness in a remark- able degree, is easily handled, and is not liable to get out of order. No part of the mechanism is exposed ; it is all enclosed within the cylinder, so there is no risk of its being broken. It is applicable to every form of rock- work, such as tunnelling, mining, quarrying, open cutting, shaft-sinking, or submarine drilling ; and in hard rock, like granite, gneiss, ironstone, or quartz, the machine will, according to size, progress at the incredible rate of four inches to twelve inches per minute, and bore holes from in. up to 5 in. diameter. It will, on an average, go through 120 ft. of rock per day, making forty holes, each from 2 ft. to 3 ft. deep, and it can be used at any angle and in any direction, and will drill and clear itself to any depth up to 20 ft. The following extract from the " Times," September 24th, 1873, gives an account of some experiments with the machine, made at the meeting of the British Association in that year, before the members of the Section of Mechanical Science : " Yesterday, considerable interest was taken in this section, as it had been announced that a * Burleigh Rock Drilling Machine ' would be work- ing during the reading of a paper by Mr. John Plant. The machine was not, however, in the room, but was placed in the grounds outside, where it was closely examined by the members after the adjournment, and seen in full operation, boring into an enormous block of granite. The aspect of the machine cannot be called formidable in any respect, for it looks like a big garden syringe, supported upon a splendid tripod ; but when at work, under about Solbs. pressure of compressed air, it would be deemed a very revolu- tionary agent indeed, against whose future power the advocates for manual labour in the open quarry, the tunnel, and even the deep mine, may well look aghast. Placed upon a block of granite a yard deep, the machine was handled and its parts moved by the fair hands of many of the lady asso- ciates of scientific proclivites ; but once the source of power was turned on, the drill began its poundings, eating holes 2 in. in diameter in the block of granite, and making a honeycomb of it as easily as a schoolboy would demolish a sponge cake. It pounds away at the rate of 300 strokes, and progresses forward about 12 in., in the minute, making a complete revolu- tion of the drill in eighteen strokes, and keeping the hole free of the pounded rock. The machine was fixed to work at any angle, almost as 254 ROCK BORING. FIG. 1 8 1. BurleigJi Rock Drill on Movable Column. readily as a fireman can work his hose ; and its adaptation to a wide range of stone-getting, by drilling for blasting, and cutting large blocks for build- ing and engineering, with a saving of capital and labour, was admitted by many members of the section. The tool is called the ' Burleigh Rock ' Drill,' invented by Mr. Charles Burleigh, a gentleman hailing from Massa- chusetts, United States. The patent is the property of Messrs. T. Brown and Co., of London. The principal feature of this new machine is, that it imitates in every way the action of the quarryman in boring a hole in the rock." Many forms of carriages and supports have, from time to time, been made to suit the work for which the ' Burleigh ; machines have been required. The machine is attached to these carriages, or supports, by means of the universal clamp, by which it can be worked in any direction and at any angle. Of these carriages we select for notice only two forms, one of which is shown in Fig. 181. This carriage can be used to great ROCK BORING. 255 advantage in adits and drifts. It consists of an upright column, with a screw clamp-nut for holding and raising or lowering the machine, the whole being mounted on a platform which can slide right across the carriage, and thus the machine can be brought to work on any point of a heading. It is secured in position by means of a jack-screw in the top of the column ; and as the carriage is mounted on wheels, it is easily moved to permit of blasting. Fig. 182 represents a carriage which is the result of many years' experience with mining machinery, and it is considered a very perfect appliance. It is constructed of wood and iron, and it runs on wheels.' FIG. 182. Burleigh Rock Drills mounted on a Carriage. The supports for the machines, four of which may be mounted at once, are two horizontal bars, the lower of which can be raised or lowered, as may be necessary. The two parallel sides of the carriage are joined only at the upper side, and there is nothing to prevent it from being run into the head- ing, though the way between the rails may be heaped up with broken rock, if only the rails are clear. Drilling, and the removal of the broken rock, may then proceed simultaneously ; for, by means of a narrow gauge inside the carriage rails, small cars may be taken right up to the debris. It is made in different sizes, to suit the dimensions of the tunnel required. To give the carriage steadiness in working, it is raised from the wheels by jack-screws, and held in position by screws in a similar manner to the car- riage represented in Fig. 181. An extremely interesting system of drilling cocks totally different from that on which the machines we have just described are constructed has, within the last few years, been introduced by Messrs. Beaumont and Appleby. What does the reader think of boring holes in rocks wieh dia- monds ? It has long been a matter of common knowledge that the dia- 256 ROCK BORING. mond is the hardest of all substances, and that it will scratch and wear down any other substances, while it cannot itself be scratched or worn by anything but diamond. In respect to wearing down or abrading hard stones, the diamond, according to experiments recentlymade by Major Beaumont, occupies a position over all other gems and minerals to a degree far beyond that which has been generally attributed to it ; for in these experiments it was found that on applying a diamond, or rather a piece of the "carbonate" about to be described, fixed in a suitable holder, to a grindstone in rapid rotation, the grindstone was quickly worn down ; but on repeating a similar experiment with sapphires and with corundum, it was these which were worn down by the grindstone. Without, on the present occasion, entering into the natural history of the diamond, we may say that there are, besides the pure colourless transparent crystals so highly prized as gems, several varieties of diamond, and that those which are tinged with pink, blue, or yellow, are far from having the same value for the jeweller. Then there is another impure variety called boort, which appears to be employed only to furnish a powder by which the brilliants are ground and polished. In the diamond gravels of Brazil, from which we derive our regular supply of these gems, there was discovered in 1842 a curious variety of dark-coloured diamond, in which the crystalline cleavage, or tendency to split in certain directions (which belongs to the ordinary stones), appears to be almost absent : and the substance might be regarded as a transition form between the diamond and graphite but for its hardness. This substance was until lately used for the same purposes as boort, which is a nearer relative of the pure crystal and like it, splits along certain planes. It received from the miners the name of *' carbonado ? 2d\& with regard to the application we are considering, it has turned out to be a sort of Cinderella among diamonds ; for its unostentatious appearance is more than compensated for by its surpassing all its more brilliant sisters in the useful property to which re- ference has been made. This Brazilian term is doubtless the origin of the English name by which the substance in question is known among the English diamond merchants, who call it " carbonate" an unfortunate word, for it is used in chemistry with an entirely different signification. " Carbonate" it is, how- ever, which supplies the requirements of the rock- drill, and the selected stones are set in a crown, or short tube, of steel, represented by c in Fig. 183. In this they are secured as follows: holes are drilled in the rim of the tube, and each hole is then cut so that a piece of the diamond exactly fits it, and when this piece has been inserted, the metal is drawn round by punches, so as almost to cover the stone, leaving only a point projecting, b b. The portions of the crown between the stones are somewhat hollowed out, as at a, for a purpose which will presently be mentioned. The crown thus set with the boring gems is attached to the end of a steel tube, by which it is made to rotate with a speed of about 250 revolutions per minute while pressed against the rock to be bored. Water is forced through the steel tube, and passing out between the rock and the crown, especially under the hollows, c c, makes its escape between the outside of the boring-tube and the rock, thus wash- ing away all the dttris and keeping the drill cool. The pressure with ROCK BORING. 257 which the crown is forced forward depends, of course, on the nature of the rock to be cut, and varies from 400 Ibs. to 800 Ibs. In this way the hardest rocks are quickly penetrated sometimes, for example, at the rate of 4 in. per minute, compact limestone at 3 in., emery at 2 in., and quartz at the rate of i in. per minute. It is found that, even after boring through hundreds of feet of such materials, the diamonds are not in the least worn, but as fit for work as before : they are damaged only when by accident one of the stones gets knocked out of its setting ; and this machine surpasses all in the rapidity with which it eats its way through the firmest rocks. This, it must be observed, is the special privilege of the diamond drill that, since the begemmed steel crown and the boring-rods are alike tubular, the rock is worn away in an annular space only, and a solid cylinder of stone is detached from the mass, which cylinder passes up with the hollow rods, where, by means of certain sliding wedges, it is held fast, and is drawn away with the rods. When the diamond drill is used merely for driving the holes for blasting, this cylinder of rock is not an important matter ; but there is an application of the drill where this cylinder is of the greatest value, furnishing as it does a perfect, complete, and easily preserved section of the whole series of strata through which the drill may pass when a bore-hole is sunk in the operation of searching for minerals (which is so significantly called in the United States " prospecting," a phrase which seems to be making its way in England in mining connections) ; for the core is uniformly cylindrical, the surface is quite smooth, and any fossils which may be present come up uninjured, so far as they are contained in the solid core, and thus the strata are readily recognized. Contrast this with the old method, where the bore- hole in prospecting is made by the reciprocating action imparted to a steel tool, and merely the pounded material is obtained, usually in very small fragments, by augers or sludge-pumps : the fossils, which might afford the most valuable indications, crushed and perhaps incapable of being recog- nized ; and instead of the beautifully definite and continuous cylinder, a mere mass of debris is brought up. In the prospecting-bores the diameter of the hole is from 2 in. to 7 in. The size adopted depends on the nature of the strata to be penetrated, and on the depth to which it is proposed to carry the boring. When the strata are soft, the operation is commenced with a bore of 7 in., and when this has been carried to an expedient depth, the danger of the sides of the hole falling in is avoided by putting down tubes, and then the diamond drill, fixed to tubes of a somewhat smaller diameter, will be again inserted, and the boring recommenced; or the hole can be widened, so as to receive the lining-tubes. Of course, in boring through hard rocks, such as compact limestones, sandstone, &c., no lining- tubes are necessary. In a very interesting paper, read before the members of the Midland Institute of Mining Engineers, by Mr. J. K. Gulland, the engineer of the Diamond Rock-Boring Company, who have the exclusive right of working the patents for this remarkable invention, that gentleman concludes by remarking that " the leading feature of the diamond drill is that it works without percussion, thus enabling the holing of rocks to be effected by a fat simpler class of machinery than any which has to strike blows. Every mechanical engineer knows, often enough to his cost, that he enters upon a new class of difficulties when he has to recognize it as a normal state of things with any machinery he is designing that portions of it are brought violently to rest. These difficulties increase very much when the power, as 17 258 ROCK BORING. in the case of deep bore-holes, has to be conveyed for a considerable dis- tance. Where steel is used a percussive action is necessitated, as, if a scraping action is used, the drill wears quicker than the rock. The extra- ordinary hardness of the diamond places a new tool in our hands, as its hardness, compared with ordinary rock, say granite, is practically beyond comparison. Putting breakages on one side, a piece of " carbonate " would wear away thousands of times its own bulk of granite. Irrespective of the private and commercial success which this invention has attained, it is a boon to a country such as ours, where minerals constitute in a great mea- sure our national wealth and greatness." The advantages of the diamond drill may be illustrated by the case of what is termed the Sub-Wealden Exploration. From certain geological considerations, which need not be entered upon here, several eminent British and continental geologists have arrived at the conclusion that it is probable that coal underlies the Wealden strata of Kent and Sussex, and that it may be perhaps met with at a workable depth. If such should really prove to be the case, the industrial advantages to the south of England would be very great, for the existence of coal so comparatively near to the metropolis would prove not only highly lucrative to the owners of the coal, but confer a direct benefit upon thousands by cheapening the cost of fuel. A number of property owners and scientific men, having resolved that the matter should be tested by a bore, raised funds for the purpose, and a 9 in. bore had been carried down to a depth of 313 ft. in the ordinary manner, when a contract was entered into with the Diamond Rock-Boring Com- pany for a 3 in. bore extracting a cylinder of rock 2 in. in diameter. The company, as a precautionary measure, lined the old hole with a 5 in. steel tube ; and in spite of some delay caused by accidents, they increased the depth of the hole to 1,000 ft. in the interval from 2nd February, 1874, to 1 8th June, 1874 the progress of the work being regarded with the greatest interest by the scientific world. Unfortunately ( the further progress of the work has been prevented by an untoward event, namely, the breaking of the boring-rod, or rather tube ; and, although the company is prepared with suitable tackle for extracting the tubes in case of accidents of this kind, and generally succeeds in lifting them by a taper tap, which, entering the hollow of the tube, lays hold of it by a few turns yet, in this instance, where there have been special difficulties, the extraction of so great a length of tubes is, as the reader may imagine, by no means an easy task. Six attempts have been made to remove the boring-rods which have dropped down ; but so difficult has this operation proved, that, all tfyese efforts having failed, it has been decided to abandon the old work and commence a new boring on an adjacent spot. A contract has been entered into with the Diamond Boring Company, who have undertaken to com- plete the first 1,000 ft. for .600, which is only ^200 more than it would have cost to completely line the old bore-holes with iron tubes an opera- tion which was contemplated by the committee in charge of the exploration. The terms agreed to by the company are very favourable to the promoters of the Sub-Wealden Exploration, although the cost of the second 1 ,000 ft. will be ,3,000 more ; and the committee are relying upon the public for contributions to enable them to carry on their enterprise. It is most pro- bable that funds will be forthcoming, and should the boring result in the finding of coal measures beneath the Wealden strata, all the nation will be the richer and participate in the advantages resulting from an undertaking carried on by private persons. Already a totally unexpected source of POCK BORING. 259 FIG. 1 84. The Diamond Drill Machinery for deep Bores. wealth has bsen met with by the old bore showing the existence of con- siderable beds of gypsum in these strata, and the deposits of gypsum are about to be worked. Whether coal be found or not found, there is no doubt that a bore-hole going down 2,000 ft. will greatly increase our geological knowledge, and may reveal facts of which we have at present no conception. The boring-tubes, it maybe remarked, are made in 6ft. lengths, and are so contrived that the joints are nearly flush that is, there is no projection 17 2 2 6o ROCK BORING. at the junctions of the tubes. Fig. 184 is engraved from a photograph of the machinery used for working the diamond drill when boring a hole for " prospecting." This looks at first sight a very complicated machine, but in reality each part is quite simple in its action, and is easily understood when its special purpose has been pointed out. We cannot, however, do more than indicate briefly the general nature of the mechanism. The reader will on reflection perceive that, although the idea of causing a rod to rotate in a vertical hole may be simple, yet in practically carrying it out a number of different movements and actions have to be provided for in the machinery. The weight of the rods cannot be thrown on the cutters, nor borne by the moving parts of the machine hence the movable disc- shaped weights attached to the chains are to balance the weight of the boring-rods as the length of the latter is increased. There must also be a certain amount of feed given to the cutters, regulated and adjusting itself to avoid injurious excess : hence a nut which feeds the drill is encircled by a friction-strap in which it merely slips round without advancing the cutter when the proper pressure is exceeded. There must be means of throwing this into or out of gear, or advancing the tool in the work and of withdraw- ing it hence the handles seen attached to the brake-straps. Water must be drawn from some convenient source, and caused to pass down the drill- tube hence the force-pump seen in the lowest part of the figure. The rods must be raised by steam power and lowered by mechanism under perfect control hence suitable gearing is provided for that purpose. The reader may be interested in learning what is the cost of " prospect- ing " with this unique machinery. The company usually undertake to bore the first 100 ft. for ^40, but the next 100 ft. cost ;8o that is, for 200 ft. ^120 would be charged ; the third looft. would cost ^120 that is to say, the first 300 ft. would cost ^240, and so on each lower 100 ft. cost- ing 40 more than the 100 ft. above it. Some of the holes bored have been of very great depth, and have been executed in a marvellously short space of time. Thus, in 54 days, a depth of 902 ft. was reached at Girrick in a boring for ironstone ; another for coal at Beeston reached 1,008 ft. ; and at Walluff in Sweden 304^ ft. were put down in one week ! These machines are peculiarly suitable for submarine boring, for they work as well under water as in the air ; and they will no doubt be put into requisition in the preliminary experiments about to be made for that great project which bids fair to become a sober fact the Channel Tunnel be- tween England and France ; and as, by the time these pages will be before the public, the work of the greatest and boldest rock-boring yet attempted will have commenced, and the scheme itself will be the theme of every tongue, the Author feels that the present article would be incomplete with- out some particulars of the great enterprise. THE CHANNEL TUNNEL. PHE notion of connecting England and France by a submarine line of railways is not of the latest novelty, but has been from time to time mooted by the engineers of both countries. The most carefully prepared scheme, however, is embodied in the joint propositions of Sir J. Hawkshaw ROCK BORING. 261 and Messrs. Brunlees and Low among English engineers ; and those of M. Gamond on the French side, which these gentlemen have prepared at the invitation of the promoters of the scheme, give the clearest and most' authentic account of the considerations on which this gigantic enterprise will be based, and from this document we draw the following passages : The undersigned engineers, some of whom have been engaged for a series of years m investigating the subject of a tunnel between France and England, having attentively considered those investigations and the facts which they have developed, beg to report thereon jointly for the informa- tion of the committee. These investigations supported the theory that the Straits of Dover were not opened by a sudden disruption of the earth at that point, but had been produced naturally and slowly by the gradual washing away of the upper chalk ; that the geological formations beneath the Straits remained in the original order of their deposit, and were identical with the formations of the two shores, and were, in fact, the continuation of those formations. Mr. Low proposed to dispense entirely with shafts in the sea, and to com- mence the work by sinking pits on each shore, driving thence, in the first place, two small parallel driftways or galleries from each country, connected at intervals by transverse driftways. By this means the air could be made to circulate as in ordinary coal-mines, and the ventilation be kept perfect at the face of the workings. Mr. Low laid his plans before the Emperor of the French in April, 1867, and in accordance with the desire of his Majesty, a committee of French and English gentlemen was formed in furtherance of the project. For some years past Mr. Hawkshaw's attention has been directed to this subject, and ultimately he was led to test the question, and to ascer- tain by elaborate investigations whether a submarine tunnel to unite the rail- ways of Great Britain with those of France and the Continent of Europe was practicable. Accordingly, at the beginning of the year 1866, a boring was commenced at St. Margaret's Bay, near the South Foreland; and in March, 1866, an- other boring was commenced on the French coast, at a point about three miles westward of Calais ; and simultaneously with these borings an exa- mination was carried on of that portion of the bottom of the Channel lying between the chalk cliffs on each shore. The principal practical and useful results that the borings have deter- mined are that on the proposed line of the tunnel the depth of the chalk on the English coast is 470 ft. below high water, consisting of 175 ft. of upper or white chalk and 295 ft. of lower or grey chalk ; and that on the French coast the depth of the chalk is 7 50 ft. below high water, consisting of 270 ft. of upper or white chalk and 480 ft. of lower or grey chalk ; and that the position of the chalk on the bed of the Channel, ascertained from the examination, nearly corresponds with that which the geological inquiry elicited. In respect to the execution of the work itself, we consider it proper to drive preliminary driftways or headings under the Channel, the ventilation of which would be accomplished by some of the usual modes adopted in the best coal-mines. As respects the work itself, the tunnel might be of the ordinary form, and sufficiently large for two lines of railway, and to admit of being worked by locomotive engines, and artificial ventilation could be applied ; or it 262 ROCK BORING. might be deemed advisable, on subsequent consideration, to adopt two single lines of tunnel. The desirability of adopting other modes of trac- tion may be left for future consideration. Such are the essential passages of the report which, in 1868, was sub- mitted to the Government of the Emperor Louis Napoleon, and was made the subject of a special commission appointed by the Emperor to inquire into the subject in all its bearings. The commission presented its report in 1869, a-nd these are the chief conclusions contained in it I. The commission, after having considered the documents relative to the geology of the Straits, which agree in establishing the continuity, homo- geneity, and regularity of level of the grey chalk between the two shores of the Channel, Are of opinion- that driving a submarine tunnel in the lower part of this chalk is an undertaking which presents reasonable chances of success. Nevertheless they would not hide from themselves the fact that its exe- cution is subject to contingencies which may render success impossible. II. These contingencies maybe included under two heads: either in meeting with ground particularly treacherous a circumstance which the known character of the grey chalk renders improbable ; or in an influx of water in a quantity too great to be mastered, and which might find its way in either by infiltration along the plane of the beds, or through cracks crossing the body of the chalk. Apart from these contingencies, the work of excavation in a soft rock like grey chalk appears to be relatively easy and rapid ; and the execution of a tunnel, under the conditions of the project, is but a matter of time and money. III. In the actual state of things, and the preparatory investigations being too incomplete to serve as a basis of calculation, the commision will not fix on any figure of expense or the probable time which the execution of the permanent works would require. The chart, Fig. 185, and the section, Fig. 186, will give an idea of the course of the proposed tunnel, which will connect the two countries almost at the nearest points. The depth of the water in the Channel along the proposed line nowhere exceeds 180 ft. little more than half the height of St. Paul's Cathedral, which building would, therefore, if sunk in the midst of the Channel, stiii form a conspicuous object rising far above the waves. But the tunnel will pass through strata at least 200 ft. below the bottom of the Channel, rising towards each end with a moderate gradient ; and from the lower points of these inclines the tunnel will rise slightly with a slope of i in 2,640 to the centre, or just sufficient for the purposes of drainage. On the completion of the tunnel a double line of rails will be laid down in it, and trains will run direct from Dover to Calais. Companies have already been formed in England under the presidency of Lord Richard Grosvenor, and in France under that of M. Michel Chevalier, and the legislation of each country has sanctioned the enterprise. Verily the real magician of our times is the engineer, who, by virtually abolishing space, time, and tide, is able to transport us hither and thither, not merely one or two almost like the magicians we read of in the " Arabian Nights," with their enchanted horses or wonderful carpets but by hundred? and by tens of hundreds. ROCK BORING. 263 FIG. 185. Chart of the Channel Tunnel. The " Daily News " of January 22nd, 1875, in presenting its readers with a chart of the proposed tunnel, offered also the following sensible and interesting comment on the subject : " This long-debated project has at length emerged from the region of 264 ROCK BORING. speculation, and is entering the stage of practical experiment. On this side the Channel a company has been formed to carry out the work, and on the other side the French Minister of Public Works has presented to the Assembly a Bill authorizing a French company to co-operate with the English en- gineers. The enterprise is one worthy of the nations which have in the present generation joined the two shores of the Atlantic by an electric cable, and cut a ship canal through the Isthmus of Suez, and of the age which has obliterated the old barrier of the Alps. All these gigantic undertakings seemed almost as bold in conception and as difficult of execution as the great work now about to commence. Those twenty miles of sea have long been crossed by telegraph lines ; they will soon be bridged, as it were, by splendid steamers ; but even our own generation, accustomed as it is to gigantic engineering works, ,has scarcely regarded the construction of a railway underneath the waves as within the reach of possi- bility. M. Thome* de Gamond, who first made the suggestion five and thirty years ago, was long re- garded as an over-sanguine person, who did not recognize the inevitable limits of human skill and power. A tunnel under twenty miles of stormy sea seemed very much like an engineer's dream, and it is only within the last few years that it has been regarded as a feasible project. Of its possibility, however, there seems now to be no manner of doubt. It is merely a stream of sea-water, and not a fissure in the earth, which divides us from the Continent Prince Metternich was right in speaking of it as a ditch. The depth is nowhere greater than one hun- dred and eighty feet ; and so far as careful soundings can ascertain the condition of the soil underneath the water, it consists of a smooth unbroken bed of ' chalk. The success of the experiment depends on this bed of chalk being continuous and whole. Should any very deep fissure exist, which is extremely im- probable, the tunnel may probably not be driven through it. But given, what every indication shows to exist, a homogeneous chalk bed some hundreds of feet in thickness, the driving of a huge bore for twenty miles through it is a mere question of time, money, and organization, and as the engineers have these resources at their command, they are sanguine, and we may even say confident, of success. "The method by which it is proposed that the excavation shall be made is in some respects simi- lar to that which was successfully employed in tunnelling the Alps. Mont Cenis was pierced by machinery adapted to the cutting of hard rock ; the chalk strata under the Channel are to be bored 1 N B ROCK BORING. 265 FlG. 187. View of Dover. by an engine, invented by Mr. Dickenson Brunton, which works in the comparatively soft strata like a carpenter's auger. A beginning will be made simultaneously on both sides of the Channel, and the effort will at first be limited to what we may describe as making a clear hole through from end to end. This small bore, or driftway as it is called, will be some seven or nine feet in diameter. If such a communication can be success- fully made, the enlargement will be comparatively easy. Mr. Brunton's machine is said to cut through the chalk at the rate of a yard an hour. We believe that those which were used in the Mont Cenis Tunnel cut less than a yard a day of the hard rock of the mountain. Two years, therefore, ought to be sufficient to allow the workers from one end to shake hands with those from the other side. The enlargement of the driftway into the completed tunnel would take four years' more labour and as many millions of money. The millions, however, will easily be raised if the driftway is made, since the victory will be won as soon as the two headways meet under the sea. One of the great difficulties of the work is shared with the Mont Cenis Tunnel, the other is peculiar to the present undertaking. The Alps above the one, and the sea above the other, necessarily prevent the use of shafts. The work must be carried on from each end ; and all the debris excavated must be brought back the whole length of the boring, and all the air to be breathed by the workmen must be forced in. The provi- sion of a fit atmosphere is a mere matter of detail. In the great Italian tunnel the machines were moved by compressed air, which, being liberated when it had done its work, supplied the lungs of the workers with fresh 266 ROCK BORING. oxygen. The Alpine engineers, however, started from the level of the earth : the main difficulty of the Submarine Tunnel seerrts to be that it must have as its starting-point at each end the bottom of a huge well more than a hundred yards in depth. The Thames Tunnel, it will be re- membered, was approached, in the days when it was a show place, by a similar shaft, though of comparatively insignificant depth. This enterprise may indeed be said to bear something like the relation to the engineering and mechanical skill of the present day which Brunei's great undertaking bore to the powers of an age which looked on the Thames Tunnel as the eighth wonder of the world. Probably the danger which will be incurred in realizing the larger scheme is less than that which Brunei's workmen faced. "It is, of course, impossible for any estimate to be formed of the risks of this enormous work. They have been reduced to a minimum by the mechanical appliances now at our disposal, but they are necessarily con- siderable. The tunnel is to run, as we understand, in the lower chalk, and there will be, as M. de Lesseps told the French Academy, some fifty yards of soil a solid bed of chalk, it is hoped between the sea-water and the crown of the arch. Moreover, an experimental half-mile is to be under- taken on each side before the work is finally begun ; the engineers, in fact, will not start on the journey till they have made a fair trial of the way. Altogether the beginning seems to us to be about to be made with a com- bination of caution and boldness which deserves success, even though it should be unable to command it. Unforeseen difficulties may arise to thwart the plans, but the enterprise, so far, is full of promise. The open- ing of such a communication between this country and the Continent will be a pure gain to the commercial and social interests on both sides. It obliterates the Channel so far as it hinders direct communication, yet keeps it intact for all those advantages of severance from the political complica- tions of the Continent, which no generation has more thoroughly appre- ciated than our own. The commercial advantages of the communication must necessarily be beyond all calculation. A link between the two chief capitals of Western Europe, which should annex our railway system to the whole of the railways of the Continent, would practically widen the world to pleasure and travel and every kind of enterprise. The 300,000 travellers who cross the Channel every year would probably become three millions if the sea were practically taken out of the way by a safe and quick com- munication under it. The journey to Paris would be very little more than that from London to Liverpool. It is, however, quite needless to enlarge on these advantages. The Channel Tunnel is the crowning enterprise of an age of vast engineering works. Its accomplishment is to be desired from every point of view, and, should it be successful, it will be as benefi- cent in its results as the other great triumphs of the science of our time." FIG. 1 88. LIGHT. / T*HE foregoing pages have been devoted to the description of inventions * or operations in which mechanical actions are the most obvious features. Some of the contrivances described have for their end and object the communication of motion to certain bodies, others the arrangement of materials in some definite form, and all are essentially associated with trie idea of what is called matter. But we are now about to enter on another region a region of marvels where all is enchanted ground a region in which we seem to leave far behind us our grosser conceptions of matter, and to attain to a sphere of more refined and subtile existence. For we 267 2 68 LIGHT. are about to show some results of those beautiful investigations in which modern science has penetrated the secrets of Nature by unfolding the laws of light " Light Ethereal, first of things, quintessence pure." The diversity and magnificence of the spectacles which, by day as well as by night, are revealed to us by the agency of light, have been the theme of the poet in every age and in every country. It cannot fail to arrest the attention to find Science declaring that all the loveliness of the landscape, the fresh green tints of early summer and the golden glow of autumn, the brilliant dyes of flowers, of insects, of birds, the soft blue of the cloud- less sky, the rosy hues of sunset and of dawn, the chromatic splendour of rubies, emeralds, and other gems, the beauties of the million-coloured rainbow, are all due to light to light alone, and are not qualities of the bodies themselves, which merely seem to possess the colours. The follow- ing quaint stanzas, in which a poet of the seventeenth century addresses " Light " have a literal correspondence with scientific truth : " All the world's bravery, that delights our eyes, Is but thy several liveries; Thou the rich dye on them bestowest, Thy nimble pencil paints this landscape as thou goest. "A crimson garment in the rose thou wearest: A crown of studded gold thou bearest ; The virgin lilies, in their white, Are clad but with the lawn of almost naked light. "The violet, Spring's little infant, stands Girt in thy purple swaddling-bands: On the fair tulip thou dost dote ; Thou clothest it in a gay and parti-coloured coat." All these beauties are indeed derived from the imponderable and invisible agent, light ; and the variety and changefulness of the effects we may con- stantly observe show that light possesses the power of impressing our visual organs in a thousand different ways, modified by the surrounding circum- stances, as witness that ever-shifting transformation scene the sky. In the skies of such a climate as that of England there are ceaseless changes and ever-beautiful effects, producing everywhere more perfect and diversi- fied pictures than the richest galleries can show. In the night how changed is the spectacle, when the sun's more powerful rays are succeeded by the soft light of the moon, sailing through the azure star-bestudded vault! What limitless scope for the artist is afforded by these innumerable modifications of a single subtile agent, in light and shade, brightness and obscurity, in the contrasts and harmonies of colours, and in the countless hues resulting from their mixtures and blendings ! It will be necessary, before attempting to explain the discoveries and inventions which prove how successfully science, aided by the powerful mathematical analysis of modern times, has acquired a knowledge of the ways of light, to discuss such of the ordinary phenomena as have a direct bearing upon the subjects to be considered. FIG. 189. Rays. SOME PHENOMENA OF LIGHT. IT may be considered as a matter of common experience that light is able to pass through certain bodies, such. as air and gases, pure water, glass, and a number of other liquids and solids, which, by virtue of this passage of light, we term transparent, in opposition to another class ol bodies, called opaque, through which light does not pass. That light tra- verses a vacuum may be held as proved by the light of the sun and stars reaching us across the interplanetary spaces ; but it may also be made the subject of direct experiment by an apparatus described below, tig. 190. Another fact, very obvious from common observation, is that light usually travels in straight lines. Some familiar experiences may be appealed to for establishing this fact. For example, every one has observed that the beams of sunlight which penetrate an apartment through any small open- ing pursue their course in perfectly straight lines across the atmosphere, in 270 LIGHT. which their path is rendered visible by the floating particles of dust. It is by reason of the straightness with which rays of light pursue their course that the joiner, by looking along the edge of a plank, can judge of its truth, and that the engineer or surveyor is able by his theodolite and staff to set out the work for rectilinear roads or railways. On a grander scale than in the sunbeam traversing a room, we witness the same fact in the effect repre- sented in Fig. 189, where the sun. concealed from direct observation, is seen to send through openings in the clouds, beams that reveal their paths by lighting up the particles of haze or mist contained in the atmosphere. It is not the air itself which is rendered visible ; but whenever a beam of sun- light, or of any other brilliant light, is allowed to pass through an apartment which is otherwise kept dark, the track of the beam is always distinctly visible, and, especially if the light be concentrated by a lens or concave mirror, the fact is revealed that the air, which under ordinary circum- stances appears so pure and transparent, is in reality loaded with floating particles, requiring only to be properly lighted up to show themselves. FIG. 190. Professor Tyndall, in the course of some remarkable researches on the decomposition of vapours by light, wished to have such a glass tube as that represented in Fig. 190, filled with air perfectly free from these floating par- ticles. When the beam of the electric lamp passed through the exhausted tube, no trace of the existence of anything within the tube was revealed, for it appeared merely like a black gap cut out of the visible rays that tra- versed the air ; thus proving that light, although the agent which makes all things become visible, is itself invisible that, in fact, we see not light, but only illuminated substances. When, however, air was admitted to the tube, even after passing through sulphuric acid, the beam of the light became clearly revealed within the tube, and it was only by allowing the air to stream very slowly into the exhausted glass tube through platinum pipes, packed with platinum gauze and intensely heated, that Professor Tyndall succeeded in obtaining air " optically empty," that is, air in which no floating particles revealed the track of the beams. The destruction of the floating matter by the incandescent metal proves the particles to be organic ; but a more convenient method of obtaining air free from all suspended matter was found by Professor Tyndall to be the passing of the air through a filter of cotton wool. It must not be supposed that it is only occasionally, or in dusty rooms, laboratories, or lecture-halls, that the air is charged with organic and other particles "As thick as motes in the sunbeams." " The air of our London rooms," says Tyndall, " is loaded with this LIGHT. 271 organic dust, nor is the country air free from its pollution. However ordi- nary daylight may permit it to disguise itself, a sufficiently powerful beam causes the air in which the dust is suspended to appear as a semi-solid, rather than as a gas. Nobody could, in the first instance, without repug- nance, place the mouth at the illuminated focus of the electric beam and inhale the dust revealed there. Nor is this disgust abolished by the reflec- tion that, although we do not see the nastiness, we are drawing it in our lungs every hour and minute of our lives. There is no respite to this con- tact with dirt ; and the wonder is, not that we should from time to time suffer from its presence, but that so small a portion of it would appear to be deadly to man." The Professor then goes on to develope a very re- markable theory, which attributes such diseases as cholera, scarlet fever, small pox, and the like, to the inhalation of organic germs which may form part of the floating particles. But we must return to our immediate subject by a few words on the VELOCITY OF LIGHT. FlG. 191. Telescopic appearance of Jupiter and Satellites. T T may be stated at once, that this velocity has the amazing magnitude -"- of 185,000 miles in one second of time, and that the fact of light re- quiring time to travel was first discovered, and the speed with which it does travel was first estimated, about 200 years ago, by a Danish astro- nomer, named Roemer, by observations on the eclipses of the satellites of Jupiter. The satellites of Jupiter are four in number, and as they revolve 272 LIGHT. nearly in plane of the planet's orbit, they are subject to frequent eclipses by entering the shadow cast by the planet ; in fact, the thVee inner satel- lites at every revolution. Fig. 191 represents the telescopic appearance of the planet, from a drawing by Mr. De La Rue, and in this we see the well- known "belts," and two of the satellites, one of which is passing across the face of the planet, on which its shadow falls, and is distinctly seen as a round black spot, while the other may be noticed at the lower right-hand corner of the cut. The satellite next the planet (lo) revolves round its primary in about 42 1 hours, and consequently it is eclipsed by plunging into the shadow of Jupiter at intervals of 42^ hours, an occurrence which must take place with the greatest regularity as regards the duration of the intervals, and which can be calculated by known laws when the distance of the satellite from the planet has been determined. Nevertheless, Roemer observed that the actual intervals between the successive immersions of lo in the shadow of Jupiter did not agree with the calculated period of rotation when the distance between Jupiter and the earth was chang- ing, in consequence chiefly of the movement of the latter (for Jupiter re- quires nearly twelve years to complete his revolution, and may, therefore, be regarded as stationary as compared for a short time with the earth). Roemer saw also, that when this distance was increasing, the observed intervals between the successive eclipses were a little greater, and that when the distance was decreasing they were a little less, than the calculated period. And he found that, supposing the earth, being at the point of its orbit nearest to Jupiter, to recede from that planet, the sum of all the re- tardations of the eclipses which occur while the earth is travelling to the farthest point of its orbit, amounts to 16^ minutes, as does also the sum of the deficiencies in the period when the earth, approaching Jupiter, is passing from the farthest to the nearest point of her orbit. While, however, the earth is near the points in her orbit farthest from, or nearest to Jupiter, the distance between the two planets is not materially changing between suc- cessive eclipses, and then the observed intervals of the eclipses coincide with the period of the satellite's rotation. The reader will, after a little reflection, have no difficulty in perceiving that the 16^ minutes represent the time which is required by the light to traverse the diameter of the earth's orbit ; or, if he should have any difficulty, it may be removed by comparing the case with the following. Let us suppose that from a railway terminus trains are dispatched every quarter of an hour, and that the trains proceed with a common and uni- form velocity of, say, one mile per minute. Now, a person who remains stationary, at any point on the railway, observes the trains passing at regular intervals of fifteen minutes, no matter at what part of the line he may be placed. But now, let us imagine that a train having that very instant passed him, he begins to walk along the line towards the place from which the trains are dispatched : it is plain that he will meet the next train before fifteen minutes he would, in fact, meet it a mile higher up the line than the point from which he began his walk fourteen minutes before ; but the train, taking a minute to pass over this mile, would pass his point of departure just fifteen minutes after its predecessor. And our imaginary pedestrian, supposing him to continue his journey at the same rate, would meet train after train at intervals of fourteen minutes. Similarly, if he walked away from the approaching trains, they would overtake him at intervals of sixteen minutes. And again, it would be easy for him to cal- culate the speed of the trains, knowing that they passed over each point of LIGHT. 273 the line every fifteen minutes. Thus, suppose him to pass down the line a distance known to be, say, a quarter of a mile ; suppose he leaves his sta- tion at noon, the moment a train has passed, and that he takes, say an hour, to arrive at his new station a quarter of a mile lower ; here, ob- serving a train to pass at fifteen seconds after one o'clock, and knowing that it passed his original station at one, he has a direct measure of the speed of the trains. Here we have been explaining a discovery two cen- turies old ; but our purpose is to prepare the reader for an account of how the velocity of light has been recently measured in a direct manner, and it certainly appears a marvellous achievement that means have been found to measure a velocity so astounding, not in the spaces of the solar system, or along the diameter of the earth's orbit, but within the narrow limits of an ordinary room ! The reliance with which the results of these direct measures will be received, will be greatly increased by the knowledge of the astronomical facts with which they show an entire concordance. In FlG. 192. taking leave of Roemer, we may mention that his discovery, like many others, and like some inventions which have been described in this book, did not for some time find favour with even the scientific world, nor was the truth generally accepted, until Bradley's discovery of the aberration of light completely confirmed it. To two gifted and ingenious Frenchmen we are indebted for independent measurements of the velocity of light by two different methods. The general arrangement of M. Fizeau's method is represented in Fig. 192, in which the rays from a lamp, L, after passing through a system of lenses, fall upon a small mirror, M N, formed of unsilvered plate-glass inclined at an angle of 45 to the direction of the rays ; from this they are reflected along the axis of a telescope, T, by the lens of which being rendered parallel, they become a cylindrical beam, B, which passes in a straight line to a station, D, at a distance of some miles (in the actual experiment the lamp was at Suresnes and the other station at Montmartre, 5^ miles distant) whence the beam is reflected along the same path, and returns to the little plate of glass at M N, passing through which it reaches the eye of the observer at E. At w is a toothed wheel, the teeth of which pass through the point F, where the 18 274 LIGHT. rays from the lamp come to a focus ; and as each tooth pksses, the light is stopped from issuing to the distant station. This wheel is capable of re- ceiving a regular and very rapid rotation from clockwork in the case, C, provided with a register for recording the number of its revolutions. If the wheel turns with such a speed that the light permitted to pass through one of the spaces travels to the mirror and back in exactly the same time that the wheel moves and brings the next space into the tube, or the second space, or the third, or any space, the reflected light will reach the spectator's eye just as if the wheel were stationary ; but if the speed be such that a tooth is in the centre of the tube when the light returns from the mirror, then it will be prevented from reaching the spectator's eye at all, so long as this particular speed is maintained, but either a decrease or an increase of velocity would cause the luminous image to reappear. Speeds between those by which the light is seen, and those by which it entirely disappears, cause it to appear with merely diminished brilliancy. It is only necessary to observe the speed of the wheel when the light is at its brightest, and when it suffers complete eclipse, for then the time is known which is required for space and tooth respectively to take the place of another space and hence the time required for the light to pass to the mirror and back is found. M. Foucault's method is similar in principle to that used by Wheatstone in the measurement of the velocity of electricity. He used a mirror which was made to revolve at the rate of 700 or 800 turns per second, and the arrangement of the apparatus was such as to admit of the measurement of the time taken by light to pass over the short space of about four yards ! More recently, however, he has modified and improved his apparatus by adopting a most ingenious plan of maintaining the speed of the mirror at a determined rate, which he now prefers should be 400 turns per second, while the light is reflected backwards and forwards several times, so that it traverses a path of above 20 yards in length. The time taken by the light to travel this short distance is, of course, extremely small, but it is accu- rately measured by the clockwork mechanism, and found to be about the f a second ! The results of these experiments of Foucault's make the velocity of light several thousand miles per second less than that deduced from the astronomical observation of Roemer and Bradley, in which the distance of the earth from the sun formed the basis of the calcu- lations ; and hence arose a surmise that this distance had been over-esti- mated. That such had, indeed, been the case was confirmed almost im- mediately afterwards by a discussion among the astronomers as to the correctness of the accepted distance, the result of which has been that the mean distance, which was formerly estimated at 95 millions of miles, has, by careful astronomical observations and strict deductions, been now esti- mated at between 91 and 92 millions of miles. The recent transit of Venus, December 9th, 187310 observe which the Governments of all the chief nations of the world sent out expeditions derives its astronomical and scientific importance from its furnishing the means of calculating, with greater correctness than has yet been attained, the distance of the earth from the sun. LIGHT. 275 REFLECTION OF LIGHT. LONG before plate glass backed by brilliant quicksilver ever reflected the luxurious appointments of a drawing-room ; long before looking- glass ever formed the mediaeval image of " ladye fair " ; long before the haughty dames of imperial Rome were aided in their toilettes by specula; long before the dark-browed beauties of Egypt peered into their brazen mirrors ; long, in fact, before men knew how to make glass or to polish metals, their attention and admiration must have often been riveted by those perfect and inverted pictures of the landscape, with its rocks, trees, and skies, which every quiet lake and every silent pool presents. Enjoyment of the spectacle probably prompted its imitation by the formation artifi- cially of smooth flat reflecting surfaces ; and no doubt great skill in the pro- JL M FIG. 194. duction of these, and their application to purposes of utility, coquetry, and luxury, preceded by many ages any attempt to discover the laws by which light is reflected. The most fundamental of these laws are very simple, and for the purpose we have in view, it is necessary that they should be borne in mind. Let A B, Fig. 193, be a plane reflecting surface, such as the surface of pure quicksilver or still water, or a polished surface of glass or metal, and let a ray of light fall upon it in the direction, I O, meeting the surface at O. it will be reflected along a line, O R, such that if at the point O we draw a line, O P, perpendicular to the surface, the incident ray, I O, and the re- flected ray, O R, will form equal angles with the perpendicular in other words, the angle of incidence will be equal to the angle of refle :tion, and the perpendicular, the incident ray, and the reflected ray, will all be in one plane perpendicular to the reflecting plane. It would be quite easy to 18 2 27 6 LIGHT. prove from this law that the luminous rays from any object falling on a plane reflecting surface are thrown back just as if they came from an object placed behind the reflecting surface symmetrically to the real object The diagrams in Figs. 194 and 195 will render this clear. In the second diagram, Fig. 195, it will be noticed that only the portion of the mirror be- tween Q and P takes any part in the action, and therefore it is not necessary, in order to see objects in a plane mirror, that the mirror 'should be exactly opposite to them ; thus the portion o Q might be removed without the eye losing any part of the image of the object A B. There are many very interesting and important scientific instruments in which the laws of reflection from plane surfaces are made use of such, for example, as the sextant and the goniometers but passing over all these, we FIG. 196. may say a word about the formation of several images from one object by using two mirrors. It has already been explained that the action of a plane mirror is equivalent to the placing of objects behind it symmetrically dis- posed to the real object The reflections, or virtual images in the mirror, behave optically exactly as if they were themselves real objects, and are re- flected by other mirrors in precisely the same manner. From this it follows that two planes inclined to each other at an angle of 90 give three images of an object placed between them, the images and the object apparently placed at the four angles of a rectangle. When the mirrors are inclined to each other at an angle of 60, five images are produced, which, with the original object, show an hexagonal arrangement The formation of these by the principle of symmetry is indicated in Fig. 196. It was these symmetri- cally disposed images which suggested to Sir David Brewster the construc- tion of the instrument so well known as the kaleidoscope, in which two or, still better, three mirrors of black glass, or of glass blackened on one side, are placed in a pasteboard tube inclined to ach other at 60 : one end of the tube is closed by two parallel plates of glass ; the outer one ground, but the inner transparent, leaving between them an interval, in which are placed fragments of variously-coloured glass, which every movement of LIGHT. 277 FlG. 197. Polemoscope. the instrument arranges in new combinations. At the other end of the tube is a small opening on applying the eye to which one sees directly the fragments of glass, with their images so reflected that beautifully symmetrical patterns are produced ; and this with endless variety. When this instrument was first made in the cheap form in which it is now so familiarly known, it obtained a popularity which has perhaps never been equalled by any scientific toy, for it is said that no fewer than 200,000 kaleidoscopes were sold in London and Paris in one month. By way of contrast to the mirrors of the kaleidoscope harmlessly pro- ducing beautiful designs, by symmetrical images of fragments of coloured glass, we show the reader, in Fig. 197, mirrors which are reflecting quite other scenes, for here is seen the manner in which even the plane mirror has been pressed into the service of the stern art of war. The mirrors are employed, not like those of Archimedes, to send back the sunbeams from every side, and by their concentration at one spot to set on fire the enemy's works, but to enable the artillerymen in a battery to observe the effect of their shot, and the movement of their adversaries, without exposing them- selves to fire by looking over the parapet of their works. The contrivance has received the appropriate name viPolemoscope (ToXe/*os, war, and cr/fOTrew., 278 LIGHT. FlG. 198. Apparatus for Ghost Illusion. to view), and it consists simply, as shown in the figure, of two plane mirrors so inclined and directed, that in the lower one is seen by reflection the loca- lities which it is desired to observe. We return once more to the arts of peace, in noticing the advantage which has been lately taken of plane mirrors for the production of spectral and other illusions, in exhibitions and theatrical entertainments, the im- provement in the manufacture of plate-glass having permitted the produc- tion of enormous sheets of that substance. Among the most popular exhi- bitions of this class was that known as " Pepper's Ghost," the arrangement of the mirrors having been the subject of a patent taken out by Mr. Pepper and Mr. Dircks jointly. The principle on which the production of the illusion depends, may be explained by the familiar experience of everybody who has noticed that, in the twilight, the glass of a window presents to a person inside of a room the images of the light or bright objects in the apartment, while the objects outside are also visible through the glass. As, by night coming on, the reflections increase in brilliancy, the darkness outside is almost equivalent to a coat of black paint on the exterior surface of the glass ; but, on the contrary, in the daylight no reflection of the in- terior of the room is visible to the spectator inside, on looking towards the window. The reflections are present, nevertheless, in the day-time as well as at night, only they are overpowered and lost when the rays which reach the eye through the glass are relatively much more powerful. Even in the day-time the image of a lighted candle is usually visible, in the absence of PLATE V. THE GHOST ILLUSION LIGHT. 279 direct sunshine, against a dark portion of the exterior objects as a back- ground. The visibility, or otherwise, of the internal objects by reflection, and of the external objects seen through the glass, depends entirely on the rela- tive intensities of the illumination, for the more illuminated side overpowers and conceals the other, just as the rising sun causes the stars " to pale their ineffectual fires." Hence, on looking through the window on a dark night, we cannot see objects out of doors unless we screen off the reflection of the illuminated objects in the room. If the rays transmitted through the glass, and those which are reflected, have intensities not very different, we see then the reflected images mixed up in the most curious manner with the real objects. It is exactly in this way that the ghosts are made to appear in the illusion of which we are speaking. The real actors are seen through a large plate of colourless and transparent glass, and from the front surface of this glass rays are reflected which apparently proceed from a phantom taking a part in the scene among the real actors. The arrangement is shown in Fig. 198, where E G is the stage, separated from the auditorium, H, by a large plate of transparent glass, E F, placed in an inclined position, and not visible to the spectators, for the lights in front are turned down, and the stage is also kept comparatively dark. Parallel to the large plate of glass is a silvered mirror, c D, placed out of the spectators' sight, and receiving the rays from a person at A, also out of sight of the spectators, and strongly illuminated by an oxy-hydrogen lime-light at B. The manner in which the rays are reflected from the silvered mirror to the plate-glass, and hence reflected so as to reach the spectators and give them the im- pression of a figure standing on the stage at G, is sufficiently indicated by the lines drawn in the diagram. The apparitional and unsubstantial cha- racter of the image is derived from its seeming transparency, and from the manner in which it may be made to melt away, by diminishing the brightness of the light which falls on the real person. The introduction of the second mirror was a great improvement, for by this the phantom is made to appear erect, while its original stands in a natural attitude. Where- as, with only the plate-glass, E F, the ghost could not be made to appear upright, unless, indeed, as was sometimes done, the plate was inclined at an angle of 45, and the actor of the ghost lay horizontally beneath it. A scene of the kind produced by the improved apparatus, is represented in Plate V. Another illusion is produced by the help of a large silvered mirror, placed at an inclination of 45, sloping backwards from the floor, and, in consequence, presenting to the spectators the image of the ceiling, which appears to them the back of the scene. The mirror is perforated near the centre by an opening, through which a person passes his head, and, all his body being concealed by the mirror, the effect produced is that of a head floating in the air. Means are provided of withdrawing the mirror, when necessary, while the curtain is down, and then the real back of the scene appears, which, of course, is exactly similar to the false one painted on the ceiling. Fig. 199 represents a scene produced at the Poly* technic by a somewhat similar arrangement of mirrors, under the manage- ment of Mr. Pepper. Plane mirrors were employed in another piece of natural magic which this gentleman exhibited to the public, who were shown a kind of large box, or cabinet, raised from the floor, and placed in the middle of the stage, so that the spectators might see under it and all round it. Inside of the box were two silvered mirrors the full height of it, and these were hinged to the farther angles, so that each one being 280 LIGHT. FIG. 199. Illusion produced by Mirrors. folded with its face against a side of the box, their backs formed the appa- rent sides, and were painted exactly the same as the real interior of the box. When the performer enters the box, the door is closed for an instant, while he, stepping to the back, turns the mirrors on their hinges until their front edges meet, where an upright post in the middle of the box conceals their line of junction. The performer thus places himself behind the mirrors in the triangular space between them and the back of the box, while the mirrors, now inclined at angles of 45 to the sides, reflect images of these to the spectators when the door is opened, and the spec- tators see then the box apparently empty, for the reflection of the sides appears to them as the back of the cabinet. The entertainment was some- times varied by a skeleton appearing, on the door being opened, in the place of the person who entered the cabinet. It is hardly necessary to say that the skeleton was previously placed in the angle between the mirrors where the performer conceals himself. LIGHT. 281 FlG. 200. A Stage Illusion. To the same inventive gentleman, whose ingenious use of plane mirrors has thus largely increased the resources of the public entertainer, is due another stage illusion, the effect of which is represented in Fig. 200 ; and, although it does not depend on reflection, it may be introduced here as show- ing how the perfection of the manufacture of plate-glass, which makes it available for the ghost exhibition, can be applied in another way in dramatic spectacles. The female form, here supposed to be seen in a dream by the sleeper, is not a reflection, although she appears floating in mid-air, strangely detached from all supports, but the real actress. This is accomplished by making use of the transparency of plate-glass, a material strong enough to afford the necessary support, and yet invisible under the circumstances of the exhibition. But it is not behind the turned-down footlights, or in the exhibitions of the showman, that we find the most beautiful illustrations of the laws of reflection. In the quiet mountain mere, amid the sweet freshness of nature, we may often see tree, and crag, and cliff, so faithfully reproduced, that it 282 LIGHT. needs an effort of the understanding to determine where substance leaves off and shadow begins, a condition of the liquid surface indicated in twc lines by Wordsworth : " The swan, on still St. Mary's Lake, Floats double, swan and shadow." The landscape painter is always gratified if he can introduce into his picture some piece of water, and it can hardly be doubted that much of the charm of lakes and rivers is due to their power of reflecting. Look on Fig. 201, a view of some buildings at Venice ; and, in order to see how much of its beauty is owing to the quivering reflections, imagine the impression it would produce were the place of the water occupied by asphalte pave- ment, or a grass lawn. The condition of the reflections here represented is perhaps even more pleasing than that produced by perfect repose: they are in movement* and yet not broken and confused : " In bright uncertainty they lie, Like future joys to Fancy's eye." FIG. 201. View of Venice Reflections. LIGHT. 283 REFRACTION. THAT light moves in straight lines is a statement which is true only when the media through which it passes are uniform ; for it is easily proved that when light passes from one medium to another, a change of direction takes place at the common surface of the media in all rays that meet this surface otherwise than perpendicularly. As a consequence of this, it really is possible to see round a corner, as the reader may convince himself by performing the following easy experiment. Having procured a cup or basin, Fig. 202, let him, by means of a little bees'-wax or tallow, attach to the bottom of the vessel, at R, a small coin. If he now places the cup so that its edge just conceals the coin from view, and maintains his eye steadily in the same position as at I, he will, when water is poured into the cup, perceive the coin apparently above the edge of the vessel in the direction I R', that is, the bottom of the cup will ap- pear to have risen higher. Since it is known that in each medium the rays pass in straight lines, the bending which renders the coin visible can therefore only take place at the common junction of the media, or, in other words, the ray, R O, passing from the object in a straight line through the water, is bent abruptly aside as it passes out at the sur- face of the water, A B, and enters the ,., air, in which it again pursues a straight * IG * 2O2< course, reaching the eye at I, where it gives the spectatoran impression of an object atR'. This experiment is also an illustration of the cause of the well-known tendency we have tounder-estimate the depth of water when we can see the bottom. The broken appearance pre- sented by an oar plunged into clear water is due to precisely the same cause. The curious exaggerated sizes and distorted shapes of the gold-fish seen in a transparent globe have their origin in the same bending aside of the rays. This deviation which light undergoes in passing obliquely from one medium into another is known by the name of refraction, and it is essential for the understanding of the sequel that the reader should be acquainted with some of the laws of this phenomenon, although their discovery by Snell dates two centuries and a half anterior to the present time. Let T O, Fig. 203, be a ray of light which falls obliquely upon a plane surface, A B, common to two different media, one of which is represented by the shaded portion of the figure, A B c D, of which c D represents another plane surface, parallel to the former. If the ray, T O, suffered no refraction, it would pursue its course in a straight line to r'\ but as a matter of fact it is found that such a ray is always bent aside at O, if the medium A B c D is more or less dense than the other. If, for example, A B c D is water, and the medium above it glass, then the ray entering at O will take the course O r ; but if A B C D is a plate of glass with water above and below it, the ray will take the course T O, o R, R B, suffering refraction on entering the glass, and again on leaving it, 284 LIGHT. so that R B will emerge fromthe glass parallel to its original direction at T O. If through the point of incidence, O, we suppose a line, O P, to be drawn perpendicular to the surface, A B, then we may say that the ray in pass- ing from the rarer medium (water, air, &c.) into the denser medium (glass, &c.) is bent towards the perpendicular, or normal, as at O ; but that on leaving the denser to enter the rarer medium, as at R, it is bent away from the perpendicular. In other words, the angle b o a is less than the angle m O T, and O R forms a less angle with R P 7 than R B' does. It is also a law of ordinary refraction that the normal, o P, at the point of incidence, the incident ray, T O, and the refracted ray, o R, are all in the same plane. Besides, there is the important and interesting law discovered by Snell and by Descartes, which may thus be explained with reference to Fig. 203. On the incident and refracted rays, T o and O R, let us suppose that any equal distances, O d and o , are measured off from o, and that from each of the points a and , perpendiculars, a m and b , are drawn to the normal, P P, which passes through o ; then it is found that, whatever may be the angle of incidence, T o P, or however it is made to vary, the length of the line a m bears always the same proportion to the line b n for the same two media. Thus, if A B c D be water, and T o enters it out of the air, the length of the line a m divided by the length of the line a b will always (whatever slope f O may have) give the quotient I "33. This number is, therefore, a constant quantity for air and water, and is called the index of refraction for air into water. The law just explained is expressed by the language of mathematics thus : For two given media the ratio of the sines of the angles of incidence and of refraction is constant. It is an axiom in optical science that a ray of light when sent in the oppo- site direction will pursue the same path. Thus in Fig. 203 the direction of the light is represented as from T towards B' ; but if we suppose B' R to be an incident ray, it would pursue the path B' R, R O, O T, and in passing out of the denser medium, A B c D at O, its direction is farther from the normal, P P, or, as the law of sines, a m will be always longer than n d, and will bear a constant ratio to it. Suppose the angle R O P to increase, then P o B will LIGHT. 285 become a right angle ; that is, the emergent ray, O T, will just graze the sur- face, A B, when the angle R O P has some definite value. If this last angle be further increased, no light at all will pass out of the medium A B C D, but the ray R o will be totally reflected at O back into the medium, A B c D, according to the laws of reflection. The angle which R o forms with o P when o T just skims the surface, A B, is termed the limiting angle, or the critical angle, and its value varies with the media. The reader may easily see the total reflection in an aquarium, or even in a tumbler of water, when he looks up through the glass at the surface of the water, which has then all the properties of a perfect mirror. The power of lenses to form images of objects is entirely due to these laws of refraction. The ordinary double-convex lens, for example, having its surfaces formed of portions of spheres, refracts the rays so that all the rays which from one luminous point fall upon the lens, meet together again at a point on the other side, the said point being termed their focus. It is thus that images of luminous bodies are formed by lenses. An explanation of the construction and theory of lenses cannot, however, be entered into in this place. One important remark remains to be made namely, that in the above statement of the laws of reflection and refraction, certain limitations and conditions under which they are true and perfectly general have not been expressed ; for the mention of a number of particulars, which the reader would probably not be in a condition to understand, would only tend to confuse, and the explanation of them would lead us beyond our limits. Some of these conditions belong to the phenomena we have to describe, and are named in connection with them, and others, which are not in im- mediate relation to our subject, we leave the reader to find for himself in any good treatise on optics. DOUBLE REFRACTION AND POLARIZATION. A BOUT two hundred years ago, a traveller, returning from Iceland, ** brought to Copenhagen some crystals, which he had obtained from the Bay of Roerford, in that island. These crystals, which are remarkable for their size and transparency, were sent by the traveller to his friend, Erasmus Bartholinus, a medical man of great learning, who examined them with great interest, and was much surprised by finding that all objects viewed through them appeared double. He published an account of this singular circumstance in 1669, and by the discovery of this property of Iceland spar, it became evident that the theory of refraction, the laws of which had been studied by Snell and by Huyghens a few years before, re- quired some modification, for these laws required only one refracted ray, and Iceland spar gave two. Huyghens studied the subject afresh, and was able, by a geometrical conception, to bring the new phenomena within the general theory of light. Iceland spar is chemically carbonate of lime (calcium car- bonate), and hence is also called calc spar, and, from the shape of the crystals, it has also been termed rhombohedral spar. The form in which the crystals actually present themselves is seen in Fig. 204, which also repre- sents the phenomenon of double refraction. Iceland spar splits up very 286 LIGHT. readily, but only along certain definite directions, and from such a piece as that represented in Fig. 204 a perfect rhombohedron, such as that shown in Fig. 206, is readily obtained by cleavage ; and then we have a solid having six lozenge-shaped sides, each lozenge or side having two obtuse angles of 101 55', and two acute angles of 78 5'. Of the eight solid corners, such as A B c, &c., six are produced by the meeting of one obtuse and two acute angles, and the remaining two solid corners are formed by the meeting of three obtuse angles. Let us imagine that a line is drawn from one of these FIG. 204. angles to the other : the diagonal so drawn forms the optic axis of the crystal, and a plane passing through the optic axis, A B, Fig. 205, and through the bisectors of the angles, E A D and F B G, marks a certain defi- nite direction in the crystal, to which also belong all planes parallel to that just indicated. Any one of such planes forms what is termed a " principal section," to which we shall presently refer. It will be observed that in Fig. 204 the white circle on a black ground seen through the crystal is doubled ; but that, instead of being white as the circle really is, the images appear grey, except where they overlap, and there the full whiteness is seen. If we place the crystal upon a dot made on a sheet of paper, or having made a small hole with a pin in a piece of cardboard, hold this up to the light, and place the crystal against it, we see apparently two dots or two holes. The two images will, if the dot or hole be sufficiently small, appear entirely detached from each other. Now, if, keeping the face of the crystal against me cardboard or paper, the observer turn the crystal round, he will see one of the images revolve in a circle round the other, which remains stationary. The latter is called the ordinary image, and the former the extraordinary image. Let us place the crystal upon a straight black line ruled on a horizontal sheet of paper, Fig. 205, and let us suppose, in order to better define the appearance, that we place it LIGHT. 287 so that the optic axis, A B, is in a plane perpendicular to the paper, A being one of the two corners where the three obtuse angles meet, and B the other, and the face, A B C D, parallel to E G H B, which touches the paper. Then, according to the laws of ordinary refraction, if we look straight down upon the crystal, we should see through it the line I. K, unchanged in position that is, the ray would pass perpendicularly through the crystal as shown by L M and, in fact, a part of the ray does this, and gives us the ordinary image, O o' ; but another part of the ray departs from the laws of Snell and Descartes, and, following the course L N Y', enters the eye in the direc- tion N Y', producing the impression of another line at I/, which is the ex- traordinary ray, E E'. If the crystal be turned round on the paper, E E' will gradually approach o o', and the two images will coincide when the principal section is parallel to the line I K ; but the coincidence is only apparent, and results from the super- position of the two images for a mark placed on the line drawn on the paper will show two images, one of which will follow the rotation of the crystal, and show itself to the right or left of the ordinary image, ac- cording as C is to the right or left of A. So that there are really in every portion of the crystal two images on the line, one of which turns round the other, and the coal- escence of the two images twice in each revolution is only apparent, for the different parts of the lengths of the images do not coincide. On continuing the revolution of the crystal after they apparently coincide, the images are again seen to separate, the extra- ordinary one being now displaced on the other side, or always towards the point, C. Thus, then, the ray, on entering the crystal, bifurcates, one branch passing through the crystal and out of it in the same straight line, just as it would in passing through a piece of glass, while the other is refracted at its entrance into the crystal, although falling perpendicularly upon its face, and again at its exit. And again, when a beam of light, R r, Fig. 206, falls obliquely on a crystal of Iceland spar, it divides at the face of the crystal into two rays, r O, and r E ; the former, which is the ordinary ray, follows the laws of ordinary refraction it lies in the plane of incidence, and obeys the law of sines, just as if it passed through a piece of plate-glass. The extraordinary ray, on the other hand, departs from the plane of incidence, except when the latter is parallel to the principal section, and the ratio of the sines of the angles of incidence and refraction varies with the incidence. The reader who is desirous of studying these curious phenomena of double refraction, and those of polarization, is strongly recommended to procure some fragments of Iceland spar, which he can very easily cleave into rhom- bohedra, and with these, which need not exceed half an inch square, or cost more than a few pence, he can demonstrate for himself the phenomena, and become familiar with their laws. He will find very convenient the simple plan recommended by the Rev. Baden Powell, of fixing one of the crystals to the inside of the lid of a pill-box, through which a small hole has been made, and through the hole and the crystal view a pin-hole in the bottom FlG. 206. 288 LIGHT. of the box, turning the lid, and the crystal with it, to observe the rotation of the image. The same arrangement will serve, by merely attaching another rhomb of spar within the box, to study the very interesting facts of the polarization to which we are about to claim the reader's attention. The curious phenomena which have just been described, although in themselves by no means recent discoveries, have led to some of the most interesting and beautiful results in the whole range of physical science. The examination and discussion of them by such able investigators as Huyghens, Descartes, Newton, Fresnel, Malus, and Hamilton, have largely conduced to the establishment of the undulatory hypothesis that compre- hensive theory of light, which brings the whole subject within the reach of a few simple mechanical conceptions. It was at first supposed that it was only one of the rays which are pro- duced in double refraction that departed from the ordinary laws, and Ice- land spar was almost the only crystal known to have the property in ques- tion. At the present day, however, the substances which are known- to produce double refraction are far more numerous than those which do not possess this property, for, by a more refined mode'of examination than the production of double images, Arago has been able to infer the existence of a similar effect on light in a vast number of bodies. Crystals have also been found which split up a ray of light entering them into two rays, neither of which obeys the laws of Descartes. It may, in fact, be said that, with the exception of water, and most other liquids, of gelatine and other colloi- dal substances, and of well-annealed glass, there are few bodies which do not exercise similar power on light. On examining the two rays which emerge from a rhomb of Iceland spar, on which only one ray of ordinary light has been allowed to fall, we find that these emergent rays have acquired new and striking properties, of which the incident ray afforded no trace ; for, if we allow the two rays emerging from a rhomb of the spar to fall upon a second rhomb, we shall find, on viewing the images produced, that their intensity varies with the position into which its second crystal is turned. Thus, if we place a rhomb of the spar upon a dot made on a sheet of white paper, we shall have, as already pointed out, two images of equal darkness. But, in placing a second rhomb of the spar upon the first, in such a manner that their principal sections coincide, and the faces of one rhomb are also parallel to the faces of the other, we shall still see two equally intense images of the dot, only the images will be more widely separated than before, and no difference will be produced by separating the crystals if the parallelism of the planes of their respective principal sections be preserved. Here, then, is at once a notable difference between a ray of ordinary light and one that emerges from a rhomb of Iceland spar ; for, in the case of rays of ordinary light, we have seen that the second rhomb would divide each ray into two, whereas it is incapable (in the position of crystals under consideration) of dividing either the ordinary or the extraordinary ray which emerges from the first rhomb. If, still keeping the second rhomb above the other, we make the former rotate in a horizontal plane, we may observe that, as we turn the upper crystal so that the planes of the principal sections form a small angle with each, each image will be doubled, and, as the upper crystal is turned, each pair of images exhibits a varying difference of intensity. The ordinary ray in entering the second crystal is divided by it into a second ordinary ray and a second extraordinary ray, the intensities of which vary accord- ing to the angle between the principal sections. When the two principal LIGHT. 289 sections are parallel to one plane, that is, when the angle between them is either o or 180, the extraordinary image disappears, and only the ordinary one is seen, and with its greatest intensity. When the two principal sections are perpendicular to each other, that is, when the second crystal has been turned through either 90 or 270, the extraordinary has, on the contrary, its greatest intensity, and the ordinary one disappears. When the principal section of the second crystal has been turned into any intermediate posi- tion, such as through 45 and 135, or any odd multiple of 45, both images are visible and have equal intensities. This experiment shows that the two rays which emerge from the first crystal have acquired new properties, that each is affected differently by the second crystal, according as the crystal is presented to it in different directions round the ray as an axis. FIG. 207. The ray of light is no longer uniform in its properties all round, but appears to have acquired different sides, as it were, in passing through the rhomb of Iceland spar. This condition is indicated by saying that the ray is polarized \ and the first rhomb of spar is termed the polarizer, while the second rhomb, by which we recognize the fact that both the ordinary and the extraordinary rays emerge having different sides, has received the name of analyser. But, in order to study conveniently all the phenomena in Iceland spar, we should have crystals of a considerable size, otherwise the two rays do not become sufficiently separated so as to make it an easy matter to intercept one of them while we examine the other. A very ingenious mode of getting rid of one of the rays was devised by Nicol, and as his apparatus is much used for experiments on polarized light, we shall state the mode of constructing NicoVs Prism. It is made from a rhomb of Iceland spar, Fig. 207, in which a and b are the corners where the three obtuse angles meet, all equal. If we draw through a and b lines bisecting the angles da c &s\&fhg, and join a , these lines will all be in one plane, which is a principal section of the crystal, and contains the axis, a b. Now suppose another plane, passing through a b, to be turned so that it is at right angles to the plane containing a b and the bisectors : this plane would cut the sides of the crystal in the lines a t, ih, bk,ka\ and in making the Nicol prism, the crystal is cut into two along 19 290 LIGHT. this plane, and the two pieces are then cemented together by Canada balsam. A ray of light, R, entering the prism, undergoes double refraction ; but the ordinary ray, meeting the surface of the Canada balsam at a certain angle greater than the limiting angle, is totally reflected, and passes out of the crystal at o ; while the extraordinary ray, meeting the layer of balsam at a less angle than its limiting angle, does not undergo total reflection, but passes through the balsam, and emerges in the direction of E, completely polarized, so that the ray is unable to penetrate another Nicol's prism of which the principal section is placed at right angles to that of the first. Among other crystals which possess the property of doubly refracting, and therefore of polarizing, is the mineral called tourmaline, which is a semi-trans- parent substance, different specimens having different tints. In Fig. 208, A,B, represent theprismatic crystals of tourmaline, and C showsacrystal whichhas been cut, by means of a lapidary's wheel, into four pieces, the planes of division being parallel to the axis of the prism. The two inner portions form slices, having a uniform thickness of about $ in., and when the faces of these have FIG. 208. been polished, the plates form a convenient polarizer and analyser. Let us imagine one of the plates placed perpendicularly between the eye and a lighted candle. The light will be seen distinctly through it, partaking, how- ever, of the colour of the tourmaline ; and if the plate be turned round so that the direction of the axis of the crystal takes all possible positions with regard to the horizon, while the plane of the plate is always perpendicular to the line between the eye and the candle, no change whatever will be seen in the appearance of the flame. But if we fix the plate of crystal in a given position, let us say with the axial direction vertical, and place between it and the eye the second plate of tourmaline, the appearances become very curious indeed, and the candle is visible or invisible according to the posi- tion of this second plate. When the axis of the second is, like that of the first, vertical, the candle is distinctly seen ; but when the axis of the second plate is horizontal, no rays from the candle can reach the eye. If the second plate be slowly turned in its own plane, the candle becomes visible or invi- sible at each quarter of a revolution, the image passing through all degrees of brightness. Thus the luminous rays which pass through the first plate are polarized like those which emerge from a crystal of Jceland spar. It is not necessary that the plates used should be cut from the same crystal of tourmaline, for any two plates will answer equally well which have been cut parallel to the axes of the crystals which furnished them. In the case of tourmaline the extraordinary ray possesses the power of penetrating the LIGHT. 291 substance of the crystal much more freely than the ordinary ray, which a small thickness suffices to absorb altogether. It may be noted that in thfe simple experiment we have just described, the plate of tourmaline next the. candle forms the polarizer, and that next the eye the analyser; and thaV until the latter was employed, the eye was quite incapable of detecting the. change which the light had undergone in passing through the first plate, for the unassisted eye had no means of recognizing that the rays emerged with, sides. The usual manner of examining light, to find whether it is polarized, is to look through a plate of tourmaline or a Nicol's prism, and observe whether any change in brightness takes place as the prism or plate is rotated. Now, it so happened that in 1808 a very eminent French man of science, named Malus, was looking through a crystal of Iceland spar, and seeing in the glass panes of the windows of the Luxembourg Palace, which was opposite his house, the image of the setting sun, he turned the crystal towards the windows, and instead of the two bright images he expected to see, he perceived only one ; and on turning the crystal a quarter of a revo- lution, this one vanished as the other image appeared. It was, indeed, by a careful analysis of this phenomenon that Malus founded a new branch of science, namely, that which treats of polarized light ; and his views soon led to other discoveries, which, with their theoretical investigations, con- stitute one of the most interesting departments of optical science, as re- markable for the grasp it gives of the theory of light as for the number of practical applications to which it has led. The accidental observation of Malus led to the discovery that when a ray of ordinary light falls obliquely on a mirror not of metal, but of any other polished surface, such as glass, wood, ivory, marble, or leather it acquires by reflection at the surface the same properties that it would ac- quire by passing through a Nicol's prism or a plate of tourmaline : in a word, it is polarized. Thus, if a ray of light is allowed to fall upon a mirror of black glass at an angle of incidence of 54 35', the reflected ray will be found to be polarized in the plane of reflection that is, it will pass freely through a Nicol's prism when the principal section is parallel to the plane of reflection ; but when it is at right angles to the latter, the reflected ray will be completely extinguished by the prism that is, it is completely polarized. If the angle of the incident ray is different from 54 35', then the reflected ray is not completely intercepted by the prism it is not com- pletely but only partially polarized. The angle at which maximum pola- rization takes place varies with the reflecting substance ; thus, for water it is 53, for diamond 68, for air 45. A simple law was discovered by Sir David Brewster by which the polarizing angle of every substance is con- nected with its refractive index, so that when one is known, the other may be deduced. It may be expressed by saying that the polarizing angle is that angle of incidence which makes the reflected and the refracted rays perpendicular to each other. The refracted rays are also found to be pola- rized in a plane perpendicular to that of reflection. Instruments of various forms have been devised for examining the phe- nomena of polarized light. They all consist essentially of a polarizer and an analyser, which may be two mirrors of black glass placed at the pola- rizing angle, or two bundles of thin glass plates, or two Nicol's prisms, or two plates of tourmaline, or any pair formed by two of these. Fig. 209 represents a polariscope, this instrument being designed to permit any desired combination of polarizer and analyser, and having graduations for measuring the angles, and a stage upon which may be placed various sub- 192 292 LIGHT. 1 stances in order to observe the effects of polarized light when transmitted through them. It is found that thin slices of crystals placed between the pola- rizer and analyser exhibit varied and beautiful effects of colour, and by such FIG. 209. Polariscope. effects the doubly refracting power of substances can be recognized, wnere the observation of the production of double images would, on account of their small separation, be impossible. And the polariscope is of great service LIGHT. 293 in revealing structures in bodies which with ordinary light appear entirely devoid of it such, for example, as quill, horn, whalebone, &c. Except liquids, well-annealed glass, and gelatinous substances, there are, in fact, few bodies in which polarized light does not show us the existence of some kind of structure. A very interesting experiment can be made by placing FIG. 210. in the apparatus, shown in Fig. 210, a square bar of well-annealed glass ; on examining it by polarized light, it will be found that before any pres- sure from the screw c is applied to the glass, it allows the light to pass equally through every part of it ; but when by turning the screw the particles have been thrown into a state of strain, as shown in the figure, distinct bands will make their appearance, arranged somewhat in the manner represented ; but the shapes of the figures thus produced vary with every change in the strain and in the mode of applying the pressure. FlG. 211. Iceland Spar showing Double Refraction, 294 LIGHT. CAUSE OF LIGHT AND COLOUR. Tl 7E have hitherto limited ourselves to a description of some of the * V phenomena of light, without entering into any explanation of their presumed causes, or without making any statements concerning the nature of the agent which produces the phenomena. Whatever this cause or agent may be, we know already that light requires time for its propagation, and two principal theories have been proposed to explain and connect the facts. The first supposes light to consist of very subtile matter shot off from luminous bodies with the observed velocity of light ; and the second theory, which has received its great development during the present century, regards luminous effects as being due to movements of the particles of a subtile fluid to which the name of " ether " has been given. Of the existence of this ether there is no proof : it is imagined ; and properties are assigned to it for no other reason than that if it did exist and possess these properties, most of the phenomena of light could be easily explained. This theory requires us to suppose that a subtile imponderable fluid per- vades all space, and even interpenetrates bodies gaseous, liquid, and solid ; that this fluid is enormously elastic, for that it resists compression with a force almost beyond calculation. The particles of luminous bodies, themselves in rapid vibratory motion, are supposed to communicate move- ment to the particles of the ether, which are displaced from a position LIGHT. 295 of equilibrium, to which they return, executing backwards and forwards movements, like the stalks of corn in a field over which a gust of wind passes. While an ethereal particle is performing a complete oscillation, a series of others, to which it has communicated its motion, are also perform- ing oscillations in various phases the adjacent particle being a little be- hind the first, the next a little behind the second, and so on, until, in the file of particles, we come to one which is in the same phase of its oscillation as the first one. The distance of this from the first is called the " length of the luminous wave." But the ether particles do not, like the ears of corn, sway backwards and forwards merely in the direction in which the wave itself advances : they perform their movements in a direction perpen- dicular to that in which the wave moves. This kind of movement may be exemplified by the undulation into which a long cord laid on the ground may be thrown when one end is violently jerked up and down, when a wave will be seen to travel along the cord, but each part of the latter only moves perpendicularly to the length. The same kind of undulation is produced on the surface of water when a stone is thrown into a quiet pool. In each of these cases the parts of the rope or of the water do not travel along with the wave, but each particle oscillates up and down. Now, it may some- times be observed, when the waves are spreading out on the surface of a pool from the point where a stone has been dropped in, that another set of waves of equal height originating at another point may so meet the first set, that the crests of one set correspond with the hollows of the other, and thus strips of nearly smooth water are produced Dy the superposition of the two sets of waves. Let Fig. 212 represent two systems of such waves pro- pagated from the two points A A, the lines representing the crests of the waves. Along the lines, b b, the crests of one set of waves are just over the hollows of the other set ; so that along these lines the surface would be smooth, while along c C the crests would have double the height. Now, if light be due to undulation, it should be possible to obtain a similar effect that is, to make two sets of luminous undulations destroy each other's effects and produce darkness : in other words, we should be able, by adding light to light, to produce darkness / Now, this is precisely what is done in a celebrated experiment devised by Fresnel, which not only proves that dark- ness may be produced by the meeting of rays of light, but actually enables us to measure the lengths of the undulations which produce the rays. In Fig. 213 is a diagram representing the experiment of the two mirrors, devised by Fresnel. We are supposed to be looking down upon the arrangement : the two plane mirrors, which are placed vertically, being seen edgeways, in the lines, M o, o N, and it will be observed that the mirrors are placed nearly in the same upright plane, or, in other words, they form an angle with each other, which is nearly 180. At L is a very narrow upright slit, formed by metallic straight-edges, placed very close together, and allowing a direct beam of sunlight to pass into the apartment, this being the only light which is permitted to enter. From what has been already said on reflection from plane mirrors, it will readily be understood that these mirrors will reflect the beams from the slit in such a manner as to produce the same effect, in every way, as if there were a real slit placed behind each mirror in the symmetrical positions, A and B. Each virtual image of the slit may, therefore, be regarded as a real source of light at A and at B ; thus, for example, it will be observed that the actual lengths of the paths traversed by the beams which enter at L, and are reflected from the mirrors, are precisely the same as if they came from A and B respectively. 296 LIGHT. The virtual images may be made to approach as near to each other as may be required, by increasing the angle between the two mirrors, for, when this becomes 180, that is, when the two mirrors are in one plane, the two images will coincide. If, now, a screen be placed as at F G, a very remark- able effect will be seen ; for, instead of simply the images of the two slits, there will be visible a number of vertical coloured bands, like portions of very narrow rainbows, and these coloured bands are due to the two sources of light, A and B ; for, if we cover or remove one of the mirrors, the bands will disappear and the simple image of the slit will be seen. If, however, we place in front of L a piece of coloured glass, say red, we shall no longer see rainbow-like bands on the screen, but in their place we shall find a number of strips of red light and dark spaces alternately, and, as before, these are found to depend upon the two luminous sources, A and B. We must, therefore, come to the conclusion that the two rays exercise a mutual effect, and that, by their superposition, they produce darkness at some FIG. 213. points and light at others. These alternate dark and light bands are formed on the screen at all distances, and the spaces between them are greater as the two images, A and B, are nearer together. Further, with the same dis- position of the apparatus, it is found that when yellow light is used instead of red, the bands are closer together ; when green glass is substituted for yellow, blue for green, and violet for blue, that the bands become closer and closer with each colour successively. Hence, the effect of coloured bands, which is produced when pure sunlight is allowed to enter at L, is due to the superposition of the various coloured rays from the white light. Let us return to the case of the red glass, and suppose that the distance apart of the two images, A and B, has been measured, by observing the angle which they subtend at C, and by measuring the distance, C O D, or rather, the distance COL. Now, the distances of A and B from the centre of each dark band, and of each light band, can easily be calculated, and it is found that the difference between the two distances is always the same for the same band, however the screen or the mirrors may be changed. On comparing the differences of the distances of A and B in case of bright bands, with those in the case of dark ones, it was found that the former could be expressed by the even multiples of a very small distance, which we will call d, thus : o, 2^,4^; 6d, 8^ . . . : LIGHT. 297 while the differences for the dark bands followed the odd multiples of the same quantity, d, thus : These results are perfectly explained on the supposition that light is a kind of wave motion, and that the distance, d, corresponds to half the length of a wave. We have the waves entering L, and pursuing different lengths of path to reach the screen at F G, and, if they arrive in opposite phases of undulation, the superposition of two will produce darkness. The undula- tions will plainly be in opposite phases when the lengths of paths differ by an odd number of half-wave lengths, but in the same phase when they differ by an even number. Hence, the length of the wave may be deduced from the measurement of the distances of A and B from each dark and light band, and it is found to differ with the colour of the light. It is also plain that, as we know the velocity of light, and also the length of the waves, we have only to divide the length that light passes over in one second, by the lengths of the waves, in order to find how many undulations must take place in one second. The following table gives the wave-lengths, and the number of undulations for each colour : Colour. Number of Waves in one inch. Number of Oscillations in one second. Red AO Q6o 514 ooo ooo ooo ooo Orange ... 43,560 5 57,000,000,000,000 Yellow 46,090 5 7 8,000,0^0,000,000 Green 49,600 62 1 ,ooo,oco ; ooo,ooo Blue 53,470 670,000,000,000,000 Indigo 56,560 709,000,000,000,000 Violet 60,040 7 50,000,000,000,000 These are the results, then, of such experiments as that of Fresnel's, and although such numbers as those given in the table above are apt to be con- sidered as representing rather the exercise of scientific imagination than as real magnitudes actually measured, yet the reader need only go care- fully over the account of the experiment, and over that of the measurement of the velocity of light, to become convinced that by these experiments something concerned in the phenomena of light has really been measured, and has the dimensions assigned to it, even if it be not actually the distance from crest to crest of ether waves even, indeed, if the ether and its waves have no existence. But by picturing to ourselves light as produced by the swaying backwards and forwards of particles of ether, we are better able to think upon the subject, and we can represent to ourselves the whole of the phenomena by a few simple and comparatively familiar conceptions. As an example of the facility with which the ether theory lends itself 3 A knowledge of their composition, he expressly asserted, could never be attained, for we could have no means of chemically examining the matter of which they are constituted. Such was the deliberate utterance of a man by no means disposed to underrate the power of the human mind in the pursuit of truth. And such might still have been the opinion of the learned and of the unlearned, but for the remarkable train of discoveries which has led us to the construction of instruments revealing to us the nature of the substances entering into the constitution of the heavenly bodies. We have now, for example, the same certainty about the existence of iron in our sun, that we have about its existence in the poker and tongs on the hearth. The last few years have seen the dawn of a new science ; and two branches of knowledge which formerly seemed far as the poles asunder namely, astronomy and chemistry have their interests united in this new science of celestial chemistry. The progress which has been made in this depart- ment of spectroscopic research is so rapid, and the field is so promising, that the well-instructed juvenile of the future, instead of idly repeating the simple lay of our childhood : "Twinkle, twinkle, little star, How I wonder what you are ! " will probably only have to direct his sidereal spectroscope to the object of his admiration in order to obtain exact information as to what the star is, chemically and physically. The results which have already been obtained in celestial chemistry, and other branches of spectroscopic science, are so surprising, and apparently so remote from the range of ordinary experience, that the reader can only appreciate these wonderful discoveries by tracing the steps by which they have been reached. A few fundamental phenomena of light have already been spoken of in the foregoing article ; and an acquaintance with these will have prepared the reader's mind for a consideration of the new facts we are about to describe. In discus- sing, in the foregoing pages, the sub- ject of refraction, we have, in order that the reader's attention might not be distracted, omitted all mention of a circumstance attending it, when a beam of ordinary light falls upon a refracting surface, such as that re- presented in Fig. 203. The laws there explained apply, in fact, to ele- mentary rays, and not to ordinary white light, which is a mixture of a vast multitude of elementary rays, red, yellow, green, &c. When such a beam falls obliquely upon a piece of glass, the ray is, at its entrance, broken up into its elements, for these, being refracted in different degrees by the glass, each pursues a different path in that medium, as represented by Fig. 216. Each elementary ray obeys the laws which have been explained, and therefore each emerges from the second surface of the plate parallel to the incident ray, and, in 3 o4 THE SPECTROSCOPE. consequence of this, the separation is not perceptible under ordinary cir- cumstances with plates of glass having parallel surfaces. But, if the second surface be inclined so as to form such an angle with the first that the rays are rendered still more divergent in their exit, then the separation of the light into its elementary coloured rays becomes quite obvious. Such is the arrangement of the surfaces in a prism, and in the triangular pieces of glass which are used in lustres. For the fundamental experimental fact of our subject, we must go back two centuries, when we shall find Sir Isaac Newton making his celebrated analysis of light by means of the glass prism. We shall describe Newton's experiment, for, although it was peformed so long ago, and is generally well known, it will render our view of the present subject more complete ; and it will also serve to impress on the reader an additional instance of the world's indebtedness to that great mind, when we thus trace the grand results of modern discovery from their source. " It is well," is the remark of a clear thinker and eloquent writer, " to turn aside from the fretful din of the present, and to dwell with gratitude and respect upon the services of 1 those mighty men of old, who have gone down to the grave with their weapons of war,' but who, while they lived, won splendid victories over ignorance." The experiment of Sir Isaac Newton will be readily understood from Fig. 217, where C is the prism, and A C represents the path of a beam of sunlight allowed to enter into a dark apartment through a small round FlG. 217. Newtorts Experiment. hole in a shutter, all other light being excluded from the apartment. In this position of the prism, the rays into which the sunbeam is broken at its entrance into the glass were bent upwards, and at their emergence from the glass they were again bent upwards, still more separated, so that when a white screen was placed in their path, instead of a white 'circular image of the sun appearing, as would have been the case had the light been merely refracted and not split up, Newton saw on the screen the variously- coloured band, D D, which he termed the spectrum. The letters in the figure indicate the relative positions of the various colours, red, orange, yellow, green, blue, &c., by their initial letters. The spectrum, or pro- THE SPECTROSCOPE. 305 longed coloured image of the sun, is red at the end, R, where the rays are least refracted, and violet at the other extremity, where the refraction is greatest, while, in the intermediate spaces, yellow, green, and blue pass by insensible gradations into each other. Newton varied his experiment in many ways, as, for example, by trying the effect of refraction through a second prism on the differently coloured rays. He found that the second prism did not divide the yellow rays, for instance, into any other colour, but merely bent them out of the straight course, to form on the second screen a somewhat broader band of yellow, and similarly with regard to the others. From these, and a number of other experiments described in his " Opticks," (A. D. 1675), Newton concludes, "that if the sun's light consisted of but one sort of rays, there would be but one colour in the whole world, nor would it be possible to produce any new colour by reflections and re- fractions, and, by consequence, the variety of colours depends upon the composition of light." " And if, at any time, I speak of light and rays, or coloured, or endued with colours, I would be understood to speak, not philosophically and properly, but grossly, and accordingly to such con- ceptions as vulgar people in seeing all these experiments would be apt to frame. For the rays, to speak properly, are not coloured. In them there is nothing else than a certain power and disposition to stir up a sensation of this or that colour. For, as sound in a bell, a musical string, or other sounding body, is nothing but a trembling motion, and in the air nothing but that motion propagated from the object, and in the sensorium 't is a sense of that motion under the form of a sound ; so colours in the object are nothing but a disposition to reflect this or that sort of rays more copi- ously than the rest : in the rays they are nothing but their dispositions to propagate this or that motion into the sensorium, and in the sensorium they are sensations of these motions under the form of colours." These memorable investigations of Newton's have been the admiration of succeeding philosophers, and even poets have caught inspiration from this theme : " Nor could the darting beam of speed immense Escape his swift pursuit and measuring eye. E'en Light itself, which everything displays, Shone undiscovered, till his brighter mind Untwisted all the shining robe of day ; And, from the whitening undistinguished blaze, Collecting every ray into his kind, To the charmed eye educed the gorgeous train Of parent colours. First the flaming red Sprung vivid forth ; the tawny orange next ; And next delicious yellow by whose side Fell the kind beams of all-refreshing green ; Then the pure blue, that swells autumnal skies, Ethereal played ; and then, of sadder hue. Emerged the deepened indigo, as when The heavy-skirted evening droops with frost, While the last gleamings of refracted light Died in the fainting violet away. These, when the clouds distil the rosy show, Shine out distinct adown the watery bow ; While o'er our heads the dewy vision bends Delightful melting on the fields beneath. Myriads of mingling dyes from these result, And myriads still remain. Infinite source Of beauty ! ever blushing ever new ! Did ever poet image aught so fair, Dreaming in whispering groves, by the hoarse brook, Or prophet, to whose rapture Heaven descends?" The spectra which Newton obtained by admitting the solar beams 20 3 o6 THE SPECTROSCOPE. through a circular aperture, were, however, not simple spectra. The cir- cular beam may be considered as built up of flat and very thin bands of light, parallel to the edges of the prism, and a simple ray would be formed by one of these flat bands ; as the round opening would allow an indefinite number of such rays to enter, each would produce its own spectrum on the screen, and the actual image would be formed of a number of spectra overlapping each other. When the aperture by which the light is admitted consists merely of a narrow slit, or line, parallel to the edges of the prism, we obtain what is termed a pure spectrum. When the prism is properly placed, an eye, viewing the fine slit through it, sees a spectrum formed, as it were, of a succession of virtual images of the slit in all the elementary coloured rays. The person who first examined the solar spectrum in this manner was the English chemist Wollaston, who, in 1802, found that the spectrum thus ob- served was not continuous, but that it was crossed at intervals by dark lines. Wollaston saw them by placing his eye directly behind the prism. Twelve years later, namely, in 1814, the German optician Fraunhofer devised a much better mode of viewing the spectrum ; for, instead of looking through the prism with the naked eye, he used a telescope, placing the prism and the telescope at a distance of 24ft. from the slit, the virtual image of which was thus considerably magnified. The prism was so placed that the in- cident and refracted rays formed nearly equal angles with its faces, in which circumstance the ray is least deflected from its direction, and the position is therefore spoken of as being that of minimum deviation. It can be shown that this position is the only one in which the refracted rays can produce clear and sharp virtual images of the slit, and therefore it is necessary in all instruments to have the prism so adjusted. Fraunhofer then saw that the dark lines were very numerous, and he found that they always kept the same relative positions with regard to the coloured spaces they crossed ; that these positions did not change when the material of which the prism was made was changed ; and that a variation in the re- fracting angle of the prism did not affect them. He then made a very careful map, laying down upon it the position of 354 of the lines out of about 600 which he counted, and indicated their relative intensities, for some are finer and less dark than others. The most conspicuous lines he dis- tinguished by letters of the alphabet, and these are still so indicated ; and the dark lines in the solar spectrum are called " Fraunhofer's Lines." These lines, as will appear in the sequel, are of great importance in our subject. A few of the more obvious ones are shown in No. i, Plate VI. Fraun- hofer found that these lines were always produced by sunlight, whether direct, or diffused, or reflected from the moon and planets ; but that the Ught from the fixed stars formed spectra having different lines from those in the sun although he recognized in some of the spectra a few of the same lines he found in the solar spectrum. The fact of these differences in the spectra of the sun and fixed stars proved that the cause of the dark lines, whatever it might be, must exist in the light of these self-luminous bodies, and not in our atmosphere. It was, however, some years afterwards ascertained that the passage of the sun's light through the atmosphere does give rise to some dark bands in the spectrum ; for it was found that certain lines make their appearance only when the sun is near the horizon, and its rays consequently pass through a much greater thickness of air. Sir D. Brewster first noticed in 1832 that certain coloured gases have the power of absorbing some of the sun's rays, so that the spectrum, when the THE SPECTROSCOPE. 307 rays are made to pass through such a gas before falling on the prism, is crossed by a series of dark lines altogether different from Fraunhofer's lines, though these are also present. The gas in which this property was first noticed is that called " nitric peroxide" a brownish-red gas, of which even a thin stratum produces a well-marked series of dark lines. The same property was soon discovered in the vapours of bromine, iodine, and a cer- tain compound of chlorine and oxygen. Each substance furnishes a system of lines peculiar to itself : thus the vapour of bromine, although it has almost exactly the same colour as nitric peroxide, gives a totally different set of lines. These, therefore, do not depend on the mere colour of the gas or vapour, and this is conclusively proved by the fact of many coloured vapours producing no dark lines whatever : the vapour of tungsten chloride, for example, although in colour so exactly like bromine vapour that the two cannot be distinguished by the eye, yields' no lines whatever. In Fig. 218 is represented a lamp for burning coal-gas, which is con- stantly used by chemists as a source of heat. It is known as " Bunsen's burner," from its inventor the celebrated German chemist. It consists of a metal tube, 3 in. or 4 in. long, and \ in. in diameter, at the bottom of which the gas is admitted by a small jet communi- cating with the elastic tube which brings the gas to the apparatus. A little below the level of the jet there are two lateral openings which admit air to the tube. The gas, therefore, becomes mixed with air within the tube, and this inflammable mixture streams from the top of the tube and readily ignites on the approach of a flame, the mixture burning with a pale bluish flame of a very high temperature. This little apparatus is not only the most useful pieces of chemical apparatus ever devised, but it furnishes highly instruc- tive illustrations of several points in chemical and physical science ; and to some of these we invite the reader's attention, as they have an immediate bearing on our present subject. Coal- gas is a mixture of various compounds of the two elementary bodies, hydrogen and carbon ; and when the gas burns, these substances are respectively uniting with the oxygen of the air, pro- ducing water and carbonic acid gas. Now, when coal-gas is burnt in the ordinary manner as a source of light, the supply of oxygen is too small to admit of the complete combustion of all its constituents ; and as the oxygen more eagerly seizes upon the hydrogen than upon the carbon, a large pro- portion of the latter thus set free from its hydrogen compound is deposited in the flame in the solid form, and is there intensely heated. The presence of solid carbon in an ordinary gas flame is easily proved by holding in it a cold fragment of porcelain, or a piece of metal, which will become covered with soot. In the flame of the Bunsen burner there is no soot, because the increased supply of oxygen, afforded by previously mixing the gas with air, enables the whole of the constituents of the gas to be completely burnt ; and this is of the greatest advantage to the chemist, who always desires to 20 2 n , 2i%.-Bunsms Burntr on a 3 o8 THE SPECTROSCOPE. have the vessels he heats free from soot, in order that he may observe what is taking place within them. The flame of Bunsen's lamp becomes that of an ordinary sooty gas flame, when the two orifices which admit the air at the bottom of the tube are closed up, and then, of course, the temperature cannot be so high as when the whole constituents of the gas are com- pletely burnt, but the flame becomes highly luminous ; whereas when the orifices are open it gives so little light, that in a dark room one cannot see a finger held 20 in. from the lamp. Plainly the cause of this difference is connected with the presence or absence of the heated particles of solid carbon. The non-luminous flame contains no solid particles ; the bright part of the other flame is full of them. To these heated particles of solid carbon we are, then, indebted for the light which burning coal-gas supplies. And, since we are able by such artificial illumination to distinguish colours, the white-hot carbon must give off rays of all degrees of refrangibility, and we should expect to find in the spectrum produced by such a flame, the red, yellow, green, and other coloured rays. And such is indeed the spectrum which these incandescent carbon particles produce : it resembles the solar spectrum, but there is an entire absence of dark lines, so that the appear- ance is that represented by No. I, Plate VI., if we suppose the Fraunhofer lines removed. If the pale blue flame of the Bunsen's burner be similarly examined, the spectrum, No. 14, Plate VI., shows that only a few rays of certain refrangibilities are emitted, forming bright lines here and there, but of little intensity, while the whole of the other rays are absent. This shows that while the highly heated solid gives off all rays from red to violet with- out interruption, the still more highly heated gases give off only a few selected rays. It has long been known that some substances impart certain colours to flames, and such substances have been long employed to produce coloured effects in fireworks, &c. But coloured flames do not appear to have been examined by the prism until 1822, when Sir John Herschel described the spectra of strontium, copper, and of some other substances, remarking that " The colours thus communicated by the different bases to flame afford in many cases a ready and neat way of detecting extremely minute quantities of them." A few years later, Fox Talbot described the method of obtaining a monochromatic flame, by using in a spirit-lamp diluted alcohol in which a little salt has been dissolved. The paper in which he describes this and other observations concludes thus : " If this opinion should be correct and applicable to the other definite rays, a glance at the prismatic spectrum of flame may show it to contain substances which it would otherwise require a laborious chemical analysis to detect." Here we have the first hint of that spectrum analysis which has provided the chemist with a method of surpassing delicacy for the detection of metallic elements. The spectra of coloured flames were also subsequently examined and described by Professor W. A. Miller, but the most complete investigation into the subject was made by Professors Kirchhoff and Bunsen, who also contrived a convenient instrument, or spectroscope, for the examination and comparison of different spectra. The instrument has received many im- provements and modifications, but the essential parts are one or more prisms ; a slit, through which the light to be examined is allowed to enter ; a tube, having at the other end a lens to. render parallel the rays from the slit ; a telescope, through which the spectrum is viewed ; and usually some appa- ratus by which the positions of the different lines may be identified. A very elegant instrument, made by Mr. John Browning, of the Strand, is PLATE VI. SPECTRA. THE SPECTROSCOPE. 309 represented in Fig. 219. It has a single prism, made of glass, of great power in dispersing the rays. The prism is supported on a little stage, placed in the middle of a horizontal circular brass table about 6 in. in diameter. On the left is seen a tube, about 1 5 in. long, at the outer extremity of which is the slit, formed of pieces of metal very accurately shaped. One of these pieces FIG. 219. Spectroscope 'with one Prism. slides in a direction at right angles to the slit, and, by means of a spring and a fine screw, can be very nicely adjusted, so that an opening of any degree of fineness can be readily obtained. In front of the slit is a small glass prism, with its edges parallel to the slit, but only half its height. The bases of this prism are formed of two sides of a square and its diagonal, and, as shown in the figure, one side is parallel to the face of the slit, and the other to the axis of the tube. Rays of light coming from a source on the left of the slit (as seen in the figure) will, therefore, enter this little prism, and be totally reflected (see page 285) by the diagonal surface, down the axis of the tube through the lower half only of the slit. This is the only office of this prism, which has nothing to do with the dispersion of the rays : the use to which it is put will be seen presently. It is fixed in such a manner that, when required, it can be turned aside with the touch of a finger, and the whole length of the slit exposed. A peculiarity in these instruments of Mr. Browning's is the admirable arrangement for deter- mining the position of any line in a spectrum. For this purpose, the eye- piece of the telescope is provided with a pair of cross-wires, and the tele- scope itself, which is about 1 8 in. in length, moves in a horizontal plane round the axis of the circular brass table, from which an arm projects, carrying a ring into which the telescope screws. This arm carries a vernier along the limb of the circular table, which is very accurately divided into thirds of degrees, so that with the aid of the vernier the angular position of the telescope can be read off to a minute, that is, to g^th of a degree. The arm carrying the telescope is provided with a screw for clamping it in any desired position while the readings are taken. On placing in front of the slit the flame of a Bunsen's burner, the spectrum produced by any substance in this flame will, when the instrument is in proper adjustment, be seen on 3 io THE SPECTROSCOPE. looking through the telescope, and the cross-wires being also in view, the point of their intersection may be brought into coincidence with any line of the spectrum, and the telescope being clamped in this position, the angular reading thus taken determines the position of the line. Thus, for example, the angular positions in which the principal Fraunhofer's lines are seen having been observed and recorded, the angular position of any line in an- other spectrum will at once determine its position among the Fraunhofer lines ; or the spectrum may be mapped by laying down the angular read- ings of the lines by means of a scale of equal parts. And, again, in the little prism in front of the slit we have the means of bringing two spectra in view at once, one being from a light directly in front, and the other from a light at the side. The two spectra are seen one above the other, and the coincidence or difference of their lines may be directly observed. When the instrument is in use, the prism and the ends of the tube are covered with a black cloth, loosely thrown over them, by which all stray light is shut out. The author has had in use for several years one of these instruments, and he cannot forbear expressing his perfect satisfaction with its powers, which he finds amply sufficient for all ordinary chemical purposes, while the accu- racy of the workmanship is really wonderful, considering the very moderate price of the instrument. The substances the spectra of which are most conveniently examined are the metals of the alkalies and alkaline earths. Small quantities of the salts of these metals, placed in a loop of fine platinum wire, impart characteristic colours to the flame of a Bunsen burner or to that of a spirit-lamp. For the examination of the spectra the former is to be preferred, as the lines come out much more vividly. Indeed, at temperatures higher than that of the Bunsen's burner, such as in the flame of pure hydrogen, or in the voltaic arc, some substances give out additional lines. In Plate VI., Nos. 2 to 9, is shown the appearance of the spectra produced by the Bunsen's burner when salts of the metals are held in the flame in the manner already men- tioned, and the spectra are examined with the instrument just described. One of the simplest of these spectra is that produced by sodium compounds, such as common salt. The smallest particle of this substance imparts an intense yellow colour to the flame, and the spectrum is found to take the form of a single bright yellow line No. 3. It has been estimated that the presence of the iooojoooo^ P art f a S ra ^ H f sodium can be detected by the production of this line. Indeed, the very delicacy of this sodium re- action renders it almost impossible to get rid o this line, for sodium is found to be present in almost everything, a fact the earlier observers of spectra were not aware of, for they attributed this yellow line to water, which was the only substance they knew to be so generally diffused. If a platinum wire be heated in the flame of the Bunsen burner until all the sodium indications have disappeared, it suffices to remove the wire, and, without allowing it to come into contact with anything, to leave it exposed to the air for a few minutes, to cause it again to give the characteristic yellow colour when again plunged into the flame. This is due to the fact that the element is contained in all the floating particles which pervade the atmosphere. The spectroscope is not required to show the presence of the sodium on the platinum which has been exposed to the air, the colour imparted to the flame being plainly visible to the eye, and it needs only the Bunsen burner and 2 in. of platinum wire to prove the fact, and also to show that mere contact with the fingers is enough to highly charge the wire with sodium compounds. Any volatile compound of potassium gives the spec- THE SPECTROSCOPE. 311 trum represented by No. 2, the principal lines being a red line and one in the extreme violet, the latter being somewhat difficult to observe. There is also a third rather ill-defined red line, and a portion of a faint continuous spectrum. Salts of strontium impart a bright red colour to the flame, and the spectrum they produce is shown by No. 6, in which are seen several bright red lines and a fainter blue one. Calcium, which also gives a red- dish colour to flame, furnishes an entirely different set of lines (No. 5). and barium salt another, containing numerous lines, especially some very vivid green ones. In all the cases we have named, and whenever bright-lined spectra are furnished by substances placed in the flame of a lamp, or in burning hydro- gen gas, or in the intensely hot voltaic arc, there is evidence that the sub- stances are converted into vapour or gas. We have already seen how hot solid carbon gives a continuous spectrum, while carbon in the state of gaseous combination gives most of the bright lines seen in the spectrum of coal-gas (No. 14). It is observed also that the more readily volatized are the salts, the more vivid are the bright lines they produce when heated in a flame. It must be understood that each element gives it own charac- teristic lines, that these are always in precisely the same position in the spectrum, that no substance produces a line in exactly the same position as another, however near two lines due to different substances may, in some cases, appear ; and also, that however the salts of the different metals are mixed together, each produces its own lines, and each ingredient may be recognized. And this is done in an instant by an experienced observer a mere glance at the superposed spectra of, perhaps, half a dozen metals, suffices to inform him which are present. There is also a peculiarity in this optical mode of recognizing the presence of bodies which gives the subject the highest interest, namely, the circumstance that the spectrum is produced and the bodies recognized, however far from the observer the luminous gas may be placed, the only condition required being that the rays reach the instrument. Until Kirchhoff and Bunsen's spectroscopic investigations, lithium was supposed to be a rare metal, occurring only in a few minerals. It happens that this substance yields a remarkable spectrum (No. 4), for it gives an extremely vivid line of a splendid red colour, accompanied by only one other, a feeble yellow line ; and the reaction is of very great delicacy, for g-ou^Tnrath of a grain can easily be detected, and an eye which has once seen the red line readily recognizes it again. A single drop of a mineral water containing lithium has been found to distinctly produce the red line, in cases where the quantity contained in a quart of the water would have escaped ordinary chemical analysis. The spectroscope has shown that lithium, so far from occurring in only four or five minerals, is a substance very widely diffused in nature. In the waters of the ocean, in mineral and river waters, in most plants, in wines, tea, coffee, milk, blood, and muscle, this metal has been found. Dr. Roscoe states that the ash of a cigar, when moistened with hydrochloric acid, and held in a platinum wire in the flame of the Bunsen's burner, at once shows the principal lines of sodium, potassium, calcium, and lithium. Salts of lithium and of strontium both impart a rich crimson tint to flames, and it is hardly possible to detect any difference in these colours with the naked eye; but, as the reader may see on comparing spectra No. 4 and No. 6, the prism makes a wide distinction. Matter for a very interesting chapter in the history of prismatic analysis has been furnished by the discovery of four new elements by means of the 3 i2 THE SPECTROSCOPE. spectroscope. In 1860 Bunsen observed that the residue, after evapora- tion, of a certain mineral water, yielded spectra with bright lines which he had not seen before. He concluded that they were due to some un- known elements, and, in order to separate these, he evaporated many tons, of the water, and was rewarded by the discovery of two alkaline metals,, caesium and rubidium. The delicacy of the spectrum reaction may be inferred from the fact of a ton weight of the water containing only three grains of the salts of each of these substances. Rubidium gives a splendid spectrum, containing red, yellow, and green lines, and also two character- istic violet lines ; while caesium has orange, yellow, and green lines, and two very beautiful blue lines, by which it is easily recognized. About the same time, Mr. W. Crookes discovered, in a mineral from the Hartz, another elementary body, the existence of which was first indicated to him by the characteristic spectrum it produces, namely, a single splendid green line (No. 8 spectrum). In 1864 two German chemists discovered, also in the Hartz, a fourth new element, which was detected by two well- defined lines in the more refrangible end of the spectrum (see spectrum FIG. 220. Miniature Spectroscope. No. 9, in the plate). This metal was named Indium, in reference to the- colour of its lines, and the names of the other three caesium, rubidium, and thallium, are also derived from the colours of their characteristic lines. Although the reader may, from such representations of the spectra as. those given in Plate VI., form some idea of their appearance, he would find his knowledge of the subject much clearer if he had the opportunity of examining for himself the actual phenomena. We have already recom- mended the performance of certain easy experiments involving no outlay, but, in the matter of spectroscopes, carefully finished optical and mechani- cal work is absolutely necessary in the appliances. It fortunately happens that one eminent optician, at least, has made it his study to produce good spectroscopic apparatus at the lowest possible cost, and if the reader be interested in this subject, and desirious of trying experiments himself, he can, for a very moderate sum, be equipped with ail the appliances for examining the phenomena we have described. He has only to procure, in the first place, a small direct-vision spectroscope, such as that represented of its actual size in Fig. 220, which is sold by Mr. Browning for twenty- two shillings ; secondly, a Bunsen's burner, a few feet of india-rubber tubing, two inches of platinum wire, and a few grains of the salts of lithium, strontium, thallium, &c. The whole expense will probably be covered by adding four shillings to the cost of the spectroscope, and the reader will then be in a position to see for himself the principal Fraunhofer lines, the spectra of the metals already referred to, and the absorption bands of the THE SPECTROSCOPE. 3 is gases which have been mentioned, as well as the absorption bands in liquids, vrhich will be spoken of in the sequel. The splitting up of a beam of light into its elements which it is the office of the prism to produce is accomplished by a single prism to a cer- tain degree only. It separates the red from the green, for example ; but the colours pass into each by insensible gradations through orange, yellow, 3 i4 THE SPECTROSCOPE. and greenish yellow. If we allow the rays to fall upon a second prism after emerging from the first, the separation is carried further ; the red, for in- stance, is spread out into different kinds of red, and so on with the rest. And the greater the number of prisms, the greater is the extension which is given to the spectrum. Now, just as by increasing the power of the tele- scope, new stars become visible, whose light was before too faint, and nebulas, or stars which before seemed single, are resolved into clusters, of individual stars so, by increasing the power of the spectroscope by em- ploying two, four, or more prisms^ lines which appear single by the less powerful instruments are, in some instances, resolved into groups of lines, and new lines come into view, which before were too faint to show them- selves. For example, if we view the Fraunhofer lines through a spectro- scope like that in Fig. 220, but having two prisms instead of one, we shall see that the D line is not really a single line, but is formed of two lines close together. If we use greater dispersive power by employing a greater number of prisms, we shall observe with solar light that when these two D lines are sufficiently separated, several other lines make their appearance between them. In this way the number of dark lines in sunlight, which have been carefully mapped by KirchhofT and others, amount to upwards of 2,000; and no doubt there are many more lines waiting a still more powerful instrument. Fig. 221 is copied from a large spectroscope made by Mr. Browning for Mr. Gassiot. It has nine or more highly dispersive glass prisms; the telescope and the tube bearing the slit have focal lengths of 1 8 in., the lenses having a diameter of \\ in. ; the telescope is provided with a slow motion for taking the angular position ; and there is a third tube provided with a micrometer, by which the position of the lines can be measured to TotWth of an inch. The instruments we have mentioned, except the miniature spectroscope, show only a portion of the spectra at once, a movement of the telescope being requisite to bring each part into view. It has been already stated that the only position of the prism which will make the lines clear and well defined is that in which the deviation is the least. In using trains of prisms it is therefore necessary to adjust each prism for the part of the spectrum which may be under observation. This is a tedious process, and it has been obviated by a useful invention of Mr. Browning's, by which the adjustment is rendered automatic that is, the movements of the telescope are communicated to the prisms in such a manner that they place them- selves into the proper position for producing clear images of the slit, what- ever may be the refrangibility of the rays under examination : Fig. 222 shows the arrangement as it appears when viewed from above. The train of six prisms can be so arranged that the ray after passing through six of them shall be totally reflected by a surface of the last prism, and pursue again its path through the six prisms in the reverse direction, becoming more and more dispersed by each prism until it emerges parallel to the axis of the telescope. The power of the instrument is, therefore, equivalent to that of one with twelve prisms ; but it can be used at pleasure with any dispersive power, from two to twelve prisms. By making use of one of the Bunsen burners, the lines which are charac- teristic of some ten or twelve metals are readily seen when one of their more volatile salts is converted into vapour. For this purpose their chlorides are usually employed, but the reactions are common to all their salts. It is necessary that the metal should exist in the flame in the state of highly heated vapour or gas, in order that its characteristic rays should be given THE SPECTROSCOPE. off. We usually introduce compounds of these metals into the flame ; but there is reason to believe that these are decomposed in the flame, and the disassociated metal takes the form of glowing gas, a small quantity of which suffices for the production of the bright lines. No doubt the other constituent of the compound, the chlorine for example, is also set free in the gaseous form ; but since the spectrum of the metal only is visible, we may infer that at the temperature of the flame, the non -metallic elements are not sufficiently luminous to produce a spectrum. When we repeat the experiments with salts of the less volatile metals, we obtain no spectra FIG. 222. Browning's Automatic Adjustment of Prisms. the temperature of the flame not being sufficiently high to convert these into vapour. Other methods have, therefore, to be resorted to, and advan- tage is taken of the fact discovered by Faraday, that an electric spark is nothing but highly heated matter. The spectroscope gives us reason to believe that this matter, which is formed of the substances between which the spark passes, is in the gaseous state; for it is found, on examining sparks passing between two pieces of each metal, that characteristic bright lines are produced. If one of the metals already named is submitted to this examination, the same lines are found which are seen in the spectra produced by the salts of the metal volatized in the flame, but in some cases additional bright lines appear in the spark spectrum. With the heavier metals the spark, or the electric arc, is, however, the only means of igniting their vapours. The usual mode of doing this is to make the discharges of a large induction coil pass between the two fine wires of the metals, placed about a quarter of an inch apart. A Leyden jar is commonly employed to condense the discharge, and thus produce a still higher temperature. Mr. Browning has contrived the neat little apparatus shown in Fig. 223, in which the jar is superseded by a more compact and convenient condenser THE SPECTROSCOPE. inside of the box, so that it is only necessary to attach one terminal of the coil to the binding-screw, seen outside of the end of the box, and place the other wire from the coil in the binding- screw of one or the other of the pieces of apparatus supported by the upright rod. Of these it is the one on the right which at present engages our attention. Within a small glass cylinder are two sliding rods, terminated by screw-clips, which hold finely- pointed pieces of the metal under examination. The slit of the spectroscope FlG. 223. Apparatus jor Spark Spectra. is placed close to the glass cylinder, and when a very rapid succession of sparks is passing, the bright lines are seen continuously. The spectra of metals examined in this way are found to yield a very large number of lines. Thus the spectrum of calcium has 75 lines, and that of iron no fewer than 450 lines. Our limits will not permit of an account of many interest- ing particulars relating to these spectra, which include those of all the 50 metallic elements. It should, perhaps, be stated that a modified mode of producing spectra by sparks is sometimes found useful. This consists in causing sparks to pass between a solution of some salt of the metal and a piece of platinum wire. The apparatus for this purpose is that shown on the left side of the upright in Fig. 223. It remains to describe the method of producing spectra of the gaseous non-metallic elements, such as oxygen, nitrogen, hydrogen, &c. For this purpose electricity is again made use of. It has been found that while an electric discharge cannot take place across a perfect vacuum, and air or gas, at ordinary densities, offers much resistance to the passage of elec- tricity, on the other hand, a highly rarefied gas permits the discharge to take place through it with great facility. This is seen in Geissler's tubes, where a succession of discharges from a RuhmkorfPs coil causes the tubes THE SPECTROSCOPE. 317 to appear filled with light due to the heating to incandescence of a very minute quantity of the gas. The eye readily recognizes difference of colour in the light given off by the different gases, and when this light is examined by the spectroscope, bright lines, characteristic of each gas, are observed. No. 12 and No. 13, in Plate VI., are the spectra of hydrogen and of nitrogen respectively, which appear when the gases are examined in the manner just described. In this manner the spectra of chlorine, bromine, iodine, oxygen, sulphur, phosphorus, &c., may be studied. Silicon and some other solid non-metallic elements present great difficulties to the spectroscopist, for these elements cannot be volatized at any temperature we can command, and the spectra of their elements can only be inferred from those of their compounds. But unfortunately the spectra are found to vary with the nature of the compound, and thus it happens that in the case of carbon, for example, no definite spectrum can be assigned to the element. The flame of coal-gas, burning in the air, as in the Bunsen burner, gives the spectrum No. 14 ; but if this is compared with the spectrum of the flame of burning cyanogen (a compound of carbon and nitrogen), the two are found to differ greatly. The cyanogen spectrum has the two pale broad bands of violet- blue, the four blue lines, the two green lines, and the brightest of the greenish yellow which are seen in the coal-gas spectrum. But it has in addition a characteristic series of violet lines, a series of bright blue, two or three crimson and red lines, and bands in the orange, and several green lines, none of which occur in the coal-gas spectrum. These additional lines are not due to nitrogen, for, with perhaps the exception of some red lines, they do not coincide in position with any of the nitrogen lines. The spectrum of hydrogen, No. 12, should be noticed, as its three lines are very distinct, and it will be observed that they exactly coincide in their position with the three Fraunhofer lines, C, F, and G, in No. I. There is another branch of this extensive subject to which we have now to invite the reader's attention. The power of certain gases to absorb or stop certain rays of an otherwise continuous spectrum has already been mentioned ; but this property is by no means confined to gases, for certain liquids and solids do this in a high degree. There is a remarkable metallic element, named didymium. It is a rare substance, and its presence can- not with certainty be detected by any ordinary tests. Its salts, however, form solutions without colour, or nearly so, which have the power of strongly absorbing certain rays. If we hold before the slit of the spectrum a small tube containing a solution of any one of the salts, and allow the rays from the sun, or from a luminous gas or candle-flame, to pass through it, we see the spectrum crossed by certain well-defined very dark bands. A spectrum of this kind is called an absorption spectrum, and the position, number, width, &c., of dark bands are found to be as peculiar to each substance as are the bright lines in the spectra of the elements. The method of observing them when produced by solutions is very simple. The liquid is contained in a small test-tube, which is placed in front of the slit ; or, more conve- niently, the liquid is put into a wedge-shaped vessel, and thus the thickness of the stratum of liquid through which the rays pass can easily be varied, so that the best results may be obtained. The absorption spectra are pro- duced by many compound substances. A striking absorption spectrum is seen when a solution in alcohol of the green colouring matter of leaves (chlorophyll] is examined ; for several distinct bands are seen, one in the red being especially well marked. Many other coloured bodies exhibit charac- teristic absorption bands, as, for example, permanganate of potash, uranic 3 i8 THE SPECTROSCOPE. salts, madder, port wine, and magenta. The bands are so peculiar for each substance, that if so-called port wine, for example, owe its colour to colouring matter other than that of the grape, such as logwood, &c., the adulteration can be instantly detected by a glance at the absorption spectrum. As, how- ever, the absorption bands are not, like the bright lines of metals, definite images of the slit, but rather broad portions of the spectra, it is very desir- able in examining such spectra to compare them directly with those o{ known substances, by throwing two spectra into one field, by means of a side reflecting prism, as already described. Perhaps one of the most interesting examples of absorption spectra is that of blood. A single drop of blood in a tea-cupful of water will show its characteristic spectrum when it is properly examined. If the blood is arterial or oxidized blood, two well-marked dark bands are visible ; but if venous or deoxidized blood be used, we see, instead of the two dark bands. a single one in an intermediate position. These differences have been E roved to be due to oxidization and deoxidization of a constituent of the lood, called hcemoglobin, and by using appropriate chemical reagents, the same specimen of blood may be made to exhibit any number of alternations of the two spectra, according as oxidants or reducing reagents are em- ployed. It would be possible by an examination of the absorption spectrum of a drop of arterial blood to pronounce that a person had died of suffoca- tion from the fumes of burning charcoal. In such case, the supply of oxygen being cut off, the haemoglobin of the whole of the blood in the system be- comes deoxidized. The beautiful delicacy of these spectrum reactions has permitted the spectroscope to be applied to the microscope with signal success by Mr, Browning, working in conjunction with Mr. Sorby, who has devoted great attention to this subject. The Sorby-Browning instrument is a direct-vision spectroscope, with a slit, lens, &c., placed above the eye-piece of the micro- scope. By receiving the light through a single drop of an absorptive liquid: placed under the object-glass of the microscope, the characteristic bands are made visible. The micro-spectroscope is also a valuable instrument for examining the absorption bands which are found in the light reflected from solid bodies, for the smallest fragment suffices to fill the field of the micro- scope. Mr. Sorby is able to obtain most unmistakably the dark bands peculiar to blood from a particle of the matter of a blood-stain weighing less than Y^njth part of a grain. It is plain from this that the spectroscope must sometimes prove of great service in giving evidence of crime from traces which would escape all ordinary observation. The micro-spectroscope, in its most complete form, is represented in. Fig. 224. As may be seen from the figure, the apparatus consists of several parts. The prism is contained in a small tube, which can be removed at pleasure ; below the prism is an achromatic eye-piece, having an adjust- able slit between the two lenses ; the upper lens being furnished with a screw motion to focus the slit. A side slit, capable of adjustment, admits, when required, a second beam of light from any object whose spectrum it is desired to compare with that of the object placed on the stage of the microscope. This second beam of light strikes against a very small prism suitably placed inside the apparatus, and is reflected up through the com- pound prism, forming a spectrum in the same field with that obtained from the object on the stage. A is a brass tube carrying the compound direct- vision prism, and has a sliding arrangement for roughly focussing. THE SPECTROSCOPE. ii FIG. 224. The Sorby-Br owning Micro-Spectroscope. B, a milled head, with screw motion to finally adjust the focus of the achromatic eye-lens. C, milled head, with screw motion to open or shut the slit vertically. Another screw, H, at right angles to C, regulates the slit horizontally. This screw has a larger head, and when once recognized cannot be mistaken for the other. D D, an apparatus for holding a small tube, that the spectrum given by its contents may be compared with that from any other object on the stage. E, a screw, opening and shutting a slit to admit the quantity of light required to form the second spectrum. Light entering the aperture near E strikes against the right-angled prism which we have mentioned as being placed inside the apparatus, and is reflected up through the slit belonging to the compound prism. If any incandescent object is placed in a suitable position with reference to the aperture, its spectrum will be obtained, and will be seen on looking through it. F shows the position of the field lens of the eye-piece. G is a tube made to fit the microscope to which the instrument is applied. To use this instrument, insert G like an eye-piece in the microscope tube. Screw on to the microscope the object-glass required, and place the object ^ whose spectrum is to be viewed on the stage. Illuminate with stage mirror .if transparent, with mirror and lieberkiihn and dark well if opaque, or by Jside reflector, bull's-eye, &c. Remove A, and open the slit by means of the Imilled head, H, at right angles to D D. When the slit is sufficiently open 3 the rest of the apparatus acts like an ordinary eye-piece, and any object can be focussed in the usual way. Having focussed the object, replace A, 3 20 THE SPECTROSCOPE. and gradually close the slit till a good spectrum is obtained. The spectrum will be much improved by throwing the object a little out of focus. Every part of the spectrum differs a little from adjacent parts in refran- gibility, and delicate bands or lines can only be brought out by accurately focussing their own parts of the spectrum. This can be done by the milled head, B. Disappointment will occur in any attempt at delicate investiga- tion if this direction is not carefully attended to. When the spectra of very small objects are to be viewed, powers of from \ in. to i-2oth, or higher, may be employed. Blood, madder, aniline dyes, permanganate of potash solution, are convenient substances to begin experiments with. Solutions that are too strong are apt to give dark clouds instead of delicate absorp- tion bands. Small cells or tubes should be used to hold fluids for exami- nation. Mr. Browning has still further improved the micro-spectroscope by the ingenious arrangement for measuring the positions of the lines, which is represented in Fig. 225, and the construction and the use of which he thus described in a paper read before the Microscopical Society : Attached to the side is a small tube, A A. At the outer part of this tube is a blackened glass plate, with a fine clear white pointer in the centre of the tube. The lens, C, which is focus- sed by sliding the milled ring, M, pro- duces an image of the bright pointer in the field of view by reflection from the surface of the prism nearest the eye. On turning the micrometer, M, the slide which holds the glass plate is made to travel in grooves, and the fine pointer is made to traverse the whole length of the spectrum. It might at first sight appear as if any ordinary spider's web or parallel wire micrometer might be used in- stead of this contrivance. But on closer attention it-will be seen that as the spectrum ; will not permit of magnification by the use of lenses, the line of such an ordinary micro- meter could not be brought to focus and rendered visible. The bright pointer of the new arrangement pos- sesses this great advantage that it does not illuminate the whole field of view. _ If a dark wire were used, the bright FIG. 225. Section of Micro-Spectro- diffused light wou id almost obscure scopt with Micrometer. the faint light of the spe c t ra, and en- tirely prevent the possibility of see- ing, let alone measuring, the position of lines or bands in the most refrangible part of the spectrum. To produce good effects with this apparatus the upper surface of the compound prism, P, must make an angle of exactly 45 with the sides of THE SPECTROSCOPE. 321 the tube. Under these circumstances the limits of correction for the path of the^rays in their passage through the dispersing prisms are very limited and must be strictly observed. The usual method of correcting by the outer surface is inadmissible. For the sake of simplicity, some of the work of the lower part of the micro-spectroscope is omitted in the engraving. As to the method of using this contrivance : With the apparatus just described, measure the position of the principal Fraunhofer's lines in the solar spec- trum. Let this be done carefully, in bright daylight. A little time given to this measurement will not be thrown away, as it will not require to be done again. Note down the numbers corresponding to the position of the lines, and draw a spectrum from a scale of equal parts. About 3 in. will be found long enough for this spectrum ; but it may be made as much longer as is thought desirable, as the measurements will not depend in any FlG. 226. way on the distance of these lines apart, but only on the micrometric num- bers attached to them. Let this scale be done on cardboard and preserved for reference. Now measure the position of the dark bands in any ab- sorption spectra, taking care for this purpose to use lamplight, as daylight will give, of course, the Fraunhofer lines, which will tend to confuse your spectrum. If the few lines occurring in most absorption spectra be now drawn to the same scale as the solar spectrum, on placing the scales side by side, a glance will show the exact position of the bands in the spectrum relatively to the Fraunhofer lines, which thus treated form a natural and unchangeable scale (see diagram, Fig 226). But for purposes of compari- son it will be found sufficient to compare the two lists of numbers repre- senting the micrometric measures, simply exchanging copies of the scale of Fraunhofer lines, or the numbers representing them will enable observers at a distance from each other to compare their results, or even to work simultaneously on the same subject. A simpler form of the micro-spectroscope is also made by Mr. Browning at a very modest price, and if the reader possesses a microscope, and desires to examine these interesting subjects for himself, he will do well to procure this instrument, instead of that represented in Fig. 220, as it will also answer better for other purposes. A section of the instrument is shown in Fig. 227. When used with the microscope it is slipped into the place of the eye-piece. There is an adjustable slit, a reflecting prism, by which two different spectra may be examined at once, and a train of five prisms for dispersing the rays. It can be used equally well for seeing the bright lines 21 322 THE SPECTROSCOPE. of metals and the Fraunhofer lines, and for viewing any two spectra simul- taneously. These direct-vision spectroscopes are better adapted for general use by those who have not several different instruments, than such forms as that shown in Fig. 229, for in the direct-vision instruments the whole extent of the spectrum is visible at one view, which is by no means the case with the larger instruments. FIG. 227. Section of Micro-Spectrosoope. CELESTIAL CHEMISTRY AND PHYSICS. \ \ 7"E now approach that portion of our subject in which its interest cul- * * minates, for however remarkable may be some of the above-named results of this searching optical analysis, they are surpassed by those which have been obtained in the field upon* which we are about to enter. The cause of the dark lines which Fraunhofer observed in the light of the sun and of certain stars remained unexplained, he only establishing the fact that they must be due to some absorptive power existing in the sun and stars themselves, and not to anything in our atmosphere. It was reserved for Professor Kirchhoff, of the University of Heidelberg, to show the full signi- ficance of the dark lines. Fraunhofer had, on his first observation of the lines, noticed that the D lines were coincident with the bright lines in the spectrum of sodium. This interesting fact may be readily observed with any spectroscope which permits of the two spectra being simultaneously viewed. The bright line (or lines if the spectroscope be powerful) of the metal is seen as a prolongation of the dark D solar line. Even with an instrument like that shown in Fig. 220 the coincidence may be noticed. Let the observer receive into the instrument the rays in diffused daylight only, when he will still see the principal Fraunhofer lines distinctly, and let him note the exact position of the D line, while he brings in front of the slit the flame of a spirit-lamp charged with a little salt. He will then see the bright yellow line replacing the dark D line, and by alternately removing and putting back the lamp he will be soon convinced of the perfectly iden- tical position of the lines. This fact remained without explanation from 1814 to 1859, when Kirchhoff accidentally found, to his surprise, that the dark D line could be produced THE SPECTROSCOPE. 323 artificially. He says : " In order to test in the most direct manner possible the frequently asserted fact of the coincidence of the sodium lines with the D lines, I obtained a tolerably bright solar spectrum, and brought a flame coloured by sodium vapour in front of the slit. I then saw the dark lines D, change into bright ones. The flame of a Bunsen's lamp threw the bright sodium lines upon the solar spectrum with unexpected brilliancy. v In order to find out the extent to which the intensity of the solar spectrum' could be increased without impairing the distinctness of the sodium lines, I allowed the full sunlight to shine through the sodium flame, and, to my astonish- ment, I saw that the dark lines, D, appeared with an extraordinary degree of clearness. I then exchanged the sunlight for the Drummond's or oxy- hydrogen lime-light, which, like that of all incandescent solid or liquid bodies, gives a spectrum containing no dark lines. When this light was allowed to fall through a suitable flame, coloured by common salt, dark lines were seen in the spectrum in the position of the sodium lines. The same phenomenon was observed if, instead of the incandescent lime, a platinum wire was used, which, being heated in the flame, was brought to a temperature near its melting point, by passing an electric current through it. The phenomenon in question is easily explained, upon the supposi- tion that the sodium flame absorbs rays of the same degree of refrangi- bility as those it emits, whilst it is perfectly transparent for all other rays." (Quoted in Roscoe's Lectures on " Spectrum Analysis.") When the light of ignited lime was similarly made to pass through flames containing the incandescent vapours of potassium, barium, strontium, &c., the bright lines which these substances would have produced had the lime-light not been present were found to be in every case changed into dark lines, occupying the very same positions in the spectrum. In such experiments the flames containing the metals in the vapourized state do all the time really give off those rays which are peculiar to each substance ; but when a more intense illumination such as the lime-light, the electric arc, or direct sunlight passes through them, the rays of the spectrum produced by the intense light overpower those given off by the relatively feebly coloured flames, and hence the portions of the spectrum which are occupied by these, appear black. But as the intense light would give a perfectly con- tinuous spectrum if the incandescent metallic vapour allowed the rays corresponding to its lines to pass through it, the inference is obvious that each vapour absorbs those particular rays which it has itself the power of emitting, but allows all others to pass freely through it. Besides the experi- mental proofs of this fact which have been already adduced, many others might be named. The flame of a spirit-lamp with a salted wick appears opaque and smoky when we look through it at a large flame of burning hydrogen, also coloured by sodium ; for the rays emitted by the latter do not penetrate the former, which, in consequence of its feebler light, appears dark by comparison. Again, if an exhausted tube containing metallic sodium be heated so as to convert the sodium into vapour, the tube viewed by the light of a sodium flame appear to contain a black smoke, and the light from the flame will no more pass through it than through a solid object ; yet the tube appears perfectly transparent when viewed by ordi- nary light, and the light from a lithium or other coloured flame would also pass freely. Kirchhoff was led by purely theoretical reasoning to conclude that all luminous bodies have precisely the same power of absorbing certain rays of light as they have of emitting them at the same temperature, and he thus brought luminous rays under the same general law which had 212 324 THE SPECTROSCOPE. previously been established for radiant heat by Prevost, Dessains, Balfour Stewart, and others. Here, then, a law was arrived at, and, abundantly confirmed by direct experiment as regards the more volatile metals, it was ready to supply the most satisfactory explanation of the coincidences which were everywhere discovered to exist between the Fraunhofer lines and those which belong to terrestrial substances. For Kirchhoff also found, when mapping the very numerous lines seen in the spark spectrum of iron, that for each of the 90 bright lines of iron which he then observed, there was a dark line in the solar spectrum exactly corresponding in posi- tion. The number of observed bright lines in the iron spectrum has been since extended to 460, and yet each is found to have its exact counterpart in a solar dark line. So many coincidences as these made it certain that these dark lines and the bright lines of iron must have a common cause, for the chances against the supposition that the agreement was merely accidental are enormous. Kirchhoff actually calculated, by the theory of probabilities, the odds against the supposition. He found it represented by i ,ocx),ooo,oc)o,ooo,ooo,ooo to i. The result arrived at in the case of sodium at once suggested the expla- nation that these lines were produced by an absorptive effect of the vapour of iron. Now, the existence of such a vapour in our atmosphere could not be admitted, while the temperature of the sun was known to be exceedingly high, far higher, indeed, than any temperature we can produce by electri- city, or any other means. Hence, Kirchhoff concluded that his observa- tions proved the presence of the vapour of iron in the sun's atmosphere with as much certainty as if the iron had been actually submitted to chemi- cal tests. By the same reasoning, Kirchhoff also demonstrated the existence in the solar atmosphere of calcium, chromium, magnesium, nickel, barium, copper, and zinc. To these, other observers have added strontium, cad- mium, cobalt, manganese, lead, potassium, aluminium, titanium, uranium, and hydrogen. It has also been demonstrated that a considerable number of the Fraunhofer lines are due to absorption in our atmosphere by its gases and aqueous vapour. This demonstration of the existence of iron and nickel in the sun is an interesting pendent to the known composition of many meteorites which reach us from interplanetary space. Kirchhoff was led to believe that the central part of the sun is formed of an incandescent solid or liquid, giving out rays of all refrangibility, just as white-hot carbon does ; that round this there is an immense atmosphere, in which sodium, iron, aluminium, &c., exist in the state of gas, where they have the power of absorbing certain rays ; that the solar atmosphere ex- tends far beyond the sun, and forms the corona ; and that the dark sun- spots, which astronomers have supposed to be cavities, are a kind of cloud, floating in the vaporous atmosphere. During total eclipses of the sun, certain red-coloured prominences have been noticed projecting from the sun's limb, and visible only when the glare of its disc is entirely intercepted by the moon. Fig. 228 represents a total eclipse, and will give a rude notion of the appearance of the red prominences seen against the fainter light of the corona, which extends to a considerable distance beyond the sun's disc. Now, two distinguished men of science .Simultaneously and independently made the discovery of a mode of seeing these red prominences, even when the sun was unobscured. M. Janssen was observing a total eclipse of the sun in India, and the examination by the spectroscope of the light emitted from the red prominences showed him that they were due to immense columns of incandescent hydrogen, for THE SPECTROSCOPE. 325 FIG. ill. Solar Eclipse, 1869. he recognized the red line and blue lines which belong to the spectrum of this gas (see No. 12, Plate VI.). Mr. Norman Lockyer at the same time also succeeded in viewing the solar prominences in "London without an eclipse. He found a red line per- fectly coinciding in position with Fraunhofer's C line and that of hy- drogen, another nearly coinciding with F, and a third yellow line near D. Soon after this, Dr. Huggins dis- covered a mode of observing the shape of the red prominences at any time, by using a powerful train of prisms and a wide slit, so that the changes in the forms of the red flames can be followed. Now, since the red prominences give off only a few rays of particular refrangibility, it is not difficult to understand that the light of the sun might be, as it were, so diluted by stretching out the spectrum, by means of a train of many prisms, that almost only the red rays, C, should enter the telescope, and occupy the field with sufficient intensity to overpower all others, and produce an image of the object from which they originated. The nature of this action may be illustrated thus : If we hold vertically a prism, and look through it at a candle-flame, we may per- ceive a lengthened-out image of the flame, showing the succession of pris- matic colours, and formed, as it were, of a red image of the flame close to a yellow one, and so on, but presenting no defined form. If, still viewing this spectrum, we introduce into the flame on a platinum wire a piece of common salt, we shall perceive a well-defined yellow image of the candle start out, because the rays which are emitted by the incandescent sodium, being all of one refrangibility, the prism simply refracts without dispersing them. The dispersion which weakens the light of the continuous spectrum by lengthening it out, does not sensibly detract from the brilliancy of the bright lines, as their breadth is scarcely increased they are refracted but not dis- persed. Hence, when a sufficient number of prisms is employed, the bright lines of the solar chromosphere may be seen in full sunshine, in spite of the greater intensity of the light emanating from the photosphere, which pro- duces the continuous spectrum. The bright C line is, of course, a virtual image of the slit produced by rays of that particular refrangibility ; but by using a very high dispersive power, the slit may be opened so wide that the C rays form in the telescope a red image of the prominence from which they issue, since their light will predominate over that of any rays belonging to the continuous spectrum. In the hands of Mr. Norman Lockyer the science of the physical and chemical constitution of the sun has made rapid progress, and new facts are continually being observed, which serve to furnish more and more defi- nite views. Mr. Lockyer considers that, extending to a great distance around the sun is an atmosphere of comparatively cooler hydrogen, or perhaps of some still lighter substance which is unknown to us. It is this which forms what is termed the corona, or circle of light which is seen surrounding the 326 THE SPECTROSCOPE. sun in a total eclipse. Immersed in this, and extending to a much smaller distance from the nucleus of the sun, is another envelope, termed the chroma- sphere, consisting of incandescent hydrogen and some glowing vapours of magnesium and calcium. The brightest part of this envelope, which lies nearest the sun, is that which gives off the red rays by which the promi- nences may be observed without an eclipse. These prominences have been shown to be tremendous outbursts of glowing hydrogen, belched up with sometimes an enormous velocity from below, since they have been observed to spring up 90,000 miles in a few minutes. Beneath the chromosphere, and nearer to the body of the sun, are enormous quantities of the vapours of the different elements sodium, iron, &c. to which the dark lines of the solar spectrum are due. This stratum Mr. Lockyer calls the reversing layer, because it reverses (turns to dark) the lines which would otherwise have appeared bright, just as KirchhofFs sodium vapour did in the experiment described on page 323. Beneath the reversing layer is the photo sphere, from which emanates the light that is absorbed in part by the reversing layer, and which there is good reason to believe is either intensely heated solid or liquid matter. In 1861 Dr. Huggins devoted himself, with an ardour which has since known no remission, to the extension of prismatic analysis to the other heavenly bodies. The difficulties of the investigations were great. There was first the small quantity of light which a star sends to the spectator ; this was obviated by the use of a telescope of large aperture, which ad- mitted and brought to a focus many more rays from the star, and therefore the brightness of the image was proportionately increased. Not so the size of the image : the case of the fixed stars for this always remains a mere point. It was, of course, necessary to drive the telescope by clock- work, so that the light of the star might be stationary on the field of the spectroscope. As the spectrum of the image of the star formed by the object-glass would be a mere line, without sufficient breadth for an obser- vation of the dark or light lines by which it might be crossed, it is necessary to spread out the image so that the whole of the light may be drawn out into a very narrow line, having a length no greater than will produce a spectrum broad enough for the eye to distinguish the lines in it. This is accomplished by means of a cylindrical lens placed in the focus of the object-glass, and immediately in front of the slit Covering one-half of the slit is a right-angled prism by which the light to be compared with that of the star is reflected into the slit. The light is usually that produced by taking electric sparks between wires of the metal in the manner already described. The dispersive power of the spectroscope was furnished by two prisms of very dense glass, and the spectrum was viewed through a tele- scope of short focal length. Dr. Muggins's observations lead him to the conclusion that the planets Mars, Jupiter, and Saturn possess atmospheres, as does also the beautiful ring by which Saturn is surrounded ; for he noticed in the spectrum of each different dark lines not belonging to the solar spectrum. Passing to the results obtained in the case of the fixed stars, we may remind the reader of the enormous distance of the bodies which are sub- mitted to the new method of analysis. Sir John Herschel gives the follow- ing illustration of the remoteness of Sirius supposed to be one of the nearest of the fixed stars : Take a globe, 2 ft. in diameter, to represent the sun, and at a distance of 215 ft. place a pea, to give the proportionate size and distance of the earth. If you wish to represent the distance of Sirius THE SPECTROSCOPE. 327 on the same scale, you must suppose something placed forty thousand miles away from the little models of sun and earth. But not only do we know with certainty some of the substances contained in Sirius, but the star spectroscope has taught us a great deal about orbs so remote, that their distance is absolutely unmeasurable. About Aldebaran we know that there are hydrogen gas and vapours of magnesium, iron, calcium, sodium, and some four or five other elements. Generally the lines indicate the presence of hydrogen in these distant suns ; but there is, at least, one FIG. 229. The Planet Saturn. remarkable exception in a Orionis, the spectrum of which yields no trace of the hydrogen lines, although it is evident that magnesium, sodium, calcium, &c., are present The spectra of celestial bodies are of several kinds. Many of the stars have, like our sun, a continuous spectrum crossed by dark lines. Such is that of Sirius, No. 10, Plate VI. Others have, however, both dark and bright lines, and some are marked by only three bright spaces. Of the spectra of the nebulae some have three bright lines (see No. n, Plate VI.), and the bodies producing them are, therefore, to be considered as masses of incandescent gas, while some give continuous spectra. One of the bright lines in the spectra of the nebulae coincides with one of the hydrogen lines, and another the brightest of the three with one of the brightest nitrogen lines ; but the third does not agree with any with which it has as yet been compared. The inference from these appearances is that the nebulae contain hydrogen and nitrogen, but the absence of the other lines of these substances has not been fully explained ; although the observation of Dr. Huggins, that when the light of incande- scent nitrogen and hydrogen is gradually obscured by interposing layers of neutral tinted glass, the lines corresponding with those in the nebulas: 328 THE SPECTROSCOPE. spectra are the last to disappear, seems to suggest a probable solution of the difficulty. There is another very interesting line of spectroscopic research in the power the prism gives us of estimating the velocity with which the distances of the stars from our system are increasing or diminishing. On closely examining the hydrogen lines cf Sirius, and comparing them with the bright lines of hydrogen rendered incandescent by electric discharges in a Geissler tube, the spectrum of which his instrument enabled him to place side by side with that of the star, Mr. Huggins was surprised to find that the lines in the latter did not exactly coincide in position with those of the former, but appeared slightly nearer the red end of the spectrum. This indicated FIG. 230. Solar Prominences, No. i. a longer wave-length, or increased period of vibration, according to the theory of light, which would be accounted for by a receding motion between Sirius and the earth, junt as the crest of successive waves of the sea would overtake a boat going in the same direction at longer intervals of time than those at which they would pass a fixed point, while, if the boat were meet- ing the waves, these intervals would, on the other hand, be shorter. H ence, if the position of the lines in the spectrum depends on the periods of vibra- tion, that position will be shifted towards the red end when the luminous body is receding from the earth with a velocity comparable to that of light, and towards the violet end when the motion is one of approach. The change in refrangibility observed by Mr. Huggins corresponded with a receding velocity of 41-4 miles per second, and when from this was subtracted the known speed with which the earth's motion round the sun was carrying us from the star at the time, the remainder expressed a motion of recession THE SPECTROSCOPE. 329 amounting to about twenty miles a second, which motion, there is reason to believe, is chiefly due to a proper movement of Sirius. These deduc- tions from prismatic observations are of the highest value astronomically, since they will eventually enable the real motions of the stars to be deter- mined, for ordinary observation could only show us that component of the motion which is at right angles to the visual ray, while this gives the com- ponent along the visual ray. In the same manner, it is inferred that Arc- _ ' FIG. 231. Solar Prominences, No. 2. turus, a bright star in the constellation Bootes, is approaching us with a velocity of fifty-five miles per second. When the solar spots are examined with the spectroscope, the dark image of the slit produced by the hydrogen line, F, is observed to show a strange crookedness when it is formed by rays from different parts of the spot. This distortion is due to the same cause as the displacement of the stellar lines, namely, motions of approach or recession of the masses of glowing hydrogen. Mr. Norman Lockyer, to whom we are indebted for the most elaborate investigations of the solar surface, has calculated, from the posi- tion of the lines, the velocities with which masses of heated hydrogen are seen bursting upwards, and those which belong to the down-rushes of cooler gas. Velocities as great as 100 miles per second were, in this way, inferred to occur in some of the storms which agitate the solar surface. Two draw- ings of a solar storm, given by Mr. Lockyer, are shown in Figs. 230 and 231. These are representations of one of the so-called red prominences, the first giving its appearance at five minutes past eleven on the morning of March I4th, 1869, and the last showing the same ten minutes after- 330 THE SPECTROSCOPE. wards. The enormous velocity which these rapid changes imply will be understood when it is stated that this prominence was 27,000 miles high. "This will give you some idea," says Mr. Lockyer, " of the indications which the spectroscope reveals to us, of the enormous forces at work in the sun, merely as representing the stars, for everything we have to say about the sun the prism tells us and it was the first to tell us we must assume to be said about the stars. I have little doubt that, as time rolls on, the spec- troscope will become, in fact, almost the pocket companion of every one amongst us ; and it is utterly impossible to foresee what depths of space will not in time be gauged and completely investigated by this new method of research." The light of comets has also been examined by the spectroscope, and many interesting results arrived at. Our limits do not, however, permit us to enter into a discussion of these interesting subjects. Fig. 232 is a section of another of Mr. Browning's popular instruments, which is named by him the " Amateur's Star Spectroscope." It exhibits very distinctly the different spectra of the various stars, nebulae, comets, &c. FIG. 232. Section of Amateur's Star Spectroscope. The reader who is desirous of learning more of this fascinating subject is referred to Dr. Roscoe's elegant volume, entitled, " Lectures on Spectrum Analysis." This work, which is embellished with handsome engravings and illustrated by coloured maps and spectra, gives a clear and full account of every department of the subject, and in the form of appendices, abstracts of the more important original papers are supplied, while a complete list is given of all the memoirs and publications relating to the spectroscope which have been published. This brief account of the spectroscope and its revelations, which is all that our space permits us to give, will not fail to awaken new thoughts in the mind of a reader who has obtained even a glimpse of the nature of the subject, especially in relation to that branch of which we have last treated, for in every age and in every region the stars have attracted the gaze and excited the imagination of men. The belief in their influence over human affairs was profound, universal, and enduring ; for it survived the dawn of rising science, being among the last shades of the long night of supersti- tion which melted away in the morning of true knowledge. Even Francis Bacon, the father of the inductive philosophy, and old Sir Thomas Browne, the exposer of " Vulgar Errors," believed in the influences of the stars ; for while recognizing the impostures practised by its professors, they still re- garded astrology as a science not altogether vain. It was reserved for the mighty genius of Newton to prove that in very truth there are invisible ties connecting our earth with those remote and brilliant bodies ties more potent than ever astrology divined ; for he showed that even the most dis- tant orb is bound to its companions and to our planet by the same power THE SPECTROSCOPE. that draws the projected stone to the ground. And now the spectroscope is revealing other lines of connection, and showing that not gravitation alone is the sympathetic bond which unites our globe to the celestial orbs, but that there exists the closer tie of a common constitution, for they are all made of the same matter, obeying the same physical and chemical laws which belong to it on the earth. We learn that hydrogen, and magnesium, and iron, and other familiar substances, exist in these inconceivably distant suns, and there exhibit the identical properties which characterize them here. We confirm, by the spectroscope, the fact partially revealed by other lines of research, that the stars which appear so fixed, are, in reality, career- ing through space, each with its proper motion. We learn also that the stars are the theatres of vast chemical and physical changes and trans- formations, the rapidity and extent of which we can hardly conceive. There is, for example, the case of that wonderful star in the constellation of the Crown, which, in 1866, suddenly blazed out, from a scarcely descernible telescopic star, to become one of the most conspicuous in the heavens, and the bright lines its beams produced in the spectroscope revealed the fact that this abrupt splendour was due to masses who can imagine how vast ? of incandescent hydrogen. This brightness soon waned, and r Cor ones Borealis reverted once more to all but telescopic invisibility. The seeming fixity of the stars is an illusion of the same nature as that which prevents a casual observer from recognizing their apparent diurnal motion, and now we have also ample evidence that permanence of physical condition, even in the stars, is impossible. Everywhere in the universe there is motion and change ; there is no pause, no rest, but a continual unfolding, an endles progression. And emulate, vaulted, " Know the stars yonder, The stars everlasting, Are fugitive also, The lambent heat-lightning, And fire-fly's flight." FIG. 233. Portrait of Professor Helmholtz. SIGHT. THE investigations of modern science have borne rich fruit, not only by vastly extending our knowledge of the universe of things around us, but also making us acquainted with the mode in which certain agents act upon our bodily organs, and by revealing, up to a certain point, what may be termed the mechanism of that most wonderful thing the human mind or, at least, that part which is immediately concerned in the perceptions of an external world. Of all the physical influences which affect the human mind, those due to light are the most powerful and the most agreeable. One of the most ancient of philosophers says, in the simple words which are appropriate to the expression of an undeniable truth, " Truly the light is sweet, and a pleasant thing it is for the eyes to behold the sun." The impression produced by light alone is a source of pleasure a cheering in- fluence of the highest order ; and there is a special character in the pleasing effects of light, from the circumstance that they do not exhaust the sense so quickly as do even pleasurable impressions on other organs such as sweet tastes, fragrant odours, or agreeable sounds. Sight is not liable to that satiety which soon overtakes the enjoyment of sensations arising 332 SIGHT. 333 from the other senses ; it possesses, therefore, a refinement of quality of which the rest are devoid. Sight converses with its objects at a greater distance than does any other sense, and it furnishes our minds with a greater variety of ideas. Indeed, our mental imagery is most largely made up of reminiscences of visual impressions ; for when the idea of anything is brought up in our minds by a word, for example, there arises, in most cases, a more or less vivid presentation of some visible appearance. Our visual impressions are also longer retained in memory or idea than any other class of sensations. The nature of the impressions we receive through the eye is extremely varied ; for we thus perceive not only the difference between light and darkness, but in the sensations of colour we have quite another class ol effects, while the lustre and sparkle of polished and brilliant objects add new elements of beauty and variety. We find examples of the latter qualities in the verdant sheen of the smooth leaf, in the splendid reflec- tions of burnished gold, in the bright radiance of glittering gems, and " in gloss of satin and glimmer of pearls." The eye is also the organ which conveys to our minds the impressions of visible motion, with all those plea- sures of exciting spectacle which enter so largely into our enjoyment of life. It likewise discriminates the forms, sizes, and distances of objects ; but by a process long misunderstood, and dependent upon a set of percep- tions which, although precisely those whence we derive our most funda- mental notions of the objects around us, have been completely overlooked in that time-honoured enumeration of the senses which recognizes only five. If such be the extent to which our minds are dependent upon the wonder- ful apparatus of the eye, it may easily be imagined what must be the com- parative narrowness of mental development in those who have never en- joyed this precious sense, and the feeling of deprivation in those, who, having enjoyed it, have unfortunately lost it. Well may our sublime poet despairingly ask Since light so necessary is to life, And almost life itself if it be true That light is in the soul The all in every part : why was the sight To such a tender ball as the eye confined, So obvious and so easy to be quenched ? " for he himself, in his own person, experienced this deprivation, and he thus touchingly, in his great work, laments his loss : " Thus with the year Seasons return ; but not to me returns Day, or the sweet approach of even or morn, Or sight of vernal bloom, or summer's rose, Or flocks, or herds, or human face divine ; But cloud instead, and ever-during dark Surround me ; from the cheerful ways of men Cut off; and for the book of knowledge fair Presented with a universal blank Of Nature's works to me expunged and rased, And wisdom at one entrance quite shut out." An organ which is the instrument of so many nice discriminations as is the eye must, of course, present the most delicate adjustment in its parts. So much has in recent times been learnt of the nature of its mechanism ; of the relation between the impressions made upon it and the judgments formed by the mind therefrom ; of the illusions which its very structure produces ; of the defects to which it is liable ; and of its wonderfully 334 SIGHT. refined physiological elements that a branch of science sufficiently exten- sive to require a large part of a studious lifetime for its complete mastery has grown up under the hands of modern physiologists, physicists, and psychologists. To some of the results of their labour we would invite the reader's attention ; and in order to render the account of them intelligible, we must, to a certain extent, describe " things new and old." THE EYE. FIG. 234. Vertical Section of the Eye. / T* HE form of the human eye and the general arrangement of its parts may * be understood by referring to Fig. 234, which is a section of the eye- ball. It has a form nearly globular, and is covered on the outside by a tough firm case, A, named the sclerotic coat, which is, for the most part, white and opaque. This covering it is which forms what is commonly termed the " white of the eye ; " but in the front part of the eyeball it loses its STGHT. 335 opacity, and merges into a transparent substance, termed the cornea, B. The cornea has a greater convexity than the rest of the exterior of the eye- ball, so that it causes the front part of the eye to have a somewhat greater projection than would result from its general globular form. This sclerotic coat with its continuation, the cornea serves to support and protect the more delicate parts within, and is itself kept in shape by the humours, which fill the whole of the interior. The greater space is occupied by the vitreous humour ', C ; but the space immediately behind the transparent cornea is filled with the aqueous humour, D. The latter is little else than pure water, and the former is like thin transparent jelly. The cavities con- taining these two humours are separated by the transparent double convex lens, E, called the crystalline lens, which, in consistence, resembles very thick jelly or soft gristle. The outward surface of this lens has a flatter curvature than the inner surface. Immediately in front of the crystalline lens is found the iris, F, which may be described as a curtain having in the middle a round hole. The iris is the part which varies in colour from one individual to another being blue, brown, grey, &c. ; and the aperture in its centre is the dark circular spot termed the pupil. The general disposition of the parts of the eye with regard to light will be most easily understood by comparing it with an optical instrument, to which it bears no little resemblance, namely, the camera obscura, so well known in connection with photography. We may picture to ourselves a still more complete resemblance, by imagining that the lens of the camera is single, that we have fixed in front of it a watch-glass, with the convex side outwards, and that we have filled with water the whole of the interior of the camera, including the space between the watch-glass and the lens. The focussing-screen of the camera corresponds with the inner surface of the back of the eyeball, about which we shall presently ha^e more to say. Now, even if the camera had no lens, but were simply a box filled with water, and having in front the watch-glass, fixed in the manner just mentioned, we could obtain the images of objects on the screen, as a consequence 01 the curvature of the watch-glass. It would, however, in this case, be neces- sary to have the camera much longer, or, in other words, the rays would be brought to a focus at a greater distance than if we put in the glass lens, which would, thus placed in the water, cause the rays to converge to a focus at a much shorter distance, although its effect when surrounded by water would be less powerful than in the air. There we see the effect of the crystalline lens of the eye in bringing the rays to a focus within a much shorter distance than that which would be required had there been present only the curved cornea, and the aqueous and vitreous humours of the eye, which are but little different from pure water in their optical properties. If vfQ focus the camera by adjusting the distance between the lens and the screen so as to get a distinct image of a near object, we should find, on directing the instrument to a distant one, that the image would be blurred and indistinct, and the lens would have to be moved nearer to the screen ; or we could get the image of the distant object distinct by replacing the lens by another lens in the same position, but having some flatter curva- ture. It is plain that the same object would be gained if our lens could be made of some elastic material, which, on being pulled out radially at its edges, could be made to assume the required degree of flatness without losing its lenticular form. Now, it is precisely with an automatic adjust- ment of this kind that the crystalline lens of the eye is provided, for the lens is suspended by an elastic ligament, G, by the tension of which its sur- 336 SIGHT. faces are more flattened than they would otherwise be ; but when the ten- sion of this ligament is relaxed, by the action of certain delicate muscles which draw it down, the elasticity of the lens causes it to assume a more convex form. These optical adjustments give, on the inner surface of the coats of the eye, a more or less perfect real image of the objects to which the eye is directed, and it is on the back part of this inner surface that the network of nerves, called the retina^ H, is spread out. The sclerotic coat, already spoken of, is lined internally with another, named the choroid, which is composed of delicate blood-vessels, inter- mingled with a tissue of cells filled with a substance of an intensely black colour. It is upon this last layer that the delicate membrane of the retina is spread out be- tween the choroid and the vitreous humour. The retina is, in part, an expansion of the fibres of the optic nerve over the back part of the eyeball. If we suppose the globe of this cut vertically into two portions, and so divide the front from the back part of the eye, the retina would be seen spread out on the concave surface of the back part, and in the middle of this part, opposite the crys- talline lens, would be seen a spot in which the retina assumes a yellowish colour, and in the centre of this, a little round pit or depression. The spot is called the macula lutea, or yellow spot, and the little central pit, which is of the highest importance in vision, is termed the fovea centralis. A little way from the yellow spot, and nearer the nose, is a point from which a number of fibres are seen to radiate, and this is, in fact, the part at which the optic nerve enters the eyeball, and from which it sends out its ramifications over the retina. This part, for a reason which will shortly appear, is called the blind spot. When the minute structure of the retina is examined by the microscope, its physio- logical elements are found to undergo very remarkable modifications at the yellow spot. In the retina, although the total thickness does not exceed the g^th part of an inch, no fewer than eight or ten different essential or nervous layers have been distinguished. Fig. 235 rudely repre- sents a section. The lowest stratum, A, which is next the choroid, and forms about a quarter of the total thickness, is formed of a multitude of little rod-shaped bodies, a, ranged side by side, and among these are the conical or bottle-shaped bodies, b. This lowest stratum of the retina is called the layer of rods and cones. At their front extremities the rods and cones pass into very delicate fibres, which, going through an extremely fine layer of fibres, B, are connected with a series of small rounded bodies, which form FIG. 235. Section of Retina. SIGHT. 337 the layer of nuclei, C, separated by a layer of nervous fibres, D, from a granular layer, E, in front of which is a stratum of still finer granules, F, underlying a layer of ganglionic nerve-cells, G, of a larger size than any of the other elements, and these ganglionic cells send out numerous branching nerve-fibres, forming the layer H. Finally, on the front surface of the retina there is a thin stratum formed of fibres, which issue from the optic nerve, K, Fig. 234, and in fact constitute the expansion of this nerve on the inner surface of the eyeball. The terminations of some, at least, of these nerve- fibres have been traced, and have been found to form junctions with those branching from the ganglionic cells. Of the part played by each of these delicate structures in exciting visual impressions little is yet known. How light, or the pulsations of ether, if such there be, is ultimately converted into sensation will probably for ever remain a mystery, although it is quite likely that the kind of visual impres- sion which is conveyed by each part of the elaborate structure of the retina FIG. 236. may ultimately be distinguished. One curious result of modern investiga- tion is that light falling directly upon fibres of the optic nerve is quite incapable of exciting any sensation whatever. Light has no more effect on this nerve and its fibres than it would have on any other nerve of the body if exposed to its action. The apparatus of rods, cones, and other structures are absolutely essential to enable light to give that stimulus to the optic nerve which, conveyed to the brain, is converted into visual sensations. So if this apparatus were absent in our organs of vision, in vain would the optic nerve proper be spread out over the interior of the eyeball : we should be no more able to see with such eyes than we are able to see with our hands. We now invite the reader's careful consideration to the diagram, Fig. 236, which is a section of the retina through the yellow spot. The upper part of the figure is the front, and the deep depression is the little pit already spoken of ihefovea centralis. The lowest dark line represents the base- ment membrane of the retina, and immediately above is seen the layer of rods and cones, and the various strata already spoken of are represented in their due order in the marginal parts of the diagram. Now observe the remarkable modifications of the nervous structures in the neighbourhood of the fovea centralis, some of which are visible in the diagram. In the first place, the cones are there much longer, more slender, and more closely set, so that there is a far greater number of them on a given surface ; but 22 33 8 SIGHT. the rods are comparatively few, and are, in fact, not found at all under the floor of the little pit. The layer of nuclei, into which the cones extend, is thinner, and is found almost immediately below the anterior surface, for all the other layers thin out in the fovea in a very curious manner. It is, however on the margin of the fovea that the stratum of ganglionic cells, G, Fig. 235, attains its greatest thickness, for there it is formed by the super- position of eight or ten cells, being here thicker than any other layer, while it is so thinned off towards the margin of the retina that it no longer forms even a continuous stratum. This layer, however, becomes much thinner in the fo^ea^ which contains, in fact, but few superposed cells. The tint of the yellow spot is said to be derived from a colouring matter, which affects all the layers except that of the cones. The centre of the yellow spot, where the fovea centralis is situated, is extremely transparent, and is so delicate that it is very easily ruptured, and has frequently been taken for an aperture. We should not have risked wearying the reader with these details con- cerning the little pit in the centre of the retina had it not possessed an extreme importance in the mechanism of the eye, a fact which he will at once appreciate when we say that of the whole surface of the retina, the only spot where the image of an object can produce distinct vision is the fovea centralis. Since this is undoubtedly true, it follows that the physiological elements which we there find are precisely those which are most essential for producing this effect. The case may be exemplified by recurring to the comparison of the eye with a photographer's camera, by supposing his screen to be of such a nature that only on one very small spot near its centre could a distinct image be possibly obtained of just one point of an object. Such a defect in his camera would render the photographer's art impossible, and this defect (if it may be so called) in the eye would render it almost equally useless, had not an adjustment, which more than com- pensates for it, been afforded in the extreme mobility of our organs of vision. This adjustment is so perfect that people in general do not even suspect that the image of each point of an object which they distinctly see must be formed on one particular spot on the retina a spot about one-tenth of the diameter of an ordinary pin-head ! We may venture, without any disrespect to the reader, to assume that the chances are that it is new to him to learn how each letter in the lines beneath his eye must successively, but momentarily, form its image in the very little pit in the centre of his retina; and the chances are at least a hundred to one that, even if aware of this, he has passively received the statement, and that he has not made the least attempt to realise the truth for himself. Yet nothing is easier. Let him request a friend to slowly peruse some printed page, while he meanwhile intently watches his friend's eyes. He will then perceive that before a single word can be read there is a move- ment of the eyeballs, which are, quite unconsciously to the person reading, so directed that the image of each letter (for the area of distinct vision is incapable of receiving more than this at once) shall fall upon the only parts of the retinae from which a distinct impression can be conveyed along the optic nerve. Thus it is that the eye, without any conscious effort of the observer, is directed in succession to the various points of an object, and it is only by an effort of will in fixing the eyes upon one spot that one be- comes aware of the blurred and confused forms of all the rest of the visual picture. Yet so readily do the eyeballs turn to any part of the indistinct picture on which the attention is fixed, that it is not improbable a person unversed in such experiments, wishing to verify our conclusions by looking, SIGHT. 339 say, at one spot on the opposite wall, will be very apt, in thinking of the features of the rest of the picture, to direct his eyes there, and then declare that he, at least, sees no such vague forms. If such be his experience, the correction is easy. He has only to ask some one to watch closely his eyes while he repeats the experiment, and after a few trials he will suc- ceed in maintaining the requisite immobility of the eyeballs a condition upon which the success of many such experiments depends. This extreme mobility of the eyeballs more than compensates for the loss of the clear and well-defined picture, for it calls into action one of the most sensitive of all the impressions of which we are capable, and one which possesses in so high a degree the power of uniting with our FIG. 237 '.Muscles of Eyes. The muscles of the eyeballs viewed from above : B, the internal rectus ; E, the external rectus ; s, the superior rectus ; T, the superior oblique, passing through a. loop of ligament at u, and turning outwards and downwards to its insertion at c. The inferior rectus and the inferior oblique are not visible in the figure : the superior rectus is removed from the right eyeball in order to show the optic nerve N. other sensations, that this sixth sense has been, as already stated, utterly overlooked, except by the more modern students of the nature of our sensations. It is usually termed the muscular sense, and to it are due some of the nicest distinctions of impressions of which we are capable. The muscles of every part of our frame take their part in producing impressions in our minds, and those of the eyeballs have a very large share in furnish- ing us with ideas of forms and motions. Fig. 237 is a diagram showing the general arrangement of these muscles ; and their anatomical designa- tions, which need not much concern us at present, are given beneath the figure. The wonder is, that the sensations arising from the relative con- ditions of parts so few, should afford us the immense variety of notions re- ferrible for their origin to these muscles only. We take one example in 222 340 SIGHT. illustration. Suppose we watch the flight of a bird, at such an elevation that no part of the landscape comes into the field of view at all ; and that, again, we follow with the eye, under similar circumstances, the path of a rocket. We can unhesitatingly pronounce the motions unlike, and yet in each case there was no visual impression present but that of the object focussed upon the yellow spot. But the movement of the muscles in one case was different from that in the other. Nay more, we can form such a judgment of the motion as to pronounce that the object followed such and such a curve we may recognize the parabola in one path, and the circle, perhaps, in the other. And this kind of discrimination arises from the fact, that when we have, maybe times without number, previously looked at parabolas and circles, in diagrams perhaps, the muscles of the eyeballs have performed just the same series of movements, as point after point of the line was made to form its image on the yellow spot. This is not the only class of impressions that these muscles are capable of affording ; there Is, for example, little doubt that they aid us in estimating distance. But space will not permit further discussion of this subject. Although the blurred and indefinite retinal picture may be compen- sated, and perhaps more than compensated, by the readiness with which the eyes move, it is, of course, possible that greater precision and delicacy of visual impression over the whole surface of the retina might be consis- tent with a still greater increase of our powers of perception. There are instances in which the absence of finish, as it may be termed, in all but one little spot in the picture, proves a real inconvenience and a sensible depri- vation. Perhaps a friend calls our attention to the fact that a balloon is sailing through the air, or some fine morning, hearing in the fields the blithe song of the sky-lark, we look up and vainly try to bring the small image upon the place of distinct vision. Now, if an image which falls upon any other part of the retina is perceived, even indistinctly, an instant suffices to direct the eyes into the exact position requisite for clear vision an ex- ample of the marvellous precision with which impressions are put in rela- tion to each other by the unconscious action of the brain. But while an image on the fovea, only ^nfe^th of an inch diameter, produces a distinct sensation, it is found that if the image falls on the retina at a point some distance from the yellow spot, the image must be 1 50 times larger in order to produce any impression ; and it is in consequence of the image of balloon or bird not having the requisite size to give any impression to the less sen- sitive portion of the retina, that we grope blindly, as it were, until by chance the image falls near the yellow spot, when the tentative motion of the eye- balls is instantly arrested, and the image fixed. On the other hand, the field of indistinct vision which the eye takes in is extremely wide, for bright objects are thus perceived, even when their direction forms an angle laterally of nearly 90 with the axis of the eye ; and, if the object be not only bright, but in motion, its presence is noticed under such circumstances with still greater ease. Thus, an observer scanning the heavens would have a per- ception of a shooting star anywhere within nearly half the hemisphere. The range is, however, less than 90 in a vertical direction. We have said that the fibres of the optic nerve, entering the back part of the eyeball, at K, Fig. 234, ramify over the anterior surface of the retina in fibres which form a layer of considerable relative thickness. The light, therefore, first encounters these nerves, and only after traversing their transparent substance does it reach the deeper seated layer of rods and tones, where it excites some action that is capable of stimulating the optic SIGHT. nerve. These rods and cones might naturally be supposed to be merely accessory to the fibres of the optic nerve, had we not the following conclu- sive evidence that the cones play a necessary part in the action, and that it is only through them that light acts upon the optic nerve : 1. The cones are more developed and more numerous in the spot where vision is most distinct 2. The " blind spot " is full of fibres of the optic nerve, but is absolutely insen- sible to light, and is without rods or cones. 3. We can distinguish an image on the fovea, having only guWh of an inch dia- meter ; but on the other parts of the retina the images must have larger dimensions. It is found that the size of the smallest distinguishable images agrees nearly with the diameters of the cones at the respec- tive parts. To some readers the fact will doubt- less be new, that a considerable por- tion of the eye is quite insensible to light, namely, that portion already de- signated as the " blind spot." A simple experiment, made by help of Fig. 238, will prove this. Place the book so that the length of the figure may be parallel to the line joining the eyes, and let the right eye be exactly opposite the white cross, and at a distance from it of about 1 1 in. If the left eye be now closed, while with the right the cross is steadily viewed so that it is always clear and distinct, the white circle will completely disappear, and the ground will appear of a uniform black colour. In order to insure success, the observer must be careful not to look at the white circle, but at the cross, and some persons find this more difficult than others. The position of the blind spot in the eye has been already mentioned, and its significance in showing the insensi- bility to light of the fibres of the optic nerve has been pointed out. In the table of the dimensions of some parts of the eye, which, for convenience of reference, is given together below, it will be seen that the diameter of the blind spot is consider- able compared with the size of the retina, its greatest diameter being about =, D in. The length on the retina of the image of a man at a distance of 6 ft. or 7 ft. is not greater than this, so that in a certain position with regard to the eye a person would, like the white circle, be quite invisible. In like manner, by looking steadily in a certain direction with one eye, the image of the full moon maybe made to fall upon the blind spot, and the luminary then FlG. 238. 342 SIGHT. becomes invisible, and would be so even if its apparent diameter were eleven times greater ; so that if we suppose eleven full moons ranged in a line, the whole would be quite invisible to a person looking towards a cer- tain point of the sky at no great angular distance from them. The following are the dimensions in English inches of some parts of the eye: In. Diameter of the entrance of the optic nerve 0*08 Distance of centre of optic nerve from centre of yellow spot 0-138 Diameter Qifovea centralis '. 0*008 Diameter of the nerve-cells of the retina 0*0005 Diameter of the nuclei o f oooc>3 Diameter of the rods o'oooo4 Diameter of the cones in yellow spot 0*00018 Length of rods 0*0016 Length of cones in yellow spot 0*0008 Thickness of retina at the back of the eye 0*0058 By means of an instrument to be presently described, the ophthalmoscope, it is possible to view directly the whole surface of the retina, and to observe the inverted images of the objects there depicted. It is thus observed that it is only on the parts near the yellow spot that the images are formed with clear and sharp definition. Away from this the definition is less perfect ; and besides the diminished sensitiveness of the retina, this circumstance contributes to the vagueness of the visual picture, although the falling off in clearness of vision at a very little distance from the yellow spot is far more marked than the loss of definition in the image there formed. Until within the last few years it has been most confidently asserted by many authors that the eye, considered as an optical instrument, is abso- lutely perfect, and entirely free from certain defects to which artificial in- struments are liable. Thus Dr. W. B. Carpenter states, in his " Animal Physiology" (1859) : "The eye is much more remarkable for its perfection as an optical instrument than we might be led to suppose from the cursory view we have hitherto taken of its functions ; for, by the peculiarities of its construction, certain faults and defects are avoided, to which all ordinary optical instruments are liable." Among the imperfections which are com- pletely corrected in the eye, he names u spherical aberration " and " chro- matic aberration" both of which give rise to certain defects in optical instruments. But by recent careful investigations it has been conclusively shown that the eye is not free from chromatic aberration ; that it has defects analogous to spherical aberration ; and that there are, besides, certain optical imperfections in its structure, which are avoided in the artificial instruments. Professor Helmholtz, one of the most distinguished of German mathematicians, physicists, and physiologists, whose great work on " Physiological Optics " is the most complete treatise on the subject which has ever appeared, is so far from considering the eye as possessed of all optical perfections that he remarks that, should an optician send him an instrument having like optical defects, he would feel justified in sending it back. The defects which may be traced in the eye, considered as an optical instrument, do not, however, he admits, detract from the excellence of the eye considered as the organ of vision. When we find that Sir Isaac Newton pointed out the chromatic aberra- SIGHT. 343 tion of the eye two centuries ago when we find that D'Alembert, in 1767, proved that the lenses of the eye might have as great a dispersive power as glass without the want of achromatism necessarily becoming noticeable when we find that the celebrated optician Dolland, the inventor of the achromatic lens, showed that the refractions which take place in the eye all tend to bring the violet rays towards the axis more than the red when we find that Maskelyne the astronomer, Wollaston the physicist, Fraun- hofer the optician, and other scarcely less distinguished men of science,, have made actual measurements of the distances of the fact in the human eye for the different rays of the spectrum when we find how these defects have so long ago been observed, examined, and measured as to their amount the persistence with which writer after writer has asserted the achroma- tism of the human eye appears so extraordinary, that it can only be accounted for by the prevalence of the preconceived notion that the eye is absolutely perfect a notion not without its reason and grounds, in the fact of the exquisite adaptation of the organ of sight to the needs of humanity. Although the want of achromatism in the eye thus escapes ordinary- notice, it is, on the other hand, easy to render it evident by simple experi- ments. If, for example, we view from a certain distance the solar spectrum projected on a white screen, it will be found that, when we see the red end quite distinctly, the violet end will, at the same time, appear vague and confused, and vice versa. The author believes that the following very simple experiment will at once convince any person that the fact is as stated. Procure a small piece of blue or violet stained glass, and another piece of red glass, and, having cut out of an opaque screen a rectangular opening, say | in. long and \ in. wide, place the glasses close to it, so that one-half the opening is covered by the red glass and the other half by the violet glass, the two being placed so that, on looking through the screen, a violet square and a red square are visible. The opaque screen may be made of black paper, cardboard, or tinfoil, and the edges of the opening must be cut perfectly even. On looking through this arrangement, held at a distance of about two feet from the eye, both squares may be seen dis- tinctly by a person of ordinary vision ; but, at a distance of five inches from the eye, he will find it impossible to see the squares otherwise than with vague and ill-defined edges. This is because the crystalline lens cannot adapt its curvature so as to bring the rays from the object to a focus on the retina. Now, by trial, the nearest distance at which each of the coloured squares becomes visible may be found, and it will be observed, that the violet square is first sharply defined at a less distance than the red, where- as, if the eye brought the red and violet rays to a focus at the same point, the smallest distance of distinct vision would coincide in both cases. The reader may observe the same fact for himself, in even a still simpler manner, by turning to Fig. 238, page 341. When the white circle is viewed by one eye, at a distance of about a foot, and an opaque screen, such as a coin, is held close to the eye, so that the pupil is half covered by it, the one side of the white circle will appear bordered by a narrow fringe of blue, and the other side by a narrow fringe of orange. If the opaque screen be shifted from one side of the pupil to the other, the colours will change places, the orange appearing always on the same side of the white circle as the screen is held before the eye. The same appearances are presented in a still more marked degree when the full moon is made the subject of the experiment. The diagram, Fig. 239, shows the course of the red and violet rays from 344 SIGHT. aluminous point, A, the refraction being supposed to take place at E l B ? . The violet rays after refraction form the cone, B lt E, Bo, and E is their focus ; the red rays form the cone, B 1? F, B 2 , and have a focus at F. The posi- tion of the retina would be intermediate between E and F, and is indicated by c x , C 2 . It will be noticed that the violet rays cross, and are received on the retina in the same circle, G G, so tha*- the colours, then blended, would be separately imperceptible ; but the point would produce a diffused cir- cular image of the blended colours. In viewing an object the moon, for example the accommodation of the eye is like that indicated in the diagram. The distinct image due to the red rays would be formed behind the retina, and that due to the violet rays would be in front of it. In the image on the retina the most intense rays such as the orange, yellow, and green are those which are blended by the adjustment of the eye, and the red and violet form images more out of focus (to use a common expression), and a very little larger than the more intense image. We might expect that a white disc would therefore appear with a fringe of colour, resulting from a mixture of red and violet ; but the fringe is too narrow, and the colour itself too feeble, to become perceptible. When, however, the pupil of the eye is half covered, the red and violet images are displaced in different directions, the position of the retina being too far forward for the one, and too far back for the other. The coincidence therefore ceasing, the colours show themselves at the margins of the image. The non-perception under ordinary circumstances of the chromatic aber- ration of the eye is largely due to the greater intensity of the colours which differ least in their refrangibilities. The clearness of our vision does not, therefore, practically suffer from this defect of the eye. Professor Helm- holtz constructed lenses which rendered his eyes really achromatic, and looking through these when the pupil was half covered, no coloured fringes were seen at the edges of dark or light objects, or when the objects were looked at with an imperfect accommodation of the eye. He was, however, unable to detect any increase of clearness or distinctness of vision by the correction. The eye is also subject to other aberrations and irregular refractions, which are special to itself; for example, with moderately illuminated objects the crystalline lens produces images apparently well defined, and nothing is visible to suggest the absence of uniformity in its structure. But when the light is intense, and concentrated in a small object surrounded by a dark field, the irregular structure of the crystalline lens shows itself in the most marked manner. Eveiy one must have noticed the appearance presented by the distant street-lamps on a dark night, and by the stars. The latter SIGHT. 345 we know to be for us mere points of light, and their images produced by perfect lenses would also be mere points ; instead of which we see what seem to be rays issuing from the star, an appearance which has given rise to the ordinary representation of a star as a figure having several rays. That no such rays actually do emanate from the real star may be easily proved: first, by concealing the luminous point from view, by means of a small object held up as a screen. If the rays had any existence outside of the eye, they would still be seen ; instead of which, the whole of them dis- appear when the luminous point, or, in the case of the street-lamp, when the flame, is covered by the screen. A second proof that the origin of the phenomenon is in the eye, and not in the object, is afforded by the fact that if, while attentively observing the rays, we incline the head, the rays turn with the eyes, so that when the head is resting on the shoulder the ray which appeared vertical becomes horizontal. The cause of these divergences from the regular image lies in the fact of the crystalline lens being built Up of fibres which have refrac- tive powers somewhat different from that of the intermediate sub- stance. These fibres are arranged in layers parallel to the surfaces of the crystalline lens, and the di- rection of the fibres in each layer is generally from the centre to the tircumference ; but towards the axis they form, by bending, a kind of six-rayed figure, as shown in Fig. 240, which represents the FlG. 240. arrangement of the fibres of the external layers of the lens. In the outermost layers the branches of the star-shaped figure are subdivided into secondary branches, which give rise to more complicated figures. When we view by night a very brilliant but small light, even these subdivisions may be traced in the radiating figure. The light which enters the eye is partly absorbed by the black pigment of the choroid, and partly sent back by diffused reflection from the retina through the crystalline lens and pupil. The image of a luminous body as depicted on the retina of another person cannot be seen by us under ordinary circumstances, because, by the principle of reversibility already mentioned as of universal application in optics, the rays which issue from the retinal images are refracted on leaving the eye, and follow the same paths by Which they entered it, so that they are sent back to the object. An ob- server cannot see the retinal image of a candle in another person's eye, unless he allows the rays to enter his own, and this cannot be done directly, because the head of the observer would be interposed between the candle and the eye observed, and the light would then be intercepted. By hold- ing a piece of unsilvered plate glass vertically, we may reflect the light of a candle into the eye of another person, and then the light thrown out from the retinal image of the candle will, on again meeting the surface of the glass, be in part reflected to its source, and in part pass through the glass, on the other side of which it may be received into the eye of an 346 SIGHT. observer. The positions of the observed and observing eye may be de- scribed as exactly opposite to and near each other, while the candle is placed to one side in the plane separating the two ayes, and the glass is held so that it forms an angle of 45 with the line Jo inm g the P u P lls - Und er these circumstances the observer may see the light at the back of the eye, but he will not be able to distinguish anything clearly, because his own eye cannot accommodate itself so as to bring to a focus the rays coming from the retina of the other, since these rays are refracted by the media through which they emerge. But, by means of suitable lenses interposed between the two eyes, the retina and all its details may be distinctly seen and examined. Such an arrangement of lenses and a reflecting surface con- FlG. 241. Ruefe's Ophthalmoscope. stitute the instrument called the ophthalmoscope (o^daX^os, the eye) of which there are many forms, but all constructed on the principle just indicated. This principle was first pointed out by Helmholtz, who described the first ophthalmoscope in 1851. Ruete's ophthalmoscope is represented in Fig. 241. The parts of the in- strument are supported on a stand, c, and about the vertical axis of this the column, D, and the arms, H and K, can turn freely and independently ; E is a concave metallic mirror, about 3 in. in diameter, and having an aperture in its centre through which the observer, B, looks. The arm, H, merely carries a black opaque screen, which serves to shield the eye of B from the light of the lamp, and to reduce, if required, the amount of light passing through the aperture in the mirror. The arm, K, which is about a foot in length, carries two uprights which slide along it, and in each of these slides a rod bearing a lens, which can thus be adjusted into any required position. The instrument is used in an apartment where all light but that of the lamp can be excluded. In the instrument just described an inverted image is obtained, which is sufficient for ordinary medical purposes, but this construction does SIGHT. 347 not allow of the examination of retinal images, which is best performed with an instrument having a plane mirror. The appearance presented by the back of the eye when viewed in the ophthalmoscope is represented in Fig. 242. The retina appears red, except at the place where the optic nerve enters, which is white. On the reddish ground the retinal blood-vessels can be distinguished ; A, A, A, branches of the retinal artery, have a brighter red colour, and more strongly reflect the light than the branches, B, B, B, of the retinal vein. Among these, and especially towards the margin, are seen, more or less distinctly, the broader vessels of the choroid. Above the optic nerve and a little to the right may be observed the fovea centralis. FIG. 242. During the last twenty years the ophthalmoscope has been the chief means of extending the knowledge of oculists regarding the diseased and healthy conditions of the eye. In this way the substance of the lens and the state of the humours can be directly seen, the causes of impaired vision can be discovered, and the nature of many maladies made out with certainty. This modern invention, by which the interesting spectacle of the interior of the living eye can be observed, has therefore been far from proving a barren triumph of science. Many insidious maladies can thus be detected, and may be successfully treated before the organ has become hopelessly diseased. In some cases the ophthalmoscope gives the most certain evidence of the existence of obscure and unsuspected diseases of other parts of the body. 34 8 SIGHT. VISUAL IMPRESSIONS. EVERYBODY knows that, however well the flat picture of an object may imitate the colours and forms of nature, we are never deceived into supposing that we have the real object before us. There must, therefore, be something different in the conditions under which we see real objects from those under which we view their pictures. The most favourable circum- stances for receiving an illusive impression of solidity from a flat picture, is when we view it from a fixed position and with one eye. This is because one means by which we unconsciously estimate distances depends upon the changes in the perspective appearances of objects caused by changes in our point of view. In many cases these changes in the perspective are the only means we have of judging of the relative distances of objects. But there is another circumstance which is still more intimately connected with our perception of solidity. Each eye receives a slightly different image of the objects before us (unless these be extremely remote), inasmuch as they are viewed from a different point. When the objects are very near, the two retinal images may differ considerably, as the reader may convince himself by viewing with each eye, alternately, objects immediately before him, while the other eye is closed, and the head all the while motionless. The nearer objects will plainly appear to shift their positions as seen against the background of the more distant objects ; and a somewhat more care- ful observation will reveal changes of perspective, or apparent form, in every one of these objects. An extreme case is presented in that of a play- ing card, or ihin book, held in the plane which divides the eyes. The back or the face, the one side or the other, will be seen, according as the right or the left eye is opened. If we close the left eye, the displacement and change of apparent form produced by a slight movement of the head are sufficiently obvious ; a movement of the head T\ in. to the left causes a decided change in the relative positions of adjacent objects. It is plain, however, that it is precisely from a point 2| in. to the left that the left eye views these objects, and hence the perspective appearance seen by the left eye must have the difference due to this shifting of the point of view. On the other hand, if one looks at a picture, or flat surface, placed imme- diately in front, no change in the relative positions of its parts is discernible by viewing it with either eye alternately. Not but that there is a difference in the retinal images in the two cases, but there is an absence of any point of comparison by which the change may be judged. If we take a photo- graph of a statue, it will, when viewed by one or the other eye, present the difference of the retinal images which is due to a flat surface ; the parts of the photographic image will be of slightly different proportions as seen by each eye. If, instead of the photograph we have before our eyes a statuette, each eye will see a quite different view : the right eye will see a portion which is invisible to the left eye, and vice versd, and, in fact, we shall see more than half round the object. Here, then, we have certain differences of the retinal pictures when solid objects are viewed, and these differences by innumerable repetitions have, unconsciously to ourselves, become asso- ciated with notions of solidity, of something having length, breadth, and depth, or thickness. The marvellous delicacy of these perceptions will be alluded to hereafter. SIGHT. 349 Let us suppose that the lenses of two cameras are fixed in the positions occupied by the two eyes, and that a photograph is taken in each camera, the subject being, for example, a statuette. It is obvious that the diffe- rences of the two photographs would correspond with the differences of the two retinal images, and that, if a person could view with the right eye only the photograph taken in the right-hand camera, and with the left eye the left-hand photograph only, there would be formed on the retinae of his eyes images very nearly corresponding with those which the actual object would produce, and the result would be, if these retinal pictures occupied the proper position on the eyes, that the impression of solidity would be pro- duced, which is called the stereoscopic effect. This may be done without the aid of any instrument, as almost any per- son may discover after some trials with nothing but a stereoscopic slide, if he can succeed in maintaining the optic axis of his eyes quite parallel. In such a case he will observe the stereoscopic effect by the fusing together, as it were, into one sensation, of the impression received by the right eye from the right photograph, with that received by the left eye from the left photo- graph. But as each eye will, at the same time, have the photograph in- tended for the other in the field of view, the observer will be conscious of a non-stereoscopic image on each side of the central stereoscopic one. FIG. 243. WheatstonJs Reflecting Stereoscope. These outside images are, however, very distracting, for the moment the attention is in the least directed to them, the optic axes converge to the one side or the other, losing their parallelism, and the stereoscopic effect vanishes, because the images no longer fall in the usual positions on the retinae. It is, in consequence, only after some practice that one succeeds in readily viewing stereoscopic slides in this manner, but the acquirement is a convenient one when a person has rapidly to inspect a number of such slides, for he can see them stereoscopically without putting them in the instrument. Many persons, however, find great difficulty in acquiring this power. In such cases it is well to begin by separating the two photographs by means of a piece of cardboard, covered with black paper on both sides. When this is held in the plane between the eyes, each eye sees only its own photograph, and the observer is not troubled with the two exterior images. After a little practice in this way, the cardboard may usually be dispensed with, and the observer will insensibly have acquired the habit of viewing the slides stereoscopically, without any aid whatever. Instruments have, however, been contrived which enable one to obtain the desired result without effort ; and one form of these is now tolerably 35 SIGHT. well known to everybody. The first stereoscope was the invention of Wheatstone. The reflecting stereoscope is represented in Fig. 243, and consists essentially of two plane metallic mirrors inclined to the front of the instrument at angles of 45, so that in each of them the observer sees only the design which belongs to it. The rays reach the eyes as if they came from images placed in front of the observer ; and the two images having the proper differences, produce together the impression of solid objects. Brewster's stereoscope -which is far more widely known than Wheat- stone's has two acute prisms, or, more usually, two portions of a convex lens are cut out, and placed with their margins or thin parts inwards, and they thus produce the same effect as would be obtained by combinations of a prism with a convex lens. Another very common form of the stereo- scope has merely two convex lenses. The effect of the convex lenses is to increase the apparent size of the images by diminishing the divergence of the rays emitted by each point, producing the appearance of larger designs seen at a greater distance. The effect of the prism is to give the rays the direction which they would have if they proceeded from an object placed in a position immediately be- tween the two designs, and an additional ele- ment by which we estimate distance, namely, the convergence of the optic axes, is made to aid in the illusion, when the rays proceeding from the two different pictures have approxi- mately the inclination that they would have if they emanated from real objects at the place where the image is apparently formed. The box or case in which the lenses or lenticular prisms are placed takes various forms. One of the most common is represented in Fig. 244, but the stand on which it is mounted is not a necessary part of the instrument, although it is sometimes convenient. A handsome form is met with as a square case, enclosing a number of photographic stereoscopic views mounted on an endless chain in such a manner that they are brought successively into view by turning a knob on the outside. When an instrument of this kind is fitted up with a series of the beautiful landscape transparencies, which are produced by certain continental pho- tographers, a more perfect reproduction of the impressions derived from nature, exclusive of colour, cannot be conceived. We seem to be present on the very spots which are so truthfully depicted by the subtile pencil of the sunbeam ; we feel that we have but to advance a foot in order to mix with the passengers in the streets of Paris or of Rome, and that a single step will bring us on the mountain -side, or place us on the slippery glacier ; at our own fireside we can feel the forty centuries looking down upon us from the heights of those grand Egyptian pyramids, and find our- selves bodily confronted with the mysterious Sphinx, still asking the solu- tion of her enigma. The truth and force with which these stereoscopic photographs reproduce the relief of buildings are such, that when one sees for the first time the real edifice of which he has once examined the stereo- scopic images, it no longer strikes him as new or unknown ; for he derives FIG. 244. SIGHT. 351 from the actual scene no impression of form that he has not already received from the image. But of all subjects of stereoscopic photography the glaciers are, perhaps, those which best show the power of the instrument as far surpassing all other re- sources of graphic presen- tation. The most careful painting fails to convey a notion of the strange glimmer of light which fills the clefts of the ice, seen through the transparent substance itself. The simple photograph com- monly presents nothing but a confused mass of grey patches ; but com- bine in the stereoscope two such photographs, each formed of nothing but slightly different grey patches, and a surprising effect is at once produced : the masses of ice assume a palpable form, and the beautiful effects of light transmitted or reflected by the translucent solid reveal themselves. An- other very beautiful class of subjects for stereoscopic slides is found in those marvellous instantaneous photographs, which seize and fix the images of the waves as they dash upon the shore. Here a scene which has tasked the power of the greatest painter is brought home to us with such force and vividness that *we all but hear the wild uproar of the breakers. But for the art of pho- tography the stereoscope ia 2 45- would not thus be ready to minister to our enjoyment, for no pictures wrought by man's handiwork could approach the requisite accuracy which the two stereoscopic pictures must possess. All attempts to produce such pictures by engraving or litho- graphy have failed, except only in the case of linear geometrical designs, such as representations of crystals. A very useful and suggestive applica* 352 SIGHT. tion of the stereoscope has been made to the illustration of a treatise on solid geometry, where the lines representing the planes, being drawn in proper perspective, the reader by placing a simple stereoscope over the plates sees the planes stand out in relief before him, and the multitude of lines, angles, &c., which in a simple drawing might be distracting even for a practised geometrician, assume a clear and definite form. The difference between the two retinal pictures of objects is so slight, that when the objects are at a little distance, ordinary observation fails to discover it without the aid of special instruments ; and an inspection of the pair of photographs in a stereoscopic slide will convince any one that, even in these, close and careful observation is required to perceive the difference. Some of the principles of stereoscopic drawings may be seen exemplified by the pair we give in Fig. 245. With this figure the reader may attempt the experiment of seeing the stereoscopic effect without the stereoscope. When he has succeeded in doing this, or when he fuses the images together by placing a simple stereoscope over the page, he will find the result very singular ; for he will receive the impression of a solid crystal of seme dark polished substance black lead, for instance placed on a surface of the same material. The edges of the solid will appear to have a certain lustre, such as one sees on the edges of a real crystal. The reason of this impres- sion being produced by two drawings, one of which is formed by black lines on a white ground, while the other has white lines on a black ground, is probably due to the circumstance that we very often see in nature the lustrous edges of an object with one eye only. That is, one eye is in the path of the rays which are regularly reflected from the object, while the other is not, a fact which may be verified in an instant by looking first with one eye and then with the other, at a polished pencil, or similar object, when placed in a certain position. There is a kind of modification of the reflecting stereoscope, known under the name of the pseudoscope, which is highly instructive, as showing how much our notions of the solidity of objects are due to the differences of the retinal images. In the pseudoscope the rays reach the eyes after pass- ing through rectangular prisms in such a manner that objects on the right appear on the left, and objects on the left appear on the right ; but the images agree by reason of the symmetry of the reflection, although the image of the objects that without the instrument would be formed in the right eye is, by the action of the prisms, formed in the left eye, and vice versa. The impressions produced are very curious : convex bodies appear concave a coin, for example, seems to have the image hollowed out, a pencil appears a cylindrical cavity, a globe seems a concave hemisphere, and objects near at hand appear distant, and so on. These illusions are, however, easily dispelled by any circumstance which brings before the mind our knowledge of the actual forms, and by a mental effort it is pos- sible to perceive the actual forms even with the pseudoscope, and indeed to revert alternately, with the same object, from convexity to concavity. This last effect is very curious, for the object appears to abruptly change its form, becoming alternately hollow and projecting, according as the mind dwells upon the one notion or the other ; but the experiment is attended with a feeling of effort, which is very fatiguing to the eyes. Professor Helmholtz has contrived another very curious instrument, depending on the same principles as the stereoscope. He terms it the telestereo scope, and while the effect of the pseudoscope is to reverse the relief of objects, the telestereoscope merely exaggerates this relief ; hence SIGHT. 353 this instrument is well adapted for making those objects which from their distance present no stereoscopic effect, stand out in relief. The distance between our eyes is not sufficiently great to give us sensibly different views of very distant objects, and what the telestereoscope does is virtually to sepa- rate our eyes to a greater distance. Fig. 246 is a horizontal section of the instrument. L and R represent the position of the eyes of the spectator ; a, d, are two plane mirrors at 45 to his line of sight ; A, B, are two larger plane mirrors, respectively nearly parallel to the former. cda'L and/> R show the paths of rays from distant objects, and it is obvious that the right eye will obtain a view of the objects identical with that which would be presented to an eye at R', while the left eye has similarly the picture of the FIG. 246. 7 he Telestereoscope. objects as seen from the point I/. The four mirrors are mounted in a box, and means are provided for adjusting the positions of the larger mirrors, as may be required. With this instrument the distant objects in a land- scapea range of mountains, for example which present to the naked eye little or no appearance of relief, have their projections and hollows revealed in the most curious manner. It is upon a similiar principle that stereoscopic views of some of the celestial bodies have been obtained. Admirable stereoscopic slides of the moon have been produced by photographing her at different times, when the illumination of the surface is the same, but when, in consequence of her libration, somewhat different views of our satellite are presented to us. Two such photographs, properly combined in the stereoscope, give not only the spherical form in full relief, but all the details of the surface : the mountains, craters, valleys, and plains are seen in their true relative projection. The telestereoscope may be inverted, so to speak, and its effect reversed; for an arrangement of mirrors similarly disposed, but on such a scale as will permit the eyes to be respectively in the lines c d and/^-, would reflect from objects in the direction L R rays which would have but little of the difference of direction to which the stereoscopic effect is due. Hence solid objects viewed with such an instrument appear exactly like flat pictures, the effect being far more marked than in simply viewing them with one eye. An ingenious method of exhibiting a stereoscopic effect to an audience 23 354 SIGHT. has been contrived by Rollmann. He draws on a black ground two linear stereoscopic designs that for the left eye with red lines, that for the right eye with blue. Each individual in the audience is provided with a piece of blue glass and a piece of red : he places the red glass before the left eye, the blue glass before the right : each eye thus receives only the picture intended for it, for the blue lines cannot be seen through the red glass, or the red lines through the blue glass. The diagrams may, of course, be projected on a screen by a magic lantern, in which case the circumstances are even more favourable. Duboscq has arranged a kind of opera-glass, so that a person may view appropriate designs on the large scale, and arrangements have been also contrived by which the stereoscopic effect may be seen in moving figures. Every student of this interesting subject should examine a few stereo- scopic images produced by simple lines representing geometrical figures, or the photographs of the model of a crystal, as these exhibit in the most striking manner the conditions requisite for the production of stereoscopic effects. A person having a little skill in perspective and geometry might construct the two stereoscopic images of a body defined by straight lines, but the drawings must be executed with extreme exactitude, for the least deviation would produce the most marked effect in the stereoscopic appear- ance. The production of stereoscopic photographs now forms a'consider- able branch of* industrial art. At first, these photographs were made by taking the two different views with the same camera at two operations. But there were difficulties in obtaining uniformity of depth in the impres- sions, and the change in the shadows produced by the earth's rotation showed itself although the interval between the two exposures might not exceed thre or four minutes. The increased shadows in such cases show themselves in the stereoscope, like dark screens suspended in the air. It was Sir David Brewster who, in 1849, first proposed the plan now univer- sally adopted, of producing the views simultaneously by twin cameras form- ing their images on different parts of the same sensitive plate, the centres of the lenses being placed at the same distance apart as a man's eyes, that is, from i\ to 3 in. This is, of course, the only manner in which instan- taneous views can be secured. Helmholtz, however, advocates the photo- graphs of remote objects being taken at a much greater distance apart, for they otherwise present little appearance of relief. By selecting from an assortment of slides, two views of the Wetterhorn, taken from different points in the Grindelwald valley, and combining these in the stereoscope, he found that a far more distinct idea of the modelling of the mountain could be thus obtained than even a spectator of the actual scene would receive by viewing the mountain from any one point. Such a mode of com- bining the photographs would produce in the stereoscope the same effect as the telestereoscope would in the landscape, but the effect would be caused to a proportionately far higher degree. The date of Wheatstone's first publication regarding the stereoscope was J 833 ; but a complete description and theory of the instrument was not published until five years afterwards. Brewster first made public, in 1843, his invention of the stereoscope with lenses, which is now sc familiar to us, and few scientific instruments have become so quickly and extensively popular ; certainly no other simple and inexpensive instrument has con- tributed so largely to the amusement and instruction of our domestic circles. And, to the philosopher who studies the nature of our perceptions, the stereoscope has been even more instructive, for, instead of vague surmises, SIGHT. 355 it provided him with the solid ground of experiment on which to found his theories. The literature of this one subject stereoscopic effect is exten- sive enough to occupy a tolerably long book-shelf. It dates from 300 B.C., when Euclid touched upon the subject in his Optics ; and after a lapse o< more than eighteen centuries it was taken up by Baptista Porta, in 1583 ; but the whole development of this subject belongs almost entirely to the last half-century. The part which the muscles of the eyes take in our perceptions of form has been already alluded to, and it may be interesting to illustrate this point by a curious example or two of illusions arising from their move- ments. If our reader will glance at Fig. 247, he will see that the lines, a b and c d, appear to be farther apart towards the centre than at the ends, while fg and h z, on the other hand, appear nearest together in the middle. He will hardly be convinced that in each case the lines are quite parallel FIG. 247. until he has actually measured the distances. A still more striking example of the same kind of illusion is shown by Fig. 248, due to Zollner. This appears a sort of pattern, in which the broad bands are not upright, but sloping alternately to the right and left, and with the spaces between the lines wider at one end than the other. The lines in the figure are, how- ever, strictly parallel. The illusion by which they appear divergent and convergent is still more strongly felt when the book is held so that the wider bands are inclined at an angle of 45 to the horizon. There is another illusion here with reference to the short lines, which will appear to be oppo- site to the white spaces on the other side of the long lines to which they are attached. That these illusions are really due to movements of the eyes may be proved by viewing the designs in any manner which entirely pre- vents the movement, as by fixing the gaze on one spot in the case of Fig. 247, when the illusion will vanish ; but this, plan is not so easily applied to Fig. 248. A convincing proof, however, will be found in the appearance of these figures when they are viewed by the instantaneous light of the electric spark, as when a Leyden jar is discharged in a dark room. The reader viewing the figures, held near the place where the spark appears, 232 356 SIGHT. FIG. 248. will see them distinctly without the illusions as to the non-parallelism of the Jines. In the absence of an electrical machine, or coil and jar, the reader may have an opportunity of seeing the figures by flashes of lightning at night, when the result will be the same. There is a property of the eye which has led to the production of many amus- ing and curious illusions. This property in itself is no new discovery, for its pre- sence and effects must have been noticed ages ago. The property in question is illustrated when we twirl round a stick or cord, burning with a red glow at the end. We seem to trace a circle of fire; but as the glowing spark cannot be in more than one point of the circle at once, it is plain that the impression produced on the eye must remain until the spark has completed its journey round the circle, and reaching each point succes- sively renews the luminous impression. Like other subjects relating to vision, this phenomenon has been carefully ex- amined in recent times, and its laws accurately determined. The fact which is obvious from such an experiment, may be thus stated: Visual impressions repeated with sufficient rapidity produce the effect of objects continually present. This persistence of the visual impressions is easily made the subject of experiment by means of rapidly rotating discs ; and in the common toy called a " colour top " we have a ready means of verifying some of the conclusions of science on this subject. Some very interesting results may be> obtained by an apparatus as simple as this, re- garding the laws of the phenomenon we are considering, and the effects of various mixtures of tints and colours. The well-known toy, the thaumatrope, depends on the same principle. In this a piece of cardboard is painted on one side, with a bird, for example, and on the other side with a cage : when the cardboard is twirled round very rapidly by means of a cord fixed at opposite points of its length, both bird and cage become visible at once, and the bird appears in the cage. A still more ingenious application of this principle we owe to Plateau, who described it in 1833, under the title of FIG. 249. \\\ephenakistiscope; and also to Stamp- fer,who independent lently devised the same arrangement about the same time, and named it the stroboscopic disc. The teader may, at almost any toy-shop, purchase one of them, provided with a number of amusing figures ; or he may easily construct for himself one which will exemplify the principle. He requires no other materials than SIGHT. 357 a piece of cardboard, and his only tools may be a sharp penknife, a pair of compasses, and a flat ruler. Let him draw on his cardboard a circle of 8 in. diameter, and divide its circumference by eight equidistant points. From, these radii should be drawn with the point of the compasses, and equal distances from the centre marked off upon them, to fix the centres of the small circles, which must all have exactly the same size (say, I in. in diameter) and be marked by a distinct line. In these are to be marked the hand of a clock-face in the positions shown in Fig. 249 ; and finally, in the direction of the radii, narrow slips are to be cut out of the cardboard as shown. If a pin be put through the centre of the disc, attaching it thus to the flat end of a cork, so that it can freely rotate in its own plane,, and the disc be turned rapidly round, as in Fig. 250, in front of a looking- glass, while the spectator looks through the slits, he will see the hand on the little dial ap- parently turning round, with rather a jerky movement it is true, somewhat like the dead- beat seconds-hand that is sometimes seen on clocks. The illusion is best when the slits are so narrow that only one of the several images is visible by reflection, namely, that which is adjacent to the slit. Thus, as the disc rotates, each little circle is visible for an instant as the slit passes in front of the spectator's eye ; and if the rotation be sufficiently rapid, the impression of the disc is permanent, as it is constantly being renewed by the successive circles, while, on the contrary, the hands, having different positions, pro- FIG. 250. FIG. 251. tfoce images in different positions, giving the appearance of a jerky rotation. The instruments sold in the shops have sometimes a thin metallic disc with the slits in it, and a series of designs printed in smaller paper discs. The paper discs may be screwed on the other disc as required, and a button on a pulley with an endless band is provided for producing the 358 SIGHT. rotation more conveniently. Fig. 251 shows one of the pictures for a disc with twelve slits, and the effect produced by it is that of a dancing figure. Another arrangement for showing the same illusion has lately become a very popular toy, and quite deservedly so, for it has the advantages of re- quiring no looking-glass, and of making the effect visible to a number of persons at the same time. This apparatus, which has been termed the Zoetrope, consists simply of a cylindrical box, like a drum with the upper end cut off. It is mounted on a pivot, which permits its revolving rapidly about its vertical axis when touched by the finger. The cylinder has a number of equidistant vertical slits round the upper part of its circumference. The figures which produce the illusion are printed on a slip of paper, which is placed in the lower part of the drum, and when this is in rapid rotation, and the figures are viewed through the slits, the illusion is produced in exactly the same manner as in the revolving disc. FIG. 252. Portrait of Sir W. Thompson. ELECTRICITY. ABOUT forty years ago a popular book was published having for its theme the advantages which would flow from the general diffusion of scientific knowledge. Great prominence was, of course, given to the utility of science in its direct application to useful arts, and many scientific inventions conducing to the general well-being of society were duly enume- rated. Under the head of electricity, however, the writer of that book men- tioned but few cases in which this mysterious agent aided in the accom- plishment of any useful end. The meagre list he gives of the instances in which he says " even electricity and galvanism might be rendered subser- vient to the operations of art," comprises only orreries and models of corn- mills and pumps turned by electricity, the designed splitting of a stone by lightning, and the suggestion of Davy that the upper sheathing of ships should be fastened with copper instead of iron nails, with a hint that the same principle might be extended in its application. At the present day the applications of electricity are so numerous and important, that even a brief account of them would more than fill the present volume. Electricity is the moving power of the most remarkable and distinguishing invention of the age the telegraph ; it is the energy employed for ingeniously mea- suring small intervals of time in chronoscopes, for controlling time-pieces, 359 360 ELECTRICITY. and for firing mines and torpedoes ; it is the handmaid of art in electro- plating and in the reproduction of engraved plates, blocks, letterpress, and metal work ; it is the familiar spirit invoked by the chemist to effect mar- vellous transformations, combinations, and decompositions ; it is a thera- Deutic agent of the greatest value in the hands of the skilful physician, .a'uch an extension of the practical applications of electricity as we have indicated implies a corresponding development of the science itself ; and, indeed, the history of electricity during the present century is a continuous record of brilliant discoveries made by men of rare and commanding genius such as Davy, Ampere, and Faraday. To give a complete account of these discoveries would be to write a treatise on the science ; and although the subject is extremely attractive, we must pass over many discoveries which have a high scientific interest, and present to the reader so much of this recently developed science as will enable him to comprehend the principles of a few of its more striking applications. The science of electricity presents some features which mark it with special characters as distinguished from other branches of knowledge. In mechanics and pneumatics and acoustics we have little difficulty in pic- turing in our minds the nature of the actions which are concerned in the phenomena. We can also extend ideas derived from ordinary experience to embrace the more recondite operations to which heat and light may be due, and, by conceptions of vibrating particles and undulatory ether, obtain a mental grasp of these subtile agents. But with regard to electricity no such conceptions have yet been framed no hypothesis has yet been ad- vanced which satisfactorily explains the inner nature of electrical action, or gives us a mental picture of any pulsations, rotations, or other motions of particles, material or ethereal, that may represent all the phenomena. Incapable as we are of framing a distinct conception of the real nature of electricity, there are few natural agents with whose ways we are so well acquainted as electricity. The laws of its action are as well known as those of gravitation, and they are far better known than those which govern chemical phenomena or the still more complex processes of organic life. Definite as are the laws of electricity, there is no branch of natural or physical science on which the ideas of people in general are so vague. Spectators of the effects of this wonderful energy as seen violently and destructively in the thunderstorm, and silently and harmlessly in the Aurora knowing vaguely something of its powers in traversing the densest mate- rials, in giving convulsive shocks, and in affecting substances of all kinds the multitude regard electricity with a certain awe, and are always ready to attribute to its agency any effect which appears mysterious or inex- plicable. The popular ignorance on this subject is largely taken advantage of by impostors and charlatans of every kind. Electric and magnetic nostrums of every form, electric elixirs, galvanic hair-washes, magnetized flannels, polarized tooth-brushes, and voltaic nightcaps appear to find a ready sale, which speaks unmistakably of the less than half-knowledge which is possessed by the public concerning even the elements of electrical science. ^ Electricity has also a special position with regard to its intimate connec- tion with almost every other form of natural energy. Evolved by mechani- cal actions, by heat, by movements of magnets, and by chemical actions, it is capable in its turn of reproducing any of these. It plays an important, but as yet an undefined, part in the physiological actions constantly going on in the organized body, and is, in fact, all-pervading in its influence over ELECTRICITY. 361 all matter, organic and inorganic a secret power strangely but universally concerned in all the operations of nature. We are compelled to regard electricity not as a kind of force acting upon otherwise inert matter, but rather as an affection or condition of which every kind of matter is capable, although we are still unable to form a conjecture of the precise nature of the action. We have now to address ourselves to the task of unfolding so much of the science as will enable the reader to understand the leading principles of such important applications as electro-plating, illumination, and the telegraph ; and this will necessarily include an account of the grand dis- covery of the identity, or at least intimate connection, of magnetism and electricity. ELEMENTARY PHENOMENA OF MAGNETISM AND ELECTRICITY. THE distinctive property of a magnet is, as everybody knows, to pieces of iron, and this property having been observed by the a: attract ic ancients in a certain ore of iron which was found near the city of Magnesia, in Asia Minor, the property itself came to be called Magnetism. A bar of steel, if rubbed with the natural magnet or loadstone, acquires the same property, and if the bar be suspended horizontally or poised on a pivot, it will settle only in one definite direction, which in this country is nearly north and south. If a narrow magnetized bar be plunged into iron filings, it will be found that these are attracted chiefly by the ends of the bar, and not at all by the centre. It appears as if the magnetic power were concentrated in the extremities of the bar, and these are termed its poles, the pole at the end of the bar which points to the north is called the north pole of the magnet, and the other is named the south pole. If a north pole of one magnet be presented to the north pole of another, they will repel each other, and the same repulsion will take place between the south poles, whereas the north pole of one magnet attracts the south pole of another. In other words, poles of the same name repel each other, but poles of opposite names attract each other, or still more concisely, like poles repel, ur.like poles attract each other. Magnetism acts through intervening non-magnetic matter with undi- minished energy. Thus, the attractions and repulsions of magnetic poles manifest themselves just as strongly when the poles are separated by a stratum of wood or stone as when merely air intervenes, and the attraction of small pieces of iron by a magnet takes place through the interposed palm of one's hand without diminution. A delicately suspended needle in even a remote apartment of a large building moves whenever a cart passes in the street. It is almost too well known to require mention here, that iron and steel are the only common substances which are capable of plainly exhibiting magnetic forces, and, indeed, there are no known substances capable of so powerful a magnetization as these. But the difference in the magnetic behaviour of iron and steel is not so well understood, and it is a point of importance for our subject, and connected with a fundamental law which governs all magnetic manifestations. A piece of pure iron is very readily cut with a file, whereas a piece of steel may be so hard that the file 362 ELECTRICITY. makes no impression upon it whatever ; and hence a piece of pure iron, or rather iron holding no carbon in combination, and possessed of no steely quality, is often spoken of as soft iron. When a piece of soft iron is placed near the pole of a magnet, the iron becomes, for the time, a magnet. If iron filings be sprinkled over it, they will arrange themselves about the parts of the iron respectively nearest and farthest from the magnet, thus showing that the piece of soft iron has acquired magnetic poles. It will be found on examining these poles that the one nearest the magnet is of the contrary name to the pole of the magnet, and the farthest is of the same name. The conversion of the soft iron into a magnet by the influence of a magnetic pole is termed induction. It need hardly be said that the inductive effect is more powerful in proportion to the shortness of the distance separating the piece of soft iron from the magnetic pole, and, of course, the effect is at its maximum when there is actual contact. Induction thus explains, by aid of the law of the poles, the attraction which a magnet exercises over pieces of iron, for it is plain that the inductive influence is accompanied by attraction between the two contiguous oppositely-named poles of the magnet, and of the piece of iron. But attraction is not the only force, for the pole developed at the farthest portion of the piece of iron being of the same name as the inducing pole, these will be mutually repulsive. The attractive force will, however, be more powerful on account of the shorter distance at which it is exerted, and will predominate over the repulsive force, particularly at short distances, because then the difference will be relatively greater. At distances from the inducing pole relatively great to the distance between the two poles of the piece of iron, the difference may be so small that its effect in attracting the piece of soft iron will be imper- ceptible, and then the piece of iroa acted on by two (nearly) equal parallel forces, will be subject to what is termed in mechanics a couple, the only effect of which is to turn the body into such a position that the opposing forces act along the same line. The definite direction assumed by a freely suspended needle may be explained by supposing that the earth itself is a magnet having a south pole in the northern hemisphere, and a north pole in the southern hemisphere, the line joining these poles being shorter than the axis of the earth, and not quite coinciding with it in position ; and the fact of the needle being turned round but not bodily attracted is then easily accounted for, the attractive and repulsive forces being reduced to a couple in the manner just explained. If the attempt be made to turn a piece of steel into a magnet, by the induction of a magnetic pole, the same results will be obtained as in the case of soft iron, but in a much feebler degree, and with this difference : the piece of steel does not lose its magnetism when the inducing magnet is withdrawn, whereas in the case of the soft iron every trace of mag- netism vanishes the instant the inducing pole is removed. And if the pole of the magnet be not only put in contact with one end of the piece of steel, but rubbed on it, the piece will acquire permanent and powerful magnetism. Hence it will be noticed that a piece of soft iron can by the mere approximation of a magnetic pole be converted in an instant into a magnet, and by the removal of the magnet can as instantly be deprived of its magnetism, and made to revert into its ordinary condition ; while steel is not so readily magnetized, but retains its magnetism permanently. The elementary phenomena of electricity are extremely simple and easy . of demonstration, and as the whole science rests upon inferences derived from these, the reader would do well to perform the following simple expe- ELECTRICITY. 363 riments for himself. Apparatus is represented in Fig. 253, but the only essential portion is a straw, B, suspended from any convenient support by a very fine filament of white silk. To one or both ends of the straw a little disc of gilt paper, or a small ball of elder-pith or of cork, should be attached, so that the straw may be balanced horizontally. Now rub on a piece of woollen cloth a bit of sealing-wax, or a stick of sulphur, or a piece of amber, or a penholder, paper-knife, or comb made of ebonite, and immediately present the substance to the ball at the end of the straw. It will be first attracted to the rubbed surface, but after coming into contact with it, repul- sion will be manifested and the ball will se- parate, and may be chased round the circle by following it with the excited body. The attraction of light bodies by amber after it has been rubbed appears to be the one soli- tary electrical observation recorded by the ancients, but it has given its name to the science, eXeKrpo*/ being the Greek name for amber. The cause, then, of this property is named electricity, and bodies which ex- hibit it are said to be electrified. The reader will remark that these words explain no- thing : they are used merely to express a certain state of matter and the entirely un- known cause of that state. Let the pith or cork ball at the end of the straw be again charged with electricity, by bringing it into contact with a piece of sealing-wax or ebo- nite which has just been electrified by fric- tion. In this condition it will, as we have just seen, be repelled by the substance which charged it, and on trial it will be found to be repelled also by all the substances we have named, after they have been excited by fric- tion. But if, while still charged with the electricity communicated to it by contact with sealing-wax, sulphur, ebonite, or amber, we present to it a warm and dry glass tube which has just been rubbed with dry silk, we shall find that the ball will be strongly attracted. After contact with the glass, repulsion will take place, and the ball will refuse again to come into contact with the excited glass. In this condition, however, it will be immediately attracted by rubbed sealing-wax or ebonite, and so on alternately : the ball when repelled by the wax is attracted by the glass, and when repelled by the glass is attracted by the wax. These simple experiments prove that, whatever electricity may be, there are two kinds of it, or, at least, it manifests two opposite sets of forces. The electricity evolved by the friction of glass with silk was formerly called vitreous electricity, and that shown by excited resin, sealing-wax, amber, &c., was named resinous electricity. These names have now been respec- tively replaced by the terms positive and negative. It must be understood that these terms imply no actual excess or defect, but are purely distin- guishing terms, just as we speak of the up and down line of a railway, with- out implying an inclination in one direction or the other. A fact of great importance in electrical theory is discovered when the substances in which FIG. 253. A simple Electro- scope. 364 ELECTRICITY. electricity is developed are carefully examined : it is found that one kind is never produced without the other simultaneously appearing. Thus, the silk which has been used for rubbing the glass in the above experiments will be found to exhibit the same electricity as sealing-wax or ebonite. And, further, the quantities of positive and negative electricity evolved are always found to be equal, or equivalent to each other ; that is, if they are put together they completely neutralize or destroy each other's effects. We have used the word " quantity," implying that electricity can be measured. No doubt, whatever electricity may be, there may be more or less of it ; but can we measure an imponderable, invisible, impalpable thing, incapable of isolation ? What we really measure when we say that we measure elec- tricity is the attractive or repulsive force : we balance this against some other force (that of gravitation, for example), and we say, so much weight lifted represents so much electricity. If we try to electrify a piece of metal by holding it in the hand and rub- bing it against woollen cloth, silk, or other substance, we shall fail in the attempt : no signs of electricity will thus be shown by the metal. Hence bodies were formerly divided into two classes those which could be elec- trified by friction, and those which could not. It was afterwards found, however, that there was no real ground for this division, but that, on the contrary, no two bodies can be rubbed together, even if they are made of the same substances, without positive electricity appearing in one, and an equi- valent quantity of negative electricity in the other. The real difference be- tween bodies which prevents the manifestation of electricity in many cases depends upon the fact that electricity is able to traverse some substances with great facility, while others prevent its passage. Thus, if we suspend horizontally a hempen cord by white silk attached to the ceiling, so that the hempen cord comes in contact with nothing but the silk, we shall find, on presenting a piece of excited ebonite to one end of the cord, that electric attraction of light bodies will be manifested at the other. If a silk cord be substituted for the hempen one, no such effect will be observed. The hemp is, therefore, said to be a conductor, and the silk a non-conductor. Again, if we substitute for one of the silk threads suspending the cord a piece of twine, or a wire, we shall fail to obtain any electric manifestations at the remote end, because the electricity will be carried off into the earth by the conducting powers of these substances. On the other hand, filaments of glass or ebonite may be used, instead of the silk, with the same effect : they do not allow the electricity to run through them to the ground, and are therefore termed, like the silk, insulators of electricity. The distinction of bodies into conductors on the one hand, arid into non-conductors or insu- lators on the other, is of paramount importance in the science and in all its applications. This distinction, however, is not an absolute one : there is no substance so perfect an insulator that it will not permit any electricity to pass, and there is no conductor so perfect that it does not offer resistance to the passage. Substances may be arranged in a list which presents a gradation from the best conductor to the best insulator. The metals are by far the best conductors, but there is great relative diversity in their con- ductive power. Silver, copper, and gold are much the best conductors among the metals, iron offering eight times, and quicksilver fifty times, the resistance of silver. Coke, charcoal, aqueous solutions, water, vegetables, animals, and steam are all more or less conductors, while among the sub- stances called insulators may be named, in order of increasing insulating .power, india-rubber, porcelain, leather, paper, wool, silk, mica, glass, wax, ELECTRICITY. 365 sulphur, resins, amber, gum-lac, gutta-percha, and ebonite. It will now be obvious why the electricity developed by the friction of a piece of metal fails to manifest itself under ordinary circumstances, as, for instance, when held in the hand : the metal and the body being both conductors, the elec- tricity escapes. But if the piece of metal be held by an insulating handle of glass or ebonite, the electrified condition may easily be observed. THEORY OF ELECTRICITY. *"F*HE few elementary facts which have been pointed out are absolutely * necessary for the foundation of what is sometimes termed the theory of electricity, but which is properly no theory, at least, not a theory in the same sense as gravitation is a theory explaining the motions of the planets, or even in the sense in which the hypothesis of the ether and its movements explains the phenomena of light. It is absolutely necessary to have a con- ception of some kind which may serve to connect in our minds the various phenomena of electricity, if it were only to enable us the more easily to talk about them. In default of any supposition which will shadow forth what actually occurs in these phenomena, we have recourse to what has been aptly termed a representative fiction : we picture to ourselves the actions as due to imaginary fluids fluids which we know do not exist, but are as much creations of the mind as Macbeth's air-drawn dagger ; not, however, like his " false creation," proceeding from " the heat-oppressed brain," but intellectual fictions, consciously and designedly adopted for the purpose of enabling us the better to think of the facts, to readily co-ordinate them, and to express them in simple and convenient language. Non-scientific persons hearing this language usually mistake its purport, and imagine that the actual existence of an " electric fluid " is acknowledged. The accounts which appear in the newspapers of the damage done by thunder-storms are often amusing from the objectivity which the reporter attributes to the " electric fluid." It is described, perhaps, as " entering the building," " passing down the chimney," then " proceeding across the floor," " rushing down the gas-pipes," " forcing its way through a crevice, and then stream- ing down the wall," &c., in terms which imply the utmost confidence of belief in the existence of the " fluid." With this intimation that the hypo- thesis of electric fluids is merely, then, a "fa<;on de parler" the reader will not be misled by the following brief explanation of the elementary facts in the language of the theory. In the natural state all bodies contain an indefinite quantity of an im- ponderable subtile matter, which may be called " neutral electric fluid." This fluid is formed by a combination of two different kinds of particles, positive and negative, which are present in equal quantities in bodies not electrified ; but when there is in any body an excess of one kind of particles, that body is charged accordingly with positive or negative electricity. Both fluids traverse with the greatest rapidity certain substances termed con- ductors; but they are retained amongst the molecules of insulating sub- stances, which prevent their movement from point to point. When one body is rubbed against another, the neutral electric fluid is decomposed the positive particles go to one body, the negative with which these positive 3 66 ELECTRICITY. particles were before united pass to the other body. The particles of the same name repel each other, but particles of opposite names attract each other ; and it is this attraction which is overcome when the electricities are separated by friction or in any other manner. It will be observed that the above is nothing but the statement of the elementary facts in the language of the hypothesis. This system of the two fluids readily lends itself to the explanation of nearly all the pheno- mena presented in what is termed static electricity that is, in those phe- nomena where the actions are conceivably due to a more or less permanent separation of the fluids. The grand discoveries in electricity turn, how- ever, upon quite another condition, namely, one in which the two hypothe- tical fluids must be imagined as constantly combining, and here the utility of the hypothesis is less marked. Inasmuch, however, as there can be no doubt regarding the identity of the agent operating in the two sets of cir- cumstances, the facts of dynamical electricity must still be expressed in the same language, with the aid of any additional conceptions which may give us more grasp of the subject ELECTRIC INDUCTION. IN all electrical phenomena an inductive action occurs, which resembles that which we have already indicated with regard to magnetism. Thus, if we take an insulated metallic conductor in the uncharged state, and bring it near an electrified body, we shall find that the conductor, while still at a considerable distance, will give signs of an electrical charge. Suppose we have a cylindrical conductor, and that we present one end of it to the electrified body, but at such a distance that no spark shall pass, we shall find, if the charge on the electrified body be strong and the conductor be brought sufficiently near, that on bringing the finger near the insulated cylinder, a spark passes. While the cylinder continues in the same position with regard to the electrified body, no further sparks can be drawn from it ; but if the distance between the two bodies be increased, the insulated cylinder will be found to have another charge of electricity, which will again produce a spark. And by repeating these movements we may obtain as many sparks as we desire by these mechanical actions, without in the least drawing upon the charge on the original electrified body. The elec- trophorus is a device for obtaining electricity by this plan, and several rotatory electrical machines have lately been invented which yield large supplies of electricity by a similar inductive action. It is found that in such a case as that we have above supposed, if the electrified body is charged with positive electricity, the uncharged conductor brought near it has its electricities separated the negative attracted and held by the attraction of the positive charge in the parts of the cylinder nearest the inducing body ; while the corresponding quantity of positive electricity is driven towards the most remote parts of the insulated conduc- tor. It is this last which gives the spark in the first case, and if it be not thus withdrawn from the conductor, it re-combines with the negative electricity when the conductor is withdrawn from the neighbourhood of the electrified body, and the conductor then reverts to the natural or unelectrified state. ELECTRICITY. 367 But the contact of a conducting body with the conductor while it is under the influence of the electrified body withdraws only positive electricity, the negative being held, as it were, by the attraction of the positive electricity of the charged body is not thus removed, and in this condition it is some- times called disguised or dissimulated electricity a term the propriety of which is doubtful. The excess of negative "fluid" which the conductor thus acquires shows itself, however, only when the inducing body has been withdrawn. Precisely similar effects will take place, mutatis mutandis, if the electrified body has a negative charge. A demonstration of inductive effects is readily afforded in the action of the gold-leaf electroscope, Fig. 254, in which two strips of gold-leaf are suspended within a glass case from wire passing through the top, and terminated in a metal plate. This instru- ment isoftenused forshowing the existence of very small electric charges. Let a stick of sealing-wax be rubbed and held, say, a foot or more from the plate of the electroscope, the leaves will diverge with negative electricity. The sealing-wax being retained in the same position, touch the plate for an instant with the finger. This will remove the negative charge, but the positive electricity will be retained on the plate by the attraction of the nega- tive of the sealing-wax. Now remove the sealing- wax, when the dissimulated charge will spread itself over the whole insulated metallic portion of the electroscope, and the leaves will diverge with a strong charge of positive electricity. If an excited glass tube is brought near the electro- scope, the leaves will now diverge still more ; if the sealing-wax is replaced in its former position, the leaves will collapse. In all these cases the electrified body parts with none of its own elec- tricity by developing electrical effects in the neigh- bouring bodies. The inductive actions we have described take place through the air, which is a non-conductor, and such actions may be made to take place through any other non-conductor. With solid non-conductors, such as glass, gutta-percha, &c., the inducing body may be brought very near to the conductor on which it is to act ; for the intervening solid substance, or dielectric, as it has been appropriately called, opposes a resistance to the combination of the opposite electricities, and the inductive effects are greatly intensified by the approximation. Faraday discovered that the amount of inductive action with a given charge is also dependent upon the nature of the dielectric, and that the electric forces act upon the particles of the dielectric, circumstances which are of the greatest importance, as we shall presently find, in practical telegraphy. The most familiar instance of induction is probably well known to the reader in the Leyden jar, Fig. 255, which is simply a wide-mouthed bottle of thin glass, covered internally and externally with tin-foil to within a few inches of the neck. The inner coat- ing communicates by means of a rod and chain with a brass knob. Such a jar admits of the accumulation of a larger quantity of electricity than the conductor of a machine will retain. A very few turns of the machine will FIG. 254. The Gold-leaf Electro- scope. 3 68 ELECTRICITY. suffice usually to charge the conductor to the fullest extent ; but if it be put in communication with the knob of a jar, a great many more turns will be required to attain the same charge in the conductor, and the excess of electricity represented by these additional turns will have accumulated within the jar an effect due to the "dissimulated" electricity of its exterior. Everybody knows the result when a metallic communication is established between the exterior and the interior of a charged Leyden jar. There is a very bright spark, a snap, and the jar is "discharged." Everybody knows, also, the sensation experienced when his body takes the place of the metallic communication, or forms part of the circuit through which the communi- cation takes place. Everybody knows that the shock then felt may also be ex- perienced at the same moment by any number of persons who join hands, under such conditions that they also form a part of the line of communi- cation. Such facts irresistibly sug- gest the notion of something passing through the whole chain, and this notion is in perfect harmony with the hypothesis of the " fluids," for we have only to suppose that it is one or both of these which rush through the circuit the instant the line of com- munication is complete. As the discharge is instantaneous, so the flow or current of electric fluid must be regarded as instantaneous also. And all the effects which such discharges produce concur to lead us to the con- clusion that, in the discharge of a Leyden jar, an instantaneous action of a kind which other dispositions of apparatus enable us to produce con- tinuously takes place. FIG. 255. The Leyden Jar. DYNAMICAL ELECTRICITY. T ET us take a vessel containing water, to which some sulphuric acid *rf has been added, Fig. 256, and in the liquid plunge a plate of copper, C, and a plate of pure zinc, z, keeping the plates apart from each other. As it is not easy to obtain zinc perfectly free from admixture of other metals, an artifice is commonly resorted to for obtaining a surface of pure metal, by rubbing a plate of the ordinary metal with quicksilver, which readily dissolves pure zinc, but is without action on the iron and other metals with which the zinc is contaminated, while the quicksilver is not acted upon by the diluted acid, but is merely the vehicle by which the pure zinc is presented to the liquid. Under the conditions we have described, no action will be perceived, no gas will be given off, nor will the zinc dis- solve in the acid. If the electrical condition of the portion of the copper- plate which is out of the liquid be examined by means of a delicate electro- ELECTRICITY. 3 6 9 scope, it will be found to possess a very weak charge o>i positive electricity, and a similar examination of the zinc plate will show the existence on it of a feeble charge of negative electricity. If the two plates be made to touch FlG. 256. A Voltaic Element. each other, or if a wire be attached to each plate, as shown in the figure, and the wires be brought into contact outside of the vessel, an action in the liquid is immediately perceptible at the surface of the copper plate, when a multitude of small bubbles of hydrogen gas will at once make their appearance, and the gas will be given off continuously from the copper plate so long as there is metallic contact through the wires, or otherwise, between the two plates, or until the acid is saturated with zinc for in this action the zinc is dissolving, and, in consequence, liberating hydrogen, which strangely makes it appearance, not at the place where the chemical action really occurs, namely, at the surface of the zinc which is in contact with the acid, but at the surface of the copper which is not acted upon by the acid. It is known that when we establish a metallic communication between two bodies charged with equivalent quantities of positive and negative electricities respectively, these combine and neutralize each other, and all signs of electricity vanish. It is obvious that the contact of the two wires has this effect, as the signs of electric charge which were before discover- able in each of the plates are no longer found while the wires are in con- tact. But the charges reappear the instant the contact is broken, the chemical action ceasing at the same time. If the wire connecting the two plates outside of the vessel be carefully examined, it will be found, so long as the chemical action is going on, to be endowed with new and very re- markable properties. If this wire be stretched horizontally over a freely suspended magnetic needle, and parallel to it, the needle will be deflected from its position, and, if the wire be placed very near it, will point nearly east and west, instead of north and south. Now, this effect is produced by any part whatever of the wire, and it instantly ceases if the wire be cut at any point. These facts at once suggest the idea of its being due to something flowing through the wire, so long as metallic continuity is pre- served. This idea is much strengthened when we find that the action of the connecting wire upon the magnetic needle is quite definite or, in other words, there are indications which correspond with the notion of direction. For when the wire, which we shall still suppose 'to be stretched 24; 370 ELECTRICITY. horizontally above the needle and parallel to its direction, is so connected with the plates immersed in the acid that the portion which approaches the south-pointing pole of the needle proceeds from the copper plate, while the portion above the north pole is in connection with the zinc plate, then the north end of the needle will always be deflected towards the west whereas, if the connections be made in the contrary manner, the deflection will be in the opposite direction ; and if the wire be below the needle, the contrary deflections will be observed with the same connections. The dis- covery of the action of such a wire on the magnetic needle was made by CErsted in 1819, and it is a discovery remarkable for the wonderful extent of the field which it opened out, both in the region of pure science and in that of practical utility. Since by such experiments as those just mentioned the notion of a current is arrived at, the mind recurs to the fiction of the " fluids," and pictures the " positive fluid " as rushing in one direction, and the " negative fluid " in the other, to seek a re-combination into " neutral fluid." But we must never lose sight of the fact that these ideas are consciously adopted as repre- sentative fictions to help our thoughts just as John Doe and Richard Roe, imaginary parties to an imaginary lawsuit, used to be named in legal docu- ments, in order to explain the nature of the proceedings. Failing, then, to I i *r V FiG. 257. Amperes Rule. find anything really flowing along our wire, it is still absolutely necessary, seeing there is something definite in its action, to assign a direction to the supposed current ; and it has been agreed that we shall represent the cur- rent as flowing from the positively charged body to the negatively charged body that is, in the case we have been considering, from the copper to the zinc through the wire. When this conventional representation has been adopted, the action on a magnetic needle can easily be defined and remembered by an artifice proposed by Ampere. In Fig. 257, let N s re- present the magnetized needle, N being the pole which points towards the north, and s the south pole. Let c be the end of the wire connected with the copper plate, and z that connected with the zinc. The current is there- fore supposed to flow in the direction indicated by the arrows in a wire above the needle and in the wire placed below. Now, suppose that a man is swimming in the current in the same direction it is flowing, and with his face towards the needle, then the north pole of the needle will always be deflected towards his left. With the direction of current represented in the figure, the pole, N, will be thrown forward from the plane of the paper, or towards the spectator. The reader who desires to study the mutual action of currents and mag- nets will find it necessary to fix this idea in his mind. He will now be able to see that if the wire be coiled round the needle, as shown by the lines and arrows, Fig. 257, so that the same current may circulate in reverse direc- ELECTRICITY. 371 tions above and below the magnet, its effects in deviating the needle will everywhere concur that is, the action of each part will be to turn the north pole towards the left. It is, therefore, plain that if the wire conveying the current be passed several times round the magnetic needle, the deflecting force will be increased ; and a current, which would, by merely passing above or below the magnet, produce no marked deflection, might be made to produce a considerable effect if carried many times round it. The ar- rangement for this purpose is shown in Fig. 258, where it will be perceived that the needle is surrounded by a coil of wire, so that the current circu- lates many times about it, and the effects of each part of the circuit concur in deflecting the needle. Such an arrangement of the wire and needle constitutes what is called the galvanometer,^ instrument used to discover the existence and direction of electric currents. 1 FlG. 258. Galvanometer. The arrangement of metals and acid which we have described is termed a voltaic couple, element, or cell; and a great controversy has long been carried on among men of science as to the place at which the develop- ment of electricity has its origin. Three-quarters of a century ago, the effect was attributed by Volta to the mere contact of the two dissimilar metals. In the experiment we have described this contact, supposing the wires to be of copper, would occur at the junction of the wire and the zinc plate. Now, by joining the copper plate of such a cell to the zinc plate of another cell, the copper of that to the zinc of a third, and so on, it is evident that the number of dissimilar contacts might be indefinitely increased, and the electric power should be proportionately augmented. It is found that this is really the case, but Volta's explanation has been opposed by another which regards the chemical action in the cells as the real origin of the electric manifestations. This last explanation, supported by many appa- rently conclusive experiments of Faraday and others, has been generally accepted. Galvanic batteries as a series of cells joined together in a cer- tain manner are termed have been constructed, in which there is no con- tact of dissimilar metals ; and no electric current can be obtained from an apparatus in which no chemical action takes place. The contact theory in a modified form has recently been revived by Sir W. Thompson and others. In this it is now maintained that some separation of electri- cities really does take place by contact of dissimilar substances, but that a current can be produced only when this separation is continually renewed by chemical actions. Be the true explanation what it may, the fact is imdoubted that by joining cell to cell, we can really obtain vastly more powerful effects. If we take a single cell, such as that represented in r ig. 256, and connect the plates with a long and thin wire, we shall find 24 2 372 ELECTRICITY. that the current flowing through each part of the circuit is much weaker than when we connect the plates with a short and thick wire. In other words, the action in the latter case, when the wire is stretched over a magnetic needle, will be more powerful than in the former. By using a long and thin wire the current may be so weakened that it becomes neces- sary to surround the needle with many coils of the wire to produce a marked deflection. Again, much depends upon the material ; thus a copper wire conveys a much more powerful current than a German silver one of the same dimensions. There thus appears to be a certain analogy between the flow of electricity along conductors to that of water through pipes. The longer and narrower are the pipes, the less is the quantity of water forced through them by a given head ; and similarly, the resistance to the passage of a current increases with the length and narrowness of the conducting wire. When all other circumstances are the same, the electrical resistance of a conductor varies directly as its length and inversely as its sectional area. Hence the current flowing in the apparatus repre- sented in Fig. 256 would be increased by making the wire thicker, and by making it shorter by bringing z and C nearer together, and by making the area they expose to the liquid larger ; for in the liquid also the current flows as indicated by the arrow, a fact which may be proved by the deflec- tion of a magnetized needle suspended above the vessel. The magnitude of the current depends, then, upon two opposing forces, namely, that which continuously separates the electricities, or drives them apart to recombine through the circuit, and that which opposes their passage. The former, which is termed the electromotive force, originates, according to some, from the mere contact of dissimilar materials, according to others from the chemical action. Now, we may increase the strength of the current in a given arrangement, either by increasing the electromotive force, or by diminishing the resistance. The increase of the strength of the current, produced by merely pouring more acid into the vessel, Fig. 256, is due, according to the chemical theory, to the former cause ; according to the contact theory, to the latter. By multiplying the cells we increase the electromotive forces : the current receives, so to speak, an onward shove in each cell, but with each cell we introduce an additional resistance. Hence, it follows, that when the resistance of the circuit outside of the cells is extremely small, the current produced by a single cell is as power- ful as that produced by a thousand. But when the external resistance is great, as when long thin wires are used, the united electromotive forces of a number of cells are needed to drive the current through the circuit. The strength of a current, c, is therefore expressible by the following simple formula, in which r stands for the internal resistance, and e for the electro- motive force in each cell ; n represents the number of cells in the battery, these being supposed exactly similar in every respect ; R is the sum of the resistances in the circuit outside of the battery. lie It is easily seen that the smaller R is made, the more nearly does the strength of the current become independent of the number of cells. But many modifications have been made in the materials and form of the cells, by which greater power and duration of action have been attained. Our space permits a description of only two forms, and these must be de- scribed without a discussion of the principles upon which their increased ELECTRICITY. efficiency depends. Daniell's constant cell is represented in Fig. 259, where D is a battery of ten such cells, A is a cylindrical vessel of copper, C is a tube of porous earthenware, closed at the bottom, and within it is suspended the solid rod of amalgamated zinc, B. The copper vessel and the zinc rod FIG. 2^.Danieirs Cell and Battery. are provided with screws by which wires may be attached. In the copper vessel is placed a saturated solution of sulphate of copper, and some crystals of the same substance are placed on the perforated shelf within the vessel. The porous tube is filled with diluted sulphuric acid. When the battery FIG. 260. Grove's Cell and Battery. is in action the zinc is dissolved by the sulphuric acid, and metallic copper is continually deposited upon the internal surface of the copper vessel. Daniell's battery, in some form or other, is much used for telegraphs and for electrotyping. Grove's cell is shown in section in Fig. 260. The external vessel is made of a rectangular form in glazed earthenware or 374 ELECTRICITY. glass. It contains a thick plate of amalgamated zinc, A, A, bent upwards, and between the two portions a flat porous cell, C, C, is placed, filled with strong nitric acid, in which is immersed a thin sheet of platinum. The outside vessel is charged with water, mixed with about ^th of sulphuric acid. D represents a battery of four such cells, in which the mode of con- necting the platinum of one to the zinc of the next may be noticed. The terminal platinum and zinc form \htpoles of the battery, and to them the wires are attached which convey the current. The substitution of plates of coke for the platinum gives the form of battery known as Bunsen's, which is also sometimes made with circular cells. Cover's and Bunsen's are much more powerful arrangements than Daniell's, but the latter has the advan- tage as regards the duration and uniformity of its action. When the current produced by a battery of a dozen or more such cells is conveyed by a wire, it is observed that this wire becomes sensibly hot, and, if the wire be thin enough, the heat may be sufficiently great to heat FIG. 261. Wire ignited by Electricity. the wire to redness. By stretching a piece of platinum wire between two separate rods which convey the current, as represented in Fig. 261, the length of wire through which the current passes may be adjusted so as to give any required amount of light, and the wire may even be heated to the fusing-point of platinum. This property of electricity has some interesting applications, as, for example, in firing mines and other explosive charges, and in some surgical operations. A still more interesting exhibition of heating and luminous effects is observed when the terminals of a battery of many cells are connected with two rods of coke, or gas-retort carbon. When the pointed ends of the rods are brought into contact, the current passes, and the points begin to glow with an intensely bright light, and if they are then separated from each other by an interval of ^th of an inch or more, according to the power of the battery, a luminous arc extends between them, emitting so intense a light that the unprotected eye can hardly support it. This luminous arc is called the voltaic arc, and it excels all other artificial lights in brilliancy, a fact due to the extremely high temperature to which the carbon particles are heated, the temperature being, perhaps, the highest we can attain. It must not be supposed that in this brilliant light we see electricity : the light is due to the same cause as the light of a candle or gas flame, namely, incandescent particles of solid carbcn. These particles are carried from one carbon point to the ELECTRICITY. 375 other, and it is found that the positive pole rapidly loses its substance, which is partly deposited on the negative pole. But in order to obtain a steady light, it is requisite to keep the pieces of carbon at one invariable distance ; and therefore the transference of the material from one pole to the other, and the loss by combustion, must be compensated by a slow movement of the carbons towards each other. Several kinds of apparatus are used for this purpose, but they all depend upon the principle of regu- lating the motions by the action of an electro-magnet, formed by the current itself, which becomes weaker as the carbons are farther apart. The move- ment is communicated to the apparatus by clockwork. Duboscq's electric FIG. 262. Duboscq's Electric Lantern and Regulator. lantern is shown in Fig. 262, with enlarged images of the carbon points projected on a screen. The mechanism of the regulator is contained with- in the cylindrical box immediately below the lantern. The supports of both carbons are moved ; that which bears the positive carbon pole being advanced twice as fast as the other, and thus the light is maintained at the same level, for the positive carbon wears away twice as fast as the other. The light is more brilliant when charcoal is used instead of coke, but then it is necessary to operate in a vacuum, to avoid the combustion of the char- coal. The voltaic arc has recently been applied to illuminate lighthouses, and for other purposes, and will probably soon be more widely employed, for a cheap and convenient mode of producing a uniform current of elec- tricity has recently been discovered and will be presently described. The current which is maintained by the chemical action taking place in the cells of the battery can also be made to do chemical work outside of the battery. When the poles of the battery are terminated by wires or plates of platinum, and these are plunged into water acidulated with sulphuric acid, bubbles of gas are seen to rise rapidly from each wire, or electrode^ 376 ELECTRICITY. as it is termed. Fig. 263 shows an arrangement by which these gases may be collected separately, and examined, by simply placing over each elec- trode an inverted glass tube, filled also with the acidulated water. The gases collect at the tops of the tubes, displacing the water, and it is found that from the wire connected with the zinc end of the battery, or negative electrode, hydrogen gas is given off, while at the positive electrode oxygen gas is liberated, in volume precisely equal to half that of the hydrogen. This being the proportion in which these two substances combine to pro- duce water, it appears that in the passage of the current a certain quantity of water is decomposed ; and the quantity thus decomposed is in reality a H 21 FlG. 263. Decomposition of Water. measure of the current, all the other effects of which are found to be pro- portional to this. When the electricity in a current is said to be measured, it is simply the power of the current to deflect a magnet, or the quantity of gas it can liberate, or some other such effect, which is in fact measured. The discharge of a Leyden jar through such an apparatus as that repre- sented in Fig. 263 would present no perceptible decomposition of the water; yet such a discharge passed through the arms and body produces, as everybody knov/s, a painful shock, and is accompanied by a bright spark and a noise, while the simultaneous contact of the fingers with the positive and negative poles of the galvanic battery occasions neither shock nor spark. Thousands of discharges from large jars must be passed through acidulated water to liberate the amount of gas which a battery current of a second's duration will produce. The electricity of the jar is often spoken about as having a higher tension than that of the battery, but the latter sets an immensely greater quantity of electricity in motion. The idea may be illustrated thus : Suppose we have a small cistern of water placed at a great height, and that this water could fall to the ground in one mass. The fall of the small quantity from a great height would be capable of producing very marked instantaneous effects, such as smashing, as with a blow, any ELECTRICITY. 377 structure upon which it might fall. This would correspond with the small quantity of electricity which passes in the discharge of a Leyden jar. Con- trast this with the case in which we allow a very large quantity of water to descend from a very small height as when the water of a reservoir is flow- ing down a. gently inclined channel. It is plain that a different kind of effect might be produced in this case ; the current might be made, for in- stance, to turn a water-wheel, which the more forcible impact of the small quantity of water in the case first supposed would have broken into pieces. It is probable that the apparent decomposition of water by the electric current is in reality a secondary effect, and that it is the sulphuric acid which is decomposed. When, instead of acidulated water, we place in the apparatus a solution of sulphate of copper, it is found that metallic copper is deposited on the negative electrode, and sulphuric acid collects at the positive electrode. The metal is deposited in a firm and coherent state, and the useful applications of this deposition of metals are of great interest and importance. For, in a similar manner, gold, silver, lead, zinc, and other metals may be made to form thin uniform layers over any properly pre- pared surface. The immense advantages which the arts have derived from electro-plating illustrate in a convincing manner the benefits which physical science can confer on society at large. The process of electro-plating may be practised by the aid of apparatus of very simple character. Fig. 264 shows all that is necessary for obtaining perfect casts in copper of seals, small medals, &c. A A is a section of a common tumbler ; B B is a tube, made by rolling some brown paper round a ruler, uniting the edge with sealing-wax, and closing the bottom by a plug of cork, round which the paper may be tied by a string, or in any other convenient manner. The tumbler contains a solution of sulphate of copper, and the tube is filled with water, to which about one-twentieth of its bulk of sulphuric acid has been added. A strip of amalgamated zinc, or a piece of thick amalgamated zinc wire, is placed in the tube, and a piece of copper bell-wire is twisted round the top of it, and has attached to its other extremity, and immersed in the copper solution, the article which is to be covered with copper. We may suppose that this is to be a cast in white wax or in plaster of one side of a medal. The cast is carefully covered with black lead by means of a soft brush, and the copper wire is inserted in such a manner as to be in contact with the black lead at some part. When the apparatus has been left for some hours in the position represented, a deposit of copper will be found over the black- leaded surface, and it will be a perfect impression of the wax cast. Such a copper cast, or any article in copper having a perfectly clean surface, can be readily covered by a film of silver by means of a similar arrangement, where a solution of cyanide of potassium, in which some chloride of silver has been dissolved, is made to take the place of the sul- phate of copper. Electro-plating with the precious metals has become a FIG. 264. Electro-plating. 378 ELECTRICITY. commercial industry of great importance ; and this process has completely superseded the old plan of covering the metallic article to be plated with an amalgam of silver or of gold, and then exposing it to heat, which vola- tized the mercury, leaving a thin film of gold or of silver adhering to the baser metal. On the large scale a battery of several cells is used for electro- plating, and the articles are immersed in the metallic solutions as the negative poles of the battery; any required thickness of deposit being given according to the length of the time they remain. At the works of Messrs. Elkington, of Birmingham, these operations are conducted on a grand scale. The liquid there employed for silvering is a solution of cyanide of silver in cyanide of potassium, and the positive pole is formed of a plate of silver, which dissolves in proportion as the metal is deposited on the nega- tive pole. As the charging of batteries is a troublesome operation, and their action is liable to variations which affect the strength of the currents, the more uniform, more convenient, and more economical mode of pro- ducing currents by magneto-electricity, which will presently be described, has been to a great extent substituted for the voltaic battery. The wire conveying a current not only affects a magnetic needle in the FlG. 265. A Current producing a Magnet. manner already described, but itself possesses magnetic properties, of which, indeed, its action on the needle is the result and the indication. If such a wire be plunged into iron filings, it will be found that the filings are attracted by it : they cling in a layer of uniform thickness round its whole circumference and along its whole length, and the moment the connection with the battery is broken they drop off. This experiment shows that every part of the wire conveying a current is magnetic, and it may be proved that the action is not intercepted by the interposition of any non-magnetic material. Thus the action of the wire upon the magnetic needle takes place equally well through glass, copper, lead, or wood. Consequently, if we cover the wire with a layer of gutta-percha, or over-spin it with silk or cotton, we shall obtain like results on our filings, and if we coil the covered wire round a bar of iron, while the non-conducting covering of the wire will compel the current to circulate through all the turns of the coil, it will not interfere with the magnetic action on each particle of the bar. Whenever this is done it is found that the iron is converted into a powerful magnet so long as the current passes. Fig. 265 represents in a striking manner the result when the current is made to circulate through numerous convo- lutions of the wire ; and as each turn adds its effect to that of the rest, magnets of enormous strength may be formed by sufficiently increasing the ELECTRICITY. 379 number of the turns. The end of the iron bar is shown projecting from the axis of the coil, and below it is placed a shallow wooden bowl, con- taining a number of small iron nails. The instant the battery connection is completed these nails leap up to the magnetic pole, and group them- selves round it in the manner shown in the cut ; and again, when the cur- rent is interrupted, the iron reverts to its ordinary condition, the magnetism vanishes, and the nails drop down in an instant. These effects may be produced again and again, as often as the current flows and is broken. A magnet so produced is called an electro-magnet, to distinguish it from the ordinary permanent steel magnets. By coiling the conducting wire round FIG. 266. An Electro-magnet. a bar of iron which has been bent into the form of a horse-shoe r very powerful magnets may be produced, and enormous weights may be sup- ported by the force of the magnetic attraction so evoked. Fig. 266 repre- sents the apparatus for experiments of this kind, in which weights exceeding a ton can be sustained. Here, then, we have a striking instance of the subtile agent electricity, evoked by the contact of a few pieces of zinc with dilute acid, showing it- self capable of exerting an enormous mechanical force. Engines have been constructed in which this force is turned to account to produce rota- tory motion as a source of power. Such engines have certain advantages for special purposes ; but the money cost for expenditure of material for power so obtained is, at least, sixty times greater than in the case of the steam engine. It is, however, in producing mechanical effects at a dis- 3 So ELECTRICITY. tance that the electric current finds the most interesting practical applica- tion of its magnetic properties. These are the actions which are so exten- sively utilized in the construction of telegraphic instruments, of clocks regulated by electric communication with a standard time-keeper, and of many ingenious self-registering instruments. The telegraph will be described in the next article, and we shall also have occasion in subse- quent articles to describe some of the other applications of electro-magnetic and electro-chemical force. INDUCED CURRENTS. '""PHESE very remarkable phenomena were discovered by the illustrious * Faraday, in 1830, and this discovery, and that of magneto-electricity, may be ranked among the most memorable of his many brilliant contribu- tions to electric science. Let two wires be stretched parallel and very near to each other, but not in contact. Let the extremities of one wire, which we shall term A, be connected with a galvanometer (page 371), so that the existence of any current through the wire may be instantly indicated. Let the two extremities of the other wire, B, be put into connection with the poles of a battery. The moment the connection is complete, and the battery current begins to rush through B, a deflection of the galvanometer needle will be observed, indicating a current of very short duration through A in the opposite direction to the battery current through B. This induced current, which is called the secondary current, does not continue to flow through A : it occurs merely at the time the primary or battery current is established ; and though the latter continues to flow through the wire, B, no further effect is produced in the other wire. When, however, the battery connection is broken, and the primary current ceases to flow, at that instant there is set up in the wire, A, another momentary secondary current, but this one is in the same direction as the battery current. This is termed the direct secondary current, in opposition to the former, which is called the inverse current. These effects are much more powerful when, instead of lengths of straight wire, or single circles of wires, we use two coils of wire, one of which, namely, that which conveys the primary currents, is placed in the axis of the other. It must be distinctly understood that the secondary currents are of momentary duration only ; they are not produced at all while the battery is flowing, but only at the time of its commencement and cessation. If, however, we make the primary coil so that it can be slid in and out of the axis of the other, then while the primary current is continuously flow- ing, we can produce secondary currents in the other coil, by causing the coils to approach or recede from each other. As we bring the coils near each other, and slide the primary into the secondary, the current in the latter is inverse j when the one coil is receding from the other, it is direct. These mechanical actions are not produced without expenditure of force, for the approaching coils repel each other and the receding coils attract each other. The setting up of the battery current in the primary coil when placed within the other is equivalent to bringing it, with the current flow- ing, from an immense distance in an extremelv small time. Similarly, ELECTRICITY. 381 when the battery current is broken, it is equivalent to an instantaneous recession. The effects, therefore, are proportionately powerful. It is found, also, and this we shall presently refer to more fully, that when, instead of the primary coil, a magnet is similarly moved into, or removed from, the axis of the secondary coil, currents in opposite directions are set up in the latter without any battery being used at all. The direction of these currents is the same as would be produced by a primary current that would form, in a piece of iron placed in the axis of the coil, an electro-magnet with poles similarly situated to those of the magnet so introduced or withdrawn. Hence, by placing a bar of soft iron in the axis of the primary coil, the secondary currents will be produced with increased force. When a long secondary coil, having the turns of its wire well insulated from each other, surrounds a primary coil provided with a core of soft iron, or still better, with a bundle of annealed iron wires, a series of powerful discharges, like those of a Leyden jar, may be obtained between the terminals of the secondary coil, when the battery contact is made and broken in rapid succession. Such induction coils have been very carefully and skilfully constructed by FIG. i&j.Rtthmkorjf's Coil. Ruhmkorff,and are therefore often called " RuhmkorfFs Coils." One of these is represented in Fig. 267. A B is the coil, and the apparatus is provided with what is termed a condenser, which consists of layers of tin-foil placed between sheets of thick paper, and alternately connected so that one set communicates with one extremity of the primary coil, and the other with the other. This condenser is conveniently contained in the wooden base of the instrument. Its introduction has greatly increased the intensity of the secondary current, and sparks of 18 in. or 20 in. in length have been obtained in the place of very short ones. It should be stated that of the two secondary currents, only one has sufficient intensity to traverse the secondary circuit when there is any break in its continuity. This is the direct secondary current, or that which is produced on breaking the primary circuit. The reason is that the com- mencing current in the primary circuit induces in the spires of its own coil an inverse current, and the battery current therefore -attains its full strength .gradually, but still in a very short time ; while, on the cessation of the battery current, the same induction sends a wave of electricity through the primary coil in the same direction, and then the current ceases abruptly. Consequently, in the latter case, the induced electricity of the secondary coil is set in motion in much less time, and therefore possesses much greater intensity. 3 82 ELECTRICITY. The magnetism of the iron core is usually made use of to break and make the current, by the attraction of a piece of iron attached to a spring, which, by moving towards the end of the core, separates from a point in connection with the battery, and, the current no longer flowing, the mag- netism ceases, and the spring again brings back the iron and renews the contact. By means of such coils many surprising effects have been produced. Perhaps one of the most beautiful experiments in the whole range of physi- cal science is made by causing the discharges of the secondary coil to take FIG. 268. Discharge through Rarefied Air. place through an exhausted vessel in the manner represented in Fig. 268. A beautiful light fills the interior of the vessel, and the terminals appear to glow with a strange radiance one being surrounded with a kind of blue halo and another with a red. On reversing the direction of the currents, which is done by the little apparatus at the right-hand end of the coil in Fig. 267, the blue and the red radiance change places. Beautiful flashes of light may also be made to appear in the vessel, having the most marked re- semblance to the streamers of the Aurora Borealis. When, instead of vessel^ almost free from common air, we repeat the experiment with tubes con- taining an extremely small residue of some other gas, such as hydrogen, carbonic acid, &c., the colour of the light and other appearances change. ELECTRICITY. 383 Geissler's tubes have already been spoken of in connection with the spec- troscope ; but, independently of that, the various beautiful appearances which such tubes have been made to present, by the introduction of fluores- cent substances and other devices, render the induction coil an instrument of the highest interest to the scientific amateur. Then there are striking physiological and other effects which the coil is capable of producing. For instance, we are able by its instrumentality to produce from atmospheric air unlimited quantities of that singular modification of oxygen which is called ozone. The electricity of the coil has been used for firing mines, torpedoes and cannons, and for lighting the gas-burners of large buildings. Mr. Apps has devoted much attention to improving the insulation of induction coils, and he recently made for the Polytechnic Institution one of astonishing power, being perhaps the largest yet constructed. This is repre- sented in Plate VII., surrounded by the somewhat scenic accompaniments which are supposed necessary to render science attractive to the multitude. Externally the coil appears an ebonite tube, about 20 in. in diameter and between 4^ ft. and 5 ft. in length. From each end smaller cylinders pro- ject, covered by ebonite, and the whole is mounted upon supports also covered with ebonite. The soft iron core 5 ft. long and 4 in. in diameter is formed of a bundle of wires. The primary coil is of pure copper wire, nearly Jjjth of an inch thick, and in length 3,770 yards, over-spun with cotton, and making about 6,000 turns round the iron core. The primary wire is covered by an ebonite tube \ in. thick, and outside of this is the secondary coil, 4 ft. 2 in. long, surrounding the ebonite tube to the depth of 6 in., and containing 150 miles of silk-covered wire, "oi^in. in diameter. The weight of this secondary wire is 606 Ibs., and so carefully have the spires been arranged that the insulation is everywhere more than ten times greater than the tension of the electricity when the coil is in action, and altogether the construction of this instrument is highly creditable to the skill and scientific knowledge of Mr. Apps. The external diameter of the secondary coil is I ft. 7 in. The condenser contains 750 square feet of tin- foil. The battery current for the primary coil is furnished by forty Bunsen cells. A contact-breaker of the ordinary kind, but detached from the coil, was first provided ; but this was soon deranged by the sparks destroying the contact points, and finally a contact-breaker on Foucault's plan was adopted, in which the contacts are made with mercury in the midst of alcohol. A large and very strong glass vessel in fact, the inverted glass cell of a bichromate battery was bored through, and the neck fitted into a cap with cement, a thick wire covered with platinum being inserted in the cap ; the platinum amalgam was poured on this, and over it a pint of alcohol ; the contact-wire was also very large, and pointed with a thick stud of platinum, and, being attached to a spring, contact was easily made and broken. Flashes of light could be seen between the amalgam and the alcohol ; but explosions did not occur, and the height of the column of the latter prevented the forcible ejection of the spirit, which no longer took fire. This break was used for eight hours in a continuous series of experiments. It was found that this great coil would give a spark 29 in. in length, and its discharge would perforate a certain thickness of plate glass. A Leyden battery of 40 square feet could be charged by three contacts of the break, and by its discharge considerable lengths of wire could be deflagrated. The appearance of the spark, with this as with other large induction coils, may be described as a thick line of light surrounded by a reddish halo of 384 ELECTRICITY. or less brilliancy, which last has an appreciable duration, while the line spark proper is instantaneous. The reddish glow may be blown aside by a current of air when a series of discharges is taking place, and separated from the denser-looking line of light. The latter is formed by intensely- heated particles of the metals between which the discharge occurs, while the former is probably due to the incandescence of the gases of the air. When one of the discharging-wires of the coils was brought to the centre of a large swing looking-glass, and the other wire connected with the amalgam at the back, the sparks were thin and wiry, arborescent, and very bright, and the crackling noise of the discharge was quite different from that of FlG. 269. Appearance of the Spark on the Looking-glass. the heavy thud or blow produced by the flaming spark. The peculiar appearance presented by these sparks is shown in Fig. 269. Some expecta- tions appear to have been formed that a source of electricity of so much power would lead to some scientific discoveries at the Polytechnic ; but these expectations have not been realized, and the coil has served merely for the occasional instruction or amusement of the marvel-seeking audiences of that popular institution. MAGNETO-ELECTRICITY. it had been shown that an electric current was capable of evoking magnetism, it seemed reasonable to expect that the reverse operation of obtaining electric currents by means of magnets should be possible. Faraday succeeded in solving this interesting problem in Novem- ber, 1831. and one of his earliest, simplest, and most convincing experi- ELECTRICITY. 385 ments for the demonstration of the production of electricity by a magnet is represented in Fig. 270. A B is a strong horse-shoe magnet, C is a cylinder of soft iron, round which a few feet of silk-covered copper wire are wound ; one end of the wire terminates in a little copper disc, and the other FIG. 270. Magneto-electric Spark. end is bent, as shown at D, so that it is in contact with the disc, but press- ing so lightly against it that any abrupt movement of the bar causes the point of the wire and the disc to separate. When the bar is allowed to fall upon the poles of the magnet, the separation occurs, and again when it is suddenly pulled off ; and on each occasion a very small but brilliant spark is observed where the contact of the wire and disc is broken. It was in allusion to this experiment that a contributor to " Blackwood's Magazine" wrote: Around the magnet, Faraday Is sure that Volta's lightnings play ; But how to draw them from the wire ? He took a lesson from the heart ; 'Tis when we meet, 'tis when we part, Breaks forth the electric fire. If a coil of fine insulated wire 'be passed many times round a hollow cylinder, open at the ends, and the extremities of the wire connected with a galvanometer at some distance, then if into the axis of the coil, A B, Fig. 271, a steel magnet be suddenly introduced, an immediate deflection of the needle takes place ; but after a few oscillations it returns to its former position. When the magnet is quickly withdrawn, the needle receives a momentary impulse in the opposite direction. The magnetization and de- magnetization of the iron core in the induction coil would, therefore, of itself cause the induced currents already described, for these actions are equivalent to sudden insertion and withdrawal of a magnet. If we suppose 25 3 86 ELECTRICITY. C, in Fig. 271, to represent, not a magnet, but a piece of soft iron the reader will remember that this soft iron can be, as often as required, mag- FIG. 271. A Magnet producing a Current. netized and demagnetized by simply bringing near one end of it the pole of a permanent magnet (see page 362). Upon this principle many ingenious FIG. 272. Clarke's Magneto- electric Machine. machines have been constructed for producing electric currents by. the relative motions of magnets and of soft iron cores surrounded by wires. ELECTRICITY. 387 Clarke's machine is shown in Fig. 272. A is a powerful steel magnet fixed to the upright. A brass spindle passing between the poles can be made to rotate very rapidly by the multiplying-wheel, E, on which a handle is fixed. There are two short cylinders of soft iron, parallel to the spindle, united together by the transverse piece of iron, D, which turns with the spindle. Each bar is surrounded by a great length of insulated copper wire, and the ends of the wires are so connected with springs which press against a portion of the spindle, which is here partly formed of a non- conducting material, that the currents generated in the coils, although in different directions as they approach a pole and recede from it, are never- theless made to flow in one direction in the external circuit. R R in the figure represent two brass handles, which are grasped by a person wish- ing to experience the shocks the machine can give when the wheel is turned. When the terminals of the coil are provided with insulating handles and connected with pointed pencils of charcoal, the electric light FIG. 273. Magneto-electric Light. can readily be produced by expenditure of mechanical effort in turning the handle. The arrangement of the points for this purpose is shown in Fig. 273, and we shall presently see what ac v intage has been drawn from this experiment on a great scale as a source of light. It will be observed that during the revolution of the armatures, as the wire-covered iron cores are termed, there are two maximum and two minimum points at which the currents are strongest and weakest. These variations may be lessened by increasing the number of armatures and of magnets, and Mr. Holmes arranged a machine with eighty-eight coils and sixty-six magnets, and the connections were so contrived that the currents always flowed in the same direction in the external circuit. This machine required i horse-power to drive it when the currents were flowing, but much less when the circuit was interrupted, and it was designed for, and successfully applied to, the production of the electric light for light- house illumination. Instead of steel magnets which gradually lose their strength, it is obvious that electro-magnets might be employed, but this source of electricity is costly, troublesome, and inconstant. Mr. Wilde" hit upon the idea of using a small magneto-electric machine with permanent steel magnets, to generate the current for exciting a larger electro-magnet, and the current from this produced a still more powerful electro-magnet, 252 3 88 ELECTRICITY. from which a magneto-electric current could be collected and applied. The same idea was subsequently applied in other forms, as by shunting off a portion of the current produced from the mere residual magnetism of an electro-magnet, to pass through its own coils and evoke a stronger magnet- ism, which again reacts by producing a more powerful current, and so on continually ; the limit being dependent only on the mechanical force em- ployed, and on the power of the wires to convey the electricity, for they become very hot, and, unless artificially cooled, the insulating material would be destroyed. The armatures used in Wilde's, Ladd's. and other machines of this kind, are quite different in arrangement from those of Clarke's machine, and are far superior. They are formed of a long bar of soft iron, of a section like this, M, and the wire is wound longitudinally between the flanges from end to end of the bar, up one side and down the other. This armature rotates about its longitudinal axis between the pairs oi the poles of a file of horse-shoe magnets, either permanent, or electro- magnets excited by the magneto-electric currents. In this case opposite poles are induced along the edges of the bar, and these poles are reversed at each half-turn. The intensity of the induced currents increases with the velocity with which the armature is made to revolve up to a certain point ; but because the magnetization of the soft iron requires a sensible time to be effected, and the poles are reversed at every half-turn, it is found that a speed increasing beyond the limit is attended by decrease of the intensity of the current. The intensity in such machines has, therefore, a definite limit. But in a modification of the magneto-electric machine, which has quite recently been invented by M. Gramme, the limit is vastly extended by the ingenious disposition of the iron core and armatures, and his machines appear to solve the problem of the cheap production of steady and powerful electric currents, so that electricity will soon be applied in processes of manufacture where the cost of electrical power has hitherto placed it out of the question. We shall now endeavour to explain the principle on which the Gramme machine depends, and describe some forms in which it is constructed. THE GRAMME MAGNETO-ELECTRIC MACHINE. T ET x, Fig. 274, be a coil of covered wire ; then while a bar magnet, B A, - 1 ' is advancing towards it and passing through it, as at M, a current will FIG. 274. flow through the coil and along a wire connecting its ends, s s. The cur- rent will change its direction as the centre of the magnet is leaving the coil ELECTRICITY. 389 to advance in the direction, B A. If A A' be a bar of soft iron, with the coil fixed upon it, we can still excite currents in the coil by magnetizing the bar inductively. If the pole of a permanent magnet be carried along from A' to M in a direction parallel to the bar, but not touching it, the part of the bar immediately opposite will be a pole of opposite name, and the advance of this induced pole towards M will be attended with a current in the coil, and its recession by an opposite current. It need hardly be mentioned that the same result is attained if the magnetic pole is stationary, and the bar with the coil upon it moved in proximity to it. Now imagine that the FIG. 2/5- Gramme Machine for the Laboratory or Lecture Table. bar is bent into a ring, the ends, A A', being united. If the ring be made to turn round its centre in its own plane, and near a magnetic pole, it is plain that when the coil is approaching this pole a current will be produced in it, and when it is receding, an opposite current. Let the number of coils be increased, and each coil in turn will be the seat of a current, or of the elec- trical state which tends to produce a current. In Fig. 275 the reader may see how this disposition is realized. The figure shows a form of the Gramme Machine adapted for the lecture-table or laboratory. A M' B M is the soft iron ring, covered with a series of separate coils placed radially, o is a com- pound horse-shoe steel magnet, s its south pole, N its north pole, each pole being armed with a block of soft iron hollowed into the segment of a circle and almost completely embracing the circle of coils. The magnetism of each pole is strongly developed in the interior faces of these armatures. The induc- tive action tends to produce two equal and opposite currents, which, like the currents of two similar voltaic batteries joined by their like poles, neutralize 39 ELECTRICITY. each other in the connected coils, but flow together through an external circuit. Fig. 276 will make clear the manner in which the coils, B B, are placed on the ring, A. The length of wire in each coil is the same, and the extremi- ties are attached to strips of copper, R R, which are fixed on the spindle of the machine. The two ends of each wire are connected with two consecu- tive strips, while the coils are insulated from each other, and thus each coil, like the element of a battery, contributes to the aggregate current. The cur- rents are drawn off, as it were, from these axial conductors at FIG. 276. Insulated Coils surrounding two opposite points of the ring, an Anmdus of Iron Wires. by springs very lightly touch- ing them on each side of the spindle, as may be seen in Fig. 275. In Fig. 277 is another arrangement of FIG. 277. Hand Gramme Machine, with Jamirts Magnet. the table apparatus with the magnet vertical, and formed according to the new plan suggested by M. Jamin, who finds the best magnets are made by tying together thin strips of steel. ELECTRICITY. But the importance of this invention consists in the facility which it affords for cheaply producing electricity on a scale adapted for industrial operations, for the deposition of metals, for artificial light, and for chemical purposes. The great importance of a cheap electric light for lighthouses prompted the British Government to permit the inventor to exhibit the light thus produced from the Clock Tower of the Houses of Parliament; for the signal light during the sittings of the House had previously been produced by a gas-light This electric light was produced by a powerful Gramme machine, such as that shown in Fig. 278, driven by a small steam engine in the vaults of the Houses of Parliament, and the ordinary carbon points, reflectors, &c., were used in the Clock Tower, where the light was exhibited ; copper wire - inch diameter being used to convey the current from the machine to the carbons. The result of these experiments may be gathered from the following extract from an official report made by the engineers of the Trinity House : " Pursuant to the instructions received from the Deputy Master to fur- nish you with my opinion on the relative merits of the electric and gas lights under trial at the Clock Tower, Westminster, I beg to submit the following report: On the evening of the ist ultimo I was accompanied by Sir F. Arrow (who kindly undertook to check my observations by his experience) to the Westminster Palace, where we met Captain Galton, R.E., Dr. Percy, and some gentlemen connected with the electric and gas appa- ratus under trial. I was informed that the stipulations under which the lights were arranged were, that they be fixed white to illuminate a sector of the town surface of 180, having a radius of three miles. I first exa- mined the Gramme magneto-electric machine, in use for producing the currents of electricity. This machine we found attached by a leather driving-belt to the steam engine belonging to the establishment. We then proceeded to the Clock Tower, where we found the electric lamp, at an elevation of 250 ft. The Wigham gas apparatus was placed at the same elevation, within a semi-lantern of twelve sides, about 8| ft. in diameter, and 10 ft. 3 in. high in the glazing. Near the centre of the lantern were three large Wigham burners, each composed of 108 jets. After the exa- mination of the apparatus, we proceeded to Primrose Hill, for the purpose of comparing the electric and gas lights at a distance of three miles. The evening, which was wet and rather misty, was admirably suited to our purpose, ordinary gas-lights being barely visible at a distance of one inile " The results of a photometric comparison of the electric and gas lights were as under, the machine making 389 revolutions per minute, and ab- sorbing 2 '66 horse-power; the illuminating power of the gas used being 25 candles, and the quantity consumed 300 cubic ft. per hour. Electric Light. Wigham Gas Burner. 108 jets. Relative intensity of lights Q4.C-C6 37crc6 Or as IOO 3Q'IQ Illuminating power i n standard sperm candles ^066 jy l y I I GO 1 .1,1 v^y 392 ELECTRICITY. "Electric Light. Total cost per session ^174 $s. od., being equal to 5-T. yd. per hour of exhibition of the light. Details shown in the full report. Gas Light. Total cost per session of one burner of 108 jets, i 59 I %s. ^d., equal to 5-r. J'4^. per hour of exhibition of light, and 296 3^. 4 ?^ G 1 more resorted to. During this process the cable parted, and Fig. 300 shows the scene on board the Great Eastern produced by this occurrence, as represented by an artist of the" Illustrated London News" who accom- panied the expedition. The broken cable was caught several times by grapnels, and raised a mile or more from the bottom, but the tackle proved 428 THE ELECTRIC TELEGRAPH. unable to resist the strain, and four times it broke ; and after the spot had been marked by buoys, the Great Eastern steamed home to announce the failure of the great enterprise. For this 5,500 miles of cable had altogether been made, and 4,000 miles of it lay uselessly at the bottom of the ocean, after a million and a quarter sterling had been swallowed up in these attempts. FIG. 299. The Instrument-Room at Valentia. But these disasters did not crush the hopes of the promoters of the great enterprise, and in the following year the Great Eastern again sailed with a new cable, the construction of which is shown of the actual size, in Fig. 301. In this there is a strand of seven twisted copper wires, as before, forming the electric conductor; round this are four coatings of gutta-percha; and surrounding these is a layer of jute, which is protected by ten iron wires (No 10, B.W.G ) of Webster and Horsfall's homogeneous metal, twisted spirally about the cable ; and each wire is enveloped in spiral strands of Manilla hemp. The Great Eastern sailed on the I3th of July, and on the 28th the American end of the cable was spliced to the shore section in Newfoundland, and the two continents were again electrically .connected. They have since been even more so, for the cable of 1865 was eventually fished up. and its electrical condition was found to be improved rather than injured by its sojourn at the bottom of the Atlantic. It was spliced to a new length of cable, which was successfully laid by the Great Eastern, and was soon joined to a Newfoundland shore cable. There were now two cables connecting England and America, and one connect- ing America and France has since been laid. At the present time up- wards of 20,000 miles ot submerged wires are in constant use in various parts of the world. Certain interesting phenomena have been observed in connection with submarine cables, and some of the notions which were formerly entertained THE ELECTRIC TELEGRAPH. 429 FIG. yx>. The Breaking of the Cable. as to the speed of electricity have been abandoned, for it has been ascer- tained that electricity cannot properly be said to have a velocity, since the same quantity of electricity can be made to traverse the same distance with extremely different speeds. No effect can be perceived in the most deli- cate instruments in Newfoundland for one-fifth of a second after contact has been made at Valentia ; after the lapse of another fifth of a second the received current has attained about seven per cent, of its greatest perma- nent strength, and in three seconds will have reached it. During the whole of this time the current is flowing into the cable at Valentia with its maximum intensity. Fig. 302 expresses these facts by a mode of repre- sentation which is extremely convenient. Along the line O X the regular intervals of time in tenths of seconds are marked, commencing from O, and the intensity of the current at each instant is expressed by the length of the upright line which can be drawn between O x and the curve. The curve therefore exhibits to the eye the state of the current throughout the whole time. If after nearly a second's contact with the battery the cable be con- nected with the earth at the distant end, the rising intensity of the current will be checked, and then immediately begin to decline somewhat more gradually than it rose, as indicated by the descending branch of the curve in Fig. 302. A little reflection will show the unsuitability for such currents of 430 THE ELECTRIC TELEGRAPH. instruments which require a fixed strength to work them. We may remark that, supposing a receiving instrument were in connection with the Atlantic Cable which required the maximum strength of the received current to work it, the sending clerk would have to maintain contact for three seconds FIG. 301. Atlantic Telegraph Cable, 1866. before this intensity would be reached, and then, after putting the cable to earth, he would have to wait some seconds before the current had flowed out. Several seconds would, therefore, be taken up in the transmission of IN SECONDS. FIG. 302. one signal, whereas by means of the mirror galvanometer about one-four- teenth of this time suffices, and the syphon recorder will write the messages twelve times as fast as the Morse instrument. The cause of the gradual rise of the current at the distant end of a submarine cable must be sought for in the fact that the coated wire plays the part of a Leyden jar, and the electricity which pours into it is partly held by an inductive action in the surrounding water. The importance of Sir W. Thompson's inventions as THE ELECTRIC TELEGRAPH. regards rapidity of signalling, upon which the commercial success of the Atlantic Cable greatly depends, will now be understood. By furnishing the means of almost instantaneous communication be- tween distant places, the electric telegraph has enabled feats to be per- formed which appear strangely paradoxical when expressed in ordinary language. When it is mentioned as a sober fact that intelligence of an event may actually reach a place before the time of its occurrence, a' very extraordinary and startling statement appears to be made, on account of the ambiguous sense of the word time. Thus it appears very marvellous that details of events which may happen in England in 1876 can be known in America in 1875, but it is certainly true ; for, on account of the difference of longitude between London and New York, the hour of the day at the latter place is about six hours behind the time at the former. It might, therefore, well happen that an event occurring in London on the morning of the ist of January, 1876, might be discussed in New York on the night of the 3 ist of December, 1875. There are on record many wonderful in- stances of the celerity with which, thanks to electricity, important speeches delivered at a distant place are placed before the public by the newspapers. And there are stories in circulation concerning incidents of a more ro- mantic character in connection with the telegraph. The American journals not long ago reported that a wealthy Boston merchant, having urged his daughter to marry an unwelcome suitor, the young lady resolved upon at once uniting herself to the man of her choice, who was then in New York, en route for England. The electric wires were put in requisition ; she took her place in the telegraph office in Boston, and he in the office in New York, each accompanied by a magistrate; consent was exchanged by electric currents, and the pair were married by telegraph ! It is said that the merchant threatened to dispute the validity of the marriage, but he did not carry this threat into execution. The following jeu d' esprit appeared a short time ago in " Nature," and, we strongly suspect, has been penned by the same hand as the lines quoted from " Blackwood," on page 385. ELECTRIC VALENTINE, (Telegraph Cterk r the small weights used by the analytical chemist. 1 1 would make admirable utensils for the more delicate operations of cooking replacing the copper ones, which render pickles and soups so poisonous. It is ex- tremely sonorous, and would make capital bells." Some difficulty in working the metal has occurred from the want of any suitable solder. This difficulty has been overcome by electrolytically coat- ing the metal with copper at the place where it has to be united with others, and then soldering the copper in the ordinary manner. Aluminium readily forms alloys with copper, silver, and iron. The alloys with copper vary in colour from white to golden yellow, according to the proportion of the metals. Some of these alloys are very hard and possess excellent working qualities. The alloy of copper with 10 per cent, of aluminium, which is called aluminium bronze, has been manufactured by Messrs. Bell in con- siderable quantities. It is made by melting a quantity of very pure copper in a plumbago crucible, and when the crucible has been removed from the furnace, the solid aluminium is dropped in. An extraordinary increase of temperature then occurs : the whole mass becomes white hot, and unless the crucible be made of a highly refractory material, it is fused by the heat developed in the combination of the two metals, although it may have stood the heat necessary for the fusion of copper. The qualities of aluminium bronze have been investigated by Lieut.- Col. Strange, who finds that the alloy possesses a very high degree of ten- sile strength, and also great power of resisting compression. Its rigidity, or power of resisting cross strains, is also very great ; in other words, a bar of the alloy, fixed at one end and acted on at the other by a transverse force tending to bend it, offers great resistance, namely, three times as much as gun-metal. An advantage attending the use of the alloy for many delicate purposes is found in its small expansibility by heat ; it is therefore well adapted for all finely-graduated instruments. It is very malleable, has excellent sounding properties, and resists the action of the atmosphere. It works admirably with cutting tools, turns well in the lathe, and does not clog the files or other tools. It is readily made into tubes, or wires, or other desired forms. The elasticity it possesses is very remarkable ; for wires made of it are found to answer better for Foucault's pendulum expe- .riment than even those of steel. These admirable qualities would seem to recommend the alloy for many applications in which it might be expected to excel other metals. It appears, however, that the demand for it has not met the expectations of the manufacturers, and the production has been somewhat diminished of late, although it is used to some extent for chains, pencil-cases, toothpicks, and other trinkets. When more than 10 per cent, of aluminium is added to the copper, the alloy produced is weaker ; and if the proportion is increased beyond a certain extent, the bronze becomes so brittle that it may be pulverized in a mortar. The metal magnesium was first prepared, in 1830, by the French chemist Bussy, by a process similar to that by which Deville obtained aluminium. NEW METALS. 511 rcelain ad been Bussy heated anhydrous magnesium chloride with potassium in a po crucible ; and when the vessel had cooled, and the soluble residue ha dissolved out by water, the metal was found as a grey powder, which could be melted into globules. The recognition of the metal as the base of magnesia is, however, due to Davy. About a quarter of a century after Bussy's dis- covery Ueville having shown that sodium could be substituted for potassium in such reductions, the metal became more cheaply producible, and soon afterwards Bunsen and Roscoe pointed out its value as a source of light. Mr. Sonstadt devoted himself to the elaboration of a method of working Deville's process on the large scale, and he succeeded in establishing a company in Manchester for the manufacture. The process as carried on at the company's works in Salford is thus described in the " Mechanics' Magazine," 3Oth August, 1867 : " Lumps of rock magnesia (magnesium- carbonate) are placed in large jars, into which hydrochloric acid in aqueous solution is poured. Chemical action at once ensues : the chlorine and the magnesium embrace, and the oxygen and carbon pass off in the form of carbonic acid. The result is magnesium in combination with chlorine, and the problem now is how to dissolve this new alliance to get rid of the chlorine and so cbtain the mag- nesium. First, the water must be evaporated, which would be easy enough if not attended with a peculiar danger. To get the magnesium chloride perfectly dry it is necessary to bring it to a red heat ; but this would result in the metal dropping its novel acquaintance with chlorine and resuming its ancient union with oxygen. To avert this re-combination, the magne- sium chloride whilst yet in solution is mixed with sodium chloride (/. when he recovers. Third, the patient may talk, laugh, or sing during the operation ; but what he says is altogether devoid of reference to what is done. Fourth, he may be conscious of what is taking place, and may look on while some minor operation is proceeding, without feeling it, or with- out feeling it painfully. This is often the condition of the patient as the effect is passing off, while some smaller.operation is still proceeding. Fifth, the patient may complain he is being hurt ; but afterwards, when the effect of the chloroform has passed off, he will assert that he felt no pain what- ever. When the chloroform has been inhaled for but a short time, the patient becomes conscious in about five minutes after its discontinuance ; but with a longer inhalation the period of unconsciousness may last for perhaps ten minutes. The return of consciousness takes place with tran- quillity : not unfrequently the patient's first speech, even after a serious operation, often being an assertion that the chloroform has not taken effect. In the strongest degree of ether and chloroform effects, all the muscles of the body are relaxed ; the limbs hang down, or rest in any position in which they are placed ; the eyelids droop over the eyes, or remain as they are placed by the finger ; the breathing is deep, regular, and automatic ; there is often snoring, and this is, indeed, characteristic of the deepest degree of unconsciousness ; the relaxation of the muscles renders the face devoid of expression, and with a placid appearance, as if the person were in a sound natural sleep. He is perfectly passive under every kind of opera- tion. The breathing and the action of the heart proceed all the while with unimpaired regularity. It is, however, known by experiments on animals that if the inhalation be prolonged beyond the period necessary to produce these effects, the respiratory functions are interfered with by the insensi- bility extending to the nerves on which they depend. The breathing of an animal thus treated becomes irregular, feeble, or laborious, and death en- sues. However nearly dead from inhalation of ether vapour the animal may be, provided respiration has not actually ceased, it always recovers when allowed to breathe fresh air. Of course, the etherization is never carried to this stage with human beings. Air containing 2 grs. of chloroform in 100 cubic inches suffices to induce insensibility ; but 5 grs. in 100 cubic inches is found a more suitable pro- portion. Dr. Snow, who strongly disapproved of the uncertain and irre- gular mode of administering chloroform on a handkerchief or sponge, contrived the inhaling apparatus already described. The air before reach- ing the mouth and nostrils of the patient passes through a vessel containing bibulous paper moistened with chloroform. This vessel he surrounds with water at the ordinary temperature of the air, in order to supply the heat absorbed by the conversion of the liquid into vapour, so that the formation ANAESTHETICS. 527 of the latter may go on regularly. The same thoughtful arrangement formed part of the ether-inhaler he had previously contrived. The extraordinary effects of ether and chloroform have introduced new and important facts into psychological science, and have illustrated and extended some of the most interesting results of physiological research. Let us trace the action of these substances, and explain it as far as may be. Nitrous oxide, ether vapour, and chloroform vapour are all soluble in watery fluids. The lungs present a vast surface bathed by watery fluids, and therefore these gases are largely absorbed ; and by a well-known pro- cess, they pass directly into the blood, through the delicate walls of the capillary vessels. The odour of ether can be detected in any blood, drawn from persons under its influence. Ether, or chloroform, thus brought into the general current of the circulation, is quickly carried to all parts of the body, and thus reaches the nerve-centres. , On these it produces character- istic effects by suspending or paralysing nervous action : why or how this effect takes place is unknown. The nervous centres are not all acted upon in an equal degree some require a larger quantity of the drug to affect them at all. The parts of the nervous system first affected are the cerebral lobes, which are known to be the seat of the intellectual powers. The cerebellum the function of which there is reason to believe is the regulation and co- ordination of movements is the next to yield to the influence. Then follow the spinal nerves, which are the seat of sensibility and motive power. This is as far as the action can safely be carried : the nervous centre called the medulla oblongata, which is placed at the junction of the brain and the spinal cord, still performs its functions one of the most important of which is to produce the muscular contractions that keep the respiratory organs in action. We have seen, by the effects of further etherization in animals, that when this part of the system is affected, the animal dies from a stop- page of the respiration. But, unfortunately, there have been instances in which death has been caused by the administration of ether and chloroform even under the most skilful management. But these occurrences were not the result of the inha- lation having been carried so far as to stop respiration : in some cases the patient has died before the first stage of insensibility. These fatal cases have all been marked by a sudden paralysis of the heart that organ has abruptly ceased to act. Why in these, certainly a very small percentage of patients, the action of the drug should at once take effect on the heart has not yet been explained. The rhythmic action of the heart depends upon nervous centres enclosed within its own substance, so that this organ is to a certain extent independent ; but it is connected with the other nervous centres by the branches of a remarkable nerve which proceeds from the medulla oblon- gata^ and also by another set of nerves which come from the chain of ganglia called the sympathetic nerve. The nerve connecting the heart with the medulla is a branch of that called the pneumo-gastric, and it is a well- established fact that the action of the heart may be arrested by irritation of this nerve. The comparatively few fatalities which have attended the use of anaesthetics may, therefore, be due either to an immediate action on the nerve-centres of the heart, or possibly to a mediate action through the medulla and the pneumo-gastric nerve. Soon after the introduction of ether the use of nitrous oxide was discon- tinued by the dentists, on account of the apparent uncertainty of its action. Within the last few years, however, its employment in the extraction of teeth has been revived by Dr. Evans, of Paris, who found that to insure '528 ANAESTHETICS. certainty in its action, the great point is the inhalation of the gas in a pure state and without admixture of air. Nitrous oxide seems now to be exten- sively used by dentists, and thus Davy's experiment of 1800 is repeated and verified daily in thousands of cases, and to the great relief of hundreds who probably never heard his name. Other bodies, such as amylene (C 5 H 10 ), carbon tetrachloride (CC1 4 ), &c., have been tried as substitutes for ether and chloroform ; but having been found less efficacious or more dangerous, their use has been abandoned. It might be instructive to reflect how much unnecessary pain would have been spared to mankind had ether and chloroform been known and ap- plied at an earlier age. We know not what other beneficent gifts chemistry may yet have in store for the alleviation of suffering, but it is unlikely that even ether and chloroform are her derniers mots. It should be remembered that the chemists who discovered and examined these bodies were attracted to the work by nothing but the love of their science. They had no idea how invaluable these substances would afterwards prove. The chemist of the present day, whose labour is often its own reward, may be cheered and stimulated in his toil by the thought that while no discovery is ever lost, but goes to fill its appropriate place in the great edifice of science, even the most apparently insignificant truth may directly lead to invaluable re- sults for humanity at large. What strange things the ancient thaumaturgists might have done had they been possessed of the secret of chloroform or of nitrous oxide ! What miracles they would have wrought what dogmas they would have sanc- tioned by its aid ! But the remarkable effects produced by the inhalation of certain gases or vapours were not altogether unknown to the ancients although these effects were then attributed to anything but their real cause. It is related that a number of goats feeding on Mount Parnassus came near a place where there was a deep fissure in the earth, and there- upon began to caper and frisk about in the most extraordinary manner. The goatherd observing this, was tempted to look down into the hole, to see what could have caused so extraordinary an effect. He was himself immediately seized with a fit of delirium, and uttered wild and extravagant words, which were supposed to be prophecies. The knowledge of the pre- sumed divine inspiration spread abroad, and at length a temple in honour of Apollo was erected on the spot. Such was the origin of the famous Oracle of Delphi, where the Pythoness, the priestess of Apollo, seated on a tripod placed over the mysterious opening, delivered the response of the god to such as came to consult the oracle. It is stated by the ancient writers, that when she had inhaled the vapour, her eyes sparkled, convul- sive shudders ran through her frame, and then she uttered with loud cries the words of the oracle, while the priests who attended took down her in- coherent expressions, and set them in order. These possessions by the spirit of divination were sometimes violent. Plutarch mentions a priestess whose frenzy was so furious, that the priests and the inquirers alike fled terrified from the temple ; and the fit was so protracted that the unfortu- nate priestess herself died a few days afterwards. FlG. 339. A Railway Cuffing. EXPLOSIVES. T^HE illustration above will serve to remind the reader of the great im- portance of explosive agents in the operations of civil industry. By reason of the more impressive and exciting spectacles which attend the use of such agents in warfare, we are rather apt to lose sight of their far more extensive utility as the giant forces whose aid man invokes when he wishes to rend the rock in order to make a road for his steam horse, or in order to penetrate into the bowels of the earth in search of the precious ore. A little reflection will show that if such work had to be done with only the pickaxe, the chisel, and the crowbar, the progress would be pain- fully slow ; and railway cuttings through masses of compact limestone, like that represented in Fig. 339, for example, would be well-nigh impos- sible. The formation of cuttings and tunnels, and the removal of rocks in mining operations, are not the only service which explosive agents render to the industrial arts ; there is, besides other uses which might be enume- rated, the preparation of foundations for buildings, bridges, harbours, and lighthouses. The use of gunpowder in all such operations as those which have been referred to is too well known to require description. But of late years gunpowder has been to a great extent superseded for such purposes by two remarkable products of modern chemistry, called gun-cotton and nitro-glycerine. Military art has also benefited by at least one of these 529 34 530 EXPLOSIVES. products ; and the use of charges of gun-cotton for torpedoes has already been described and illustrated in these pages. It is not a little curious that the two most terribly powerful explosives known to science should be prepared from two most harmless and familiar substances. The nice, soft, clean, gentle cotton-wool, in which ladies wrap their most delicate trinkets, becomes, by a simple chemical transformation, a tremendously powerful explosive ; and the clear, sweet, bland liquid, glycerine, which they value as a cosmetic for its emollient properties, be- comes, by a like transformation, a still more terrifically powerful explosive than the former. It is, perhaps, even more curious that having undergone the transformation which confers upon it these formidable qualities, neither cotton-wool nor glycerine is changed in appearance. The former remains white and fleecy ; the latter is still a colourless syrupy-looking liquid. The fibres which form cotton, linen, paper, and wood, are composed almost entirely of a substance which is known to the chemist as cellulose or cellu- lin. That this substance, as it exists in the fibres of linen and in sawdust, could be converted into an explosive body by the action of nitric acid, ap- pears to have been first observed by the French chemist, Pelouze, in 1838. The action with cellulose in the form of cotton-wool was more fully examined by Professor Schonbein, of Basle, who, in 1846, first described the method of preparing gun-cotton, and suggested some uses for it. He directs that one part of finely-carded cotton-wool should be immersed in fifteen parts of a mixture of equal measures of strong sulphuric and nitric acids ; that after the cotton has remained in the mixture for a few minutes, it should be re- moved, plunged in cold water, and washed until every trace of acid has been removed, and then carefully dried at a temperature not exceeding the boiling-point of water. After Professor Schonbein had demonstrated the power of the new agent in blasting, and its projectile force in fire-arms, its manufacture on a large scale was undertaken at several places. Messrs. Hall commenced to make it at their gunpowder works at Faversham, and a manufactory was also established near Paris. In July, 1847, a fearful explosion of gun-cotton occurred at the Faversham works, which was believed to have been caused by the spontaneous detonation of that substance. This induced Messrs. Hall to discontinue the manufacture as too dangerous ; and they even destroyed a large quantity of the product which they had in hand by burying it in the ground. The making of gun-cotton was soon afterwards discontinued also by the French, who did not find the substance to possess all the qualities fitting it for military use. The Prussian Government also began to make gun-cotton ; but the experiments were put a stop to by the explosion of their factory. An eminent artillery officer in the Austrian service, General von Lenk, undertook a thorough examination of the manu- facture and properties of gun-cotton for military purposes. He introduced several improvements into the processes of the manufacture ; and the Austrian Government established works at Hirtenberg, with a view to .the adoption of gun-cotton as a substitute for gunpowder in fire-arms. It has some undoubted advantages over powder, for it neither heats the gun nor fouls it. and it produces no smoke. Notwithstanding this the Austrians have not abandoned the use of gunpowder in favour of gun-cotton. Gun-cotton, as a military agent, has a strenuous advocate in Professor Abel, who presides over the Chemical Department of the British War Office. To this gentleman we are indebted for great improvements in the manufacture of gun-cotton, and for a more complete investigation of its EXPLOSIVES. 531 properties. Professor Abel's processes were put in practice at a manufac- tory which the Government established at Waltham Abbey ; and Messrs. Prentice also set up works at Stowmarket. Some details of the mode in which the manufacture of gun-cotton was carried on at Stowmarket may be of interest. The cotton was first tho- roughly cleansed and carefully dried ; and these operations are of great importance, for unless they are well performed, the product is liable to explode spontaneously. The cotton was then weighed out in charges of i lb., and each charge was completely immersed in a separate vessel, con- taining a cold mixture of sulphuric and nitric acids. After a short immer- sion the cotton was removed from the liquid, and with about ten times its own weight of acids adhering to it, each charge was placed in a separate jar, where it was allowed to remain for forty eight hours. The vessels were kept cool during the whole period by being placed in a trough through which cold water was flowing. On removal from the jars, the cotton was freed from adhering acid by being placed in a centrifugal drying machine. It was then drenched with a large quantity of cold water, and dried, washed again in a stream of cold water for forty-eight hours, and the operations of alternately washing for forty-eight hours and drying were repeated eight times. The drying was effected by placing the material in cylinders of wire-gauze, which were whirled round by a steam engine at the rate of 800 revolutions per minute, so that the water was expelled by centrifugal force. The cotton was next reduced to a pulp by a process similar to that which is employed in paper-making, and the moist pulp was rammed into me- tallic cylinders by hydraulic pressure, in order that it might be brought into forms suitable for use in blasting, &c. The pulp was put into these moulds while wet, but the water was nearly all expelled by the compression. The cylinders of gun-cotton thus obtained were then covered with paper-parch- ment, and finally dried at a steam temperature, with many precautions. The compression of the cotton pulp, by bringing a large quantity of the material into a smaller bulk, causes a greater concentration of the explo- sive energy, and this is a matter of great importance in blasting. We may now consider what chemistry has to teach concerning the nature of the action by which cotton-wool is converted into gun-cotton. Cotton itself is nearly pure cellulose. The chemical composition of cellulose may be represented most simply by the formula C ft H 10 O 5 . Nitric acid is a powerful oxidizing agent, and is constantly used in chemistry to fix oxygen in various substances ; but another kind of action exerted by nitric acid in certain cases consists in the substitution of a portion of its atoms for hy- drogen, by which the residue of the particle of nitric acid is converted into water. The formula for nitric acid may be written HO NO 2 , and it will be seen that by changing NO 2 for H, water, HOH, would be pro- duced. This is precisely the kind of action which occurs when cellulose is converted into nitro-cellulose. Two or three, or more, atoms of hydrogen may be taken out of cellulose, and replaced by two or three, or more, groups NO 2 , and the result will be a different kind of nitro-cellulose, according to the number of atoms in the molecule replaced by NO 2 . Several varieties of gun-cotton are known, these being doubtless the result of the differences here alluded to. The action producing di-nitro-cellulose is represented by this equation : CH 10 5 + 2HN0 3 = C 6 H 8 (N0 2 ) 2 5 + 2 H 2 O. Cellulose. Nitric acid. Di-nitro-cellulose. Water. 34 2 53 2 EXPLOSIVES. The equation shows that water is produced by the reaction, and the sul- phuric acid which is used in the preparation performs no further part than to take up this water, which would otherwise go to dilute the rest of the nitric acid. The union of sulDnuric acid and water is attended with great heat, hence the necessity of cooling the vessels in making the gun-cotton. Quite other products would be formed if the mixture became heated. The action of nitric acid on glycerine is of the same kind as that on cellulose. When glycerine is allowed to drop into a cooled mixture of nitric acid and sulphuric acid, the eye can detect little or no difference between the appearance of the liquid which collects in the bottom of the vessel and the glycerine dropped in. The product of the action is, however, the ter- rible nitto- glycerine, a heavy, oily-looking liquid, which explodes with fearful violence. Even a single drop placed on a piece of paper, and struck on an anvil, detonates violently and with a deafening report. The chemical change which is effected in the glycerine (C 3 H 8 O 3 ), is the substitution of three NO 2 groups for three of hydrogen, producing C 3 H 5 (NO 2 )3O 3 , or tri-nitro-gfycerine. The general reader may perhaps marvel that the che- mist should be able not only to count the number of atoms which go to make up the particles of a compound body, but to say that they are arranged so and so : that the atoms do not form an indiscriminate heap, but that they are connected in an assignable manner. The reader is no doubt aware that these compound particles are extremely small, and he may reasonably wonder how science can pronounce upon the structure of things so small. He may be more perplexed to learn that a calculation made by Sir W. Thompson shows that the particles of water, for instance, cannot possibly be more than the 250000000^ of an inch in diameter, and may be only i-2Oth of that size. The truth is that the very existence of atoms and molecules is an assumption. Like the undulatory ether, it is an hypothesis which is adopted to simplify and connect our ideas, and not a demonstrated reality. But the atomic hypothesis has so wide a scope that some philosophers hold the existence of atoms and molecules as almost a known fact Be that as it may, the chemist in assigning to a body a certain molecular formula really does nothing but express the results of certain experiments he has made upon it. With one re-agent it is decomposed in this manner, with another in that. By certain treatment it yields an acid, a salt ; so much carbonic acid, such a weight of water, is acted on or remains unaltered ; gives a precipitate or refuses to do so. Such are the facts which the che- mist conceives are co-ordinated and expressed by the formula he gives to a substance. The best formula is that which accords with the greatest number of the properties of the body which includes as many of the facts as possible. It follows, therefore, that a formula which aims at expressing more than the mere percentage composition of the body which, in the language of the atom hypothesis, seeks to represent the mode in which the atoms are grouped in the molecule, but which in reality represents only reactions, is written according as the chemist considers this or that group of reactions more important. These remarks might be illustrated by filling this page with the different formulae (a score or more) which have been proposed as representing the constitution (reactions ?} of one of the best- known of organic compounds, namely, acetic acid. Whether atoms really exist, and their arrangement in the particles of bodies can be deduced from the phenomena, or not, the fact is undeniable that these ideas have given chemists a wonderful grasp of the facts of their science. The consistency and completeness of the explanation afforded EXPLOSIVES. 533 by these theories are ever being extended by modifications which enable them to embrace more and more facts. Some of the properties of the substance we are now considering confirm in a remarkable manner the theoretical views which are expressed in its constitutional formula. We may first consider the nature of gunpowder, and by comparing it with nitro- glycerine, endeavour to explain the greater power of the latter substance. Gunpowder is a mixture of charcoal, sulphur, and nitre, the latter consti- tuting three-fourths of its weight. Nitre supplies oxygen for the combus- tion of the charcoal, which is thus converted into carbonic acid, and the sulphur, which is added to increase the rapidity of the combustion, is also oxidized. The products of the action are, however, numerous and compli- cated, but the important result is the sudden generation of a quantity of carbonic acid, nitrogen, carbonic oxide, hydrogen, and other gases, which at the oxidizing temperature and pressure of the air would occupy a space 300 times greater than the powder from which they are set free ; but the intense heat attending the chemical action dilates the gases, so that at the moment of explosion they would occupy a space at least 1,500 times greater than the gunpowder. The materials of which gunpowder is com- posed are finely powdered, in order that each portion shall be in immediate contact with others, which shall act upon it. Plainly, the more thorough the incorporation of the materials that is, the more finely ground and intimately mixed they are the more rapid will be the inflammation of the powder. Looking now at the crude formula of nitro-glycerine, C 3 H 5 N 3 O tt , the reader will remark that the molecule contains more than sufficient oxygen to form carbonic acid with all the carbon atoms, and water with all the hydrogen atoms ; for the C 6 in two molecules of nitro-glycerine would take only O 12 to form 6CO 2 ; and the H 10 , to be converted into 5H 2 O, would only need O 5 ; thus there would be an excess of oxygen. Now, it may occur to the reflective reader that in every molecule of nitro-glycerine the carbon and hydrogen are already associated with as much oxygen as they can take up : that they are, in fact, already burnt, and that no further union is possible. But from chemical considerations it has been deduced that in the nitro-glycerine molecule the oxygen atoms, except only three, which are partially and imperfectly joined to carbon, are united to nitrogen atoms only. The constitution of the molecule may be represented by arranging, as below, the letters which stand for the atoms, and by joining them with lines, which shall stand for the bonds by which the atoms are united. O H H H O II III II N- O C C C O N II III II O H O H O 0-N-O We see here that the hydrogen atoms are completely, and the carbon atoms partially, detached from the oxygen atoms; and therefore these atoms are in the condition of the separated carbon and oxygen atoms in gunpowder. Only the pieces of matter which lie side by side in gunpowder are in size to the molecules of nitro-glycerine as mountains to grains of sand. The mixture of the materials is then so much more intimate in nitro-glycerine, since atoms which can rush together are actually within 534 EXPLOSIVES. the limits of the molecules ; and these molecules have such a degree of minuteness, that 25 millions, at least, could be placed in a row within the length of an inch. We know that the finer the grains and the more inti- mate the mixture, the quicker will gunpowder inflame ; but here we have a mixture far surpassing in minute subdivision anything we can imagine as existing in gunpowder. Hence the combination in the case of nitro-glycerine must be instantaneous, whereas that in gunpowder, quick though it be, must still require a certain interval. If it take a thousandth of a second for the gases to be completely liberated from a mass of gunpowder, and only one-millionth of a second for a vast quantity of carbonic acid, nitro- gen, and steam to be set free from nitro-glycerine, the destructive effect will be much greater in the latter case. Again, the volume of the gases liberated from nitro-glycerine in its detonation have at least 5,000 times the bulk of the substance. We have entered into these chemical conside- rations, at some risk of wearying the reader, with the desire of affording him a clue to the singular properties of nitro-glycerine and gun-cotton, which we are about to describe. The nature of the chemical changes which maybe set up in an explosive substance, and the rapidity with which these changes proceed throughout a mass of the material, are greatly modified by the conditions under which the action takes place. If a red-hot wire be applied to a small loose tuft of gun-cotton, it goes off with a bright flash without leaving any smoke or any other residue. Thus, when the substance is quite unconfined, no ex- plosion occurs. If the cotton-wool be made into a thread, and laid along the ground, it will burn at the rate of about 6 in. per second ; if it be twisted into a yarn, the combustion will run along at the rate of 6 ft. per second ; but if the yarn be enclosed in an Indian-rubber tube, the ignition proceeds at the rate of 30 ft. in a second. If to a limited surface of gun-cotton, such as one end of a length of gun-cotton yarn, a source of heat is applied the temperature of which is high enough to set up a chemical change, but not high enough to inflame the resulting gases (carbonic oxide, hydrogen, &c.) the cotton burns comparatively slowly, rather smouldering than inflam- ing. If, however, aflame be applied, the gun-cotton flashes off with great rapidity, because the heat applied sets fire to the gaseous products of the chemical action. But if the gun-cotton be confined so that the gases can- not escape, the combustion becomes rapid however set up. The reason is that if the gases escape into the air, they carry off so much of the heat pro- duced by the smouldering gun-cotton, that the temperature does not rise to the extent required to produce the flaming ignition, in which the pro- ducts are completely oxidized. If a mass of gun-cotton be enclosed in a capacious vessel from which the air has been removed, and the gun-cotton be ignited by means of a wire made hot by electricity, the cotton will at first only burn in the slow way without flame ; but as the gases accumulate and exert a pressure which retards the abstraction of heat accompanying their formation, the temperature will rise and attain the degree necessary for the complete and rapid chemical changes involved in the flaming com- bustion. Thus, the more resistance is offered to the escape of the gases, the more rapid and perfect is the combustion and explosive force produced by the ignition. Now, the explosion of gun-cotton has been found to be too rapid when it is packed into the powder-chamber of a gun, for its ten- dency is to burst the gun before the ball has been fairly started. Hence a material like gunpowder, in which the combustion is more gradual, is better suited for artillery. The ignition of gunpowder, though rapid, is not EXPLOSIVES. 535 instantaneous, and therefore we can speak of it as more or less gradual. Indeed, in even the most violent explosives, some time is doubtless re- quired for the propagation of the action from particle to particle. This extreme rapidity of combustion, and consequent rending power, which is so objectionable in a gun-chamber, makes gun-cotton a most powerful bursting charge for shells, and, when it is enclosed in strong receptacles, for torpedoes also. But by the researches of Nobel, Professor Abel, and others, it has been discovered and this is, perhaps, the most remarkable discovery in con- nection with explosives that gun-cotton, nitro-glycerine, and other explo- sive bodies, are capable of producing explosions in a manner quite different from that which attends their ignition by heat. The violence of this kind of explosion is far greater than that due to ordinary ignition, for the action takes place with far greater rapidity throughout the mass, and is, indeed, practically instantaneous. It appears to be produced by the mere mechani- cal agitation or vibrations which are communicated to the particles of the substance. Turning back to the representation of the molecule of nitro- glycerine on page 533, it will not be difficult to imagine that this may be an unstable kind of structure ; that the atoms of oxygen are prevented from rushing into union with those of hydrogen and carbon only by the inter- position of the nitrogen ; and that an agitation of the structure might shake all the atoms loose, and leave them free to re-combine according to their strongest affinities. Nitro-glycerine is by no means so ready to itiflame as is gun-cotton : it is said that the flame of a match may be safely extin- guished by plunging it into the liquid ; and when a sufficient heat is applied to a quantity of the liquid in the open air, it will burn quietly and without explosion. Even when nitro-glycerine is confined, the application of heat cannot always be made to produce its explosion ; or, at least, the circum- stances under which it can do scare not accurately known, and the opera- tion is difficult and uncertain. On the other hand, nitro-glycerine explodes violently even when freely exposed to the air if there be exploded in con- tact with it a confined charge of gunpowder, or a detonating compound such as fulminating powder. Gun-cotton possesses the same property of exploding by concussion, which appears indeed to be a general one belong- ing to all explosive bodies. According to recent researches, even gunpowder is capable of a detonative explosion. A mass of gunpowder confined with a certain proportion of gun-cotton, which is itself set off by fulminate of mercury, is said to exert an explosive force four t.mes greater than that developed by the ignition of the gunpowder in the ordinary manner. It has also been found that wet gun-cotton can be exploded by concussion, and the force of the explosion is unimpaired even when the material is saturated with water. This makes it possible to use gun-cotton with greater safety, as it may be transported and handled in the wet condition without risk, and it preserves its properties for an indefinite period without being deteriorated by the water. Some experiments illustrating the extraordinary force of the detonative explosions of gun-cotton and nitro-glycerine will give the reader an idea of their power. A palisade, constructed by sinking 4 ft. into the ground trunks of trees 1 8 in. in diameter, was completely destroyed in some experiments at Stow- market by the explosion of only 15 Ibs. of gun-cotton. Huge logs were sent bounding across the field to great distances, and some of the trees were literally reduced to match-wood. The gun-cotton, be it observed, was simply laid on the ground exposed to the air. The destructive powers of 53 6 EXPLOSIVES. nitro-glycerine are even greater. A tin canister, containing only a few ounces of nitro-glycerine, is placed, without being in any way confined, on the top of a smooth boulder stone of several tons weight ; it is exploded by a fuse containing fulminating powder, which is fired from a distance by electricity. There is a report, and the stone is found in a thousand frag- ments. The last experiment shows one of the advantages of nitro-glycerine over gunpowder as a blasting material, beyond its far greater power, which is about ten times that of gunpowder. A charge of gunpowder inserted in a vertical hole tends to force out a conical mass, the apex of which is at the space occupied by the charge. With nitro-glycerine, and also with gun-cotton, which last has almost six times the force of gunpowder, a powerful rending action is exerted below as well as above the charge. Again, in blasting with gunpowder the charge must be confined, and the hole is filled in above the charge with tightly rammed materials, forming what is termed the tamping. But nitro-glycerine requires no tamping : a small, thin metallic core containing the charge is simply placed in the drill-hole, or the liquid itself is poured in, and a little water placed above it. The effect of the explosion of nitro-glycerine in " striking down," when apparently no resistance is offered, will seem very strange to the reader who is oblivious of certain fundamental principles of mechanics. The force of the explosion is due entirely to the sudden production of an enor- mous volume of gas, which at the ordinary pressure would occupy several thousand times the bulk of the material from which it is produced. This gas, by the law of the equality of action and reaction, presses down upon the stone with the same force that it exerts to raise the superincumbent atmosphere. The pressure of the gas at the moment of its liberation is enormous ; but the atmosphere cannot instantaneously yield to this, for time is required to set the mass of air in motion, and the wave of com- pression advances slowly compared with the rapidity of the explosion. Hence the air acts, practically, like a mass of solid matter, against which the gases press, and which yields less readily than the rock, so that the blow which is struck takes visible effect on the latter. Now, with gun- powder, the evolution of gas is less rapid, the atmosphere has time to yield, and the reaction has not the same violence. The rapidity of the evolution of gas from the exploding nitro-glycerine is so great, that the gases, though apparently unconfmed, are not so in reality ; for the atmosphere acts as a real and very efficient tamping. When nitro-glycerine first came into use for blasting purposes, it was used in the liquid form under the name of " blasting oil;" but the dangers attending the handling of the substance in this state are so great, that it is now usual to mix the liquid with some powdered substance which is itself without action, and merely serves as a vehicle for containing the nitro- glycerine. To mixtures of this kind the names " dynamite" " dualine? " lithojracteur? &c., have been given. It is now hardly necessary to point out that the discovery of these new explosives has largely extended our power over the rocks, enabling works to be executed which would have been considered impracticable with less powerful agents. It is true that the most fearful disasters have been acci- dentally produced by the new explosives ; but such occasional devastation is the price exacted for the possession of powerful agents. And just as in other cases steam, for example where great forces are dealt with, so these new powers must be managed with unceasing care, and placed in the ha*nds of only skilful and intelligent men. FIG. 340. View on the Tyne. MINERAL COMBUSTIBLES. /CERTAIN mineral combustibles may fairly claim attention in a work ^-^ treating of the discoveries of the nineteenth century, not because these bodies have been known and used only in recent times, but for other rea- sons. The true nature of coal that most important of all combustibles its relation to the past history of the earth, and to the present and future interests of mankind ; the work it will do ; the extent of the supply still existing in the bowels of the earth ; the innumerable chemical products which it yields are subjects on which the knowledge gained during the present century forms a body of discovery of the most interesting and important kind. Another substance we have to mention, though not a modern discovery, has lately been found in far greater abundance, and is now so largely used for various purposes, that it has become an important article of commerce. COAL. TV/TOST persons know, or at least have been told, that coal is fossil 1Y1 vegetable matter, the long-buried remains of ancient forests. But 537 538 MINERAL COMBUSTIBLES. probably many receive the statement, not perhaps with incredulity, but with a certain vague notion that it is, after all, merely a daring surmise. And, indaed, nothing is at first sight more unlike stems, or leaves, or roots of plants than a lump of coal. Then everybody knows that coal is found thousands of feet beneath the surface of the earth, whereas plants can grow only in the light of the sun. One begins to understand the matter only when the teachings of geology have shown him that, so far from the crust of the earth being, as he is apt to suppose, fixed and unchangeable, it is in a state of constant fluctuation. Changes in the levels of the ground are always going on : in one place it is rising, in another sinking ; here a FlG. 341. Fossil Trees in a Railway Cutting. tract of land is emerging from the ocean, there a continent is subsiding beneath the water. The extreme slowness with which these changes pro- ceed causes them to escape all ordinary observation. The case may be compared to the hour-hand on the dial, which a casual spectator might pronounce quite stationary, since the observation of a few seconds fails to detect its movement. As the whole period comprehended in human annals counts but as a second of geological time, it cannot be wondered at that it required a vast accumulation of facts, and much careful and patient deduction from them, before a conclusion was reached so apparently con- tradictory of experience. It is, indeed, startling to learn that u the sure and firm-set earth " is in a state of flow and change. Even the " everlast- ing hills" give evidence that their materials were collected at the bottom MINERAL COMBUSTIBLES. 539 of the sea, and we know that the water which runs down their sides is slowly but surely carrying them back particle by particle. Of the magni- tude of the changes which the surface of the earth has undergone m times past, and which are still imperceptibly but constantly proceeding, the ordi- nary experience of mankind can of itself give no example. But such changes have sufficed to entomb a vast quantity of relics of the innumerable forms of vegetation which flourished and waved their branches in the sun, ages upon ages before the advent of man. It may be thought impossible that vegetable matter should have so changed as to become a dense, black, glistening, brittle mass, showing no obvious forms of leaves or texture of wood. But no one who has seen how a quantity of damp hay closely pressed together will, after a time, become heated and change in colour to black, can have any difficulty in comprehending how chemical and mechanical actions may completely alter the aspect of vegetable matter. We have, however, the most direct evidence of the vegetable origin of coal in the numberless unquestion- able forms of trees and plants met with in all coal strata. Sometimes the trunks of the trees fossilized into stony mat- ter are found upright in the very situation in which they grew. Thus in Fig. 341 is re- presented the appearance ex- hibited by the trunks and roots of some fossil trees, which were exposed to view in the formation of a railway cut- ting between Manchester and Bolton. In every coal-field also beautiful impressions of the stems and leaves of plants are met with one common form of which is shown in Fig. 342. Most of the plants so found belong to the flowerless division of the vegetable kingdom. Some are closely allied to the ferns of the pre- sent day to the common " mare's-tail " (Eqmsetinri}, to the club-moss, and to other well-known plants. The firs and pines of the coal age are scarcely distinguishable from existing species. If a fragment of ordinary coal be ground to a very thin slice so thin as to be transparent and placed under the microscope, it will show a number of minute rounded bodies, which are, there is good reason to believe, nothing else than the spores or seeds of plants, closely resembling the existing club-mosses. The spores of the* club-moss (Lycopodiitm) are so full of resinous matter, that they are used for making fireworks and the flashes of lightning at theatres. It is, therefore, extremely probable that the bitumen of coal is due to the resin of similar spores, altered by the effects of subterranean heat. The FlG. 342. Impression of Leaf found in Coal Measures (Pecopteris). 540 MINERAL COMBUSTIBLES. FIG. 343. Possible Aspect of the Forests of the Coal Age. immense abundance of these little spores in the coal is a proof that they accumulated in the ancient forests as the mosses grew, and therefore the matter of coal was not accumulated under water or washed down into the sea ; for these little spores are extremely light, and they cannot be wetted by water, and therefore they would have floated on the surface, and would not have been found so diffused throughout the coal. Fig. 343 is a picture of the possible aspect of the ancient forests of the coal age. In the humid atmosphere which probably prevailed at that period, the large tree-ferns and gigantic club-mosses, which are conspicuous in the picture, must have flourished luxuriantly. The immense importance of coal for domestic purposes will be obvious MINERAL COMBUSTIBLES. 54i from the fact that it is estimated that in the United Kingdom alone no less than 30,000,000 tons are annually consumed in house fires. Another great use of coal is in the smelting, puddling, and working of iron, and this pro- bably consumes as much as our domestic fireplaces. Then there is the vast consumption by steam engines, by locomotives, and by steamboats* Another purpose for which coal is largely used is the making of illumina- ting gas ; and to the foregoing must also be added the quantity which goes to feed the furnaces necessary in so many of the arts such as in the manufacturing of glass, porcelain, salt, chemicals, &c. The quantity of coal raised in Great Britain was not accurately known until 1854, when it was ordered that a register should be kept, and an annual return made. The following figures in round numbers represent the returns which have been published since that date : Year. Coal raised, in Tons. 1854 64,661,000 1855 64,453,000 1856 66,645,000 1857 65,395,000 1858 65,008,000 1859 71,979,000 1860 83,208,000 1861 85,635,000 1862 83,638,000 1863 88,292,000 Year. Coals raised, in Tons. 1864 92,787,000 1865 98,150,000 1866 101,630,000 1867 104,500,000 1868 103,141,000 1869 107,427,000 1870 110,289,000 1871 117,352,000 1872 123,497,000 1873 127,017,000 The first return showed our annual produce to be 64,661,000 tons. The amount did not greatly vary until 1859, when there was an increased production of nearly seven millions of tons ; in 1860 a further increase of eleven millions of tons more. Since then the quantity annually raised has been increasing. Comparing the quantity which has been raised in any year after 1 863 with that raised ten years before, we see that the increase in ten years is nearly half as much again ; or, that at the present rate of increase the amount annually raised doubles itself at least every twenty years. Now, the question arises, How long can this go on ? However great may be the store of coal, it must sooner or later come to an end. Is it possible to calculate how long our coals will last ? and what are the results of such calculations ? These calculations have been made ; but there are great discrepancies in the results, for the estimates of the amount of available coal still remaining vary greatly, and different views are held regarding the rate of consumption in ihe future. A very liberal estimate, by an excellent authority, of the quantity of coal remaining under British soil, makes it 147,000 millions of tons. With a consumption stationary at the present rate, this will last 1,200 years ; with an increase of consumption of 3,000,000 tons a year, 276 years ; but if the consumption continues to increase in the same geometrical ratio it has hitherto followed, the supply will scarcely last 100 years. It cannot be conceived, however, that this last will be the real case, for the increasing depth to which it will be neces- sary to go will soon cause a great increase in the cost, and thus effectually check the consumption. Great Britain will, however, be compelled to retire from the coal trade altogether, by the cheaper supplies which other countries will yield, long before the absolute exhaustion of her own coal- fields. It is calculated that the coal-fields of North America contain thirteen times as much as those of all Europe put together. Coal is also found 542 MINERAL COMBUSTIBLES. abundantly in India, China, Borneo, Eastern Australia, and South Africa; and it is believed that these stores will supply the world for many thousand years. Seeing, then, that our supply of coal has a limit, and that at the present increasing rate of consumption, the chief source of the wealth of Great Britain must necessarily be exhausted in a few more centuries, it behoves us to turn our mineral treasures to the best account, and to adopt every possible means of obtaining from our coal its whole available heat and force. The amount of avoidable waste of which we are guilty in the con- FIG. 344. The Fireside. sumption of coal is enormous. This is especially the case in its domestic use, where probably nine teen- twentieths of the heat produced is absolutely thrown away sent off from the earth to warm the stars. In England people look upon the wide open fireplace as the image and type of home comfort. No doubt there are, from long use and habit, many pleasing as- sociations which cluster round the domestic hearth ; but we, to whom it is given to " look before and after," must think what it takes to feed that wide- throated chimney. All but a very small fraction of the heat thus escapes, and is lost to man and the world for ever ; and surely we shall deserve the curses of our descendants if we continue recklessly to throw away a treasure which, unlike the oil in the widow's cruse, is never renewed for there is no contemporaneous formation of coal. Thanks to the enhanced price of coal during the last few years, some attention has been directed to con- MINERAL COMBUSTIBLES. 543 trivances for the economical consumption of coal in its domestic, as well as in its manufacturing, applications. A time, however, will sooner or later come, when the whole available coal shall have been consumed. What will then be the fuel of the engines, and steamboats, and locomotives of the future ? The reader may think that then it will only be necessary to burn wood. But wood is already being consumed from the face of the earth much more rapidly than it is produced. How, then, can it be available when coal fails? The truth is, we are now consuming not merely the wood which the sun-rays are build- ing up in our own time, but in hewing down the forests we are using the sun-work of a century, while in coal we have the forests of untold ages at our disposal the accumulated combustible capital stored up during an immense period of the earth's existence. Upon this stored-up capital we are now living, our current receipts of sun-force being wholly inadequate to meet our expenditure. The coal is the sun-force of former ages ; and it is from this we are now deriving the energy which performs most of our work. George Stephenson long ago declared that his loc omotives were driven by sunshine by the sunshine of former ages bottled up in the coal. And he was right. The mechanical energy of our steam engines, and the chemical energy of our blast furnaces, are derived from the combustion of vegetable matter, in which the heat and light of the sun our present sun or that of the coal ages are in someway stored up. The burning of wood or coal is, chemically, the reverse action to that performed by the sun- light : by the former carbon and oxygen are united, by the latter they are separated. We foresee, then, a future period however distant may be that future in which the world's capital shall have been exhausted, and the energies which are now employed in doing the world's work will no longer be avail- able. But the reader will perhaps think that by improvements in the steam engine, and in other ways, means will be found of getting more and more work out of coal. It is true that we obtain from coal only a fraction of its available energy ; but the whole work which could, by any possible pro- cess, be done by the combustion of coal is definite and limited, although its amount is large. A pound of coal burnt in one minute sets free an amount of energy which would, if it could all be made available, do as much as 300 horses working in the same time. But, again, the reader may think, even if at some distant future the supplies of fuel for the steam engines of our remote posterity should fail, that before that time some other form of force than steam or heat engines will have been discovered some applica- tion of elect' icity, for example. Now, it will appear, from principles which will be discussed in a subsequent article, that not only is there no proba- bility of such a discovery, but that now, when the relations of the whole available energies of the globe have been traced and defined, Science can find no ground for admitting such a possibility under the present condition of the universe. PETROLEUM. V\7HEN coal is heated in closed vessels, there are given off, as we shall * * presently see, a number of gaseous and volatile products many being compounds of carbon and hydrogen which condense to liquids or 544 MINERAL COMBUSTIBLES. solids at ordinary temperatures. Carbon is by far the largest constituent of coal, which commonly contains only about 10 per cent, of other sub- stances, although the proportions vary very widely. Another important constituent of coal is its hydrogen, and the value of coal as a source of heat depends almost entirely upon the carbon and hydrogen it contains. Carbon is one of the most remarkable of all the elements of the globe for its power of entering into an enormous number of compounds. Thus, for example, the compounds of carbon with only hydrogen are innumerable ; but they are all definite, and their composition is expressible by the admir- able system of chemical symbols, of which the reader has now, it is hoped, some definite notion. Perhaps these hydro-carbons are among the best evidences which could be adduced that modern science has obtained a grasp of certain conceptions which have a real correspondence with the actual facts of nature, even as regards the intimate constitution of matter. This is not the place to enter into a complete exposition of this subject. We may, however, irvite the reader's attention to a few simple facts. A very large number of compounds of carbon and hydrogen are known. If the percentage compositions of these be compared together, it is only the eye of a most expert arithmetician which can detect any relation between the proportions of the constituents in the various compounds. The che- mist, however, by associating such of these compounds as resemble each other in their general properties, finds that they can be arranged in series, in which the composition is accurately expressed by multiples of the pro- portions: C=i2, H i. And not only so, the different series themselves form a series of series, having a simple relation to each other. Thus, con- fining ourselves to some of the known hydro-carbons, we have the following : A B C D E F C H 4 C H 2 C 2 H 6 C 2 H 4 C 2 H 2 C 3 H 8 C 3 H 6 cX C 3 H 2 4^10 C 4 H 8 cX cX C 4 H 2 ^5^12 C 5 H 10 C 5 H 8 C 5 H 6 C 5 H 4 C 5 H 2 C 6 H 14 C 6 H 12 C 6 H 10 C 6 H 8 C 6 H 6 ClHj &c. &c. &c. &c. &c. &c. C H 2 -j-2 CH 2 CH 2 _ 2 C H 2 4 C H 2 6 C H 2 s This table might be indefinitely extended, but enough is given to enable the intelligent reader to discover the laws connecting these formulas. The series headed B, it will be observed, have all the same percentage compo- sition. Why, then, one formula rather than another ? The answer to this question is the statement of a theoretical law upon which the whole science of modern chemistry is based; for it has the same relation to that science as gravitation has to astronomy. It is a matter of fact that all gases, what- ever their chemical nature, expand alike with the same application of heat, and all obey the same law, which connects volumes and pressures. These are very remarkable uniformities, for gases in this respect exhibit the most decided contrast to liquids and solids. The volume of each solid and of each liquid has its own special relations to temperature and pressure : here MINERAL COMBUSTIBLES. 545 there is endless diversity. The volumes of all gases have one and the same relation to temperature and pressure : here there is absolute uniformity. As an explanation of these and other facts relating to gases, Amedeo Avogadro, in 1811, put forward this hypothesis Equal volumes of all gases, under like circumstances of temperature and pressure, contain the same number of molecules. This hypothesis was revived by Ampere a few years later, and sometimes is called his. A necessary consequence of this law is that the weights of the molecules of gases are proportional to their densities or specific gravities. Hence when the percentage composition of a hydro-carbon has been determined, by burning or oxidizing it in such a manner as to obtain and weigh the products, carbonic acid and water, the next thing the chemist does is to obtain the weight of a volume of the gas. The number of times this exceeds the weight of hydrogen gas, under the same conditions, expresses how many times the molecule is heavier than the hydrogen molecule. Now, the chemist's unit of weight in these inquiries is the weight of a single atom of hydrogen ; and, as there are grounds for believing that the molecule of hydrogen consists of two atoms of that sub- stance, its weight = 2. Now, if the molecule of marsh gas, the first hydro- carbon in the above list, has the composition assigned, it will be 12+4=16 times heavier than the atom of hydrogen, and *-=8 times heavier than the molecule of hydrogen. Hence, if Avogadro's law be correct, marsh gas should be just eight times heavier than hydrogen gas ; which is really the fact. The formula expressing the composition of the molecule of a hydro-carbon, or of any chemical compound whatever, is always so fixed that the same relations may hold ; and almost the first thing a chemist does in examining a new body is to endeavour to obtain it in the state of gas. The first four members of the series headed A are gases at ordinary temperatures, the fifth is a gas at temperatures above the freezing-point, and a liquid at lower temperatures ; the next following members are liquids which boil (that is, are converted into gases) at temperatures rising with each additional carbon atom about 20 F. The specific gravities and boiling- points of these liquids augment as we pass from one hydro-carbon to the next, and the lower members of the series are solids, fusing at tempera- tures higher and higher as the number of carbon atoms is greater. Similar gradations of properties are exhibited by the other series of hydro-carbons. Petroleum or rock-oil is the name given to liquid hydro-carbons found in nature, and consisting chiefly of compounds belonging to the series marked A in the above list. Some varieties of petroleum hold in solution other hydro-carbons, and in some cases paraffin is extracted from the oils by exposing the liquid to cold, when the solid crystallizes out. Paraffin is a solid belonging to the B series, and it is for the most part obtained by heating certain minerals. Deposits of liquid hydro-carbons, perhaps formed by a kind of natural subterranean distillation from coal or other fossil organic matter, exist in various localities. These deposits have long been known and utilized at Rangoon, in Burmah, and on the shores of the Caspian Sea. At Rangoon the mineral oil is obtained by sinking wells about 60 ft. deep in a kind of clay soil, which is saturated with it. The oily clay rests upon a bed of slate also containing oil, and underneath this is coal. It may be supposed that subterranean heat, acting upon the coal, has distilled off the petroleum, which has condensed in the upper strata. This petroleum, when distilled in a current of steam, leaves about 4 per cent of residue, and the volatile portion contains about one-tenth of its weight of a substance (paraffin) 35 54 6 MINERAL COMBUSTIBLES. which is solid at ordinary temperatures. After an agitation with oil of vitriol, and another distillation, rock oil or naphtha is obtained, which, however, is still a mixture of several distinct chemical compounds. Mineral oils have also been found in China, Japan, Hindostan, Persia, the West India Islands, France, Italy, Bavaria, and England. In one of the Ionian Islands there are oil-springs which have flowed, it is said, over 2,000 years. But it is the recently discovered and extremely copious springs and wells in Pennsylvania and Canada which have given a vastly extended import- ance to the trade in mineral oiL Rock oil is now used in enormous quan- tities as the cheapest illuminating oil, and that which furnishes the most intense light. Its consumption as a lubricating oil for machines has also been very large. Mineral oil was occasionally found at various places in the United States, and sometimes used by the inhabitants of the locality before the recent discoveries; but it was not until August, 1859, that it was met with in large quantities. About this time a boring which was made at Oil Creek, Pennsylvania, reached an abundant source, for 1,000 gallons a day were drawn from it for many weeks. The news of the discovery of this copious oil-spring spread rapidly: thousands of persons flocked to the neighbourhood in hopes of easily making a fortune by " striking oil." Be- fore the end of 1 860 more than a thousand wells had been bored, and some of these had yielded largely. The regions of North America in which petroleum has been found cover a large part of the States, and comprise Pennsylvania, New York, Ohio, Michigan, Kentucky, Tennessee, Kansas, Illinois, Texas, and California. In the vicinity of Oil Creek the bore-holes are usually about 3 in. or 4 in. in diameter, and are often 500 ft. deep, and even 800 ft. is not uncommon. To make a bore-hole 900 ft. deep, and pro- cure all the requisites steam engines, barrels, &c., for pumping the oil costs about $5,000. In 1869 many of these wells still yielded regularly 300 barrels a day, but the supply has not continued with the same abun- dance. One of the luckiest wells flowed at its first opening at the rate of about 25,000 barrels a day. The apparatus used for working the oil- wells is very simple a rude derrick, a small steam engine, a pump, and some barrels and tubs being all that is necessary. Fig. 345 will give the reader an idea of the scene presented by a cluster of oil-wells in the Oil Creek region. Oil Creek received its name before the petroleum trade was established, from the oil found floating on the surface of the water. It is on the Alleghany River, about 150 miles above Pittsburg, and here at its mouth is situated Oil City. There is a wharf in Pittsburg for the oil traffic, and the barrels are brought down the river in flats, or the oil is poured into very large flat boxes, divided into compartments, which are then closed, ancl the boxes floated down in groups of twenty or more. The refining process consists in placing the crude oil into a large iron retort, connected with a condenser formed of a coil of iron pipes, surrounded by cold water. Heat is applied, and the lighter hydro-carbons (naphtha) come over first. After the naphtha, the oils which are used for illuminating purposes distil off. A current of steam is then forced into the retort, and this brings over the heavy oils which are used for greasing machinery. A black tarry oil yet remains ; and, finally, after the separation of this, a quantity of coke. The products are subjected to certain processes of purification, which need not here be described. The magnitude of the American oil trade may be inferred from the fact that in the second year of its existence, from January to June, 1862, more than 4,500,000 gallons were exported from four sea- ports. This can hardly be wondered at, considering the extremely low MINERAL COMBUSTIBLES. 547 FIG. 345. View on Hyde and Egbert's Farm, Oil Creek. price at which this excellent illuminating and lubricating agent can be pro- duced. Refined petroleum can be bought at Pittsburg for 16 cents, per gallon. It is believed by some that the supplies of petroleum which exist in various localities are so abundant that they will furnish illuminating oils to the whole world for centuries. PARAFFIN. T N the course of some researches on the substances contained in the tar, * which is obtained by heating wood in close vessels, Reichenbach found a white translucent substance, to which he gave the above name, because it was not acted upon by any of the ordinary chemical reagents, such as sulphuric acid, nitric acid, &c. This substance, which is composed of carbon and hydrogen only, is not unlike spermaceti ; it is colourless, trans- lucent, and without smell or taste. But when slightly warmed, it becomes very plastic, and may then be moulded with the greatest ease and in this respect it differs from spermaceti. Paraffin melts at from 88 to 150 F., to a colourless liquid, which is so fluid that it may be filtered through paper like water, and at a higher temperature it can be distilled unchanged. Paraffin does not dissolve in water, and is but slightly soluble in alcohol. In ether, naphtha, turpentine, benzol, and sulphide of carbon, it dissolves very readily. When heated with sulphur, it is decomposed : the sulphur seizes upon its hydrogen, sulphuretted hydrogen is given off, and the carbon is separated ; and this action has been proposed as a ready means of obtaining pure sulphuretted hydrogen for laboratory use. It is probable 35 2 54 8 MINERAL COMBUSTIBLES. that paraffin is a mixture of various hydro-carbons, having a composi- tion expressible by the formula, C H 2 ; for different specimens fuse at different temperatures, according as the paraffin has been obtained from one or the other source. In the year 1847, Dr. Lyon Playfair directed the attention of Mr. James Young, then of Manchester, to a dense petroleum which issued from the crevices of the coal in a Derbyshire mine It was soon found that this substance yielded a distillation a pale yellow oil which, on cooling, de- posited solid paraffin. Mr. Young, recognizing the importance of this dis- covery, had an establishment at once erected on the spot, and the work of extracting paraffin was carried on until the supply of the petroleum had become nearly exhausted. Forced to seek for other sources of paraffin, Mr. Young was fortunate enough, after many trials, to discover that a species of bituminous coal, which occurs at Boghead, near Bathgate, in the county of Linlithgow, yielded by distillation annually large quantities of paraffin. In 1850 he procured a patent for " treating bituminous coals to obtain paraffin, and oil containing paraffin, therefrom." This method consisted in distilling the coal in an iron retort, gradually heated up to low redness, and kept at that temperature until the volatile products ceased to come off. Under this patent, Mr. Young developed the manufacture of paraffin into a new and important branch of industry. The oil which first comes over in the distillation of the Boghead mineral is largely used for illuminating purposes under a variety of names besides that of paraffin oil, which term is, we believe, chiefly applied to a less volatile portion, exten- sively used for lubricating machinery, and consisting ofliquid hydro-carbons of the same percentage composition as solid paraffin, which substance it also holds in solution. Mr. Young's process consisted in placing the mineral in a retort encased in brickwork an arrangement which caused the tem- perature of the retort to be more uniform than if the heat of the furnace had been applied to it directly. The retorts were placed vertically, and they were fed with the mineral by a hopper at the top. The products of the distillation passed through a worm tube surrounded by cold water into a cooled receiver. The result of the first distillation was a crude oily matter, differing from tar in being lighter than water, and in not drying-up when exposed to the air. This crude oil was then several times alternately treated with sulphuric acid and caustic potash, and distilled ; and when about two- thirds of the oil had been separated from the rest, as an oil for burning and lubricating purposes, the residue yielded paraffin, or " paraffin wax," as it is sometimes called. It is estimated that in Scotland no less than 800,000 tons of shale are annually distilled for mineral hydro-carbons, with a consumption of 500,000 tons of fuel. It is believed that about 25,000,000 gallons of crude oil are thus obtained, and from this 350,000 gallons of illuminating oil, 10,000 tons of lubricating oil, and 5,800 tons of solid paraffin are produced. Among the products exhibited in the International Exhibition of 1 862, was a block of beautifully translucent paraffin, of nearly half a ton weight. Paraffin is also obtained on the continent by distilling a variety of coal termed lignite. The tar which comes over is distilled, until nothing but coke remains. The condensed products are then treated with caustic soda, , in order to remove carbolic acid and other substances. After washing with water, the oils are treated with sulphuric acid, in order to remove basic substances. The oil is again washed, and is then rectified by another dis- tillation. The products which successively come over are, if necessary, MINERAL COMBUSTIBLES. 549 separated by being collected in different vessels ; but sometimes they are mixed together, and sent into the market as illuminating oils under various names, such as " photogen," " solar oil," &c. Oils having a specific gravity about 0-9 are collected apart, and are placed in tanks in a very cool place. In the course of a few weeks the solid paraffin, which is dissolved in the other hydro-carbons, crystallizes out. The liquid oils are drawn off, and the crude paraffin, which is of a dark colour, is freed from adhering oil by a centrifugal machine, and afterwards by pressure applied by hydraulic power. It then undergoes several other processes of purification before it is obtained as a colourless translucent solid. Several thousand tons of paraffin are annually consumed for making candles, which is the most important application of the material. The variation in the fusing-points of different specimens is doubtless due to admixtures in greater or less proportion of other more easily fusible hydro- carbons. It was on account of the imperfect separation of these that the candles first made from paraffin were so liable to soften and bend, and felt greasy to the touch. Paraffin for candle-making is sometimes mixed with a certain proportion of other substances, such as palmitic acid, &c. Among the patented applications of paraffin are the lining of beer-barrels, and the preserving of fruits, jams, and meat. Some kinds of paraffin are also used in the manufacture of matches. Liebig once expressed a wish that coal-gas might be obtained in a solid form : " It would certainly be esteemed one of the greatest discoveries of the age if any one could succeed in condensing coal-gas into a white, dry, odourless substance, portable and capable of being placed in a candlestick or burned in a lamp." Now, it is curious that paraffin has nearly the same composition as good coal-gas : it burns with a bright and smokeless flame, and beautiful candles are formed of it, which burn like those made of the finest wax. When the fused paraffin first assumes the solid form, it is trans- parent like glass ; and if it could be retained in that condition, we might have the pleasing novelty of transparent candles. But the particles seek to arrange themselves in crystalline forms, and the substance soon takes on its white semi-opaque appearance. The great richness of the Boghead mineral in paraffin, which appears to exist in it ready formed, prevented for many years any successful com- petition by the working of other sources of supply. But paraffin is an abundant constituent of Rangoon petroleum, and considerable quantities may be obtained by distilling peat, and other fossil substances. All petro- leums and paraffins are, in fact, mixtures of a number of hydro-carbons, whicli in many cases cannot be entirely separated from each other. The accidents which have from time to time occurred with some of these com- bustibles, and have caused legislative enactments with regard to them, are due to the imperfect removal by distillation of the more volatile bodies, which rise in vapour at ordinary temperatures. Explosions of the hydro- carbons can occur only under the same circumstances as with coal-gas ; that is to say, the application of a flame to a mixture of the vapour with atmospheric air. FIG. 346. View of the City of London Gas-works. COAL-GAS. WHEN coal is burning in a common fire, we may see jets of smoky gas issuing from the pieces of coal before they become red hot. This vapour, coming in contact with flame in another part of the fire, may often be observed to ignite, thus supplying an instance of gas-lighting in its most elementary form. In the ordinary fire the air has free access, and the inflammable gases and vapours continue to burn with flames more or less bright, and when these have ceased the cabonaceous portion continues afterward to glow until nearly the whole has been consumed, except the solid residue which we call the ashes. These ashes in general contain a portion of unconsumed carbon, mixed with what is chemically the ash, namely, certain incombustible salts, constituting the white part of the ashes. If, however, we heat the coal in a vessel which prevents access of air, and allows the gases to escape, the coal is decomposed much in the same way as when it is burnt in the open fire ; but the products formed are no longer burnt, the supply of oxygen being cut off. Every one knows the familiar experiment of filling the bowl of a common clay tobacco-pipe with powdered coal, then covering it with a dab of clay, and placing it in a fire. The gas which soon comes from the stem of the pipe does not take fire unless a light be applied, when it may be seen to burn with a bright flame, and after the flow of gas has ceased, nearly the whole of the carbon of the coal will be found unconsumed in the bowl of the pipe. This simple expe- riment illustrates perfectly the first step in the manufacture of coal-gas, namely, the process of heating coal to redness in closed vessels, by which operation the substances originally contained in coal are destroyed, and their elements enter into new combinations. 550 COAL-GAS. 551 These elements are few in number ; for, except the very small portion which remains as incombustible white ash, coal is constituted of carbon, hydrogen, oxygen, nitrogen, and a little sulphur. All the varied and inte- resting products obtained by the destructive distillation of coal are com- binations of two or more of these four or five elements. Illuminating gas is far from being the only product when coal is heated without access of air ; for of the numerous substances volatized at the red heat of the gas- retort a great number are not only incapable of affording light, but liable to generate noxious compounds when burnt Besides this there are nume- rous bodies which, though leaving the retort in the gaseous form, imme- diately assume the liquid or solid state at ordinary temperatures. All such substances must be separated before permanent gases are obtained fit for illuminating purposes and capable of being carried through pipes to dis- tant places. Thus an important part of the apparatus for gas manufacture consists in arrangements for separating the condensible bodies, and for removing useless or injurious gases from the remainder. FIG. 347. Section of Gas-making Apparatus. The products resulting from the destructive distillation of the coal may, therefore, be classified as a> solids left behind in the retort ; b, solids and liquids condensed by cooling the vapours which issue from the retort ; c, coal-gas a mixture of gases from which certain useless and noxious con- stituents must be removed. Fig. 347 is intended to give a diagrammatic view of the apparatus employed in the generation, purification, and storage of gas, the various parts being shown in section. A is the furnace contain- ing several retorts, of which B is one. From each retort a tube, d, rises vertically, and curving downward like an inverted Uj it enters a long hori- zontal cylinder, yj half filled with water, beneath the surface of which the open end of the recurved tube dips. The cylinder containing water passes horizontally along the whole range of furnaces in the gas-works, and is known as the hydraulic main. It is here, then, the tar and the moisture first condense, and the pipe is always kept half full of these liquids, so that the ends of the pipes, d, from the retorts, dipping beneath its surface, form 552 COAL-GAS. traps or water-valves, which allow any retort to be opened without per- mitting the gas to escape. As the tar accumulates in the hydraulic main, it flows over through a pipe, g, leading downwards into the tar-well, H. The gases take the same course ; but while the tar flows down the vertical tube, R, ths gases pass on through/ into the condensers or refrigerators. Gas cannot escape from the open end of the tube, for it is always closed by the liquids tar and ammoniacal liquor which accumulate and flow over the top of the open inner vessel into the cistern, s, from which they are drawn off from time to time by the stop-cock, i. Although when the gas has arrived at this cistern much of its tar and ammoniacal vapours have been condensed, a portion is still retained by reason of the high tempera- ture of the gas ; and this has to be removed before it is permitted to enter the purifier. This is the object of passing the gas through the series of pipes, //, forming the condenser. These are kept cool by the large surface they expose to the air, and, when necessary, cold water from the cistern, K, may be made to flow over them. The tar and other liquids condense in the iron chest, T, which is so divided by partitions as to compel the gas to pass through the whole series of tubes ; and as the liquid accumulates, it also overflows into the tar-well. The cooled gas then enters the purifier, L L, in which are layers of slaked lime placed on a number of shelves. By contact with the extensive surface of slaked lime the gas has its sulphur- etted hydrogen, carbonic acid, and some other impurities, removed ; and it then, through the tube , enters the gasholder, in which it is stored up for use. Hydrated oxide of iron is now much used for purifying coal-gas. The oxide is mixed with sawdust, and placed in layers 10 in. thick. Sulphide of iron and water is formed ; and when the mixture has ceased to absorb any more, it is removed and exposed to a current of air ; the hydrated oxide is thus reproduced and sulphur set free. The process may be re- peated many times in succession, until the absorbent power is impaired by the accumulation of sulphur. The gasholder or " gasometer," as it is often improperly named is an immense cylindrical bell, made of wrought iron plates, and inverted in a tank of water, in which it rises or falls. It is counterpoised by weights attached to chains passing over pulleys, so as to press the gas with a small force in order to drive it along the main, which communicates with the pipes sup- plying it to the various consumers. The pressure impelling the gas through the mains does not in general exceed that of a column of water two or three inches high. It will be necessary, after this slight outline describing the essential parts of the apparatus, to enter more fully into the details of the several parts. The retorts are constructed of wrought iron, cast iron, or earthenware, and in shape are cylindrical, with a diameter of 12 in. to 18 in., or more, and a length of 6 ft. to 10 ft. Though sometimes circular in section, other forms are commonly used such as the elliptical, and especially the Q- shaped. The retorts are closed except at the mouth-end, Fig. 348, from the top of which rises the stand-pipe, A, which has usually a diameter of about 5 in. When the charge has been introduced, the mouth is closed by a plate of iron, B, closely and securely applied by means of a screw, c, as shown in the figure a perfectly tight joint being obtained by a luting of lime mortar spread on the part of the lid which comes into contact with the mouth of the retort. The retorts are always set horizontally in the furnace each COAL-GAS. 553 furnace usually including a set of five retorts. The charge of coals is in- troduced on a tray of sheet iron adapted to the size of the retort, which, when properly pushed in, is inverted so as turn out the contents, and then withdrawn. The time required to completely expel the volatile constituents from the charge in a gas retort varies very much, because there are great diversities in the composition of the different kinds of coal employed. Some varieties of coal, such as cannel, are easily decomposed, and the operation may be complete in about three hours ; while other kinds may require double that time. The quantity of gas procurable from a given weight of coal also varies according to the kind of coal made use of. Thus, while a hundredweight of cannel may give 430 cubic feet of gas, the same weight of New- castle coals will yield but 370 cu- bic feet. The nature of the gases given off from a retort will be different at the different stages of the operation. The scene presented by the retort-house of a large gas manu- factory, when viewed at night, is a singular spectacle. The strange lurid gleams which shoot out amid the general darkness as the retorts are opened to withdraw the coke, and the black forms of the workmen partially illumi- nated by the glare, or flitting like dark shadows across it, form a picture which might engage the pencil of a Rembrandt. In Plate IX. is depicted the retort-house at the Imperial Gas Works, King's Cross. Here the retorts are ar- ranged in several tiers the coal being brought, and the cokewith- drawn, by the aid of an iron car- riage running on rails parallel to the line of furnaces. In the process of heating, a proper regulation of the temperature is of the highest importance. It is found that when the retorts are heated to bright cherry-red, the best results are obtained. At a lower temperature a larger quantity of condensable vapours are given off, which collect in the gasholders and distributing pipes as solid or liquid, and occasion much inconvenience, while the quantity of gas obtained is decreased. On the other hand, if the temperature be too high, some of the gases are decom- posed, and the quantity of carbon contained in the product is so much diminished as seriously to impair the illuminating power. Again, every second the gases after their production remain in the red-hot retort dimi- nishes their light-giving value ; for those hydro-carbons on which the luminiferous power of the gas depends, are then liable to partial decom- position ; a portion of their carbon is deposited on the walls of the retort in a dense layer, gradually choking it up, while the liberated hydrogen does FIG. 348.- The Retort. 554 COAL-GAS. not add to the illuminating but to the heating constituents of the gas. A plan has been patented by Mr. White, of Manchester, for rapidly removing the illuminating gases from the retort by sweeping them out by means of a current of what has been termed " water gas." This water gas is pro- duced by causing steam to pass over heated coke, and is a mixture of car- bonic acid, carbonic oxide, and hydrogen. Though only two of these are combustible gases and even they do not yield light by their combustion, and, by adding to the bulk of the gas, serve rather to dilute it yet it has been found that in some cases twice the amount of light is obtainable by White's process than the same weight of coal supplies when treated in the ordinary manner. The hydraulic main, as already mentioned, being kept half full of tar into which the lower ends of the dip-pipes descend, prevents the gas from escaping through the stand-pipes when the lid of a retort is removed for the introduction of a fresh charge. The hydraulic main is from 12 to 18 in. diameter, and the dip-pipes pass into it by gas-tight joints. Various forms of purifiers are in use besides the simple one already mentioned. Some of these have arrangements for agitating the gas with a purifying liquid by mechanical means, the motion being supplied by a steam engine. The gasholder, as it sinks in the water of the cistern, presses with less force on the contained gas, and unless this inequality of pressure were counteracted there would be very unequal velocities in the flow of gas from the burner. The equality of pressure is obtained by making the weight of the chains by which the gasholder is suspended equal to half the weight the gasholder loses in the same length of its motion. Gas- holders are also constructed without chains or counterpoises, as these are found to be unnecessary where the height of the gasholder does not exceed half its width. In such cases, especially when the vessel is very large, the difference of pressure at the highest and lowest position is quite incon- siderable, and nothing more is necessary than that upright guides or pillars be placed to preserve the vertical motion of the vessel. Another improve- ment, which enables a lofty gasholder to be used without increasing the depth of the tank, consists in forming the gasholder of several cylinders, which slide in and out of one another like the draw-tubes of a telescope. Each cylinder has a groove formed by turning up the iron inside the rim, and at the top of the next cylinder the edge is turned outwards so as to drop in the groove or channel, which thus forms a gas-tight joint, for it is of course filled with water as it rises. The pressure is, however, more accu- rately regulated by an apparatus called the governor, through which the gas passes in before it enters the mains. The construction and action of the regulator will be understood from Fig. 349, where A represents a kind of miniature gasholder, inverted in the cistern, B. From the centre of the interior of the bell hangs a cone, C, within the contracted orifice of the inlet-pipe. If this cone be drawn up, the size of the orifice, D, is reduced, and, on the other hand, by its descent it enlarges the opening through which the gas passes outward. By properly adjusting the weights of the counterpoise, E, such a position of the cone may be found that the gas passes into the mains at an assigned pressure. Suppose, now, that from any cause the pressure of gas in F increases, that pressure acting upon the inverted bell, A, causes it to rise and carry with it the cone, which, by narrowing the orifice of the outlet, checks the flow of gas. Similarly, a decrease of pressure in the mains would be followed by the descent of the cone, and consequently freer egress of gas. In hilly towns it is necessary PLATE IX. RETORT HOUSE CF THE IMPERIAL GAS-WORKS, KINGS CROSS, LONDON. COAL-GAS. 555 to fix regulators of this kind at certain heights in order to equalize the pressure. It is found that a difference of 30 ft. in level affects the pres- sure of gas in the same main to about the same amount as would a column FIG. 349. The Gas Governor. of water one-fifth of an inch high, the pressure being least at the lowest point. Coal-gas is a mixture of several gases, and these may be classified as, first, the light-giving gases, or those which burn with a luminous flame ; secondly, gases which burn with a non-luminous flame, and which there- fore contribute to the heat, and not to the light, of a gas-flame, and have the effect of diluting the gas ; third, gases and vapours which are properly termed impurities, as they are either incombustible or by their combustion give rise to injurious products. Of the first kind the principal is olefiant gas, a gas which burns with a brilliant white flame without smoke. It is a compound of hydrogen and carbon, six parts by weight of carbon being combined with one part by weight of hydrogen. Besides olefiant gas other gaseous hydrocarbons are found in smaller quantities. These contain a larger proportion of carbon than olefiant gas. The second class contains hydrogen, light carburetted hydrogen, and carbonic oxide. Hydrogen is one element of water, of which it forms one-ninth of the weight. It burns with a flame giving singularly little light, but having intensely heating power ; in fact, one of the brightest lights we can produce is obtained by allowing the flame of burning hydrogen to heat a piece of lime. Light carburetted hydrogen, like olefiant gas, is a compound of hydrogen and carbon, but the proportion of carbon to hydrogen is only half what it is in olefiant gas, namely, three parts to one. This gas enters largely into the 55 6 COAL-GAS. composition of coal-gas, and occurs naturally in the coal seams, being, in fact, the dreaded fire-damp of the miner. It is much lighter than defiant gas, for while that gas is of nearly the same specific gravity as atmospheric air, light carburetted hydrogen is only a little more than half that specific gravity. It is this ingredient of coal-gas which renders it so light as to be available for inflating balloons. It burns with either a bluish or a slightly yellow flame, yielding hardly any light. Olefiant gas and the other lumi- niferous hydro-carbons, when exposed to a bright red heat, split up for the most part into this gas and carbon. This explains the importance of rapidly removing the gas from the retort in which it is generated, a point which has been referred to above. Carbonic oxide is a gas which one may often see burning with a pale blue flame above the glowing embers of a common fire, the flame giving, however, little light. It is a compound of carbon and oxygen, containing only one-half the quantity of oxygen which its carbon is capable of uniting with, and therefore ready to unite with another pro- portion, which it does in burning, carbonic acid being the product. The third class of constituents of coal-gas the impurities are those which the manufacturer strives to remove by passing the gas over lime, milk of lime, oxide of iron, &c. Sulphuretted hydrogen, a compound of sulphur and hydrogen, has an extremely nauseous odour resembling that of rotten eggs. It is always formed in the distillation of coal, and if not removed from the gas in the process of purification, it has a very objection- able effect ; for one product of its combustion is sulphurous acid, and in a room where such gas is burnt much damage may be done by the acid vapours; for example, the bindings of books, &c., soon become deteriorated from this cause. The detection of sulphuretted hydrogen in coal-gas is quite easy, for it is only necessary to hold in a current of the gas a piece of paper dipped in a solution of the acetate of lead. If in a few minutes the paper becomes discoloured the presence of sulphuretted hydrogen is indicated. But the bete noire of the gas-maker is a substance called " sulphide of carbon," which is formed whenever sulphur and carbonaceous matters are brought together at an elevated temperature* Sulphide of carbon is, in the pure state, a colourless liquid, of an intensely offensive odour, resem- bling the disagreeable effluvia of putrefying cabbages. The liquid is ex- tremely volatile, and coal-gas usually contains some of its vapour. When too high a temperature is used in the generation of the gas, it contains a large quantity of this deleterious ingredient, especially if the amount of sulphur contained in the coal is at all considerable. This sulphide of carbon vapour is very inflammable, and one product of its combustion is a large quantity of sulphurous acid. This substance cannot be removed from coal-gas by any process sufficiently cheap to admit of its application on the large scale. It is said, however, that by passing the gas over a solution of potash in methylated spirit, the sulphide of carbon vapour can be completely got rid of. The price of these materials renders the process available in special cases only, where the damage done by the sulphurous acid would be serious, as in libraries, &c. Besides the impurities we have already enumerated, many others are present in greater or less quantity. Carbonic acid the gas resulting from the complete combustion of carbon should be entirely removed by the lime purifiers, as the presence of even a small percentage detracts materially from the illuminating power. This gas is not inflammable and cannot support combustion. It has decided acid properties, and readily unites with alkaline bases forming carbonates : COAL-GAS. 557 it is upon this behaviour that its removal by lime depends. The illumi- nating power of coal-gas containing only I per cent, of carbonic acid is reduced thereby by about one-fifteenth of its whole amount. The proper mode of burning the gas so as to obtain the maximum amount of light it is capable of yielding requires a compliance with certain physical and chemical conditions. The artificial production of light de- pends upon the fact that by sufficiently heating any substance, it becomes luminous, and the higher the temperature the greater the luminosity. The light emitted by solid bodies moderately heated is at first red in colour ; as the teniperature rises it becomes yellow, which gradually changes to white when the heat becomes very intense. The widest difference exists, how- ever, in the temperature required to render solids or liquids luminous, and that needed to cause gases to give off light. In all luminous flames the light is emitted by solid particles highly heated. Every luminous gas- flame contains solid particles of carbon, as may be easily shown by the soot deposited on any cold body such as a piece of metal introduced into the flame. On the other hand, the flame of burning hydrogen, which pro- duces only aqueous vapour, furnishes no light, but a heat so intense, that a piece of lime introduced into the jet becomes luminous to a degree hardly supportable by the eye. The conditions requisite, therefore, for burning illuminating gas are, first, just such a supply of air as will prevent particles of carbon from escaping unconsumed in the form of smoke, and yet not enough to burn up the carbon before it has separated from the hydrogen, and passed through the flame in the solid state ; second, the attainment of the highest possible temperature in the flame, compatible with the former condition. When the supply of oxygen is not in excess, the hydrogen of the gaseous hydro-carbon appears to burn first ; the carbon is set free, and its solid particles immersed in the flame of the burning hydrogen are there intensely heated ; but ultimately reaching the outer part of the flame, they enter into combination with the oxygen of the air, producing carbonic acid ; or if present in excessive quantity, they are thrown off as smoke. If the pur- pose of burning the gas is to obtain heating effects only, this is accomplished by supplying air in such quantities, that the carbon enters into combination with oxygen in the body of the flame, with- out a previous separation from the hydro- gen with which it is combined. In this case a higher temperature is attained^ and the flame is wholly free from smoke; so that vessels of any kind placed over it remain perfectly clean and free from the least de- posit of soot. The last result is of great ad- vantage in chemical processes, especially where glass vessels require to be heated, for the chemist retains an uninterrupted view of the actions taking place in his flasks and retorts. No better illustration of the nature of the combustion in a gas-flame can be found FIG. QQBunserts Burner. than is furnished by Bunsen's burner, Fig. 350, now universally employed as a source of heat in chemical laboratories. In this burner the gas issues from a small orifice at the level of a, near the bottom of the tube, b, which is open at the top, and is in free communication at the bot- 558 COAL-GAS. torn with openings through which air enters and mixes with the gas, as they rise together in the tube and are ignited at the top. If the pressure of the gas be properly regulated, the flame does not descend in the tube, but the mixture burns at the top of the tube, producing a pale blue flame incapable of emitting light, but much hotter than an ordinary flame, for the combus- tion is much quicker. If the openings at a be stopped, the supply of air to the interior of the tube is cut off, and then the gas burns at the top of the tube, ^, in the ordinary manner, giving a luminous flame. Ordinary gas-jets burning in the streets, at open stalls or shops, may be seen on a windy night to have their light almost extinguished by the increased supply of oxygen, carried mechanically into the body of the flame, the white light instantly changing to pale blue. The disappearance of the light in such cases is due, as in Bunsen's burner, to the supply of oxygen in sufficient quantity to combine at once with the carbon as well as the hydrogen of the hydro-carbons. FIG. 351. Faraday 's Ventilating Gas-burner. The burners now chiefly used for the consumption of coal-gas for illu- minating purposes are the bat's-wing, the fish-tail, and various forms of Argahd. The bat's-wing burner is simply a fine slit cut in an iron nipple, and it produces a flat fan-like flame. The fish-tail is formed by boring two holes so that two jets of gas inclined at an angle of about 60 infringe on each other and produce a flat sheet of flame. The Argand, in its simplest form, consists of a tubular ring perforated with a number of small holes from which the gas issues. Many modifications of this kind of burner have been devised, in all of which a glass chimney is requisite to obtain a current of air sufficient to consume the gas without smoke, and it is important that the height of the chimney should be adapted to the amount of light required if the gas is to be used economically. Argand COAL-GAS. 559 burners are specially advantageous where a concentrated light is required. Fig. 351 represents a ventilating gas-burner, contrived by Faraday, the object being to remove from the apartment the whole of the products of the combustion of the gas. A is the pipe conveying the gas to the Argand burner, B, the flame of which is enclosed in the usual cylindrical glass chimney, c c, open at the top. This is enclosed in a wider glass cylinder closed at the top by a double disc of talc, D D, and opening at its base into the ventilating tube, E E. The direction of the currents produced by the heat of the flame is shown by the arrows. The whole is entirely enclosed by a globe of ground glass. Means are provided for regulating the draught in the pipe, E E, which, when heated, creates of itself a strong current of air through the apparatus. The illuminating power of coal-gas may be measured directly by com- paring the intensity of the light emitted by a gas-flame consuming a known quantity of gas per hour with the light yielded by some standard source. The standard usually employed is a spermaceti candle burning at the rate of 120 grains of sperm per hour. It is not necessary that the candle actu- ally used should consume exactly this amount, but the consumption of sperm by the candle during the course of each experiment is ascertained by the loss of weight, and the results obtained are easily reduced to the standard of 120 grains per hour. An instrument is used for determining the relative intensities of the illumination, called Bunsen's photometer. It consists of a graduated rule, or bar of wood or metal, about 10 ft. long. At one end of this bar is placed the standard candle, at the other is the gas- flame. A stand slides along the rule supporting a circular paper screen at the level of the two flames, and at right angles to the line joining them. This paper screen is made of thin writing-paper, which has been brushed over with a solution of spermaceti, except a spot in the centre, or, more simply, a grease-spot is made in the middle of a piece of paper. In conse- quence the paper surrounding the spot is much more transparent; yet when it is placed so that both sides are equally illuminated, a spectator will not perceive the spot in the centre when viewing the screen on either side. When the screen has been placed by trial in such a position between the two sources of light, it is only necessary to measure its distance from each flame in order to compute the number of times the illuminating power of the gas-flame exceeds that of the candle. This computation is based on the fact that the intensity of the light from any source diminishes as the square of the distance from the source. Thus, if a sheet of paper be illu- minated by a candle at 2 ft. distance, it will receive only one-fourth of the light that would fall upon it were its distance but I ft., and if removed to 3 ft. distance it has only one-ninth of the light. In the instrument used for measuring the illuminating power of gas the rule is graduated in accord- ance with this law, so that the relative intensities may be read off at once. The gas passes through a meter for measuring accurately the quantity per minute which is consumed by the burner, and there is also a gauge for ascertaining the pressure. Another mode of estimating the illuminating power of coal-gas is by determining the quantity of carbon contained in a given volume. For, in general, the richness of the gas in carbon is a fair index of the quantity of its luminiferous constituents. This may be readily effected by exploding the gas with oxygen, and measuring the amount of carbonic acid produced. Still more accurate determinations of the illu- minating value of gas may be obtained by a detailed chemical analysis. The illuminating power of any gas is so calculated that it represents the 560 COAL-GAS. number of times that the light emitted by a jet of the gas, burning at the rate of 5 cubic feet per hour, exceeds the light given off by the standard sperm candle burning 1 20 grains of sperm per hour. For example, when it is said that the illuminating power of London gas is 13, it is meant that when the gas is burnt in an ordinary burner at the rate of 5 cubic feet per hour, the light is equal to that given by thirteen sperm candles burning together 13 X 120 grains per hour. The quality of gas varies very much, as it depends upon the kind of coal employed, and upon the mode in which the manufacture is conducted. The following are the results of experi- ments made to determine the illuminating power of the gas supplied to several large towns : Candles. London 12*1 Paris 12*3 Birmingham 15*0 Berlin 15-5 Candles. Carlisle 16*0 Liverpool 22*0 Manchester 22-*o Glasgow 28*0 The relative quantities of tar, ammonia water, and coke yielded in vari- ous gas manufactories also vary very considerably for the same reasons. In the early days of gas illumination the consumers were charged accord- ing to the number of burners ; but this arrangement proved so unsatisfac- tory that the gas-meter became a necessity, and already in 1817 meters had been devised, which were not essentially different from those now in use. Although gas is used in so many houses, there are few persons who have any notion of the mechanism of the gas-meter. Our space will not allow full details of the construction, but the following particulars may be men- tioned. In the ordinary " wet " meter there is a drum divided into four compartments by radiating partitions ; this drum revolves on a horizontal axis, and the lower half of the drum, or rather more, is beneath the surface of water contained in the case, the water being at the same level inside and outside the drum. The gas enters one of the closed chambers formed between the surface of the water and a partition of the drum. Its pressure tends to increase the size of the chamber, hence the drum revolves. The preceding division of the drum being filled with gas, this is driven into the exit pipe by the motion of the drum, as it is included in a space comprised between the water and a partition. Each division in turn comes into com- munication with the gas-main, and as it is filled passes on towards the posi- tion in which a passage is opened for it to the exit-pipe. Each turn of the drum, therefore, carries forward a definite quantity of gas, and the only thing necessary is a train of wheels, to register the number of revolutions made by the drum. The " wet " meter is much inferior in almost every respect to the " dry " meter, in which no water is used. The principle of the " dry" meter is very simple. The gas pours into an expanding chamber, partly constructed of a flexible material, and which may be compared to the bellows of a circular accordion. The expansion is made to compress another similar chamber, already filled with gas, which is thus forced through the exit-pipe. When the first chamber has expanded to a definite volume, it moves a lever, and this reverses the communications. The ex- panded chamber is now opened to the exit-pipe, and the other to the entrance-pipe, and so on alternately. A train of wheels registers the number of movements on a set of dials. FIG. 352. Apparatus for making Magenta. COAL-TAR COLOURS. COAL-TAR is an exceedingly complex material, being a mixture of a great number of different substances. The following table shows the chemical name of many of the substances obtainable from the coal-tar. It must not be supposed that these substances exist ready formed in the coal, and that they are merely expelled by the heat. We can understand better how heat, acting upon an apparently simple substance like coal, and one containing so few elements, is able to produce so large a variety of different bodies, if we remember that heat is the agent most often employed to effect chemical changes, and that from even two elements, variously combined, bodies differing entirely from each other are producible. SUBSTANCES FOUND IN COAL-TAR. a. COMPOUNDS OF CARBON AND HYDROGEN. Hydrides of amyl, hexyl, heptyl, nonyl, and decyl. Amylene, hexylene, heptylene, octylene, nonylene, decylene ; (paraffin). Benzol, toluol, xylol, cumol, cymol. Naphthalene. Anthracene. Pyrene. Chrysene. 561 36 COAL-TAR COLOURS. b. COMPOUNDS OF CARBON, HYDROGEN, AND OXYGEN. Phenol, cresol, phlorol. Rosolic acid, brunolic acid. C. COMPOUNDS OF CARBON, HYDROGEN, AND NITROGEN. Aniline, toluidine. Pyridine, picoline, lutidine, collidine, parvoline, coridine, rubidine, viridine. Leucoline, lepidine, cryptidine. Cespitine, pyrrol. This list contains only the names of substances which have actually been found in the coal-tar, and it is certain that a number of products must have escaped notice. It is obvious, too, that by using coal of different kinds, and by varying the temperature and pressure at which the operation of dis- tilling the coal is effected, we shall probably be able to increase the number of possible constituents of coal-tar almost indefinitely. The list above pre- sents to the non-chemical reader a string of quite unfamiliar names ; but, though the system of nomenclature in chemistry is far from perfect, yet each of these names has a meaning for the chemist beyond the mere de- signation of a substance. The chemical name aims at showing, or at least suggesting, the composition of a body and the general class to which it belongs. This may be illustrated by the names of hydro-carbons in the above list. The five compounds headed by benzol have many properties in common, and each one is entirely different in its chemical behaviour to those which follow amylene. The Greek numerals enter into the names of the latter, in order to express, in this case, the number of atoms of carbon which are supposed to be contained in each ultimate particle of the body. We write down in parallel columns the names of these two classes of bodies, together with the symbols which represent their composition, reminding the reader that the letter C represents carbon ; the letter alone indicating one atom of that element, but, when followed by a small figure, it implies that number of carbon atoms ; in like manner H, N,and O represent atoms of hydrogen, nitrogen, and oxygen respectively. Benzol C 6 H 6 Toluol C 7 H 8 Xylol C 8 H 10 Cumol C Q H 12 Cymol C 10 H 14 Hexylene C 6 H 12 Heptylene C 7 H 14 Octylene C 8 H 16 Nonylene C Q H 18 Decylene C 10 H 20 If these lists be carefully examined, it will be observed that there is a regular progression in the constituent atoms, so that each set of substances forms a series, the differences being always the same. The various bodies contained in the coal-tar are separated from each other by taking advan- tage of the fact that each substance has its own boiling-point ; that is, there is a certain temperature, different for each body, at which it will rise into vapour quickly and continuously. Benzol, for example, boils at 82 C., toluol at 1 14 C., and phenol at 188 C. ; so that, if we apply heat to a mix- ture of these three substances, the benzol will boil when the temperature reaches 82, and will pass away in vapour, carrying off heat, so that the temperature will not rise until all the benzol has been driven off ; then, when the temperature reaches 114, the toluol will begin to come off, but not COAL-TAR COLOURS. 563 until that has all passed over into the receiver will the temperature rise above 114; and the phenol remaining will distil, only at 188. Another mode of separating bodies when mixed together is by treating them with a liquid which acts on, or dissolves out, some of the constituents, but not the rest. The coal-tar, as it is received from the gas-works, is placed in large stills, capable, perhaps, of holding several thousand gallons, and usually made of wrought iron. Stills sufficiently good for the purpose are commonly constructed from the worn-out boilers of steam engines. The application of heat, of course, causes the more volatile substances to come over first. These are condensed and collected apart until products begin to come off which are heavier than water. The first portion of the distillate, containing the lighter liquids, is termed " coal naphtha." The pro- cess is continued, and heavier liquids come over, forming what is called in the trade the " dead oil." Pitch remains behind in the retort, from which it is usually run out while hot, but sometimes the distillation is carried a step further. The chief colour-producing substances contained in coal-tar are benzol, toluol, phenol, naphthalene, and anthracene. The aniline which is present in the tar is very small in amount, and if this ready-formed aniline were our only supply, it would be impossible to make colours from it on an industrial scale. The first of the above-named substances, benzol, was discovered by Faraday, in 1825, in liquid produced by strongly compressing gas obtained from oil. He called it bicarburet of hydrogen ; but afterwards another chemist, having procured the same body by distilling benzoic acid with lime, termed it benzine. It readily dissolves fats and oils ; and is used domestically for removing grease-spots, cleaning gloves, &c, and in the arts as a solvent of india-rubber and gutta-percha. It is a very limpid, colour- less liquid, very volatile, and, when pure, is of a peculiar but not disagree- able odour. It boils at 82 C, and, cooled to the freezing-point of water, it solidifies into beautiful transparent crystals, a property which is sometimes taken advantage of to separate it in a state of purity from other liquids which do not so solidify. Benzol is very inflammable, and its vapour produces an explosive mix- ture with air. The vapour, which is invisible, will run out of any leak in the apparatus, like water, and flow along the ground. Accidents have occurred from this cause, and a case is on record in which the vapour having crept along the floor of the works, was set on fire by a furnace forty feet away from the apparatus, the flame, of course, running back to the spot from which the vapour was issuing. Benzol is a dreadful sub- stance for spreading fire should it become ignited, for, being lighter than water, it floats upon its surface, and therefore the flames cannot be extin- guished in the ordinary way. The discovery of the presence of benzol in coal-tar was made by Hofman in 1845. ^ * s obtained from the light oil of coal-tar by first purifying this liquid by alternately distilling it with steam and treating with sulphuric acid several times. The product so obtained is a colourless liquid, sold as " rectified coal naphtha," which, however, has again to be several times re-distilled with a careful regulation of the tem- peratvwe, so that the benzol may be distilled off from other substances, boil- ing at a somewhat higher temperature, with which it is mixed. Even then the resulting liquid (commercial benzol) contains notable quantities of toluol. If benzol be added in small quantities at a time to very strong and warm nitric acid, a brisk action takes place, and when after some time water is added, a yellow oily-looking liquid falls to the bottom of the vessel. The 363 564 COAL-TAR COLOURS. FIG. 353. Iron Pots for making Nitro-Benzol. benzol will have disappeared, for nitric acid under such circumstances acts upon it by taking out of each particle an atom of hydrogen, which it re- places by a group of atoms of nitrogen and oxygen, and, instead of benzol, we have the yellow oil, nitro-benzol. Chemists are accustomed to repre- sent actions of this kind by what is called a chemical equation, the left-hand side showing the symbols representing the constitution of the bodies which are placed together, and the right hand the symbols of the bodies which result from the chemical action. Here is the equation representing the action we have described : C 6 H 6 Benzol. N0 2 OH Nitric acid. Nitro-benzol. HOH Water. Nitro-benzol has a sweet taste and a fragrant odour. It is known in commerce under the names of artificial oil of bitter almonds and essence of mirbane, and it has been used for perfuming soap. The chemical action between benzol and concentrated nitric acid is so violent that, when nitro- benzol first had to be manufactured on the large scale, great difficulty was experienced on account of the serious explosions which occurred. The apparatus now used in making nitro-benzol on the large scale is represented COAL-TAJt COLOURS. 565 in Fig. 353, which shows some of the cast-iron pots, of which there is usu- ally a long row. These pots are about 4! ft. in diameter, and the same in depth. Each is provided with a stirrer, which is made to revolve by a bevil-wheel, c, on its spindle, working with a pinion on a shaft, b, driven by a steam engine. A layer of water is kept on the tops of the lids, the water being constantly passed in and drawn off through the pipes, d, in order to keep it cool. For the chemical action is, as usual, attended with heat, which vaporizes some of the benzol, but the cold lid re-condenses the FIG. 354. Section of Apparatus for making Nitro-Benzol. vapour, which would otherwise escape with the nitrous fumes that pass off by the pipe, a. There is at e an opening, through which the material may be introduced, and in the bottom of the vessel is an aperture through which the products may be drawn off. Fig. 354 shows a section of one of the cast-iron vessels, and exhibits the mode in which the spindle of the stirrer passes through the lid. In the cup, rt, filled with a liquid, a kind of in- verted cup, which is attached to the spindle, turns round freely. It would not do to choose water for the liquid in this cup, for water would, by absorb- ing the nitrous fumes, form an acid capable of attacking and destroying the spindle. Nothing has been found to answer better for this purpose than nitre-benzol itself. The charge introduced into these vessels is a mixture 566 COAL-TAR COLOURS. of nitric and sulphuric acids together with the benzol. During the action, which may last twelve or fourteen days, no heat is applied, for the mixture becomes hot spontaneously, and in fact care must be taken that it does 355. Apparatus for making Aniline. not become too hot. The nitro-benzol thus obtained is purified by washing with water and solution of soda. If nitro-benzol were brought into contact with ordinary hydrogen gas, no action whatever would take place. But it is well known to chemists that gases which are just being liberated from a compound have at the instant of their generation much more powerful chemical properties than they pos- sess afterwards. Gases in this condition are said to be in the nascent state. COAL-TAR COLOURS. 567 If we submit nitro-benzol to the action of nascent hydrogen we find a remarkable change is produced. This change consists, first, in the hydro- gen robbing the nitro-benzol of all its oxygen atoms ; second, in the addi- tion of hydrogen to the remainder ; third, in some re-arrangement of the atoms, by which a new body is formed. Not that these changes are suc- cessive, or that we actually know the movement of atoms, but we are thus able to form ideas which correspond with the final result. The new sub- stance is named aniline. It is regarded by chemists as a base ; that is, a substance capable of neutralizing and combining with an acid to form a salt. Its composition is represented by the symbols C 6 H 5 H 2 N. Aniline was found in coal-tar in 1834, and even its colour-producing power was noticed, for its discoverer named it kyanol, in allusion to the blue colour it produced with chloride of lime. Later it was obtained by distilling indigo with potash, and hence received its present name from anil, the Portu- guese for indigo. The quantity of aniline contained in the tar is quite insignificant. Aniline is prepared from nitro-benzol on the large scale by heating it with acetic acid and iron filings or iron borings, a process which rapidly changes the nitro-benzol into aniline. The equation representing the change is QH 5 N0 2 Nitro-benzol. H 6 = Hydrogen. Aniline. 2H 2 0. Water. The operation is effected in the apparatus represented in Fig. 355. It consists of a large iron cylinder, within which works a paddle on a vertical revolving spindle, which, being hollow, is also a pipe to convey high pres- sure steam within the apparatus. Fig. 356 is a section of the hollow spindle, in which / is the pivot at the bottom of the cylinder on which it turns ; d is the stirring paddle; e is an aperture admitting the steam from the pipe, c, form- ing the shaft of the paddle, which is made to revolve by the bevil- wheel. The steams enters by the elbow-pipe, which has a nozzle ground to fit the head of the ver- tical revolving pipe, upon which it is pressed down by the screw. When the materials have been introduced into the cylinder, the stirrer is set in motion, and super- heated steam is sent down the pipe ; the aniline is volatilized and FIG. 356. Section of Hollow Spindle, Aniline Apparatus. passes with the steam through the pipe, which is connected with a worm surrounded by cold water. The aniline is purified by another distillation over lime or soda. When pure, aniline is a colourless, somewhat oily-looking liquid, of a feeble aromatic odour. Under the influence of light and air it becomes of a brownish tint, in which condition it usually presents itself in commerce. It scarcely dissolves in water, but is readily soluble in alcohol, ether, &c. It was Mr. Perkin who, in 1856, first obtained from aniline a substance 568 COAL-TAR COLOURS. practically available for dyeing. Let it be noticed that when Mr. Perkin discovered aniline purple, he was not engaged in searching for dye-stuffs, but was carrying on a purely scientific investigation as to the possibility of artificially preparing quinine. With this view, having selected a substance into the composition of which nitrogen, hydrogen, and carbon enter in exactly the same proportions -as they occur in quinine, but differing from it by containing no oxygen, he thought it not improbable that by oxidizing this body he might obtain quinine. In this he was disappointed, for the result was a dirty reddish-brown powder. Being desirous, however, of understanding more fully the nature of this reddish powder, he proceeded to try the effects of oxidation on other similarly constituted but more simple bodies. For this purpose he fortunately selected aniline, which, when treated with sulphuric acid and bichromate of potash, he found to yield a perfectly black product. Persevering in his experiments by exami- ning this black substance, he obtained, by digesting it with spirits of wine, the now well-known " aniline purple." Mr. Perkin, having determined to make the aniline purple on the large scale, patented his process, and suc- ceeded in overcoming the many obstacles incident to the establishment of a new manufacture requiring as its raw material products not at that time met with as commercial articles. The process is now carried on on the large scale by mixing sulphuric acid and aniline in the proportions in which they combine to form the sulphate of aniline, and dissolving by boiling with water in a large vat. Bichromate of potash is dissolved in water in another large vat. When both solutions are cold, they are mixed together in a still larger vessel and allowed to stand a day or two. A fine black powder settles on the bottom of the vessel in large quantities ; this is collected in filters, washed with water, and dried. This powder is not aniline purple alone, but a mixture of this with other products, presenting a very unpro- mising appearance; but when it has been digested for some time with diluted methylated spirit, all the colouring matter is dissolved out, and is obtained from the solution by placing the latter in a still, where the spirit is distilled off and collected for future use, while all the colouring matter remains behind, held in solution by the water. From this aqueous solution the mauve is thrown down by adding caustic soda. It is collected, washed, and drained until of a pasty consistence, in which condition it is sent into the market. It can be obtained in crystals, but the commercial article is seldom required in this form, as the additional expense is not compensated by any superiority in the practical applications of the colour. Mauve is readily soluble in spirits of wine, but not very soluble in water. Its tinctorial power is so great that one-tenth of a grain suffices to impart quite a deep colour to a gallon of water. Silk and woollen fabrics have an extraordinary attraction for this colouring matter, which attaches itself very firmly to their fibres. If some white wool is dipped into even a very dilute solution, the colour is quickly absorbed. Mauve is more permanent than any other coal- tar colour, being little affected by the prolonged action of light. Mauve is chemically a salt of a base which has been termed " mauve- ine." Mauveine itself is a nearly black crystalline powder, which forms solutions of a dull blue-violet tint, but when an acid is added to such a solution the tint is at once changed to purple. Mauveine is a powerful base, displacing ammonia from its compounds. The commercial crys- tallized mauve is the acetate of mauveine. The process by which Mr. Perkin orginally obtained mauve from aniline evidently depends upon the well-known oxidizing property of bichromate COAL-TAR COLOURS. 569 of potash, and experiments were accordingly made with other oxidizing bodies and aniline ; in fact, patents were taken out for the use of nearly every known oxidizing chemical. Three years after Mr. Perkin's discovery of mauve, M. Verguin, of Lyons, obtained, by treating crude aniline with chloride of tin, the bright red colouring matter now known as magenta. It was found also that crude > aniline, when treated with other metallic chlorides, nitrates, or other salts, which are oxidizing agents less powerful than bichromate of potash, yields this bright red colouring matter. A process patented by Medlock, in 1860, in which arsenic acid is the oxi- dizing agent, has almost entirely superseded, in England at least, all the others yet proposed for the manufacture of magenta. It is not a little remarkable that magenta would not have been discovered had M. Verguin and others operated on pure aniline instead of on the ordinary commercial article. For it was found subsequently by Dr. Hofman that pure aniline can- not be made to yield magenta : the presence of another body is necessary. A reference to the table of coal-tar constituents will show that there is a hydro-carbon named " toluol." This substance is of a similar nature to benzol, and has a boiling-point so little above that of benzol, that in the rough methods of separation usually employed, a notable quantity of toluol is carried over with the benzol, and is always present in the commercial article. In the processes which benzol undergoes for conversion into ani- line, the toluol accompanies it in a series of parallel transformations, re- sulting in the production of a base termed " toluidine " similar to aniline being, however, in its pure state a solid at ordinary temperatures. We write down the symbols representing the composition of the bodies formed in the two cases in order to clearly show this : Benzol C 6 H 6 Nitro-benzol C 6 H 5 (NO 2 ) Aniline C 6 .H 5 NH 2 Toluol C 7 H 8 Nitro-toluol C 7 H 7 (NO 2 ) Toluidine C 7 H 7 NH 2 This aniline prepared from commercial benzol always contains some toluidine ; and it is essential for the production of magenta that this sub- stance should be operated on along with the aniline. Whether the presence of some toluidine is also necessary for the production of mauve and other colours is not yet known, but they are always prepared from commercial benzol. It is certain that pure aniline yields no magenta, neither does pure toluidine ; but a mixture supplies it in abundance. For the preparation of magenta the best proportions for this mixture would be about three parts of aniline to one of toluidine ; but, in practice, it is not necessary to obtain the two substances separately, as benzol, mixed with a sufficient quantity of toluol, may be obtained by regulating the distillation. The apparatus used in the production of magenta is shown in Fig. 352. It consists of a large iron pot set over a furnace in brickwork, and having a lid with a stuff- ing-box, through which passes a spindle carrying a stirrer. A bent tube rises from the lid, and is connected with a worm surrounded by cold water, for the purpose of condensing the aniline which is vapourized in the process. The aniline, containing a due amount of toluidine, is mixed in this apparatus with about one and a half times its weight of a saturated solution of arsenic acid (H 3 AsO 4 ). The fire is lighted and kept up for several hours : water first, and lastly aniline, distil over. When the operation is ended, steam is blown through the apparatus, thus carrying off an additional portion of aniline. The crude product is then boiled with water, the solution filtered, 570 COAL-TAR COLOURS. and common salt added, which precipitates an impure magenta. This is afterwards dissolved and recrystallized several times. The crystals of this magenta like those of many of the coal-colour products have a peculiar greenish metallic lustre ; they dissolve in warm water, forming a deep purplish-red solution. The chemical composition of magenta has been in- vestigated by Dr. Hofman, who found it to be a salt of an organic base, to which he gave the name of " rosaniline." This rosaniline is easily ob- tained from magenta by addition to its solution of an alkali. While all its salts are intensely coloured, rosaniline itself is a perfectly colourless sub- stance, becoming reddened by exposure to the air, as it absorbs carbonic acid, thus passing to the condition of a salt. Rosaniline, then, displays its chromatic powers only when it is combined with an acid. This pro- perty is sometimes shown at lectures in a striking manner by dipping a piece of paper into a colourless solution of rosaniline, and exposing it to the air, when, as the rosaniline absorbs carbonic acid, the paper changes from white to red. A more elegant form of the same experiment is to dip a white rose into a solution of rosaniline containing a little ammonia. As the ammonia escapes, or is expelled by a current of warm air, the same kind of action occurs, and the white rose changes to red as if by magic, the emblem of the House of York is transformed into the badge of Lan- caster ! The chemical nature of rosaniline is regarded as analogous to that of ammonia it is, in fact, looked upon by chemists as a sort of am- monia, in each particle of which some atoms of hydrogen have been replaced by certain groiips of carbon and hydrogen atoms some of these groups be- ing derived from the aniline and others from the toluidine. The particular salt of rosaniline which constitutes the crude product of the action on the aniline and toluidine, depends on the substance employed to effect the oxidation. If a chloride, the resulting product is chloride of rosaniline ; if a nitrate, it is the nitrate ; and so on. The magenta which is formed in the first instance by the process we have described is an arseniate of rosani- line ; but in the subsequent processes, it is converted into the chloride the salt usually sold as magenta. Other salts of rosaniline are made on the large scale especially the acetate, the beautiful crystals of which have the advantage of being very soluble. Magenta attaches itself strongly to animal fibres, but the colour is some- what fugacious under the action of sunlight. It is used not only as a dye, but more largely as the raw material from which a number of other beauti- ful colours are obtained. For this reason it is manufactured on an enormous scale, thousands of tons being produced annually, and the money value of the colour produced from it must be reckoned by thousands of pounds. Yet aniline was a few years ago merely a curiosity never met with out of the laboratory of the scientific chemist. It is stated that a single firm now makes more than twelve tons of aniline weekly, and on its premises may be seen tanks, in each of which 30,000 gallons of magenta solution is depositing its crystals. If a salt of rosaniline be heated with aniline, the colour changes gradually through purple to blue, while ammonia is at the same time given off. This is the colour known as aniline blue, " bleu du Lyons," &c. In its preparation it has been found that the best results are obtained by employing the salt of some weak acid acetate of rosaniline, for example and pure aniline, that is, aniline free from toluidine. The operation is conducted in iron pots very similar to those used in making magenta, but smaller. These pots are not set over a fire, but a number of them are placed in a large vessel containing oil, by which they can be COAL-TAR COLOURS. 57 1 maintained at a regulated temperature when the oil is heated. The crude product undergoes several purifications, and the aniline blue is supplied in commerce in powder, or dissolved in spirits of wine. It is insoluble in water, and this has been an obstacle to its employment ; but recently a similar substance has been obtained in a soluble form, and is extensively used for dyeing wool, under the name of " Nicholson's blue." Other blues have been similarly prepared, and from the same two substances, magenta and aniline, a colour known as " violet imperial " was formerly made in very large quantities, but it has been superseded by the colours about to be described. It may be well to mention that these blues and violets have been found to contain bases formed of rosaniline, in which one, two, or three atoms of hydrogen are replaced by the group C 6 H 5 . This group of atoms will be noticed to belong to aniline, and chemists have named it phenyl, and, therefore, bases of these coloured salts are respectively named phenyl-rosaniline, di-phenyl-rosaniline, tri-phenyl-rosaniline. But Dr. Hofman found that other groups of atoms besides C 6 H 5 may be made to take the place of H in rosaniline. By acting on rosaniline or its salts with iodides of ethyl, C 2 H 5 I, or iodide of methyl, CH 3 I, he obtained a beautiful series of violets, of which many shades could be produced,. vary- ing from red-purple to blue. These are the colours so well known as Hofman's violets, and are prepared on the large scale by heating a solu- tion of magenta (chloride of rosaniline) in alcohol or wood spirit, with the iodide of ethyl or the iodide of methyl. The nature and proportions of the ingredients are regulated according to the tint required. The vessels are hermetically closed during the heating, which is accomplished by steam admitted into a steam-jacket surrounding the vessel. The crude product has to be separated from the substances with which it is mixed, and the colouring matter is finally obtained, presenting in the solid state the peculiar semi-metallic lustre so characteristic of these products. Like the other colours, Hofman's violets are salts of colourless bases, which, as indicated above, are substitution products of rosaniline. The tints they produce incline to red, violet, or blue, according as one, two, or three hydrogen atoms are replaced by the ethyl or methyl groups. Colours have also been obtained from mauve and iodide of ethyl for example, the dye known in commerce as " dahlia." Other colours are procured from magenta by treating it with various compounds : one such is the " Britan- nia violet," discovered also by Mr. Perkin, who procures it from magenta and a hydrocarbon-bromide derived from the action of bromine or common turpentine. This is a very useful colour, and is largely used in dyeing and printing violets, of which any shades may be obtained. Another derivative of rosaniline is the aniline green. It is obtained by dissolving the rosaniline salt in dilute sulphuric acid, adding crude alde- hyde (a substance obtained by acting with oxidizing agents on alcohol). The mixture is heated until a sample dissolves in acidulated water with a blue tint ; it is poured out into boiling water containing in solution hypo- sulphite of sodium, boiled, the liquid filtered ; and the green dye, if required in the solid state, is precipitated by carbonate of sodium. Aniline green dyes wool and silk, the latter especially, of a magnificent green ; perhaps as beautiful a colour as any of the coal-tar series, and one which has the sin- gular advantage among greens of looking as beautiful in artificial light as in daylight. The manner in which this dye was discovered is somewhat curious. It is related by Mr. Perkin of a dyer, named Chirpin, that he was trying to render permanent a blue colouring matter, which had been found 572 COAL-TAX COLOURS. could be produced from rosaniline by the action of aldehyde and sulphuric acid. After a number of fruitless attempts at fixing it, he confided his perplexities to a photographic friend, who evidently thought that if it was possible to fix a photograph, anything else might be fixed in like manner, for he recommended his confidant to try hyposulphite of sodium. On making the experiment, however, the dyer did not succeed in fixing his blue, but converted it into the splendid aldehyde green. Like other colour- ing matters we have described, this is a salt of a colourless base containing sulphur. Like rosaniline, the colourless base takes on the characteristic colour of its salts by merely absorbing, carbonic acid from the air. Again, by a modification of the process for producing the Hofman violets, another green of an entirely different constitution may be obtained. It is bluer in tint than the former, and is much used for cotton and silks, under the name of " iodine green." In' the manufacture of magenta there is formed a residuum or bye-product, consisting of a resinous, feebly basic substance, from which Nicholson ob- tained a dye, imparting to silk and wool a gorgeous golden yellow colour. This dye cannot be obtained directly, but is always produced in greater or less quantity when magenta is made on the large scale, and is separated during the purification. By first dyeing the silk or wool with magenta, and then with this dye, which is commercially known as " phosphine," brilliant scarlet tints are obtained. The yellow colours have been found to be salts of a base termed chrysaniline, a sort of chemical relative of rosaniline, as may be seen in comparing the formulas which represent their constitution, with which we place also the symbol for another substance obtained by submitting rosaniline to the influence of nascent hydrogen. This body, leucaniline, again yields rosaniline very readily when the hydrogen is re- moved by oxidizing agents. It will be noticed that the three bodies form a series the members of which differ only by H 2 , thus indicating their close relationship. C 20 H 17 N 3 Chrysaniline. C 20 H 1Q N 3 Rosaniline. C 20 H 21 N 3 Leucaniline. Some idea will have been obtained from the foregoing particulars of the great colour-supplying capabilities of aniline ; but we have not yet exhausted the utility of this interesting substance. It is probable that the letters on the page now under the reader's eye owe their blackness to an aniline product. For after all the salts furnishing the lovely tints we have mentioned have been extracted, there is in their manufacture a final residuum, and from this an intense black is obtained, which is largely used in the manufacture of printing-ink. We have mentioned phenol as a substance yielding colours. Phenol is the body now so well known as a disinfectant under the name of " carbolic acid," a name given to it by its discoverer, Runge, who prepared it from coal-tar, in 1834. Phenol forms colourless crystals, which dissolve to some extent in water, and very readily in alcohol. It is a powerful antiseptic, that is, it arrests the process of pul refaction in animal or vegetable bodies, and it is also highly poisonous. The constitution of phenol is given by the formula C 6 Hg OH, in which the reader will recognize the same group of atoms already indicated as entering into the aniline derivatives. From some of these phenol may in fact be obtained, and although it cannot be formed directly from benzol, phenol can be made to furnish benzol. When COAL-TAR COLOURS. 573 crude phenol is treated with a sulphuric acid and oxalic acid, a substance is obtained which presents itself as a brittle resinous mass of a brown colour, with greenish metallic lustre. This substance is called rosolic acid by chemists, but in commerce it is known as aurine, and is used for dyeing silk of an orange colour, which, however, is not very permanent. But by heating rosolic acid with liquid ammonia, a permanent red dye is procured which has been termed peonine, and has been much used for woollen goods. But it lately had the reputation of exerting a poisonous action, producing blistering and sores when stockings or other articles dyed with it were worn in contact with the skin. It is now, therefore, less extensively em- ployed. Coralline, another body identical with or very similar to the former, is similarly prepared from rosolic acid by heating it with ammonia under pressure. Again, by heating coralline with aniline, a blue dye, known as " azurine," or "azuline," was formerly made in large quantities ; but it has been sup- planted by the aniline blues already described. When phenol is acted upon by nitric acid new compounds are produced, standing in the same relation to phenol as nitro-benzol does to benzol. The final result of the action of nitric acid on phenol is picric acid, called also " carbazotic acid," and, more systematically, " tri-nitro-phenol ; " for it is regarded as phenol in which three of the hydrogen atoms have been re- placed by the group NO 2 thus, C 6 H 2 (NO a )3 OH. It forms bright yellow- coloured crystals, and its solution readily imparts a bright pure yellow colour to wool, silk, &c. It received the name of picric acid (iriKpos, bitter) from the exceedingly bitter taste of even an extremely diluted solution. It is said that picric acid is employed as an adulterant in bitter ale instead of hops. Now, the colouring power of picric acid is so great, that even the minute quantity which could be used to impart bitterness to beer is recog- nizable by dipping a piece of white wool into the beer, when, if picric acid be present, the wool acquires a clear yellow tint. Besides its employment as a yellow, it is useful for procuring green tints by combination with the blues. Picric acid again furnishes, by treatment with cyanide of potassium, a deep red colour, consisting of an acid which, when combined with ammonia, furnishes a magnificent colouring material which is, in fact, murexide, a dye identical with the famous Tyrian purple of the ancients, and formerly obtainable only from certain kinds of shell-fish. Naphthaline another of the colour-yielding substances of coal-tar is, like benzol, a hydro-carbon, but one belonging to quite another chemical series. Its formula is C 10 H 8 , and it has an interest to chemists altogether apart from its industrial uses, from having been the subject of the classic researches of the French chemist, Laurent researches which resulted in the introduction of new and fertile ideas into chemical science, con- tributing largely to its rapid progress. Naphthaline forms colourless crystals, which, like camphor, slowly volatilize at ordinary temperatures, and are readily distilled in a current of steam. It is thus sufficiently vola- tile to escape complete deposition in the condensers of the gas-works, and to be partly carried over into the mains, where its collection occasions some trouble. Nitric acid acts upon naphthaline in a manner analogous to that in which it acts on benzol, forming nitro-naphthaline, which, in its turn, submitted to the action of iron filings and acetic acid, is transformed into a base called " naphthylamine." The salts of naphthylamine are coloured products which, in some cases, have been found available as dyes. There is a crimson colour, and a yellow largely used under the name of 574 COAL-TAR COLOURS. "Manchester yellow," for imparting to silk and wool a gorgeous golden yellow colour. Another coloured derivative of naphthaline, called " carmi- naphtha," was discovered by Laurent in the course of his researches. It would be easy to fill this volume with descriptions of the properties, and modes of preparing the numerous colouring matters that have been obtained from coal-tar products. In order to give the reader an idea of the extent to which the tar products have been made to minister to our sense of the beautiful, a list is here given of the principal colouring matters from these sources that have been employed in the arts. The various names under which a product has been commercially known are in most cases given. It must be understood that the same name is frequently applied to products chemically distinct, and some of the names which appear as synonyms may also in reality indicate different substances. . LIST OF COAL-TAR COLOURS. I. COLOURS DERIVED FROM ANILINE AND TOLUIDINE. Blues and Violets. Mauve, aniline purple, Perkin's violet, violine, mauve, rosaniline, indisine, &c. Aniline blue, rosaniline blue, Hofman's blue, bleu de Paris, bleu de Lyons, bleu de Mulhouse, bleu de Mexique, bleu de nuit, bleu lumiere, night blue. Hofman's blue. Nicholson's blue, soluble blue. Hofman's violet, rosaniline violet. A long series of red and blue violets, bearing Hofman's name and distin- guished in commerce by adding R or B, according to the redness or the blueness of the tint, ranging from RRRR to BBBB. Dahlia. Toluidine blue. Violet de Paris. Mauvaniline. Violaniline. Regina blue, opal blue, bleu de Fayolle, violet de Mulhouse. Britannia violet Violet imperial. And many others. Reds. Aniline red, new red, magenta, solferino, anileine, rouge", roseine, azaline. Rubine, rubine imperial. Chrysaniline red. (The above are all salts of rosaniline.) Xylidine, tar red, soluble red. Yellows. Chrysaniline, phosphine, aniline yellow, yellow fuschine. Chrysotoluidine. Dinaline. Field's orange. COAL-TAR COLOURS. 575 Greens. Aldehyde green, aniline green, viridine, emeraldine. Iodine green, iodide of methyl green, iodide of ethyl green. Perkin's green. Browns. Havanna brown. Bismarck brown, aniline brown, Napoleon brown, aniline maroon. Greys and Blacks. Aniline grey, argentine. Argentine black. II. COLOURS DERIVED FROM PHENOL. Blues and Violets. Isopurpuric acid, Grdnat Azuline, azurine. Reds. Picramic acid. Coralline, peonine. Red coralline. Yellows. Picric acid, carbazotic acid. Aurine, rosolic acid. Green. Chloropicrine. Browns. Picrate of ammonia. Isopurpurate of potash. Phenyl brown, phenicienne. III. COLOURS DERIVED FROM NAPHTHALENE. Reds. Pseudoalizarine, naphthalic red. Roseonaphthaline, carminaphtha. Yellows. Binitronaphthaline, naphthaline yellow, golden yellow, Manchester yellow. And others. The introduction of aniline colours into dyeing and calico-printing has caused quite a revolution in these arts, the processes having become much more simple, and the facilities for obtaining every variety of tint largely increased. The arts of lithography, type-printing, paper-staining, &c., have also profited by the coal-tar colours. For such purposes the colour 57 6 COAL-TAR COLOURS. is prepared by fixing it on alumina, a process in which much difficulty was at first experienced, for the colours are themselves almost all of a basic nature. The desired result is now attained by fixing them on the alumina with tannic or benzoic acid. These lakes produce brilliant printing- inks, which are extensively used. The aniline colours are also employed for coloured writing-inks, tinted soaps, imitations of bronzed surfaces, and for a variety of other purposes. Not many years ago coal-tar was a valueless substance : it was actually given away by gas-makers to any one who chose to fetch it from the works. It was then " matter in the wrong place;" but Mr. Perkin's dis- covery led to its -being put in the right place, and it has become the raw material of a manufacture creating an absolutely new industry, which has developed with amazing rapidity. This industry dates from only 1856, and in 1862 the annual value of its products was more than ,400,000. Dr. Hofman, in reporting on the coal-tar colours shown at the Paris Exhi- bition of 1867, computed the value at that time at about ^1,250,000, al- though the products were much cheaper than before. Large manufactories have been established in Great Britain, in France, Germany, Switzerland, America, and other countries. The possibility of such an industry is an interesting illustration of the manner in which the progress made in any one branch of practical science may lead to unexpected developments in other quarters. The quantity of aniline obtained from coal-tar is very small compared to the amount of coal used, as may be seen from the following table, in which the respective weights of the various products required in the manufacture of mauve are arranged as given by Mr. Perkin for the produce of 100 Ibs. of coal. Ibs. oz. Coal loo o Coal-tar 10 12 Coal-tar naphtha o 8|- Benzol o 2| Nitro-benzol o 4^ Aniline o 'z\ Mauve o oj From this we may perceive that had not the manufacture of gas been greatly extended, so as to yield a large aggregate produce of tar, the re- quisite supply for the manufacture of aniline would not have been attain- able ; and the industrial application of the previously worthless bye-product reacts upon gas manufacture by cheapening the price of that commodity, thus tending still more to extend its use. Although anthracene has already been named as one of the colour- producing substances found in coal-tar, we have not in the list of coal-tar colours included the colouring matter which anthracene is capable of yield- ing. The reason is that this case stands apart in some respects from the rest. The colours derived from aniline and the other substances already enumerated are instances of the production of bodies not found in nature mauve, magenta, &c., do not, so far as we know, exist in nature. Their artificial formation was a production of substances absolutely new. The colour of which we have now to treat is, on the other hand, found in nature, and from its occurrence in the rubia tinctoria^ the roots of that plant have for ages been employed as a source of colour, and are well known in this country as "madder." The plant is grown largely in Holland, in France, COAL-TAR COLOURS. 577 in the Levant, and in the south of Russia.* Madder is used in enormous, quantities for dyeing reds and purples : the well-known " Turkey red " is due to the colouring matter of this root. The total annual value of the madder grown is calculated to reach nearly 2| million pounds sterling. More than forty years ago it was discovered that the madder- root yielded a colouring substance, to which the name of " alizarine " was bestowed, from alizari, the commercial designation of madder in the Levant. The aliza- rine does not exist in the fresh root, but is produced in the ordinary pro- cesses of preparing the root and dyeing with it, in consequence of a peculiar decomposition or fermentation. Alizarine may be procured from dried madder by simply submitting it to sublimation, when beautiful orange needle-shaped crystals of alizarine may be obtained. It is nearly insoluble in water, but readily dissolves in hot spirits of wine. Acids do not dissolve it, but potash dissolves it freely, striking a beautiful colour ; with lime, barytes, and oxide of iron, it forms purple lake, and with alumina a beauti- ful red lake. According to Dr. Schunck, of Manchester, to whose investi- gations we are indebted for much of our knowledge of madder, the root contains a bitter uncrystallizable substance called " rubian," which, under the action of certain ferments, and of acids and alkalies, is decomposed into a kind of sugar, and into alizarine and other colouring matters. The ferment, which in the process of extracting the colouring matter from the roots causes the formation of alizarine, is contained in the root itself. We have already seen how an investigation relating to a question of pure chemical science accidentally led Mr. Perkin to the discovery of mauve the precursor of the long range of beautiful colours already de- scribed. The mode of artificially preparing alizarine, so far from being an accidental discovery, was sought for and found in 1869 by two German chemists, Graebe and Liebermann. The researches of these chemists were conducted in a highly scientific spirit. Instead of making attempts to pro- duce alizarine by trying various processes on first one body, then another, to see if they could hit upon some tar product, or other substance, which would yield the desired product, they began by operating analytically on alizarine itself. Just as a mechanic ignorant of horology, required to make a watch, would be more likely quickly to succeed in his task by taking a watch to pieces to see how it is put together, than if he had tried all man- ner of arranging springs and wheels until he hit upon the right way ; so these chemists set themselves to take alizarine to pieces, in order to see from what materials they might be able to put it together. They decom- posed alizarine, and among the products found a hydro-carbon identical in all its properties with anthracene. Anthracene was discovered in coal-tar by Laurent in 1832, and its properties were investigated by Anderson in 1862. It may be remarked that such investigations were not conducted with a view to any industrial uses of anthracene, but merely for the sake of chemistry as a science. Certainly no one could have supposed at that time that the slightest rela- tion existed between anthracene and madder. Anthracene is a white solid hydro-carbon, which comes over only in the last stages of the distil- lation of coal-tar, accompanied by naphthaline, from which it is easily separated by means of spirits of wine, by which the naphthaline is readily * The natural Order to which the madder plant belongs is interesting from the number of its members which supply us with useful products. That valuable medicine, quinine, is obtained from plants belonging to this family, as is also ipecacuanha, and other articles of the materia medtca. Coffea arabica, which furnishes the coffee-berry, is another member. 37 57 8 COAL-TAR COLOURS. dissolved, but the anthracene scarcely. Anderson, in 1861, discovered, among other results, that anthracene, C 14 H 10 , by treatment with nitric acid became changed into oxy-anthracene, C 14 H 8 O2 ; an d this reaction we shall see is a step in the process of procuring alizarine from anthracene. Phenol, as already mentioned, can be made to yield benzol, by a process of deoxidization. With a view to similarly obtaining a hydro-carbon from alizarine, Graebe and Liebermann passed its vapours over heated zinc filings, and thus produced anthracene from alizarine. It now remained to find a means of reversing this process, that is, so to act on anthracene as to produce alizarine, and this was effected by treating anthracene with bromine, forming a substance which, on fusing with caustic potash, yielded alizarate of potash, from which pure alizarine resulted by treat- ment with hydrochloric acid. A much cheaper method was, however, necessary for manufacturing purposes, and it was found in a process by which oxy-anthracene, C 14 O 8 H 2 , is treated at a high temperature with strong sulphuric acid, and the product so formed heated with a strong solution of potash, yielding alizarate of potassium as before. Many other interesting substances appear to be formed in the reactions, but the nature of these bodies has as yet been imperfectly investigated. No doubt what- ever can be entertained of the identity of natural with artificial alizarine ; and the production of this substance, the first instance of a natural colour- ing matter made artificially, may be regarded as a great triumph of chemical science. It was not long ago supposed that the chemical bodies found in plants or animals, or produced by vital actions, could not possibly be formed by any artificial process from their elements. The laws which presided at their formation were, it was conceived, wholly different from those which governed the chemicals of the laboratory, for they were held to act exclusively under the influence of a mysterious agent, namely, " vital force." It was supposed, for example, that from pure carbon, oxygen, and hydrogen, no chemist would ever be able to produce such a compound as acetic acid. Accordingly the domain of chemical science, previous to the end of the first quarter of the present century, was divided by an impass- able barrier into the two regions of organic and inorganic chemistry. Now, however, the chemist is able to build up in his laboratory from their very elements a great number of the so-called organic bodies. And it is quite possible to do this in the case of alizarine ; that is, a chemist having in his laboratory the elements, hydrogen, carbon, oxygen, &c., could actually build up the substance which gives its value to madder. The quantity of anthracene procurable from coal-tar is, unfortunately, comparatively small, for it is found that from the distillation of 2,000 tons of coal only one ton of anthracene can be obtained. The use of artificial alizarine would doubtless entirely supplant the employment of madder- root if anthracene could be obtained in larger quantities ; and the change would be highly advantageous to this country, for as no madder is grown in Great Britain, and we consume nearly half the whole annual growth, it follows that every year a million pounds sterling go out of the country for this commodity. When anthracene is produced from coal in sufficient abundance, this sum will be available for the support of our own popula- tion. In the meantime, the manufacture of artificial alizarine is restricted only by the supply of its raw material. FIG. is?. James Prescott "Joule, F.R.S. THE GREATEST DISCOVERY OF THE AGE. THE indulgent reader who may have followed the course of the fore- going pages, will perhaps peruse the title of this article with some little bewilderment. His attention has been drawn to one after another of a series of remarkable and important discoveries, and he will naturally wonder what can be the discovery which is greater than any of these. Now, a discovery is great in proportion to the extent and importance of the results that flow from it. These results may be immediate and practical, as in the case of vaccination ; or they may be scientific and intellectual, as in Newton's discovery of the identity of the force which draws a stone to the ground with that which holds the planets in their orbits. Such dis- coveries as most enlarge our knowledge of the world in which we live, by embracing in simple laws a vast field of phenomena, are precisely those which are most prolific in useful applications. If we admit, as we must, the truth of Bacon's aphorism, which declares that " Man, as the minister and interpreter of nature, is limited in act and understanding bv his obser- 579 580 THE GREATEST DISCOVERY OF THE AGE. vation of the order of nature ; neither his understanding nor his power extends farther,"* then it would be easy to show that the discovery of which we have to treat, more than any other, must be of immense practical service to mankind in every one of the ways in which a knowledge of the order of nature can be of use, viz. : " First, In showing in how to avoid attempting impossibilities. Second, In securing us from important mistakes in at- tempting what is, in itself, possible, by means either inadequate or actually opposed to the end in view. Third, In enabling us to accomplish our ends in the easiest, shortest, most economical, and most effectual manner. Fourth, In inducing us to attempt, and enabling us to accomplish, objects which, but for such knowledge, we should never have thought of under- taking." t A great principle, like that which we are about to explain to the reader, is too vast in its bearings for its discovery and elaboration. to have been the work of an individual. This truth, and indeed the whole of our know- ledge, is but the result of the development and growth of pre-existing knowledge. In fact, every discovery, however brilliant every invention, however ingenious, is but the expansion or improvement of an antecedent discovery or invention. In strictness, therefore, it is impossible to say where the first germ of even our newest notions may be found. Our latest philosophy can be shown to be the result of progressive modifications of ideas of remote ages. Hence every great truth, every grand invention, has in reality been the offspring of many minds ; but we record as the discoverers and inventors those men who have made the longest strides in the path of progress, and whose genius and labours have overcome obstacles defying ordinary efforts. The extent of the field which is covered by the principle we have in view is so vast embracing, as it does, the whole phenomena of the universe that it will not be possible to do more within our limits than give the reader a general notion of the principle itself. It may be useful to instance a truth which has a similar generality and significance, and which has also acquired the force of an axiom, because it is verified every hour. It is that greatest generalization of chemistry, affirming that in all its transfor- mations matter is indestructible, and can no more be destroyed than it can be called into being at will. This truth is so well established, that some philosophers have asserted that an opposite state of things is inconceivable. But it was not always known ; and there are at the present day untutored minds which not only believe that a substance destroyed by fire is utterly annihilated, but what they find inconceivable is the continued existence of the substance in an invisible form. The candle burns away, its matter vanishes from our view; but if we collect the invisible products of the com- bustion, we find in them the whole substance of the candle in union with the atmospheric oxygen. We may, in imagination, follow the indestructible atoms of carbon in their migrations, from the atmosphere to the plant, which is eaten by the animal and goes to form its fat, and from the tallow, by combustion, back into the atmosphere again. The notion of the real identity of matter under changing forms has been expressed by our great dramatist in a well-known passage, which is remarkable for its philosophic insight, when we consider the age in which it was written : " Homo naturae minister et interpres, tantum facit et intelligit quantum de naturae ordine re vel mente observaverit : nee amplius scit aut potest." Novum Organum, Afihor. i. 1Sir J. Hersche'. THE ORE A TEST DISCO VER Y OF THE AGE. 581 HAMLET. To what base uses we may return, Horatio ! Why may not imagination trace the noble dust of Alexander, till he find it stopping a bung-hole ? HORATIO. 'Twere to consider too curiously to consider so. HAMLET. No, faith, not a jot; but to follow him thither with modesty enough, and likelihood to lead it. As thus : Alexander died, Alexander was buried, Alexander returneth to dust ; the dust is earth ; of earth we make loam ; and why of that loam, whereto he was converted, might they not stop a beer-barrel ? Imperial Caesar, dead, and turned to clay, Might stop a hole to keep the wind away ; O, that the earth, which kept the world in awe, Should patch a wall to expel the winter's flaw ! Now the greatest discovery of our age is that force, like matter, is inde- structible, and that it can no more be created than can matter. The reader may perhaps think the statement that we cannot create force is in contradiction to experience. He will be disposed to ask, What is the steam engine for but to create force ? Do we not gain force by the pulley, the lever, the hydraulic press ? And are not tremendous forces produced when we explode gunpowder or nitro-glycerine ? When the principle with which we are here concerned has been developed and stated in accurate terms, it is hoped the reader will see the real nature of these contrivances. We are, however, aware that it is quite impossible within the limits of a short article to do much more than indicate a region of discovery abounding with results which may be yet unfamiliar to some. Into this, if so minded, they should seek for further guidance, which they will pleasantly find in the pages of Dr. Tyndall's " Heat considered as a Mode of Motion," and in a little work by Professor Balfour Stewart, entitled " The Conservation of Energy," and quite fascinating from the clearness and simplicity of its style. We may continue our humble task of merely illustrating the general nature of this, in reality the most important, subject which we have had occasion to bring under the reader's notice. Perhaps the first step should be to point out the fact of the various forces of nature mechanical action, heat, light, electricity, magnetism, chemical action being so related that any one can be made to produce all the rest directly or indirectly. Some examples of the conversion of one form of force into another occur in the foregoing pages. Thus, on page 363 an experiment is described in which electricity produces a mecha- nical action ; electricity is also shown, on page 374, to produce heat ; on page 376 chemical action ; on page 378 magnetism. Then, as instances of the inverse actions, there is on page 366, in the first paragraph on " Electric Induction," an account of the mode in which mechanical movements may give rise to electricity; and in the experiments in pages 385, 386, and particularly in the account of the Gramme machine, page 388, it is shown how mechanical movements can, through magnetism, produce electricity. The voltaic element, page 369, and the galvanic batteries, are instances of chemical action supplying electricity. On page 325 a striking instance is mentioned of changes in the forms of force. Every lighted candle is a case of chemical action giving rise to light ; and interesting examples of the inverse relation are referred to on page 447. On page 116 is represented the conversion of arrested motion into heat and light. We have, indeed, sufficient examples to arrange a series of these conversions of forces in a circle. Thus, chemical action (oxidation in the animal system) supplies muscular power, this sets in motion a Gramme machine, the motion is con- verted into electricity, the electricity produces the electric light, and light causes chemical action, and with this the cycle is complete. In the steam engine heat is converted into mechanical force, and many cases will pre- 582 THE GREATEST DISCOVERY OF THE AGE. sent themselves to the reader's mind in which mechanical actions give rise to heat. The doctrine of a mutual dependence and convertibility among all the forms of force was first definitively taught in England by Mr. (now Justice) Grove, in 1842 ; and almost simultaneously Dr. Meyer promulgated similar views in Germany. Mr. Grove subsequently embodied his doctrine in a treatise, called " The Correlation of the Physical Forces," which has seen several editions. But this teaching included much more than a mere connection between the various forces, for it extended to quantitative relations. It declared that a given amount of one force always produced a definite amount or another . that a certain quantity of heat, for example, would give rise to a certain amount of mechanical action, and that this amount of mechanical action was the equivalent of the heat which produced it, and would in its turn reproduce all that heat. These last doctrines, however, rested on a specula- tive basis, until Mr. James Prescott Joule, of Manchester, carried out a most patient, laborious, and elaborate experimental investigation of the subject. His labours placed the truth of the numerical equivalence of forces on a foundation which cannot be shaken ; and he accomplished for the principle of the indestructibility of force what Lavoisier did for that of the inde- structibility of matter he established it on tha incontrovertible basis of accurate and conclusive experiment. His determination of the value of the mechanical equivalent of heat especially is a model of experimental research ; and subsequent investigators have, by diversified methods, con- firmed the accuracy of his results. A great part of his work consisted in finding what quantity of heat would be produced by a given quantity of work. Before we proceed to give an indication of one of Dr. Joule's methods of making this determination, we may point out that if a weight be raised a certain height, the work which is done in raising it will be given out by the weight in its descent. If you carry a I Ib. weight to the top of the London Monument, which is 200 ft. high, you perform 200 units of work. When the weight is at the top, the work is not lost ; for let the weight be attached to a cord passing over a pulley, and it will, as it descends, draw up to the top another i Ib. weight. If you drop the weight so that it falls freely, it descends with a continually increasing velocity, strikes the pavement, and comes to rest. Still your work is not lost. The collision of the weight and the pavement develops heat, just as in the case of the experiment depicted on page 116, but to a less degree the increase of temperature might not be sensible to the touch, but could be recognized by delicate instruments. Your work, then, has now changed into the form of heat the weight and the pavement are hotter than before. This heat is carried off by contigu- ous substances. But still your work is not lost, for it has made the earth warmer. The heat, however, soon flows away by radiation from the earth, and is diffused into space. The final result of your work is, then, that a certain measurable quantity of heat has been sent off into space. Is your work now finally lost ? Not so : in reality, it is only diffused throughout the universe in the form of radiant heat of low intensity. Yet it is lost for ever for useful purposes ; for from this final form of diffused heat there is no known or conceivable process by which heat can be gathered up again. Dr. Joule arranged paddles of brass or iron, so that they could turn freely in a circular box containing water or quicksilver. From the sides of the box partitions projected inwards, which contained openings that permitted the divided arms of the paddle to pass, and preventing the liquid from, 7 HE GREATEST DISCOVERY OF THE AGE. 583 moving en masse, thus caused a churning action when the paddle was turned. Now, every one who has worked a rotatory churn knows that a considerable resistance is offered to this action ; but every one does not know that under these circumstances the liquid becomes warmer. It was Dr. Joule's object to discover how much the temperature of his liquid was raised by a measured quantity of work. He used very delicate thermo- meters, and had to take a number of precautions which need not here be described ; and he obtained the definite quantity of work by the descent of a known weight through a known distance, a cord attached to the weight being wound on a drum, which communicated motion to the paddle. The experiments were conducted with varying circumstances, to avoid chances of error, and were repeated very many times until uniform and consistent indications were always obtained. The result of the experiments showed that 772 units of work (foot-pounds) furnished heat which would raise the temperature of I Ib. of water from 32 to 33 F., which is the unit of heat. This number, 772, is a constant of the greatest importance in scientific and practical calculations, and is called " the mechanical equivalent of heat" The amount of work it represents is sometimes called a " Joule," and is always represented in algebraical formulas by " J." Mr. Joule's first paper appeared in 1843, an d soon afterwards various branches of the subject of " The Equivalence and Persistence of Forces " were taken up by a number of able men, who have advanced its principles along various lines of inquiry. Among the most noted contributors to this question we find the names of Sir William Thompson, Helmholtz, James Thomson, Rankin, Clausius, Tait, Andrews, and Maxwell. In the steam engine the case is the inverse of that presented by the above- named experiment of Dr. Joule's. Here we have heat producing work. Now, the quantity of steam which enters the cylinder of a steam engine may be found, and the temperature of the steam can be determined, and from these the amount of heat which passes into the cylinder per minute, say, can be calculated. A large portion of this heat is, in an ordinary engine, yielded up to the condensing water, and another part is lost by conduction and radiation from the cylinder, condenser, pipes, &c. But both these quantities can be estimated. When the amount is compared with that entering the cylinder in the steam, a difference is always found, which leaves a quantity of heat unaccounted for. When this quantity is compared with the work done by the engine in the same interval (which work can be measured as described on page 10), it is always found that for every 772 units of work a unit of heat has disappeared from the cylinder. The numerical relation between work and heat which is established in these two cases has been tested in many quite different ways ; and, within the limits of experimental errors, always with the same numerical result. But equally definite quantitative relations are known to exist among all the other forms of force ; and the manner in which these are convertible into each other has already been indicated, although want of space prevents full illustration of this part of the subject. It may, however, be seen that each form of force can be mediately or immediately converted into mecha- nical effect, hence each is expressible in terms of work. That is to say, we can assign to a unit of electricity, for example, a number expressing the work which it would do if entirely converted into work ; and the same number also expresses the work which would be required to produce the unit of electricity. An ounce of hydrogen in combining with 8 oz. of oxy- gen produces a certain measurable quantity of heat. If that heat, say = H, 584 THE GREATEST DISCOVERY OF THE AGE. were all converted into work, we now know that the work would = H J. Hence we can express a definite chemical action in terms of work. The same is generally true of all physical forces, though in some cases, such as light, vital action, &c, the quantitative relations have not yet been defi- nitely determined. Since, then, all the forces with which we are acquainted are expressible (though the exact relations of some have yet to be discovered) in terms of work, it is found of great advantage to consider the power of doing work as the common measure of doing all these. Thus, if we define energy as that which does, or that which is capable of doing, work, we have a term extremely convenient in the description of some aspects of our subject. Thus we can now speak of the energies of nature, instead of the forces. And all forces, active or passive, may be summed up in one word energy. And, further, the great discovery of the conservation of forces under definite equivalents, may be summed up very briefly in this statement THE AMOUNT OF ENERGY IN THE UNIVERSE IS CONSTANT. To make this statement clear requires that a distinction between two forms of every kind of energy be pointed out To recur to the example before imagined : if you carry the i Ib. weight up the Monument, and deposit it on the ledge at the top, it might lie there for a thousand years before it was made to give back the work you had performed upon it. That work has been, in a'manner, stored up by the position you have given to your weight. Now, in taking up the weight, you expended energy you really performed work : that is an in- stance of energy in operation, and may be termed " actual energy." In what form does the energy exist during the thousand years we may suppose your weight to lie at the top of the Monument ? It is ready to yield up your work again at any moment it is permitted to descend, and it possesses therefore during the whole period a potential energy equal in amount to the actual energy you bestowed upon it. A similar distinction between actual and potential energy exists with regard to every form of force. If by any means you separate an atom of carbon from an atom of oxygen, you exert actual energy. The process is analogous to carrying up the weight. The atoms when separated possess potential energy, they can rush together again, like the weight of the earth, and in doing so will give out the work which was expended on their separation. A parallel illustra- tion might be drawn from electrical force. A typical example of the storing up of energy is furnished by a cross- bow. The moment a man begins to bend the bow he is doing work, be- cause he pulls the string in opposition to the bow's resistance to a change in its form ; and it is plain that the amount of energy thus expended is measurable. Suppose, now, the bow has been bent and the string caught in the notch, from which it is released by drawing the trigger when the discharge of the bow is desired. The bow may be retained for an indefi- nite period in the bent condition, and in this state it possesses, in the form of potential energy, all the work which has been expended in bending it, and which it will, in fact, give out, in some way or other, whenever the trigger is drawn. To fix our ideas, let us suppose that to draw the string over the notch required a pull of 50 Ibs. over a space of 6 in. ; that is equi- valent to 50X^ = 25 units of work. Now let the bow be used to shoot an arrow weighing \ Ib. vertically upwards. The height in feet to which the arrow will rise multiplied into its weight in pounds will be the work done upon it by the bow. Now, we say that experiment proves that in the case supposed the arrow would rise just 100 ft., so that the work done by the THE GREATEST DISCOVERY OF THE AGE. 585 bow (f X 100= 25) would be precisely that done upon it. For the sake of simplicity, we keep this illustration free from the mention of interfering causes, which have to be considered and allowed for when the matter is put to the real test of quantitative experiment. The instance of the cross- bow brings into notice a highly instructive circumstance, which is this : the bow, which it may have taken the strength of a Hercules to bend, will shoot its bolt by the mere touch of a child on the trigger. In the same way, when a man fires a gun, he merely permits the potential energy con- tained in the charge to convert itself into actual, or kinetic, energy. The real source of the energy, in the case of the child discharging the cross- bow, is the muscular power of the man who drew it ; the real source of the energy in exploding gunpowder is the separation of carbon atoms from oxygen atoms, and that has been done by the sun's rays, as truly as the string was pulled away from the bow by muscular power. If we turn our attention to nitro-glycerine or to nitro-cellulose, we can, by following the chemical actions giving rise to these substances, in like manner trace their energies to our great luminary. The unstable union by which oxygen and nitrogen atoms are locked up in the solid and liquid forms of nitro-cellu- lose and nitro-glycerine is also the work of the sun ; for nitrogen acids, or rather nitrates, are produced naturally under certain electrical and other conditions of the atmosphere, which are due, directly or indirectly, to the sun's action ; and they cannot be formed artificially, except by imitating the natural conditions, as by passing electric sparks through air, &c. It will now be understood, as regards the wonderful relations between animal and vegetable life, which have already been alluded to more than once, how the sun, by expending actual energy, separates atoms of carbon from atoms of oxygen in the leaves of plants, and confers upon these a posi- tion of advantage, i.e., potential energy; and how animals, absorbing the separated carbon in the form of food, and inhaling the separated oxygen in the air they breathe, cause the conversion of the potential into actual energy, which appears in the heat, movements, and vital functions of the animal body. In coal we have the energy which plants absorbed from the sun ages ago, stored up in a potential form. The carbon atoms are ready to rush into union with oxygen atoms, and convert their energy of position into the energies developed by chemical action, viz., heat, light, &c. Energy is thus constantly shifting its form from actual to potential, and vice versa, and exhibiting itself under the various transformations of force, as when sun-force changes to chemical action, chemical action to heat, heat to electricity, &c. Energy is, indeed, the real modern PROTEUS constantly assuming different shapes, difficult to grasp if not held in fetters; now taking on the form of a lion, now of a flame of fire, a whirlwind, a rushing stream. As sober, literal matter of fact we catch glimpses of energy under these very forms. The greatest discovery of the age has, as already indicated, immediate and important practical bearings. The amount of thought which, even in the present day, is devoted by unscientific mechanics to the old problem of perpetual motion is far greater than is generally supposed. The principle of the conservation of energy shows that this is an impossibility ; that the inventor who seeks to create force might just as well try to create matter ; that the production of a perpetually moving self-sustaining machine is as far removed from human power as the bringing into existence of a new planet. In force, as in matter, the law is inexorable ex nihilo nihil fit. Again, knowing the definite amount of energy obtainable from the combus- 586 THE GREA TEST DISCO VER Y OF THE A GE. tion of a pound of coal, we can compare the amount we actually procure from it in our steam engines with this theoretical quantity as the limit to- wards which our improvements should bring us continually nearer, but which we can never exceed, or, indeed, even reach. The schemers of perpetual motion are not the only class of speculators who pursue objects which are incompatible with our principle. There are many who seek to accomplish desirable ends by inadequate means : who, for example, are aiming perhaps to accomplish the reduction of ores by a quantity of fuel less than that mechanically equivalent to the work, or who conceive that by adding to coal some substance which itself is unchanged, an indefinitely greater amount of heat may be liberated by the combustion. Enough has been said to show that the energies of animal life can be traced to the sun as their source. The sun builds up the plant, separating oxygen from carbon. The animal directly or mediately by devouring other animals takes the carbonaceous matter of the plant, and reunites it with oxygen. In" the plant the sun winds up the spring which gives life to the animal mechanism ; for the winding-up of a spring and the separation of the atoms having chemical affinities are alike instances of supplying poten- tial energy. In the animal there is a running-down of the potential into actual energy. It is plain also that of the total energy radiated from the sun in every direction, the earth receives but a very small part (^^oogbooou)' By far the larger part is diffused into space, where, for all such purposes as those with which we are concerned, it is lost. The heat which the sun sends out in a year is calculated to be equal to that which would be pro- duced by the combustion of a layer of coal 17 miles thick over the whole surface of the luminary. Is the sun, then, a flaming fire? By no means. Combustion is not possible at its temperature ; and as we know the sub- stances which enter into its composition are the same as those we find in the earth, we know that the chemical energies of such substances could not supply the sun's expenditure. Passing over as unsatisfactory an explana- tion which might occur to some minds namely, that the sun was created hot at the beginning, and has so continued there are two theories which attempt to account for the sun's heat. One is that of Meyer, who supposes the heat is due to the continual impact of meteorites drawn to the sun by its gravity ; and the other is that of Helmholtz, who attributes the heat to the continual condensation of the substance of the sun. Helmholtz calcu- lates that a shrinking of the sun's diameter by only T ooTjth of its present amount, would supply heat to last for two thousand years ; while the con- densation of the substance of the sun to the density of the earth would cover the sun's expenditure for 17,000,000 of years. There is great proba- bility that both theories may be correct, and that the cause of the sun's heat may be considered as due in general terms to aggregation of matter, by which the original potential energy of position is converted into the actual energy of heat and light. Now, however immense may be our plane- tary system, the sun being continually throwing off this energy into space, there must come a time when the supplies of meteorites will fail, and when the great globe of the sun will have shrunk to its smallest dimensions. We see, then, that heat and light are produced by the aggregation of matter ; the heat and light are radiated into space ; the small fraction intercepted by our globe is the source of almost every movement the original stuff, so to speak, out of which all terrestrial forces are made. The sun produces the winds, the thunderstorms, the electric currents of the Aurora, the phe- nomena of terrestrial magnetism, and is the source of vegetable and animal THE GREATEST DISCOVERY OF THE AGE. 587 life. The waves, the rains, the mountain torrents, the flowing rivers, are the work of the sun's emanations. In the illustration of the energy expended on raising a weight afterwards dropped, we traced that energy into the final form of heat of a low tempera- ture radiated into space. It would be easy to show that all energy ultimately takes the same form. Now, although it is easy to convert work into heat, there is no conceivable process by which uniformly-diffused heat can again be made to do any kind of work. The case may be compared to water, which in moving down from a higher to a lower level may be made to per- form any variety of work. But when all the water has passed down from the higher level to the lower, it can no longer do any work. Whenever work is done by the agency of heat, there is always a passing from a higher tempera- ture to a lower a transference of heat from a hotter body to a colder. If the condenser of the steam engine had the same temperature as the stea'm, the machine would not work. Not only do all the energies in operation on the face of the earth continually run down into the form of radiant heat sent off by the earth into space ; but our sun's energy, and that of the suns of other systems, are also continually passing off into space ; and the final effect must be a uniform diffusion of heat in a universe in which none of the varied forms of energy we now behold in operation will be possible, because all will have run down to the same dead level of uniformly-diffused heat. This startling corollary from the principle of the conservation of energy has been worked out by Sir W. Thompson under the title of " The Dissipation of Energy." It leads us to contemplate a state of things in which all light and life will have passed away from the universe a con- dition which the poet's terrible dream of darkness, " which was not all a dream," seems to shadow forth " The bright sun was extinguished, and the stars Did wander darkling in the eternal space, Rayless and pathless ; and the icy earth Swung blind and blackening in the moonless air. ****** The world was void, The populous and the powerful was a lump, Seasonless, herbless, treeless, manless, lifeless A lump of death a chaos of hard clay. The rivers, lakes, and ocean all stood still, And nothing stirred within their silent depths. * * * * * * The waves were dead ; the tides were in their grave, The Moon, their mistress, had expired before ; The winds were withered in the stagnant air, And the clouds perished ; Darkness had no need Of aid from them She was the Universe." The doctrine of this persistence and dissipation of energy completely harmonizes with the grand speculation termed the " nebular hypothesis," which regards the universe as having originally consisted of uniformly dif- fused matter, which, being endowed with the power of gravitation, aggregated round certain centres. This process is still going on ; and, according to modern speculations, light and life and motion are but manifestations of this primaeval potential energy being converted into actual energy, and degrading ultimately into the form of universally-diffused heat. To quote the closing sentences of the eloquent passage in which Professor Tyndall concludes the work mentioned above, " To nature nothing can be added, from nature nothing can be taken away ; the sum of her energies is con- stant, and the utmost man can do in the pursuit of physical truth, or in the 586 THE GREATEST DISCOVERY OF THE AGE. tion of a pound of coal, we can compare the amount we actually procure from it in our steam engines with this theoretical quantity as the limit to- wards which our improvements should bring us continually nearer, but which we can never exceed, or, indeed, even reach. The schemers of perpetual motion are not the only class of speculators who pursue objects which are incompatible with our principle. There are many who seek to accomplish desirable ends by inadequate means : who, for example, are aiming perhaps to accomplish the reduction of ores by a quantity of fuel less than that mechanically equivalent to the work, or who conceive that by adding to coal some substance which itself is unchanged, an indefinitely greater amount of heat may be liberated by the combustion. Enough has been said to show that the energies of animal life can be traced to the sun as their source. The sun builds up the plant, separating oxygen from carbon. The animal directly or mediately by devouring other animals takes the carbonaceous matter of the plant, and reunites it with oxygen. Iri the plant the sun winds up the spring which gives life to the animal mechanism ; for the winding-up of a spring and the separation of the atoms having chemical affinities are alike instances of supplying poten- tial energy. In the animal there is a running-down of the potential into actual energy. It is plain also that of the total energy radiated from the sun in every direction, the earth receives but a very small part (^oooVoorra)' By far the larger part is diffused into space, where, for all such purposes as those with which we are concerned, it is lost. The heat which the sun sends out in a year is calculated to be equal to that which would be pro- duced by the combustion of a layer of coal 17 miles thick over the whole surface of the luminary. Is the sun, then, a flaming fire? By no means. Combustion is not possible at its temperature ; and as we know the sub- stances which enter into its composition are the same as those we find in the earth, we know that the chemical energies of such substances could not supply the sun's expenditure. Passing over as unsatisfactory an explana- tion which might occur to some minds namely, that the sun was created hot at the beginning, and has so continued there are two theories which attempt to account for the sun's heat. One is that of Meyer, who supposes the heat is due to the continual impact of meteorites drawn to the sun by its gravity ; and the other is that of Helmholtz, who attributes the heat to the continual condensation of the substance of the sun. Helmholtz calcu- lates that a shrinking of the sun's diameter by only T7 njut)th of its present amount, would supply heat to last for two thousand years ; while the con- densation of the substance of the sun to the density of the earth would cover the sun's expenditure for 17,000,000 of years. There is great proba- bility that both theories may be correct, and that the cause of the sun's heat may be considered as due in general terms to aggregation of matter, by which the original potential energy of position is converted into the actual energy of heat and light. Now, however immense may be our plane- tary system, the sun being continually throwing off this energy into space, there must come a time when the supplies of meteorites will fail, and when the great globe of the sun will have shrunk to its smallest dimensions. We see, then, that heat and light are produced by the aggregation of matter ; the heat and light are radiated into space ; the small fraction intercepted by our globe is the source of almost every movement the original stuff, so to speak, out of which all terrestrial forces are made. The sun produces the winds, the thunderstorms, the electric currents of the Aurora, the phe- nomena of terrestrial magnetism, and is the source of vegetable and animal THE GREATEST DISCOVERY OF THE AGE. 587 life. The waves, the rains, the mountain torrents, the flowing rivers, are the work of the sun's emanations. In the illustration of the energy expended on raising a weight afterwards dropped, we traced that energy into the final form of heat of a low tempera- ture radiated into space. It would be easy to show that all energy ultimately takes the same form. Now, although it is easy to convert work into heat, there is no conceivable process by which uniformly-diffused heat can again be made to do any kind of work. The case may be compared to water, which in moving down from a higher to a lower level may be made to per- form any variety of work. But when all the water has passed down from the higher level to the lower, it can no longer do any work. Whenever work is done by the agency of heat, there is always a passing from a higher tempera- ture to a lower a transference of heat from a hotter body to a colder. If the condenser of the steam engine had the same temperature as the stea'm, the machine would not work. Not only do all the energies in operation on the face of the earth continually run down into the form of radiant heat sent off by the earth into space ; but our sun's energy, and that of the suns of other systems, are also continually passing off into space ; and the final effect must be a uniform diffusion of heat in a universe in which none of the varied forms of energy we now behold in operation will be possible, because all will have run down to the same dead level of uniformly-diffused heat. This startling corollary from the principle of the conservation of energy has been worked out by Sir W. Thompson under the title of " The Dissipation of Energy." It leads us to contemplate a state of things in which all light and life will have passed away from the universe a con- dition which the poet's terrible dream of darkness, " which was not all a dream," seems to shadow forth " The bright sun was extinguished, and the stars Did wander darkling in the eternal space, Rayless and pathless ; and the icy earth Swung blind and blackening in the moonless air. **** The world was void, The populous and the powerful was a lump, Seasonless, herbless, treeless, manless, lifeless A lump of death a chaos of hard clay. The rivers, lakes, and ocean all stood still, And nothing stirred within their silent depths. * * * * * The waves were dead ; the tides were in their grave, The Moon, their mistress, had expired before ; The winds were withered in the stagnant air, And the elouds perished ; Darkness had no need Of aid from them She was the Universe. " The doctrine of this persistence and dissipation of energy completely harmonizes with the grand speculation termed the " nebular hypothesis," which regards the universe as having originally consisted of uniformly dif- fused matter, which, being endowed with the power of gravitation, aggregated round certain centres. This process is still going on ; and, according to modern speculations, light and life and motion are but manifestations of this primaeval potential energy being converted into actual energy, and degrading ultimately into the form of universally-diffused heat. To quote the closing sentences of the eloquent passage in which Professor Tyndall concludes the work mentioned above, " To nature nothing can be added, from nature nothing can be taken away ; the sum of her energies is con- stant, and the utmost man can do in the pursuit of physical truth, or in the 5 88 THE GREATEST DISCOVERY OF 7 HE AGE. applications of physical knowledge, is to shift the constituents of the never- varying total. The law of conservation rigidly excludes both creation and annihilation. Waves may change to ripples, and ripples to waves ; mag- nitude may be substituted for number, and number for magnitude ; aste- roids may aggregate to suns, suns may resolve themselves into florae and faunae, and florae and faunae melt in air : the flux of power is eternally the same. It rolls in music through the ages, and all terrestrial energy the manifestations of life as well as the display of phenomena are but the modulations of its rhythm." The discoveries to which we have here endeavoured to attract the reader's attention thus give rise to conceptions of the utmost grandeur and interest. We see that the sum of Nature's energies is constant ; that all the mani- festations of force are but the transference of power from one position to another. And we have recognized the material source of all our terrestrial energies in the sun. It is but the small fraction of the total energy pour- ing from our luminary that reaches us, yet it suffices to maintain the life of the globe. It is the energy indirectly derived from the sun which drives our steam engines, impels the missiles from our guns, wafts our ships over the ocean, and blasts the rocks for our roads and mines. INDEX. Abel, Professor, 530, 535 Adhesion of locomotive, 18 Advantages of present age, 2 Air, 523 Albert Bridge, Saltash, 194 Alizarine, 577 Aluminium, 507 bronze, 510 Ampere's hypothesis, 545 Ampere's rule, 370, 399 Amphioxus 490 Anaesthetics, 520 Anemometer, 475 Angle, limiting, or critical, 285 Aniline, 567 black, 572 blue, 570 green, 571 purple, 568 Anthea cereus, 489 Anthracene, 576, 577 Applegath printing machine, 208 and Cowper, ditto, 204 Apps's anemometer, 475 induction coil, 383 Aquaria, 484 Arago, 440 Argand gas-burners, 558 lamps, 436 Armour, ships', strengths of, no Armstrong's, Sir W., guns, 118 - hydraulic crane, 229 Atoms, 522, 532 Aurora, 382 Australian gold, 497 Austrian torpedoes, 149 Axolotl, 495 Bacon, Francis, 579 Baxter House experiments, 36 Bell Rock lighthouse, 434 Bells, electro-magnetic, 404 Benzol, 563 Bessemer, 22, 25 Channel steimer, 93 converter, 38 iron, 36 steel, 37 Blanchard lathe, 54 Blast furnace, 28 Blind spot in eye, 341 Blood spectra, 318 Blowing apparatus, 31 Boilers of steam engines, 13 Boring for coals, &c., 257 Bourdon's pressure gauge, 12 Box girders, 191 Breech-loading rifles, 131 Brewster, Sir D., 291, 306, 350, 354 Brighton Aquarium. 491 Britannia Bridge, 191 . raising tubes. 232 Browning microspectroscopes, 320 spectroscope, 308, 318 Brunei, 194 Bullets, explosive, 145 , machinery for making, 226 Bunsen and Kirchhoff, 308 and Roscoe, 512 Bunsen's battery, 374 burner, 307, 557 Caesium, 312 Calico printing machines, 217 California, discovery of gold in. 497 "Cape Horn," 78 Captain, H.M.S., 106 Carbonic oxide, 29, 31 Carcel lamp, 436 Carpenter. Dr. W. B., 342 Carriages, railway, 69 for rock drills, 254 Carriers in pneumatic tubes, 240 Cars, Pullman, 70 Castalia, steamship, 96 Cast iron, composition of, 30 Catoptric lighthouse apparatus, 438 Cause of light and colour, 294 Celestial Chemistry and Physics 322 Central Telegraph Office, London, 424 Centres of gravity and buoyancy, 109 Centrifugal force, 65 Chains, 226 Chain-testing machine, 225 Channel Tunnel, the, 260 Chassepot rifle, 135 Chemical action ot' light, 447 equations, 523 nomenclature, 562 symbols, 522 work of electricity, 375 Chloroform, 525 Chromatic aberration of eye, 342 Chromo-lithography, 465 Chronograph) _ Wfr ,v ... Chronoscope j electnc 475 Cincinnati Suspension Bridge, 198 Clarke, hydraulic lift graving dock, 227 Clay process, stereotyping by, 460 Clifton Suspension Bridge, near Bristol, 196 , Niagara, 198 Coal, 537 Coal-gas, 550 Coal-tar colours, 561 Code, telegraphic, of American War Dept., 407 , Morse's, 410 -, Wheatstone's dot, 415 Colesberg, 503 Cold-short iron, 35 Collodion process, 457 589 59 INDEX. Colours not in the objects, 290 Colour printing, 466 Comets, spectra of, 330 Composition, rollers, 292 Condie's steam-hammer, 24 Copying principle, 44 Corona, 324 Couple, mechanical, 109 Cramp gauge, 63 Crystalline lens, 335 Crystal Palace, an example of use of iron in architecture. Plate II. Crystal Palace Aquarium, 486 Cup and cone, 31 Current, electric, 370 , induced, 380 Currents in submarine cables, 429 Daguerre, 448 Daguerreotype, 449 Dallmeyer, 456 Daniell's battery, 373 Delphi, oracle at, 528 Dial telegraphs, 416 Diamondiferous, 504 Diamond rock drill, 255 Diamonds, 501 Dioptric lighthouse apparatus, 439 D lines of sodium spectrum, 311, 322 Discoveries, progressive, 580 Dissipation of Energy, 586 Distinct vision, 338 Domestic consumption of coal, 541, 542 Double refraction and polarization, 285 Dowlais (Iron-works), 31, 34 Dredges, Suez Canal, 169 Drilling machine. 48 Dry digging, 503 Duboscq's electric lamp, 375 Dynamical electricity, 368 Earth circuit, 424 Ebonite, 517 Eccentric, 9 Eclipse of sun, 324 Eddystone lighthouse, 433 Electricity. 359 Electric current, 570 induction. 366 - light, cost of, 392 in lighthouses, 437 telegraph, 397 torpedo, 149 Electro-magnet, 378 Electromotive force, 372 Electro-plating, 377. 395 Electrode, 375 Electrotyping. 461 Elementary bodies, 507 ' phenomena of Magnetism and Elec- tricity, 302 Energy, 584 Ether, 524 . the luminiferous, 294 Exhaustion of coal, 541, 542, 543 Expansive working of steam, 6/17 Explosion by concussion, 534 ~ of locomotive, 18 ~' of torpedoes, 149 Explosives, 529 Explosive bullets, 145 Eye, 334 , dimensions of some parts of, 342 not optically perfect, 342 Eyeballs, muscles of, 340 Fairbairn, Sir W., 234 Faraday, 384, 385, 524 Faraday's ventilating gas-burner, 558 Field telegraphs, 405 Fizeau. 273 Fire-arms, 117 Fish-plates. 63 Fly-wheel, 7, 34 Fluids, electric, 365 Force, conservation of. 584 , electromotive. 372 Foucault, 274 Fovea centralis, 336, 337 Fraunhofer's lines, 306. 322 Fresnel's mirrors, 205 measurement of velocity of light, 439 Fribourg Suspension Bridges, 197 Froment's dial telegraph, 417 Galvanic batteries. 371, 372 Galvanometer, 371 , mirror, 420 Gas governor, 554 holder, 552 making apparatus, 551 meters, 560 pressure. 554 retorts, 552 Gases of blast furnace, 31 Gatling battery gun, or mitrailleur, 137 Gauges, broad and narrow, 64 Bourdon's pressure, 12 Geissler's tubes. 383 Ghost, Pepper's, 278 Giffard's injector, n Girder Bridges, 191 Glass, strains in, 293 Glatton, H.M.S., in Gold and Diamonds, 496 Gold. 496 Gold-mining operations, 409 Goodyear, Mr., 516 Governor of steam engines, 6 Gower Street Station, 72 Gramme Magneto-Electric Machine. 388 Graphotype, 471 Grove, Sir W. R.. 582 's battery, 373 oun-cotton, 536 torpedoes, 153 Great Eastern, 87, 102, 226, 426, 428 Greatest Discovery of the Age, 579. Gunpowder, 533 Guns, submarine, 160 Gutta-percha, 517 Hancock, Mr. Charle, 518 Mr. Thomas, 514 INDEX. 59* Harvey's torpedoes, 154 Heat produced by electric current, 380 Heat spectrum, 452 Helmholtz, 342, 344, 352, 354 Hercules, H.M.S., 100 Hippocampus, 493 Hoe's printing machines, 212, 214 Holophotal light, 443 Holyhead and Kingston steamers, 90 Horse-power, 10 Hot blast. 30 Hough's meteorograph, 473 Hughes's printing telegraph, 410 Hydraulic power. 220 Iceland spar, 285 Illuminating power of gas 559 Illusion by movement of eye, 355 by persistence of vision, 356 , stage, 200 Images formed by lenses, 285. 455 Inconstant, H.M.S., 102 Indian-rubber and gutta-percha, 513 Indian-rubber, 513 Indicator, 9 Induced currents, 513 J nduction coils, 381 Injector, n Iron, 25 bridges, 187 cast, 29 lighthouses, 435 ores, 26 ships, 87 , smelting, 27 , wrought, 34 Jackson, 401 Jamin's magnet, 390 Joule, 582, 583 Jupiter, 271 Kaleidoscope, 276 Kirchhoff, 322 Konig's printing machine, 204 Kb'nig m Kaiser Willielm ironclad, 114 Krupp's guns, 127 steel, 42 works, 126 Lathe at Woolwich, 121 , B Ian chard, 54 , screw-cutting, 45 Lap of slide-valve, 9 Lens, formation of image by a, 455 in steps, 439 , photographic, 455 Lepidosiren, 494 Letterpress printing, 202 Leydenjar, 368 Light, 267 .electric, 374 Light, invisible, 270 Lighthouses, 432 Limiting angle, 285 Link motion, 16 Lithium, 311 Lithography, 463 Liverpool and Manchester Railway. 14 Madder, 576 Magnesium, 511 Magnetism produced by current. 378 Magneto-electricity, 384 Magneto-electric machines, 387, 388 I Malus, 291 I Manufacturing ~v. making, 43 !Map, Channel Tunnel, 260 , Pacific Railway, 75 I , Suez Canal, 167 i Martini-Henry rifle. 132 i Matter indestructible, "588 I Measuring machines. 46 Menai Straits bridges, 191, 19; I Meteorites. 26 Meteorographs, 473 Meteorology, importance of, 483 Mirror galvanometer, 420 Mirrors, plane, 275 .illusions by, 279 Meters, gas, 560 Mitrailleurs, or machine guns, 136 Monarch, H.M.S., 106 Molecules, 522, 532 Moncrieff's gun-carriages, 129 Mont Cenis Tunnel, 247 Montigny mitrailleur, 140 Morse's code, 410 instruments, 408 telegraphic line, 406 transmitting key, 411 plate, 412 Napier's platen machine, 217 Naphthaline, 573 Nasmyth's steam-hammer, 22 Nature knowledge, i - printing, 467 Needle telegraphs, 403 Negretti and Zambra's recording thermometer, 478 New metals, 505 Newton's prism experiment. 304 Niagara Suspension Bridges, 198 Nicol's prism, 289 Niepce, J. N.,448 de Saint- Victor, 450. 454 Nitro-benzol, 564 Nitrogen and oxygen compounds, 521 Nitro-glycerine, 533 CErsted's experiments, 399 Oil springs, 546 Ophthalmoscope, 346 Optical apparatus of lighthouses, 437 Orders of lighthouse apparatus, 441 Organic bodies, 578 Oscillating engines, 14 592 INDEX. Paddle-wheels. 84 Papier-mache stereotype process, 460 Paraffin, 547 oils, 548, 549 Parallel motion, 8 Pascal principle, 221 Pattern printing, 217 Pepper, J. H., 179, 278, 279 Petroleum, 543 Phenakistiscope, 356 Photograph engraved on glass, 185 Photographic camera, 454 Photography, 446 in colours, 453 in the daik, 452 Photolithography, 471 Photozincography, 471 Planes, Whitworth's, 52 Planing machines, 50 Plants in coal measures, 539 Plaster of Paris, stereotype process, 460 Pneumatic dispatch, 236 force, 243 Pniel, 502 Points, railway, 66 Polariscope, 291 Polarizer, 289 Polytechnic, Regent Street. 184. 383 Portable telegraphic instruments, 406 Portrait, Davy, 505 , Helmholtz, 332 , Joule, 569 , Kirchhoff, 302 , Lesseps, 162 , Morse. 397 , Senefelder, 459 , Simpson, 520 , Thompson, 359 , Watt, 3 , Whitworth, 43 Port Said, 164 Post-office railway van, 69 Potassium, 506 Power, horse, ip , hydraulic. 220 Powers, mechanical, 222 Power of a steam-engine, 9 of locomotive, 18 Pressure gauge, 12 transmitted in fluids, 220 Printing machines, 201 processes, 459 telegraphs, 420 Progress of mankind, 2 Projectiles, 116, 125 , deviation of, 122 , long range of Whitworth, 124 , speed of, measured, 478 Prospecting, 257 Proteus anguinus, 493 , the modern, 585 Pseudpscope, 353 Puddling, 32 Railways, 59 Great Western, 64 , Metropolitan, 72 , Midland, 70 > London and Manchester, 60, 82 Railways. London and Woolwich, 62 , Pacific. 74 , Stockton and Darlington, 59 Rangoon petroleum, 545 Rays polarized, 287 Recording instruments, 314 Red-short iron, 35 Reflection in water, 282 of light, 275 , total, 285 Refraction, 283 , double, 285 Resistance, electrical, 372 Retina, 336 Rifled cannon, 118 Rifling guns, 122 Rifle, Chassepot, 135 , Martini-Henry, 132 , Snider-Enfield, 131 Rock boring, 245 "Rocket." 14 Rock drilling machines, 251 Rolling iron, 33 mill, accident at a, 34 Ronalds' telegraph, 398 Roscje, 311, 323, 330 and Bumen, 512 Royal Gun Factory, Woolwich, 23 Ruete's ophthalmoscope, 346 Ruhmkorff's coil, 381 Saltash Bridge, 194 Sand, 179 blast, 1 8