8 175 ERSITY OF CALIFORNIA LIBRARY OF THE UNIVERSITY OF CALIFOR 'ERSITY OF CALIFORNIA LIBRARY OF THE UNIVERSITY OF CALIFOR I ,X*v37/ Ch >~ OF THE UNIVERSITY OF CALIFORNIA QJS LIBRARY OF THE UNIVERSITY OF THE UNIVERSITY OF CALIFORNIA /ft) LIBRARY OF THE UNIVERSITY METEOROLOGY PRACTICAL AND APPLIED .Sanitatg Series, $0. METEOKOLOGY PRACTICAL AND APPLIED BY JOHN WILLIAM MOOEE B.A., M.D., M.Ch., UNIV. DUEL. ; F.R. C.P.I. ; FELLOW OF THE ROYAL METEOROLOGICAL SOCIETY J DIPLOMATS IN STATE MEDICINE AND EX-SCHOLAR OF TRINITY COLLEGE, DUBLIN LONDON F. J. REBMAN, 11 ADAM STREET, STRAND 1894 Entered at Stationers' Hall o PEEFACE THE writing of this book has been to me a labour of love. Should the reader derive some pleasure as well as information from the perusal of its pages, the task set before me will not have been undertaken and completed in vain. It may be objected that a work on Meteorology should more fitly have been written by an author distinguished for scientific attainments, and whose life-work lay amid the precise sciences. A physician, it may be said and said with truth is daily and hourly exposed to distractions of all kinds in the practice of his profession. He is, in consequence, placed at a disadvantage when he discusses a purely scientific topic. In answer to such an objection it may be urged with some force that the physician of all men has the fullest opportunities of observing the far-reaching influence of weather and climate upon human health, happiness, and longevity. If he utilises these oppor- tunities with intelligence and zeal, he is bound to make of such topics a peculiar study. That this often happens, a reference to the roll of Fellows of the Royal Meteorological Society or of the Scottish Meteorological Society will abundantly prove. In my own case, more than thirty years ago I was already vi METEOROLOGY a systematic observer of the weather, and such I continue to be. My " Second Order Station " and the lessons it has taught through all these years afford the needed foil to my more serious professional studies and pursuits. Hence it happens that I have been able to bring no small practical experience of meteorology to bear in the writing of the pages which follow. The work is divided into four parts. A brief introduction is succeeded by a full account of the methods which are employed in practical meteorology. The third part treats of climate and weather neces- sarily in a somewhat condensed and concise manner. Finally, I have endeavoured to point out, in the fourth and concluding portion of the book, a few of the practical bearings of the subject. In those closing chapters the grave question of the influence of weather and season upon disease is in some measure discussed. The time seems opportune for the publication of a popular yet scientific Text-book of Meteorology. The marvellous advances of Preventive Medicine within recent years, the institution of a registrable qualification in State Medicine in the United Kingdom of Great Britain and Ireland, the establishment of a new order of public servants drawn from the ranks of the medical profession I allude to Medical Officers of Health the vast development of international telegraphy in modern times, the hearty co-operation of the various national Weather Bureaux all these things have done much within the last quarter of a century to raise Meteorology to the rank of a science, and have given a wonderful impetus to the continuous or the periodic study of the weather. The literature of the subject is PREFACE Vll therefore daily increasing, and volume after volume is being added to the list of standard works on Meteorology and Climatology. Primarily intended for the use of my professional brethren at home and abroad, this book has been so written as to claim the attention of a much wider circle of readers. Its chief object is to convey a clear idea of the science of meteorology to any one of ordinary mental capacity and fair education who had been previously quite unacquainted with so attractive a study. Technical and scientific terms have been as far as possible explained. No pains have been spared to make the description of the different instruments used by meteorological observers as clear as can be. In most instances drawings of these instruments have been interpolated in the text. Thanks to a generous publisher, and to the great kindness of many scientific friends, the following pages will be found copiously illustrated. For valuable assistance in this direction, I desire to express my grateful acknowledgments to Mr. Greenwood Pirn, M.A., for his beautiful photograph of the Matterhorn, which forms the frontispiece to this volume ; to Mr. F. J. Eebman, publisher of this book, for the interest- ing photograph illustrating a burst of sunshine through an early morning mist ; to Mr. E. H. Scott, F.E.S., Secretary to the Meteorological Council, Mr. George J. Symons, F.E.S., Editor of British Rainfall, and Mr. William Marriott, Secretary of the Eoyal Meteorological Society, for permission to use various illustrations reproduced in the book. I would also thank MM. Eichard Freres, of Paris ; Messrs. Negretti and Zambra, of London ; Messrs. Yeates and Son, of Dublin ; and b X METEOROLOGY CHAPTER III THE COMPOSITION OF THE ATMOSPHERE Components of the Atmosphere A mechanical Mixture of Oxygen and Nitrogen Nature's cleansing Operations Volumetric Analysis of Air Proofs that Air is a Mechanical Mixture and not a Chemical Combination Sources of the Carbon Dioxide of the Atmosphere Its Distribution Action of Chlorophyll upon it Ozone Other Gaseous Constituents of the Atmosphere Graham's Law of the Diffusion of Gases Mineral Constituents of the Atmosphere Micro- organisms, or Microbes Permanganate of Potassium Test for Organic Matter in the Air Aqueous Vapour . . . Pages 19-26 PAET II. PRACTICAL METEOROLOGY CHAPTER IV BRITISH METEOROLOGICAL OBSERVATIONS Merle's Journal of the Weather, 1337-1344 Admiral FitzRoy's Charts and Forecasts Synoptic Weather Charts Meteorology of the British Isles Stations of the First Order Anemographic Stations Stations of the Second Order Telegraphic Reporting Stations "Isobars " and " Isotherms " Extra Stations Royal Meteorological Society Scottish Meteorological Society Daily Weather Report Weekly Weather Report Monthly Summary Accumulated Tem- perature Numerical Scales for Telegraphic Weather Reports Code System Meteorological Conditions which influence Disease and the Death-rate Equipment of a Second Order Station . Pages 27-38 CHAPTER V HISTORY, ORGANISATION, AND WORK OF THE UNITED STATES WEATHER BUREAU Pages 39-48 CHAPTER VI HISTORY, ORGANISATION, AND WORK OF THE UNITED STATES WEATHER BUREAU (continued) Pages 49-67 CONTENTS xi CHAPTER VII HISTORY, ORGANISATION, AND WORK OF THE UNITED STATES WEATHER BUREAU (continued) Pages 68-80 CHAPTER VIII HISTORY, ORGANISATION, AND WORK OF THE UNITED STATES WEATHER BUREAU (continued and concluded) Pages 81-93 CHAPTER IX AIR TEMPERATURE AND ITS MEASUREMENT Temperature the most important Meteorological Factor Meaning of the word Thermometer History of the Instrument Fahrenheit's Scale Why Mercury was selected as the Medium for measuring Tempera- ture Celsius' Scale The Centigrade Thermometer Reaumur's Scale Relations of the three Scales to one another Rules for reduc- ing Readings of one Scale to those of another Melting Point of Ice Freezing Point of Water Boiling Point of Water varies with Atmo- spheric Pressure and with Altitude Definition of a Thermometer Steps in the Construction of this Instrument . _'. Pages 94-99 CHAPTER X THERMOMETERS Standard Thermometers Ordinary Thermometers " Displacement of Zero" Registering Thermometers Phillips's Maximum Thermo- meter Negretti and Zambra's Maximum-Thermometer Rutherford's Minimum Thermometer Objections to Spirit Thermometers Six's combined Maximum and Minimum Thermometer Casella's mer- curial Minimum Thermometer Exposure of Instruments Steven- son Thermometer Stand and Screen The " Wall Screen "Method of Reading the Instruments The Sling Thermometer (Thermometre fronde) Self-recording Thermometers, or "Thermographs" Elec- trical and Photographic Thermographs Radiation Thermometers- Mean Temperature Average Mean Temperature Pages 100-113 CHAPTER XI RADIATION Heat : how transmitted Conduction Convection Radiation Solar Radiation Terrestrial Radiation Effect of Altitude on Temperature : xii METEOROLOGY how brought about Kadiation Thermometers Black -bulb Thermo- meter in vacua Bright-bulb Thermometer in vacuo Southall's Helio-pyrometer Herschel's Actinometer Pouillet's Pyrheliometer Grass Minimum Thermometers Earth Temperatures Critical Temperature at depth of 4 feet Duration of bright Sunshine Sun- shine Recorders . . . . . . Pages 114-128 CHAPTER XII ATMOSPHERIC PRESSURE The Barometer and its Uses Galileo's Observation Torricelli's Discovery The "Torricellian Vacuum" Principle involved in Nature's abhorrence of a Vacuum Fluids used in constructing a Barometer : Mercury, Water, Glycerine Jordan's Glycerine Barometer Esti- mation of the Height of the Atmosphere Pascal's Experiments Estimation of Mountain Altitudes by means of the Barometer Scaling and Lettering of the Barometer The "Weather Glass," or Wheel Barometer, of Robert Hooke History of the Barometer Pages 129-135 CHAPTER XIII THE BAROMETER The Mercurial Barometer Extreme Limits of Atmospheric Pressure at Sea-Level "Torricellian Vacuum" Attached Thermometer Mount- ing of the Mercurial Barometer Two Difficulties in the Construction of this Instrument How they are Surmounted " Error of Capacity " Capacity Correction The Fortin Barometer The Kew Barometer (Adie) The Siphon Barometer (Gay-Lussac) The "Gun Baro- meter" (FitzRoy) The Wheel Barometer, or "Weather Glass" (Hooke) Self-registering Barometers, or "Barographs" King's Mechanical Barograph Ronalds's Photographic Barograph Redier's Mercurial Registering Barometer Wheatstone's Electrical Barograph Transmission of Barometric Indications by Electricity (J. Joly) Substitutes for Mercurial Barometers : the Aneroid Barometer Its Altitude Scale Bourdon's Metallic Barometer Measurement of Heights The "Engineering Aneroid" Sympiesometer Hypso- meter ' . . Pages 136-150 CHAPTER XIV BAROMETRICAL READINGS Attached Thermometer Mounting of Barometer Method of taking a Barometrical Observation " Capillary Action "Capillarity The CONTENTS Xin Vernier Graduation of British Barometers Corrections to be applied to Barometrical Readings : Index Error, Capacity, Capillarity, Temperature, Altitude Verification of a Barometer The Catheto- meter Kew Certificate Schumacher's Formula for Reduction of Barometer Readings to 32 Ordnance Datum for Great Britain Ordnance Datum for Ireland Table of Corrections for Altitude con- trolled by existing Air Temperature and Pressure Laplace's Formula for finding the difference in Height between two Places Determina- tion of Mountain Heights by the Barometer . Pages 151-163 CHAPTER XV BAROMETRICAL FLUCTUATIONS Periodic and Non-periodic Variations in Atmospheric Pressure Regular and Irregular Variations Regular : Diurnal and Annual Irregular : Cyclonic and Anticyclonic Distribution of Diurnal Variations Explanation Dove's Theory Mr. Strachan's Objections Diurnal Range of Pressure Bibliography Annual Variations of Pressure Periodical Anticyclonic and Cyclonic Systems Explanation of their Formation Trade Winds Irregular Variations in Pressure Isobars : Primary and Secondary Shapes Cyclonic and Anticyclonic Secondary or Subsidiary Depression V-shaped Depression Straight Isobars "Wedge" of High Pressure "Col " of High Pressure Cyclonic and Anticyclonic Systems contrasted "Radia- tion Weather" "Intensity" Path of Cyclonic Systems Weather Changes accompanying their Passage "Veering," "Hauling," "Backing" of the Wind Anticyclonic Weather: (1) in Winter; (2) in Summer Pages 164-178 CHAPTER XVI THE ATMOSPHERE OF AQUEOUS VAPOUR Aqueous Vapour, or Water in a Gaseous or Aeriform State Its Elastic Force or Tension Influence of Aqueous Vapour on the Barometer, on the Weather Evaporation Fogs Capacity of the Atmosphere for Aqueous Vapour Atmometry or Atmidometry Hygrometry Hyetometry Determination of the Amount of Evaporation Evapori- meters, Atmometers, or Atmidometers Saturation Heat made latent in Evaporation Uses of Coolness produced by Evaporation- Amount of Evaporation Babington's Atmidometer Von Lament's Atmometer De la Rue's Evaporimeter Richards' Self-recording Evaporation Gauge Mr. Symons's Evaporimeter Observations on Evaporation in Ireland by Mr. James Price, M. Eng.,Univ. Dubl., C.E. Pages 179-185 XIV METEOROLOGY CHAPTEK XVII THE ATMOSPHERE OF AQUEOUS VAPOUR (continued] Direct and Indirect Hygrometers Organic and Inorganic Hygrometers Dew Point Direct Hygrometers : Daniell's, Regnault's, Dines's Indirect Hygrometers : Saussure's Hair Hygrometer Chemical Hygrometer Mason's Dry and Wet-Bulb Hygrometer The Psychro- meter Apjohn's Formula for calculating the maximal Vapour Tension for the Dew Point Glaisher's Hygrometrical Tables Greenwich Factors Examples of their Use Relative Humidity Management of the Wet-Bulb Thermometer Distribution of Aqueous Vapour in the Atmosphere "Absolute Humidity" History of Hygrometers .... . . . Pages 186-196 CHAPTER XVIII THE ATMOSPHERE OF AQUEOUS VAPOUR (continued) Hyetometry includes Seven Modes of Condensation of the Watery Vapour of the Atmosphere Dew Two forms of Dew: "Serein" and "Rosee" Hoar Frost Silver Thaw (Glatteis, Verglas} Interference with Formation of Dew Measurement of Dew Mist and Fog "Scotch Mist" Formation of Fogs, Clouds, and Rain Mr. John Aitken's Researches The Influence of Atmospheric Dust Mr. Aitken's Koniscope The Number of Dust Particles in the Air How Fog or Mist forms Dry Town Fogs Haze Pages 197-212 CHAPTER XIX THE ATMOSPHERE OF AQUEOUS VAPOUR (continued) Definition of 'a "Cloud "Height of Clouds The " Cloud Line "Luke Howard's Classification of Cloud Forms Upper Clouds : Cirrus, Cirro-cumulus, Cirro-stratus Pallium (Poey) The Mackerel Sky- Lower Clouds : Stratus, Cumulus, Cumulo-stratus, Nimbus Roll- cumulus "Pocky" or "Festooned" Cloud Cloud-slopes in Cumulus Various meanings of the word " Nimbus " Scud Cloud Observa- tionsScale for the amount of Cloud Characters of Thunder-clouds Their rapid changes in Formation, Shape, and Density Pages 213-221 CONTENTS XV CHAPTER XX THE ATMOSPHERE OF AQUEOUS VAPOUR (continued and concluded] Relative Amount of Precipitation as Dew and as Rainfall Excessive Pre- cipitation in the Khasi Hills, Assam Rainfall Observations in the Seventeenth Century Weighing the Rainfall Hooke's Rain Gauge (1695) Exhibition of Rain Gauges by the Royal [Meteorological Society in 1891 Chief Gauges in use : Meteorological Office Gauge, Symons's Snowdon Gauge, the Mountain Gauge, Symons's Storm Gauge, Crosley's Registering Gauge, Yeates's Registering Gauge, Richards' Self-recording Rain Gauge Measurement of Rain Time and Method of Observing Variation of Rainfall with Elevation, how explained Physical Cause of Rain Snow Sleet Hail Measure- ment of Snow Theory of the Formation of Hail Examples of Hail- storms Soft Hail Relative Size and Weight of Hailstones Distri- bution of Rain : (1) Geographical, (2) Seasonal, (3) Diurnal Weight and Bulk of Rain Pages 222-248 CHAPTER XXI ANEMOMETRY AND ANEMOMETERS Wind : what it is and how produced Force or Velocity of Wind depends on Barometric Gradients Estimation of Wind Force Beaufort Scale Wind Direction Bearings should be true Variation of the Com- pass Position of the Pole Star Equation of Time A Windrose : how determined Anemometers : Pendulum, Bridled, Pressure Plate, Pressure on a Fluid, Velocity, Evaporation or Temperature, Suction, Direction, Inclination, Musical, Dines's Helicoid, MM. Richards' Anemo - Cinemographe Lambert's Formula for determining Mean Direction of the Wind Mr. Dines's Comparative Experiments with Anemometers Casella's Self-recording Anemometer Pages 249-275 CHAPTER XXII ATMOSPHERIC ELECTRICITY Identity of Atmospheric Electricity and that obtained from an Electric Machine The Electroscope The Nature of Electricity Atmospheric Electrical Phenomena Electrical Density, Force, and Potential Use of the Electroscope The Collector The Electrometer Coulomb's Torsion Balance The Electrophorus The Replenisher The Diurnal and Annual March of Atmospheric Electricity Its Distribution Thunderstorms : Professor Mohn's Classification Cyclonic and Heat Thunderstorms Geographical Distribution of Thunderstorms Pages 276-289 xvi METEOROLOGY CHAPTER XXIII ATMOSPHERIC ELECTRICITY (continued and concluded) Lightning Thunder Varieties of Lightning : zigzag, or forked ; diffused, or sheet ; globular, or ball lightning Fulgurites Rapidity of Light- ning St. Elmo's Fire Hail The Aurora : how caused ; its height ; its colour Ozone Lightning-conductors . . Pages 290-303 PART III. CLIMATE AND WEATHER CHAPTER XXIV CLIMATE Meaning of the term " Climate " Definition of Climate Accumulated Temperature Effect of Temperature on the Animal and Vegetable Kingdoms Principal Factors of Climate : Latitude, Altitude, Relative Distribution of Land and Water Presence of Ocean Currents Pages 304-315 CHAPTER XXV CLIMATE (continued and concluded) Proximity of Mountain Ranges, Soil, Vegetation, Rainfall, Prevailing Winds Cold Winds : East Wind of Spring in the British Isles, Mistral, Tramontana, Nortes of Gulf of Mexico, Pamperas, Tormentos, Etesian Winds Hot Winds : Scirocco, Solano, Leveche, Harmattan, Khamseen, Simoom, Hot Wind of Australia, Fohn, Leste Pages 316-328 CHAPTER XXVI THE CLIMATE OF THE BRITISH ISLANDS Division of Climates into (1) Insular, or Moderate ; (2) Continental, or Excessive Distribution of Atmospheric Pressure according to Season Climate of the British Isles: (1) in Summer; (2) in Winter Isothermals in July and January Features in the Climatology of Great Britain and Ireland Sea Temperatures : in Winter and in CONTENTS xvii Summer Air Temperatures: arrangement of the Isotherms in January, April, July, and October Atmospheric Pressure : its Monthly Distribution Equinoctial Gales . . Pages 329-345 CHAPTER XXVII THE CLIMATE OF THE BRITISH ISLANDS (continued and concluded) Distribution of Kainfall in the British Islands Regions of heaviest Rain- fall How determined : Prevalent Winds, Exposure to these Winds, Mountains Regions of least Rainfall Geological Formation Its Local Influence on Temperature Permanent Elevation of Surface Pebble Beds, Sands, and Sandstones Clays and Shales Limestones Crystalline Rocks, whether Slates or Schists Climatological Tables for Dublin " ' . Pages 346-356 PART IV. THE INFLUENCE OF SEASON AND OF WEATHER ON DISEASE CHAPTER XXVIII ACUTE INFECTIVE DISEASES Introductory Remarks Dr. William Heberden's Observations on Weather and Disease in 1797 Meteorological Factors which influence the Prevalence and Fatality of Disease : Mean Temperature, Rainfall, Humidity Acute Infective Diseases : Influenza, Cholera, Diarrhceal Diseases Dr. Edward Ballard's Researches Influence of the Temperature of the Soil The Critical Subsoil Temperature of 56 F. at a depth of Four Feet Dr. E. Meinert's Observations on Cholera Infantum Dr. Ballard's working Hypothesis . Pages 357-375 CHAPTER XXIX ACUTE INFECTIVE DISEASES (continued and concluded) Influence on the Prevalence of Enteric Fever of (1) Season ; (2) Tempera- ture and Moisture ; (3) Soil and Underground Water Outbreaks at Terling and in Trinity College, Dublin Seasonal Mortality from Enteric Fever in Dublin for Twenty Years, 1872-91 Typhus Fever a Disease of Winter and Spring Influence of Season, Overcrowding, xviii METEOROLOGY Defective Ventilation, Temperature and Atmospheric Moisture- Seasonal Mortality from Typhus in Dublin for Twenty Years, 1872-91 Seasonal Prevalence of (1) Smallpox ; (2) Measles ; (3) Scarlatina Pages 376-393 CHAPTEE XXX THE SEASONAL PREVALENCE OF PNEUMONIC FEVER Pages 394-406 APPENDIX I LIST OF THE REGULAR STATIONS OF THE UNITED STATES WEATHER BUREAU Pages 407-409 APPENDIX II THE MORE IMPORTANT PUBLICATIONS OF THE UNITED STATES METEOROLOGICAL SERVICE Pages 410-419 APPENDIX III HYGROMETRICAL TABLES Pages 420, 421 INDEX OF SUBJECTS AND PLACES Pages 423-440 INDEX OF PKOPER NAMES Pages 441-445 LIST OF ILLUSTBATIONS PLATE I. Cloud Formation on the Matterhorn ' . . Frontispiece PAGE II. Dispersion of Early Morning Mist. . . . . 212 CHARTS A. and B. Weather Signals of U. S. Weather Bureau . 82 C. Distribution of Extreme and Mean Temperature . .;.- 328 FIG. 1. Kew Observatory Thermometer . ..... 101 2. Phillips's and Negretti and Zambra's Maximum Thermometers 102 3. Rutherford's Minimum Thermometer . '. ."'.". 104 4. Six's Thermometer . . . . . 105 5. Casella's Mercurial Minimum Thermometer ". . 107 6. Stevenson's Thermometer Stand . . -. . 108 7. Underground Thermometer . . . .115 8. .... 115 9. Solar Radiation Thermometer Stand . . . 119 10. Richards' Actinometer . . .121 11. Casella's Bifurcated Grass Minimum . . . 122 12. Symons's Earth Thermometer . .123 13. Campbell-Stokes Sunshine Recorder . . . 125 14. Whipple-Casella Universal Sunshine Recorder . 126 15. Jordan Photographic Sunshine Recorder . . 127 16. Improved Jordan Photographic Sunshine Recorder . 128 17. Torricelli's Experiment . 130 18. Readings of the Jordan Barometer . . 133 19. Fortin Barometer . . . .139 20. Wallis's Arrangement for adjusting the Ivory Point . 140 21. Kew Barometer I 41 xx METEOROLOGY FIG. PAGE 22. Gay-Lussac Air-Trap . 141 23. Siphon Barometer . . . . .142 24. Alfred King's Barograph . .144 25. Aneroid Barometer . . . . .147 26. Aneroid Barometer, extra small, in silver case . . 147 27. Field's Engineering Aneroid Barometer . . . 147 28. Extra-sensitive Aneroid Barometer . . ..,.. . 1 . 147 29. Adie's Sympiesometer . . \{ . - J . 149 30. Portable Leather Case for holding Casella's Hypsometer . 150 31. Casella's Hypsometer .- . . TTT7.". ^0 32. Method of reading the Vernier (Case 1) . . 154 33. , (Case 2) . . - . 154 34. Cathetometer constructed for the Indian Government . 157 35. Cathetometer, as used at Kew Observatory . ' . 158 36. Cathetometer, 6 feet in height . . . . 158 37. Diurnal Oscillation of the Barometer in Various Latitudes 168 38. Cyclonic and Anticyclonic Isobars . ." . 174 39. Daniell's Hygrometer . . ' '. . 187 40. Regnault's Hygrometer . . ... . 188 41. Dines's Hygrometer . . . . . 189 42. Vertical View of Dines's Hygrometer . * . . 189 43. Mason's Hygrometer . . . . . 191 44. Hooke's Rain Gauge . V" .. . . 224 45. Meteorological Office Rain Gauge .... 226 46. Snowdon Rain Gauge . . . . " . 226 47. Mountain Rain Gauge . . . . . 227 48. Symons's Storm Rain Gauge (Second Pattern) . . 228 49. Crosley's Self-registering Rain Gauge . ' . ' . 229 50. Yeates's Electrical Self-registering Rain Gauge . . 230 51. Registering Part of Yeates's Rain Gauge . . . 230 52. Richards' Self-recording Rain Gauge (Float Pattern) . 231 53. Richards' Self-recording Rain Gauge (Balance Pattern) . 232 54. Casella's Self-recording Anemometer or Anemograph . 254 55. Recording Cylinder of Casella's Anemograph . . 255 56. Pressure Plate Anemometer .. " . . . . 257 57. Lind's Anemometer ..... 258 58. Robinson's Anemometer . 259 LIST OF ILLUSTRATIONS XXI FIG. PAGE 59. Negretti and Zambia's Improved Robinson's Anemometer 261 60. Air-meter ....... 262 6lA. Yane of Casella's Altazimuth Anemometer . . 266 6 IB. Casella's Altazimuth Anemometer . . .267 62. Vane of Richards' Anemo-Cinemographe . . . 270 63. Richards' Anemo-Cinemographe . . ' '. . 271 64. Richards' Anemo-Cinemographe (Second Form) . . 272 65. Anemo-Cinemographe in position . . .V . 273 66. Gold-leaf Electroscope . . . .279 67. Thomson's Portable Electrometer . ' . . . 283 68. Diagram illustrating the Effect of the perpendicular and the oblique falling of the Sun's rays . . . 307 DIAGRAM 1. Enteric Fever (22 Years, 1869-90) . - .- . .377 2. Smallpox (50 Years, 1841-90) . . . . 384 3. Measles (50 Years, 1841-90) . . . .388 4. Scarlet Fever (30 Years, 1861-90) . \ ' .392 METEOROLOGY PART L INTRODUCTORY CHAPTER I METEOROLOGY Meaning and History of the Term Eelations of Meteorology to Medicine Buys Ballot's Law Theory of the Winds Cyclonic and Anticyclonic Systems Direction and Force, or Velocity, of the Wind Barometri- cal Gradients Seasonal Variations of Temperature and Air Move- mentsThe Claim of Meteorology to be regarded as a Science. THE term Meteorology is more than two thousand years old. It was first used by the philosopher Plato four hundred years before Christ, when he described Socrates as "a sage, both a thinker on supra-terrestrial things, and an investigator of all things upon the earth beneath." (" Sco/cpar^ o-o(bs dvtjp, rd T [JieT(Dpa (ppovTLcmijs^ Kot TO, vTTo yrjs awavTa ave^T^KW?." Apologia Socratis, cap. ii.) In his Pkcedrus, the same author employed the very word r\ /xerew/ooAoyt'a in the sense of a dis- cussion of ra /xeTw/)a, that is, things in the air, natural pheno- mena, the heavenly bodies Cicero's "supera atque caelestia." Fifty years later the philosopher Aristotle wrote a treatise which he styled rot /xerew/ooAoytKa, in which he discussed the subjects of air, water, and earthquakes, in this way approach- ing the modern signification of the word. Originally applied 2 METEOROLOGY to appearances in the sky, whether atmospheric or astron- omical in their character, the term Meteorology is at present used in a much stricter and more scientific sense to denote that branch of natural philosophy which deals with weather and climate. It includes the study of the physical properties of the atmosphere, a description of the instruments of precision employed in that study, and the application of the knowledge so obtained to the elucidation of the problems of Physical Geography, the advancement of Agriculture, and the promotion of Health as well as the prevention of Disease. To the physician the securing of these last-named ends is of course of paramount importance. From the earliest times the relations existing between medicine and meteorology have been most intimate. The far-seeing Fathers of Medicine were not slow to perceive how sensitive to the changes of the weather the delicate human organism is, and what an important bearing the study of weather - phenomena should exercise on the practice of medicine. Two thousand two hundred years ago Hippocrates wrote: " Whoever wishes to study the healing art properly must do this first, he must attentively consider the seasons of the year," etc. 1 In his other writings, also, he often recurs to this subject. Celsus, again, in the second book of his treatise, De Medicind, says : " Saluberrimum Ver est : proxime deinde ab hoc Hiems, periculosior ^Estas, Autumnus longe periculosissi- mus" a statement which to this day is true to the letter as regards Southern Italy, where the words were penned. What can be more graphic than his description of the effects of a north wind, or as we may say of a " nor'-easter " : "Aquilo tussim movet, fauces exasperat, ventrem adstringit, urinam sup- primit, horrores excitat, item dolores lateris et pectoris, sanum tamen corpus spissat, et mobilius atque expeditius reddit ! " 1 IIe/>l 'Ad/jaw, 'TSdrwi/, T6iruv. " 'iTjTpt/cV fora jSotfXerat 6p6ws rde XP*) voteiV irpurov pv ii>0vn4c<70tu rds wpas rov freos, K.T.X. METEOROLOGY 3 There is reason to believe that the suggestions thrown out by these illustrious Greek and Latin physicians were allowed to remain almost a dead letter. Certain it is that their doctrines as to the close relation of meteorology and of climatology to medicine became dimmed by the rust of time, and were neglected or forgotten. Within comparatively recent years, however, a keen interest has been awakened in the subject. Every medical man who enters Her Majesty's service is required to study the broad facts relating to meteorological observation, and afterwards is called upon to aid in building up a science of climatology. Men like Edmund A. Parkes and Ballard in England, Stark and Sir Arthur Mitchell in Scotland, Quetelet in Belgium, Pettenkofer and Buhl in Germany, have studied the weather, and published researches on it and upon its bearing upon health. And now meteorology, than which no science is more closely analogous to that of medicine, takes its proper place in the curriculum and in the examination for the Diploma in State Medicine, Public Health, or Sanitary Science, which has been made a registrable qualification under Section 21 of the Medical Act, 1886 (49 and 50 Viet. cap. xlviii.) In 1854, the Rev. Humphrey Lloyd, D.D., Provost of Trinity College, Dublin, demonstrated the cyclonic character of most of the gales experienced in Ireland, 1 and so fore- shadowed what is now universally known as Buys Ballot's Laiv a law on which the whole of modern meteorology turns. As applicable to the northern hemisphere, except close to the equator, this law may be concisely stated in the following terms : " Stand with your back to the wind, and the baro- meter will be lower on your left hand than on your right hand." Similarly, for the southern hemisphere, except close to the equator, the rule holds good : " Stand with your back to the wind, and the barometer will be lower on your right hand than on your left hand." 1 " Notes on the Meteorology of Ireland," Royal Irish Academy Trans- actions^ vol. xxii., "Science," 1854. 4 METEOROLOGY So long as the atmosphere is in a state of equilibrium the air is, of course, motionless or " calm " ; but the moment the equilibrium is disturbed, an aerial current, which we call " wind," is generated the air moving, or the wind blowing, from the district of greater towards that of less pressure, with the object of restoring absolute equilibrium. This, however, is in nature hardly ever attained. "The prime cause of atmospheric disturbance," says the Rev. W. Clement Ley, M.A., "is found in the unequal distribution of solar heat over the earth's surface; in the changes, diurnal and seasonal, in that distribution ; and in the unequal effects thus produced on the tension of the air itself and of the vapour suspended in it." 1 Experience and reflection have alike proved that air currents flowing in towards an area of low atmospheric pressure do so, not along straight lines, but in curves, so that a gyratory movement is developed round the low-pressure area. The determining cause of this phenomenon is the rotation of the earth upon its axis. A given point on the equator travels round at an infinitely greater speed than a similar point near either of the poles, because the equatorial point has to perform a journey of some 25,000 miles in the same space of time (namely, twenty-four hours) that a circum- polar point takes wherein to leisurely traverse a distance of perhaps only 100 miles. The actual speed at which a point is carried round with the earth as it spins on its axis is at the equator, 1040 miles an hour (namely, 24,900 miles -f- twenty- four hours) ; in latitude 30, 900 miles an hour ; and in latitude 60, only 520 miles an hour, or but one -half the equatorial velocity. The result of this is that air flowing northwards from the equator outstrips the earth's surface over which it is blowing because of its greater initial velocity, and accordingly trends .towards the north-eastward. In this way a south wind is 1 Aids to the Study and Forecast of Weather, p. 9, London : J. D. Potter. 1880. METEOROLOGY 5 deflected into a south-west wind. Conversely, air flowing southwards from the north pole lags behind the earth's surface, which is travelling from west to east with increasing speed according as the latitude diminishes ; and so a north wind is deflected into a north-east wind. Supposing, then, an area of low pressure to exist between such south-west and north- east winds, it is evident that these winds must make for that centre so as to fill up its vacuum by curving : the south- west wind through south to south-east and east to the right- hand side, or the eastward, of the low-pressure area; the north-east wind through north to north-west and west to the left-hand side, or the westward of that area. In this way a circulation in a direction against the hands of a watch is developed round a low-pressure area, the point of lowest pressure in the cyclonic system so formed always lying (in the northern hemisphere) on the left-hand side. In the case of the southern hemisphere the reverse of all this holds good. Air flowing southwards from the equator, that is, a north wind, travels faster than the surface over which it is blowing, and so it trends towards the south-east- ward, becoming a north-west wind. Conversely, air flowing northwards from the south pole lags behind the earth's surface, which, as before stated, is travelling from west to east with ever-increasing speed as the latitude diminishes, and so a south wind is deflected into a south-east wind. These north- west and south-east winds, thus formed, will curve into a vacuum or low-pressure area, in a direction with the hands of a watch: the north-west wind through north to north-east and east to the right-hand side, or the eastward, of the low pressure area ; the south-east wind through south to south- west and west to the left-hand side, or the westward of that area. Thus, the point of lowest pressure in the cyclonic system so formed always lies (in the southern hemisphere) on the right-hand side. Similar considerations will show that, when air flows out in all directions from an area of high atmospheric pressure 6 METEOROLOGY a gyratory movement, or circulation, will be developed, which will be in opposite directions to those just described in the case of each hemisphere north and south of the equator. To these high-pressure systems and circulations the term "anti- cyclonic " is applied, because they are the opposites, or anti- theses, of the cyclonic systems already described. "From these considerations," writes K. H. Scott, 1 "we gather that round an area of low pressure in the northern hemisphere the wind will circulate, having the lowest pressure on its left, or in a direction against the hands of a watch. Round an area of high pressure in the same hemisphere it will circulate in the opposite direction, or with watch hands. "In the southern hemisphere these conditions will be exactly reversed : the wind will move round an area of low barometer readings with watch hands, and round an area of high readings against watch hands." It is now forty years since Professor Adolf Erman first drew attention, in Poggendorf's Annalen (vol. Ixxxviii. 1853, p. 260), to these relations between wind and atmospheric pressure. But to the late Professor H. Buys Ballot, Director of the Royal Meteorological Institute of the Netherlands, Utrecht, belongs the credit of having first insisted on their constancy and importance hence the law which expresses them is called "Buys Ballot's Law." The application of this law teaches us that, in the northern hemisphere, the wind will be more or less easterly at a given station when the barometer is higher to the north than to the south of it ; more or less southerly when the barometer is higher to the east than to the west ; more or less westerly when pressure is higher to the south than to the north ; and more or less northerly when pressure is higher to the west than to the east. The qualifying words " more or less " are used, because the wind seldom blows directly along, or parallel to, the "isobars" (Greek, ftros, equal; and fidpos, weight), as the 1 Elementary Meteorology, p. 254. London : Kegan Paul. Trench, and Co. 1883. METEOROLOGY 7 lines of equal barometrical pressure are called. To these lines the wind is often inclined at an angle of some 30 or even 40. From the foregoing considerations it follows that the direction of the wind, or the point from which the wind is blowing, is determined by differences in atmospheric pres- sure, which are recorded and gauged by differences in the height of the barometer. But further, the velocity or force of the wind measured by the Beaufort scale, which will be afterwards explained is found to depend mainly on the amount of those differences, or on what are called the "barometrical gradients." The term " gradient " is borrowed from the language of engineer- ing. Engineers measure the steepness of a slope or " incline " by the relation which its vertical height bears to its horizontal length. If the ground rises or falls one foot in a distance of 60 feet, they speak of the gradient as 1 in 60. It is to Mr. Thomas Stevenson, C.E., of Edinburgh, that we owe the application of the term " gradient " to differences of atmospheric pressure as measured by barometrical ob- servations at neighbouring or even distant stations. But barometrical gradients differ from engineering gradients in a very important particular, namely, that their vertical and horizontal units of scale are not of the same kind. Their "vertical scale," says the Hon. Ealph Abercromby, F.R. Met. Soc., " is expressed in units of barometrical read- ings, and the horizontal scale in units of geographical measure- ment." * In the Meteorological Office, London, barometrical gradients are now expressed in decimal parts of an inch of mercury per 15 nautical miles, or about 17 statute miles, the line joining the points of barometrical observation necessarily running at right angles to the isobars. This unit of distance 15 nautical miles was adopted by the Permanent Com- mittee of the International Meteorological Congress, held at Vienna in 1873, in order to secure uniformity between the 1 Principles of Forecasting by Means of Weather Charts, p. 4. London : Edward Stanford. 1885. 8 METEOROLOGY gradients of the British scale and those expressed in terms of the metric system. As a hundredth of an inch is nearly equal to a quarter of a millimetre, the English gradient given above corresponds closely with a French gradient expressed in milli- metres per 60 nautical miles, or 1 of latitude. Barometrical gradients are regarded as slight or moderate when they are below '01 inch, but steep when they exceed 02 inch. They seldom exceed "04 inch or *05 inch in the British Islands. An example of the use of these barometrical gradients may be given : when it is said that on a given day there is between Dublin and Holyhead a gradient of '025 for northerly winds, it is implied that the isobars run north and south between those stations, and that the barometer stands as nearly as possible a tenth of an inch higher in Dublin than at Holyhead there being a difference in pressure in favour of Dublin amounting to '025 inch for each unit of 15 nautical miles between the two stations (15 x 4 = 60 nautical miles) ('025 inch x 4 = '100 inch, or one-tenth of an inch). In general, the steeper the gradient, or the closer the isobars are on a weather chart, the greater the velocity or the force of the wind. But the direct relation between these two factors the gradient and wind force is often interfered with by inequalities of the earth's surface, variations of temperature and of humidity, the existence of cross-currents in the higher strata of the atmosphere, and probably also the actual height of the barometer. Buys Ballot's Law, as originally formulated, was supposed to apply only to those ephemeral and varying systems of atmospheric pressure which are called " cyclones " and " anti- cyclones " ; but it is found to be equally applicable to the far vaster and more permanent seasonal variations of pressure and wind which depend on the alternate heating and cooling of large continents, and the periodical disturbance of the balance of temperature over their surface and that of neigh- bouring oceans, such as the Atlantic and the Pacific. This topic will be more fitly considered at length in connec- METEOROLOGY 9 tion with the subject of Barometrical Fluctuations (see Chapter XV.) The foregoing reflections will, I trust, vindicate the claim of Meteorology to be regarded as a science not, indeed, an exact science in the sense that mathematics and physics are exact sciences. The phenomena with which it deals are too many and too complex for that ; our knowledge of those phenomena is so imperfect that we cannot systematise it so as to predict or " forecast " with certainty. But for this very reason, perhaps, the study of the weather and the elucidation of the laws which govern it possess an interest amounting to fascination, which is quite unfelt by the student of the exact sciences. CHAPTEE II THE PHYSICAL PROPERTIES OF THE ATMOSPHERE The Atmosphere : what it is, and its Effects The Inclination of the Earth on its Axis The Revolution of the Earth round the Sun Atmospheric Air is Transparent, but has both Substance and Colour Weight of the Atmosphere Its gaseous Condition It is capable of Compression and of Expansion Professor Dewar's Experiments on the Liquefaction of Gases by Cold Liquid Air Liquid Oxygen Frozen Air Boyle's and Mariotte's Law The Law of Charles, or Regnault's Law Bearing of these Laws of Compression and Expan- sion of Gases on practical Meteorology Elasticity of the Air Rare- faction of the Atmosphere at great Altitudes Atmospheric Pressure is exerted equally in all Directions Otto von Guericke's Experiment with the Magdeburg Hemispheres Transparency of the Atmosphere Its Diathermancy and Athermancy. THE gaseous or aerial envelope which surrounds the earth is called the Atmosphere (Greek, ar/uds, vapour ; a-faipa, a globe or sphere). It profoundly influences animal and vege- table life, modifies and retains the heat derived from the sun, facilitates the transmission of sound, causes twilight or the gradual shading of day into night, and is intimately concerned in the production of weather phenomena and geological changes of all kinds. Before we proceed to pass in review the properties of the atmosphere, it is necessary to remember that the earth's axis of rotation is inclined at an angle of 2327'44" to its orbit of revolution round the sun, or as it is technically called, the plane of the ecliptic, that is, the apparent annual path of the PHYSICAL PROPERTIES OF THE ATMOSPHERE n sun. round the heavens, or the real path of the earth as seen from the sun. As the earth revolves round the sun year after year, the plane of the ecliptic cuts the plane of the equator (or the great circle which is equi-distant from the poles and perpendicular to the earth's axis of rotation) at two points which are diametrically opposite to each other. This happens on March 21, when the sun is on the equator and going northwards, and on September 23, when the sun is again on the equator, but going southwards. The points where the ecliptic and the equator intersect are called the equinoctial points, and the times when this occurs are called the equinoxes, because day and night are then of exactly equal length, the sun being twelve hours above and twelve hours below the horizon. From March 21 to June 21 the sun is getting farther and farther north of the equator, and remains longer and longer than twelve hours above the horizon at all places in the northern hemisphere. On June 21 the sun appears to traverse the heavens at an angular distance north of the equator, amounting at present to 2327'44". He "stands still," as it were, at the Tropic of Cancer and at the summer solstice, before beginning a retrograde journey to the equator and ultimately to the southward of it. Hence the terms " sol- stice "(that is, solis statio) and " tropic " (from the Greek T/x>7nj, a turning round). On December 21 the sun in like manner reaches his greatest southern declination in latitude at the Tropic of Capricorn, and, in time, at the winter solstice. The expression T/OOTTOU r/eAioio occurs both in Homer and in Hesiod, the latter first using the phrase as a note of time midsummer or midwinter. The reason why it has been necessary to enter into these particulars about the inclination of the earth on its axis, and the revolution of the earth round the sun, is that the change of seasons on all parts of the earth's surface depends chiefly upon the relations of these two factors to each other. Long days and more or less vertical suns produce summer ; short 12 METEOROLOGY days and more or less horizontal suns, on the other hand, produce winter. Summer merges into winter through autumn ; winter yields to summer through spring. Although invisible to the eye, owing to its transparency, atmospheric air has both substance and colour. That it has substance is evident from the mechanical effects which it produces when in motion. The windmill, the sailing vessel, and the anemometer alike illustrate this. The pres- sure anemometer has been called upon, in the gusts of great storms, to bear pressures up to 36 or even 40 Ibs. on the square foot ; for example, a pressure of 42 Ibs. was recorded at Glasgow on January 24, 1868, and one of 53 Ibs. at Greenwich on October 14, 1881. The extraordinary pres- sure of over 70 Ibs. per square foot was registered at Bidston Observatory, near Liverpool, on February 1, 1871. This, however, must have been quite a local phenomenon, as a pressure of 49 Ibs. equals a velocity of 110J. miles per hour, and means a "hurricane that tears up trees and throws down buildings " (Rouse). The principle upon which the parachute is constructed, or the boomerang of the aborigines of Australia, has reference to the substantial nature of air, which is capable of resisting these bodies when passing through it. Air, again, has weight, and can be weighed. At a temperature of 32 F., the barometer standing at 2 9 '9 2 inches, 100 cubic inches of air weigh 32*6 grains nearly. The weight of a cubic foot of air is 573 '5 grains. At a temperature of 60 F., and with the barometer at 30'00 inches, the corresponding weights are 30*93 grains, and 534-47 grains respectively. According to Dr. Robert J. Mann, 1 13 cubic feet, or a quadrangular block measuring 24 inches in two directions and 39 inches in the third, weighs exactly 1 Ib. ; a room 10 feet square contains 77 Ibs. of air ; while Westminster Hall holds 75 tons. He adds that air is about 760 times lighter, bulk for bulk, than water. 1 Modern Meteorology, p. 3. London: Edward Stanford. 1879. PHYSICAL PROPERTIES OF THE ATMOSPHERE 13 Under ordinary circumstances, the atmosphere exists in a gaseous state. A gas may be defined as a body whose molecules are in a constant state of repulsion. It is to Lord Kelvin (Sir William Thomson), the President of the Royal Society, and Professor of Natural Philosophy in the University of Glasgow, that we are particularly indebted for a molecular analysis of air. He believes that the atoms of air are so minute that five hundred millions of them would fit into an inch if arranged in a line. They float at some distance apart from one another, repelling each other very energetically whenever an attempt is made to drive them mechanically together. Like other gases, and, indeed, in consequence of this loose arrangement of the atoms of which it is composed, atmospheric air can be readily compressed to one -half its original volume by doubling the pressure to which it is subjected. This fixed law of compression of gases was dis- covered in the seventeenth century (1662) by the Hon. Robert Boyle, F.R.S., and afterwards, independently, in 1679, by Edme Mariotte, a priest who lived at Dijon, in Burgundy. Hence it is known as " Boyle's and Mariotte's Law." It may be concisely stated thus : The temperature remaining the same, the volume of a given quantity of gas is inversely as the pressure which it bears. 1 It will be seen, from the statement of this law, that the elastic resistance of air becomes greater and greater the more it is attempted to be compressed, whether mechanically or by means of cold. Notwithstanding this, science has triumphed, and in January, 1893, Professor Dewar, in a discourse on the "Liquefaction of Gases by Cold," delivered at the Royal Institution of Great Britain, demonstrated the liquefaction of atmospheric air. For the purpose of this experiment a temperature of not less than 182 Centigrade (3 2 7 '6 Fahrenheit) below the melting point of ice (32 F.) is required ; in other words, the temperature must be reduced to - 295 '6 F. If a vessel containing air is 1 Essentials of Physics, p. 61. By Fred. J. Brockway, M.D. Phila- delphia : W. B. Saunders. 1892. I 4 METEOROLOGY chilled to this extent, " the air will condense, trickle down the sides, and accumulate as a liquid at the bottom " (Sir Kobert Ball, LL.D., F.K.S.) At this lecture, Professor Dewar actu- ally succeeded in pouring half a pint of liquid air from one vessel to another. The professor's subsequent experiments have been still more marvellous. He has frozen air into a viscid, jelly-like mass, something between a liquid and a solid. It is supposed that frozen air assumes this form because oxygen an element which appears to resist independent solidification successfully is probably entangled in a liquid state in the frozen nitrogen of the air. It is worth noting that liquid oxygen, also shown by Professor Dewar at his lecture, displays a beautiful blue tint, suggesting, according to Sir Robert Ball, a possible explana- tion of the colour of the sky that is, of the atmosphere. To this coloration of the air allusion has already been made above. Not only is atmospheric air capable of compression, but, in common with all gases, it is capable of expansion also. As compression of air takes place in accordance with a fixed law, so also does its expansion. Like other gases, air in- creases its volume, or expands, by the gr^rd of its bulk for every degree of the Centigrade thermometer, or the ^^th of its bulk for every degree of Fahrenheit's scale (Regnault). When air is heated from the melting point of ice to the boil- ing point of water, it is found that 1000 cubic inches become 1366-5 cubic inches. The fraction |||^, or i- ^^ (nearly JJ) therefore represents the amount of the expansion of a volume of air when raised from 32 to 212 F. ; that is, through 180. But, as the expansion is equal for each degree, the amount of the expansion for one degree is ^XTW = 1 54oo 5 > which, when reduced, becomes -^\-^ as above ; or, in decimals, '00203 6 for each degree. It is to be noted that the increase of the unit of volume of a gas for one degree is called its coefficient of expansion. Gases are not only the most expansible of all bodies, but they all have the PHYSICAL PROPERTIES OF THE ATMOSPHERE 15 same coefficient of expansion, namely, -^ for 1 C., or j-gyry for 1 F. Except at very high temperatures their expansion is uniform, no matter what the temperature or the pressure may be. The fixed formula or law just stated, by which the expanding effect of heat on a gas is expressed, was first laid down in 1787 by M. Charles, then Professor of Physics in the Conservatoire des Arts et Metiers, Paris. It was subse- quently arrived at independently by John Dalton, a distin- guished meteorologist, physicist, and chemist (1766-1844), whose name is especially identified with the Atomic Theory, which elevated chemistry into a science. In 1802 Louis Joseph Gay-Lussac published a memoir on the subject, and later still Regnault improved upon Gay-Lussac's experiments, so that the law is sometimes spoken of as Regnault's Law as well as the Law of Charles. The property which air possesses of contracting in bulk when exposed to pressure, and of expanding again on the removal of that pressure, constitutes what is called elasticity a property which air possesses in no ordinary degree. These laws of the compression and expansion of gases have a most important and direct bearing upon meteorology. According to the law of Charles a volume of air at a constant pressure is proportional to its absolute temperature. Accord- ing to the law of Boyle and Mariotte the density of air varies inversely as its volume. From these two facts it follows that the density of the atmosphere is inversely as its absolute temperature in other words, hot air is specifically lighter than cold air. This aphorism gives a clue to the origin of those great movements of the atmosphere which we call "winds." Wherever the air becomes heated on the earth's surface it expands, and the barometer falls. Wher- ever the air is chilled it contracts, and the barometer rises. By these changes in atmospheric pressure are brought about the great wind circulations which have been mentioned in Chapter I. 1 6 METEOROLOGY Another important consequence of Charles's or Regnault's Law is rarefaction of the atmosphere at great altitudes. Speaking in general terms, the atmosphere exerts at the sea level a pressure of about 15 Ibs. (strictly speaking, 14 '7 3 Ibs.) to the square inch. That is to say, a column of air one inch square, if built up from the earth's surface to the extreme limit of the atmosphere, or to a height of some 200 miles, would weigh about 15 Ibs. (14 '73 Ibs.) This weight is equal to 2160 Ibs., or nearly 1 ton on a square foot, or 1 kilogramme on a square centimetre, or 263,000,000 tons on a square mile. A pressure of 15 Ibs. upon the square inch is technically spoken of as a pressure of one, atmosphere. A column of mercury 30 inches in height and 1 square inch in section is found to weigh 14'73 Ibs., and so is equivalent to the weight of a column of atmospheric air of the same section. The principle of the construction of the barometer is based upon this fact, as we shall see in a subse- quent chapter. Now, if we ascend 2 '7 miles, or to a little below the summit of Mont Blanc (15,781 feet), half the weight of the atmosphere will have been left below, and the baro- meter will read not 30 but only 15 inches. In accordance with Charles's or Regnault's Law, a given bulk of air will, at the height mentioned, expand to twice the volume it would have at sea-level. At a height of 5 '4 miles its volume would be again doubled, and the barometer would read only 7 '5 inches, and so on until at 60 miles above sea-level "the air is probably as rare as the best vacuum that can be produced by the air-pump " (R. J. Mann). The outward limit of the atmosphere must be determined by a counterpoise between gravity on the one hand and centrifugal force and the repul- sive action of the aerial molecules on the other. Where that limit lies cannot be stated with certainty, but investigations upon the duration of twilight assign to the atmosphere a height of 45 miles at the lowest estimate, and of 190 or possibly 212 miles at the highest. The latter great elevation is inferred by M. Liais, from observations upon the influence PHYSICAL PROPERTIES OF THE ATMOSPHERE 17 of the rarer regions of the atmosphere upon twilight at Kio Janeiro. 1 Experiments prove that atmospheric pressure is exerted equally in all directions downwards, as already shown ; but also upwards and horizontally or laterally. Otto von Guericke's classical experiment with the Magdeburg Hemi- spheres, first performed in 1650 before the Imperial Diet at Katisbon, is conclusive on this point. Hence it is that objects near the earth's surface are not crushed by the pres- sure of the atmosphere a pressure so tremendous that an average-sized man sustains a weight of some 1 5 tons. In this pressure of air in all directions we have a further and effective cause of air movement or wind. A column of cold air being heavier than an equal volume of warm air, its lower strata are pushed towards the area where atmospheric pressure is less, or towards the area of warm and therefore lighter air. The wind, in other words, blows from the area of high barometer towards that of low barometer, not, indeed, in a straight line, but anticyclonically, as explained in Chapter I. The atmosphere, when pure and dry, possesses two further remarkable properties transparency and diathermancy. Transparency means that pure air is permeable to the vibra- tions of light. In consequence, we are able to scan the heavens in one direction and to study the effects of light as broken up into colour on the earth in the other direction. When aqueous vapour intervenes, we gaze with admiration on the glories of sunrise and of sunset, which are due to diffraction of light, absorption of the blue rays of the spec- trum taking place because their wave-lengths are small, while the yellow and red vibrations of greater length are allowed to pass through the aqueous vapour and so are reflected to earth. Diathermancy (Gk. Sta#e/D//,os, thoroughly warm) is the property of transmitting radiant heat, and this property dry air possesses in a remarkable degree ; it is so freely perme- 1 Comptes Rendus, tome xlviii. p. 109. C !8 METEOROLOGY able to radiant heat that both at great altitudes, as on the snow-covered Alps, and in high latitudes, as within the Arctic Circle, the sun's heat may be of extraordinary power, pro- vided only that the atmosphere is extremely dry. Mr. K. H. Scott says : 1 " The observation is as old as the time of Scoresby, that on board a whaler you may see the pitch bubbling out of the seams of the ship where the sun shines on them, while ice is forming on the side of the ship which is in shade." It has been computed that the sun's rays lose, under ordinary circumstances, 20 per cent of their heat by absorption while passing vertically through the earth's atmo- sphere. The percentage of loss increases as the path of the heat rays becomes more and more horizontal, until soon after sunrise, or shortly before sunset, a condition of complete, or almost complete, athermancy is reached that is, the power of stopping radiant heat (corresponding to opacity as regards light), is greatest, the heat rays being entirely intercepted by the dense, damp strata of the atmosphere, at sunrise and sunset. The heat waves from the sun are long and short. The long waves are absorbed as they pass through the atmosphere towards the earth. The short waves reach the surface, whence they are reflected or radiated back again in length- ened waves, to meet their fate at last in absorption by the aqueous vapour of the atmosphere. In this way radiation into space is checked, and life is preserved upon the face of the globe. In the British Islands diathermancy is most decided during the prevalence of clear skies and dry easterly winds in spring and early summer ; it is least marked during the prevalence of damp fogs and mists in late autumn and the winter season of the year. 1 Elementary Meteorology, p. 57. 1883. CHAPTEE III THE COMPOSITION OF THE ATMOSPHERE Components of the Atmosphere A mechanical Mixture of Oxygen and Nitrogen Nature's cleansing Operations Volumetric Analyses of Air Proofs that Air is a mechanical Mixture and not a chemical Combination Sources of the Carbon Dioxide of the Atmosphere Its Distribution Action of Chlorophyll upon it Ozone Other gaseous Constituents of the Atmosphere Graham's Law of the Diffusion of Gases Mineral Constituents of the Atmosphere Micro- organisms, or Microbes Permanganate of Potassium Test for organic Matter in the Air Aqueous Vapour. CAREFUL volumetric analysis shows that atmospheric air consists almost entirely of a mechanical mixture of oxygen and nitrogen together, with a small and variable quantity of carbon dioxide or carbonic acid (CO 2 ). There is also present in the air moisture or aqueous vapour, the amount of which varies, especially with the temperature. Peroxide of hydro- gen and nitrous and nitric acids are occasional components ; so is sulphurous acid in the vicinity of large towns. Besides the foregoing, very minute traces of ammonia, as well as sulphide of hydrogen or its ammonia compound, and a variable quantity of organic matter derived from the animal, vegetable, and mineral kingdoms, are commonly present in those strata of the atmosphere which are nearest the earth's surface at sea level. The air is purest on the summits of lofty mountains, on open prairies or moorlands, in Arctic regions, and in mid-ocean. 20 METEOROLOGY It is temporarily purified by gales and thunderstorms, down- pours of rain, copious dews, and heavy falls of snow or hail all of them great cleansing operations of nature, which the Germans expressively call " Niederschlage " (precipitants). (Cornelius B. Fox). The most elaborate volumetric analyses of air have been made by Dr. Angus Smith, Bunsen, and Regnault. In a series of fifteen analyses, Bunsen found the oxygen by volume to vary from 20*970 to 20*840 per cent. Regnault's examin- ations of air from different parts of the world gave very similar results 20*940 to 20*850 per cent. In country air he occasionally found the percentage volume of oxygen to rise to 21 '000. On one occasion the air of Paris yielded 20 - 9 9 9 of oxygen by volume per cent. Angus Smith, in twenty-two examinations, found 20 '9 38 per cent of oxygen in the most crowded parts of Perth, while the air of the heath and of the seashore gave 20*999. For all practical purposes the percentage volume of nitrogen in the air may be found by subtracting the foregoing figures from 100, for carbon dioxide is present only in quantities ranging from *025 to 045 per cent, or '25 to *45 per 1000 parts by volume. In order easily to remember the composition of the atmo- sphere by volume and by weight we may say that in 100 parts there are of Volumes. Grains weight. Oxygen 20*96 23 '10 Nitrogen 79*00 76 '84 Carbon dioxide . . . . 0'04 0*06 100-00 100-00 It is right also to mention that, in analyses by weight, the percentage weights of oxygen and nitrogen may be translated into percentage volumes by dividing the respective specific gravities of these gases into their respective percentage weights. The specific gravity of oxygen is 1*10561 ; that of nitrogen is 0*97135. THE COMPOSITION OF THE ATMOSPHERE 21 Atmospheric air is not a chemical combination of oxygen and nitrogen. It is simply a mechanical mixture, in which the molecules of oxygen are separate and distinct from those of nitrogen, through which they vibrate at inconceivable speed without let or hindrance of any importance. Only when the nitrogen molecules are so compressed by cold as to form a liquid or a solid is the free play of the oxygen mole- cules so far interfered with as to lead to the formation of the viscid jelly-mass which represents atmospheric air when frozen solid. That air is a mechanical mixture and not a chemical com- bination is proved by the following considerations : 1. There is no chemical formula for air, for the relative proportions of oxygen and of nitrogen present in it are not those of their combining weights, or of any simple multiple of those weights. 2. When air is artificially made by mixing oxygen and nitrogen together in proper proportions, no change of volume takes place, nor is heat or electricity disengaged as in the production of ordinary chemical combinations. 3. Air is slightly soluble in water, but oxygen dissolves more readily than nitrogen. If water, in which air has been dissolved, is boiled, the air which is expelled is found to con- tain nearly 35 per cent of oxygen, instead of only 21 per cent. The air is oxygenated to the amount of 14 per cent. This could not happen if air was a stable chemical compound. 4. The refraction of air is the mean of the refraction of oxygen and of that of nitrogen. If air was a chemical compound it would have a refraction of its own, not the mean refraction of its constituent gases. Carbon dioxide is a normal constituent of the atmosphere. The table given above shows that it forms four out of every 10,000 volumes of air, and weighs six grains out of every 10,000 grains of air. If it exceeded this amount to any great extent, it would poison animal life ; if it fell short of this amount, the vegetable kingdom would starve. 22 METEOROLOGY The carbon dioxide of the atmosphere is derived from 1. The soil and subterranean sources generally. 2. The respiration of animals. 3. Combustion. 4. Fermentation and decomposition. 5. The burning of limestone in lime-kilns. 6. Carbonated natural mineral waters. Experiments prove that on land the quantity of carbon dioxide in the air is greater by night than by day, because so much of the gas is exhaled by plants at night. It increases after rain and towards midday. At sea, it is greater by day (5 volumes per 10,000) than by night (3 volumes per 10,000). M. Mene l found that the highest percentage of the gas in the air was in October, and that its amount falls to a mini- mum in December, January, and August. Kisler 2 arrived at somewhat analogous results from investigations at Nyon, Switzerland. Frankland, 3 Angus Smith, and M. G. Tissan- dier 4 all found larger quantities of the gas at considerable elevations than lower down at medium heights. Frankland's experiments were made at the summit of Mont Blanc ; Tis- sandier's in a balloon. At moderate elevations, however, the quantity of carbon dioxide is not so great as on the ground or at sea level. The composition of the atmosphere, as regards oxygen and carbon dioxide, is maintained by the action of chlorophyll the green granular matter formed in the cells of the leaves of plants which, under the influence of sunlight, has the extra- ordinary power of splitting carbon dioxide up into its two constituents carbon which it retains, and oxygen which it exhales (Wynter Blyth). Ozone (Gk. 6'oo, / have a smell) is a colourless, gaseous 1 Comptes Rendus, Ivii. p. 155. 2 Comptes Rendus, xciv. pp. 1390, 1391. 3 Journal of the Chemical Society, 1861. 4 Comptes Rendus, April 12, 1875. THE COMPOSITION OF THE ATMOSPHERE 23 substance, with a peculiar smell like weak chlorine, which is developed as the immediate result of electrical disturbances. Houzeau has experimentally demonstrated its amount in country air to be one volume in 700,000 volumes of air. It is absent in cities, in crowded dwelling -rooms, and over marshes. Unfortunately, the tests for it react to other sub- stances in the atmosphere, such as hydrogen dioxide (peroxide of hydrogen) and nitric acid. The whole subject, however, of ozone and of ozone-testing will more fittingly be considered in Chapter XXIII. on Atmospheric Electricity (see p. 296). The other gases which are more or less constantly present in the atmosphere have already been named. Nitric acid is generally present in minute quantities. Sulphurous acid is derived from the combustion of coal in large towns. From experiments undertaken at Lille, A. Ladureau * found that it increased in amount during calm weather, while it equally decreased on stormy days. Mr. Horace T. Brown estimates the normal amount of ammonia present in the air to be about six parts per 1,000,000. Heavy rain lessens the amount for a time. The presence of hydrogen sulphide in the atmosphere of large towns is proved by the tarnishing of silver plate and coins. It may be present as ammonium sulphide. Besides carbon dioxide, marsh-gas is always present in the air, although in minute quantities. Its chemical formula is CH 4 . It is a product of decomposition of organic matter in stagnant pools. Hence its name. It is also called methane, and constitutes the Jire-damp of coal mines. These gases diffuse freely through the atmosphere in obedience to a fixed law, which is that the diffusibility of two gases varies in the inverse ratio of the square roots of their densities. This law of the diffusion of gases, commonly called " Graham's Law," is based upon a consideration of the size and velocity of repulsion of the molecules of each gas. If one molecule, say of oxogen, weighs sixteen times as 1 Ann. C/iem. Phys. 5, xxix. pp. 427-32. 24 METEOROLOGY much as another, say one of hydrogen, then the latter has to move four times as fast as the former in order to strike as effective a blow. Hydrogen, a light gas, diffuses four times as fast as oxygen, a heavy one. The rate of diffusion is in this instance inversely as the square roots of one and sixteen. "The process of diffusion," says Professor Miller in his Chemical Physics, "is one which is continually performing an important part in the atmosphere around us. Accumulations of gases which are unfit for the support of animal and vege- table life are by its means silently and speedily dispersed, and this process thereby contributes largely to maintain that uniformity in the composition of the aerial ocean which is so essential to the comfort and health of the animal creation. Respiration itself, but for the process of diffusion, would fail of its appointed end, in rapidly renewing in the lungs a fresh supply of air in place of that which has been rendered unfit for the support of life by the chemical changes which it has undergone." Among mineral constituents of the atmosphere common salt (sodium chloride) is the most frequently met with, espe- cially in the lower strata of the air. Spectroscopic analysis of the Bunsen flame invariably gives the sodium line in con- sequence of the presence of salt. Metallic dust of various kinds abounds in the vicinity of manufactories and in the air of mines. Vast numbers of micro-organisms, or microbes, infest the air. These belong both to the pathogenic and to the non- pathogenic groups. They are infrequent in the air of mid- ocean and on high mountains, but abound in the air of towns, swarming in that of ill-ventilated dwelling-rooms. Mr. J. B. Dancer, 1 F.R.A.S., examined the solid particles of the air of Manchester microscopically, and came to the conclusion " that 37J millions of these bodies [particles of both organic and 1 Proceedings of the Literary and Philosophical Society of Manchester, vol. iv. Series 3. 1867-68. THE COMPOSITION OF THE ATMOSPHERE 25 inorganic origin], exclusive of other substances, were collected from 2495 litres = 88 cubic feet of the 'air of Manchester,' a quantity which would be respired in about ten hours by a man of ordinary size when actively employed." According to Mr. A. Wynter Blyth, 1 the best chemical method of estimating organic matter in the air is its approxi- mate estimation by means of permanganate of potassium. A known bulk of air is drawn through a little distilled water, and the amount of oxygen consumed is determined by the Forchammer process. Ten cubic centimetres of a standard solution of permanganate of potassium, 2 and ten cubic centimetres of sulphuric acid, diluted to one-third, are added to a known bulk of water, say a litre. The whole is then heated for four hours to 80 F. (26 '6 C.) At the end of that time the water is titrated that is, has its strength determined with a hyposulphite solution made by dissolving one part of crystallised sodic hyposulphite in a litre of water, using iodide of potassium and starch as an indicator. The value is obtained by running a control with distilled water. One of the most important constituents of the atmosphere is aqueous vapour, or water in a gaseous or aeriform state. A vapour, like a gas, is subject to the laws of expansion and of compression, which have been already discussed in these pages but only uithin certain limits. "If," says Mr. R. H. Scott, 3 " these limits be overpast, i,e. if the pressure becomes too great or the temperature falls too low, a portion of the vapour will pass into the state of liquid. Under any circum- stances of pressure and temperature, a given space can contain only a given quantity of vapour. This is as true of vapour mixed with air as of vapour by itself." The overwhelming influence of aqueous vapour in practical meteorology arises in part from its sensitiveness to the action 1 A Manual of Public Health, p. 96. 1890. 2 Made by dissolving '395 grm. of potassic permanganate in a litre of water. Each c.c. contains '0001 grm. of available oxygen. 3 Elementary Meteorology, p. 95. 1883. 26 METEOROLOGY of heat even moderate changes of temperature causing it to expand or contract, to evaporate or condense, with great facility ; but more especially from its liability to pass from the gaseous or vaporous, to the liquid or even solid form, at temperatures of everyday occurrence in nature. It is, how- ever, to the marvellous heat-absorbing powers of aqueous vapour that the attention of the practical meteorologist must in particular be directed, when he seeks an explanation of the phenomena of what we call "Weather." PAET II. PRACTICAL METEOKOLOGY CHAPTER IV BRITISH METEOROLOGICAL OBSERVATIONS Merle's Journal of the Weather, 1337-1344 Admiral FitzRoy's Charts and Forecasts Synoptic Weather Charts Meteorology of the British Isles Stations of the First Order Anemographic Stations Stations of the Second Order Telegraphic Reporting Stations " Isobars " and " Isotherms " Extra Stations Royal Meteorological Society Scottish Meteorological Society Daily Weather Report Weekly Weather Report Monthly Summary Accumulated Tem- perature Numerical Scales for Telegraphic Weather Reports Code System Meteorological Conditions which influence Disease and the Death-rate Equipment of a Second Order Station. THE earliest known Journal of the Weather was that kept at Oxford by the Rev. William Merle, Fellow of Merton College and afterwards Rector of Driby, Lincolnshire, during the seven years 1337-1344. His " Consideraciones Temperiei pro 7 Annis " were discovered in a MS. 1 in the Bodleian Library, Oxford, in 1891, and were immediately afterwards reproduced and translated under the supervision of Mr. G. J. Symons, F.R.S., to whom British meteorology owes so much. The observations are climatological and are written in Latin in Old English characters. They represent the primeval stage of weather study, in which popular weather prognostics came to 1 Digby MS. 176, fol. 4. 28 METEOROLOGY be drawn from daily scanning of the heavens, untiring observ- ation of the movements of animals, including the arrival and departure of migratory birds, of the leafing and flowering of trees and shrubs, of the ripening of harvests and fruits, and of the fall of the leaf in autumn. These phenological observations in bygone days conferred a marvellous power of forecasting weather, and so they do at the present day also, so far as local districts are concerned. Since the discovery of the barometer in the seventeenth century, isolated observations on atmospheric pressure crude, no doubt, and unreduced to any standard altitude or tem- perature afforded an increased power of weather forecasting. It was not, however, until 1861 that the application of telegraphy to the synchronous study of the weather at distant stations revolutionised meteorology and raised it to the dignity of a science. The service of Daily Weather Charts and Forecasts, which was inaugurated by the late Admiral FitzRoy in the year named, has been amplified and improved since then, but to him belongs the credit of organising a system of weather study which now extends over the whole civilised world. Every country in Europe ; Canada, and the United States ; India, Australia, and New Zealand all have their Meteorological Offices or Weather Bureaux, at which synoptic weather charts and forecasts are prepared at least once a day. The word "synoptic" (Gk. CTWOTTTIKOS from o-wo^is, a seeing all together, a general view) signifies that the weather chart has been prepared from observations taken at the same moment of time over a large tract of country, and that it illustrates the type of weather prevailing throughout the district embraced in the chart at the hour of observation. The following account of the method in which the land meteorology of the British Islands is studied through the medium of the Meteorological Office, London, will apply mutatis mutandis to the Weather Bureaux of the British Colonies and of Foreign States. BRITISH METEOROLOGICAL OBSERVATIONS 29 The observatories in the United Kingdom in connection with the Central Office may be arranged in five classes : 1. Stations of the First Order, or Self-recording Observa- tories, which are furnished with self-registering instruments by which all the principal meteorological phenomena are recorded continuously. These alone afford the materials necessary for the study of the periodic variations of the meteorological elements. There are seven such stations at present : three in England Falmouth, Kew, and Stonyhurst ; three in Scotland Glasgow, Aberdeen, and Fort- William ; and only one in Ireland Valentia Island, in Kerry. The observa- tory at Armagh was relegated to Class II. some years ago. 2. Anemographic Stations, furnished with instruments registering the wind only. At Armagh, rainfall and sun- shine are in addition recorded. The observations from these stations are important in connection with storms, and afford evidence available in courts of law relative to collisions at sea and damage done by wind either on land or at sea. 3. Stations of the Second Order, or Climatological Observa- tories, of which upwards of 100 are now at work. At the end of March 1892 the total number of these stations was 103, including 17 belonging to the Royal Meteorological Society and 19 belonging to the Scottish Meteorological Society. The stations are distributed as follows : 45 in England, 4 in Wales, 28 in Scotland, and 26 in Ireland. Reports from eleven of the Irish stations are regularly supplied to the Registrar -General for Ireland for his Weekly and Quarterly Returns. At all of these climatological stations regular eye observations are taken twice daily, at 9 A.M.. and 9 P.M., of atmospheric pressure, temperature (dry bulb and wet bulb), wind, cloud, and weather, with the daily maxima and minima of temperature, the daily rainfall, together with general remarks on the weather. The observers at these stations are all volunteers. All the stations are regularly inspected by inspectors from the Meteorological Office. 4. Telegraphic Reporting-Stations fifty-nine in number .30 METEOROLOGY at wliicli the observations are taken by eye at certain hours determined by the requirements of the telegraphic system. In some cases the eye observations are supplemented by self- recording aneroid barometers, etc. The foreign reporting-stations, twenty-eight in number, ex- tend along the entire western coast of the continent of Europe, from Bodo in lat. 67 N. to Lisbon in lat. 38 N., and include four stations on the shores of the Baltic, three in Germany, and two in the Mediterranean. The remaining thirty-one telegraphic reporting-stations are scattered throughout Great Britain and Ireland and the adjacent islands. At these stations observations are taken at 8 A.M. and 6 P.M. Greenwich time, and are telegraphed to London according to a special cipher code. In addition, observations are taken at 2 P.M. daily at fifteen home and two foreign stations (Skudesnaes in Norway, and Rochefort on the west coast of France) and at once telegraphed in cipher to London. As the reports come in, the information is entered on a chart, which is preserved in the Office, and from which the Daily Weather Report is prepared. This Report fills four large quarto pages, and is arranged as follows : Page 1 contains the whole of the fifty-nine reports from which the maps for the day (given on page 2) are prepared, and the 6 P.M. reports of the previous day, together with the maximum and minimum temperatures of the air, and the rainfall for the previous twenty-four hours. Page 2 contains (1) a map of North - Western Europe showing, for 8 A.M. on the date of publication, the distribu- tion of pressure, the prevalent winds, and the sea disturbance, with necessary explanations, together with a table showing the mean atmospheric pressure for the month at twenty -five stations during the twenty years 1871-1890 ; (2) a similar map showing the distribution of temperature at 8 A.M., the weather at each station, and the distribution of rainfall during the past twenty-four hours, together with a table of the mean tempera- BRITISH METEOROLOGICAL OBSERVATIONS 31 ture of the air and of evaporation at 8 A.M., the means of the daily maximum and minimum temperatures, and the mean rain- fall for the month at the same twenty-five stations in the United Kingdom. The mean monthly rainfall values are those for the twenty-five years 1866-1890. The other means are for the twenty years 1871-1890. On these maps lines are drawn through the places where atmospheric pressure and tempera- ture are respectively equal. The lines of equal pressure are called "isobars" (Gk. ro?, equal; /3dpos, weight):, those of equal temperature are called "isotherms" (Gk. t'o-o?, equal; Oep/Ar), warmth). Isobars are by far the most important element in forecasting, while isotherms play a very subordinate part. The direction of the wind is marked by arrows, which fly with the wind. They carry a number of " fleches " propor- tional to the force of the wind estimated by the Beaufort scale (see p. 35). Page 3 contains (1) notes on the "general situation" at 8 A.M., and a statement as to the probable changes in the system of weather then prevalent ; and (2) the forecasts drawn up for each district of the British islands eleven in number at 11 A.M., and an explanation of the meaning of the storm signals exhibited on our coasts. These forecasts refer to the weather likely to be experienced during the twenty-four hours ending at noon of the day succeeding that of publication. Page 4 contains the reports for 2 P.M. of the previous day, an account of the distribution of pressure, temperature, wind and weather experienced over the whole of continental Europe on the previous day, and, on Mondays, a brief summary of the weather experienced during the previous week. The standing portion of the reports, such as the maps and mean tables, is printed in blue, while the information for each day is in black. 5 Extra Stations, furnishing returns with less complete- ness and detail than those of the Second Order. These 32 METEOROLOGY returns are not published by the Office, but some of them are used in the checking of Storm Warnings. The rainfall values at these extra stations also are copied and supplied to Mr. George J. Symons, F.K.S., for publication in British Rainfall, an annual volume, of which he is the indefatigable compiler and editor. A continuous record of the amount of bright sunshine is received from forty-four stations in the British Isles, of which some are First or Second Order Stations, whilst from others the sunshine record is alone received. Such is the official machinery for the scientific study of the weather in the British Isles. It should be mentioned that the Meteorological Office, situated at 63 Victoria Street, Westminster, London, S.W., is under the management of the Meteorological Council. The Council, which was registered as a corporate body under the "Companies Act, 1867," on October 8, 1891, also administers an annual parliamentary grant for meteorological purposes. It was 15,300 in each of the financial years 1891 and 1892. The Secretary of the Council is Mr, Robert H. Scott, M.A. Univ. Dubl., F.R.S., and the Marine Superintendent is Navigating -Lieutenant C. W. Baillie, R.K, F.R.A.S. Besides the Meteorological Office, the Royal Meteoro- logical Society and the Scottish Meteorological Society have covered the United Kingdom with a network of climato- logical and phenological stations. 1 Each of these societies also publishes a journal containing many valuable papers on meteorological subjects. Mention should also be made in this connection of the wonderful system of rainfall observa- tion which Mr. Symons, F.R.S., has organised through years of patient and untiring labour. In 1891 ftie number of per- fect rainfall returns published in British Rainfall amounted to 27992091 in England, 168 in Wales, 359 in Scotland, and 181 in Ireland. England had about one observer for 1 Phenological stations are those at which a registry is kept of natural periodic phenomena connected with the animal and vegetable kingdoms. BRITISH METEOROLOGICA L OBSER VA TIONS 33 each 25 square miles, Scotland about one for each 80 square miles, and Ireland only about one for each 180 square miles. In relation to the study of the meteorology of the British Isles, allusion should be made to two periodical publications of the Meteorological Office apart from the Daily Weather Report. These are (1) The Weekly Weather Report; (2) the Monthly Summary, issued as a supplement to the same. The Weekly Weather Report has appeared since the begin- ning of February 1878. It is published regularly on Thurs- days, and is illustrated by three maps for each day of the week. The maps show for 8 A.M. the temperature, weather, and sea disturbance ; and for 8 A.M. and 6 P.M. the distribu- tion of atmospheric pressure and the winds over and on the coasts of Europe. The information given on the first and second pages of each Report is based upon observations of tempera- ture and rainfall made at seventy-six stations, and of sun- shine records taken at forty -seven stations. The Reports contain also tables of " Accumulated Temperature." These are designed to give persons engaged in agriculture better means of estimating the manner in which vegetation is affected by temperature than those afforded by the more usual methods of treating the readings of the thermometer. The tables show for each week, and for the whole period from the beginning of the year, the weekly and progressive values, respectively, of the combined amount and duration of the excess or defect of the air temperature, above or below a suitably fixed standard or base temperature. The base value adopted is 42 F., as being nearly equivalent to 6 C., which has been considered by continental writers on these subjects to be the critical value, the temperature above which is mainly effectual in starting and maintaining the growth, and in completing the ripening of agricultural crops in a European climate. This base is also convenient as being precisely 10 F. above the freezing point, or melting point of ice. Accumulated Temperature is expressed in Day Degrees a day degree signifying 1 F. of excess or defect of temperature 34 METEOROLOGY above or below 42 F. continued for twenty-four hours, or any other number of degrees for an inversely proportional number of hours. It has been ascertained, by calculation from a con- siderable series of hourly observations at various places, that the accumulated temperature may be computed, with a very close approximation to the truth, from the observed daily maximum and minimum temperatures alone. The Monthly Summary of the Weekly Weather Report gives for each calendar month the mean and principal values for the different elements pressure, temperature, rainfall, and sunshine and the differences between these and the averages for the corresponding months in a long series of past years. There are also four maps, showing the distribution of atmo- spheric pressure, wind, temperature, and rainfall, and the movements of the principal barometrical depressions observed during the month, as well as some brief notes as to the chief features exhibited. To return to the Daily Weather Report, it may interest the reader to learn something about the composition of the weather telegrams which are sent at 8 A.M. by Greenwich time daily to the Meteorological Office, London. The telegraphic reports, which are addressed simply to " WEATHER, LONDON," consist of two parts. The first part is composed entirely of figures, arranged in groups of five each, in accordance with the Code approved by the International Meteorological Congress, held at Utrecht in September, 1874. The second part consists mainly of words, occasionally mingled with figures in groups, and is designed to throw additional light on the information given in the first part of the message. The following numerical scales are used in drawing up the telegrams : Wind Direction. The thirty-two different points of the compass are supposed to be numbered, beginning with 01 = N. by E. and 02 = N.N.E. (true bearings), to 08 correspond- ing to E., 16 to S., 24 to W., and 32 to N. According to BRITISH METEOROLOGICAL OBSERVATIONS 35 this scale, S.S.E. would be telegraphed " 14," and W. by N. "25." Wind Force. This is estimated in accordance with the annexed scale, commonly known as the "Beaufort Scale," because it was originally drawn up by Admiral Sir F. Beaufort, in command of H.M.S. Woolwich in 1805. The wording has been altered so as to suit the present use of double topsails. Added to the table also is a column giving the average value in miles per hour travelled by the wind during the prevalence of the different forces. TABLE I. BEAUFORT SCALE OF WIND FORCE. Miles per hour. 0. Calm . ......... 3 1. Light Air . Just sufficient to give steerage way . . 8 2. Light breeze . 'That in which a well-" 1 to 2 knots . 13 conditioned man-of- 3. Gentle breeze . - war, with all sail set, and clean full, would 3 to 4 knots . 18 go in smooth water 4. Moderate breeze k. from 5 to 6 knots . 23 5. Fresh breeze . -\ Royals, etc. . . 28 6. Strong breeze . Topgallant sails . 34 7. Moderate gale . That to which she Topsails, jib, etc. . 40 8. Fresh gale could just carry in - Reefed upper top- chase, full and by . sails and courses 48 9. Strong gale Lower topsails and , . courses . .56 10. Whole gale . That with which she could scarcely bear lower main topsail and reefed foresail . 65 11. Storm . . That which would reduce her to storm staysails . . . . . .75 12. Hurricane . That which no canvas could withstand . 90 As storms in the British Islands are rarely, if ever, so violent as those in tropical latitudes, great caution should be used in the insertion of extreme figures in the telegraphic reports, such as 12 for the wind and 9 for the sea. 3 6 METEOROLOGY TABLE II. SCALE OF SEA DISTURBANCE AND WEATHER. = Dead calm. 1 = Veiy smooth. 2 = Smooth. 3 = Slight. Sea Disturbance. 4 = Moderate. 5 = Kather rough. 6 = Rough. 7 = High. 8 = Very high. 9 = Tremendous. Weather. = Sky quite clear. 1 = Sky a quarter clouded. 2 = Sky half clouded. 3 = Sky threequarters clouded. 4 = Sky entirely overcast. 5 = Rain falling. 6 = Snow falling. 7 = Haze. 8 - Fog. 9 = Thunderstorm. Any other phenomena must be reported in words after the groups of figures, such as " lightning last evening," " heavy dew," "aurora." In this scale the values to 4 refer to the amount of cloud, not to its density. Time. 00 or 24 stands for midnight ; 01 for 1 A.M., and so on every hour to 11 P.M., which is represented by 23. Armed with the foregoing scales, the observer having taken the readings at 8 A.M. (Greenwich time) transmits a telegraphic message to "Weather, London," consisting of six groups of five figures each. Here is an example : 97622 09549 96228 06253 50046 64485 The first group contains the reading of the barometer (omitting the first figure of the value), reduced to 32 F. and the mean sea level, for 6 P.M. on the previous day, and the direction of the wind (true, not magnetic) at the same hour. 97622 is thus resolved into: Barometer, 29*76 inches; wind, W.S.W. The second group gives the force of the wind at 6 P.M. on the previous day, the weather and air temperature at the same hour. 09549 thus becomes : Wind force, 9, or a strong gale ; weather, rainy ; air temperature, 49. The third group supplies the reading of the barometer at 8 A.M., reduced to 32 F. at mean sea-level, and also the BRITISH METEOROLOGICAL OBSERVATIONS 37 direction of the wind. Thus 96228 becomes : Barometer, 29'62 inches; wind, N.W. The fourth group gives the wind force, weather, and air temperature at 8 A.M., for 06253 = wind force, 6, or a strong breeze ; 2, half-clouded sky ; dry-bulb thermometer, 53. The fifth group contains the reading of the wet-bulb thermometer at 8 A.M., and the amount of precipitation or rainfall, including melted snow and hail, during the last twenty-four hours, in inches, tenths, and hundredths, omitting the decimal point. For example : 50046 = wet-bulb tem- perature, 50 ; rainfall = "046 inch. The sixth group gives the maximum and minimum temper- atures in the last twenty-four hours, together with the amount of sea disturbance at 8 A.M. At inland stations the last figure is of course always 0. Thus, 64485 means that the maximum temperature in the twenty-four hours ending 8 A.M. has been 64, the minimum temperature has been 48, and the sea is " rather rough " at 8 A.M. (i.e. sea disturbance, 5). Certain selected stations send in additional reports in three groups only at 2 P.M. and 6 P.M. daily. These groups corre- spond closely with the third, fourth, and fifth groups in the 8 A.M. report. Special telegrams in threatening weather are sent in a similar way at other times as required. By the adoption of this code system, a very full report can be condensed into what is equivalent to only six words for, under the Post-Office Regulations, five figures, or a letter preceding or following a group of figures, are counted as only one word, and charged for accordingly. The foregoing information has been culled from the official instructions for meteorological telegraphy, prepared for the use of observers exclusively, in accordance with the International Code adopted at Utrecht in September 1874. The meteorological conditions which possess the greatest interest and value for Medical Officers of Health, from their influence on the prevalence of disease and on the death-rate are, undoubtedly, temperature, humidity, and rainfall. But 3 8 METEOROLOGY as these depend to a large extent on the state of the barometer, the direction and force of the wind, and the condition of the sky as regards cloud, fog, and mist or haze, it is necessary to study the whole group of meteorological phenomena. In subsequent chapters, then, a detailed description of the various instruments required by the observer will be given, and the practical application of the information afforded by them will be explained. The instruments required for a Second Order Station are a standard barometer, maximum and minimum thermometers, dry and wet bulb thermometers, and a rain gauge. All the four thermometers named should be suspended in a properly placed Stevenson thermometer screen (see p. 108). These instruments barometer, four thermometers, and rain gauge are indispensable, but besides them it is desirable to have also a black-bulb maximum thermometer in vacua, a bright-bulb maximum thermometer in vacuo, and a minimum thermometer (graduated on the stem, without attached scale) for terrestrial radiation, one or more earth thermometers, an anemometer, and a sunshine recorder. All the thermometers should be graduated on the stem, and only such instruments should be used as have been verified at Kew Observatory, so that the " index-error " may be known. CHAPTEE V HISTORY, ORGANISATION, AND WORK OF THE UNITED STATES WEATHER BUREAU 1. Historical Sketch THE development of interest in meteorology in the United States dates from the earliest times of its history. It was apparently Benjamin Franklin who first called attention to the progression of weather from west to east. He noted that a north-easterly storm appeared earlier at Philadelphia than at Boston. It was Thomas Jefferson, afterwards President of the United States, who first undertook in that country simul- taneous meteorological observations. From 1772 to 1777 he carried on such observations at Monticello with Mr. (after- wards Bishop) Madison, who was located at Williamsburg, both in Virginia, and about 120 miles apart. With the invention of the telegraph by Morse, in 1837, came almost immediately the idea of collecting at one place simultaneous observations from different parts of the States ; and on this followed, in about ten years, the idea of charting these instantaneous observations, and deducing from this chart some conclusions as to the future weather. This idea was proposed and vigorously canvassed by Commodore Maury in the early fifties. It was put into operation by Professor Henry about the same time, and continued until the breaking 40 METEOROLOGY out of the Civil War, when it was discontinued. Professor Henry used a wall map, with movable symbols. Each day, w r hen the observations were received, they were entered on this map, and from the appearance of the map he drew certain conclusions about the weather, which were trans- mitted to Congress, and attracted much attention. On the occurrence of the late Civil War the matter was dropped until 1869, when Professor Cleveland Abbe, then Director of the Cincinnati Observatory, undertook the collection of data and the forecasting of the weather for the Cincinnati Board of Trade. The data was collected free of cost by the Western Union Telegraph Company, and the map employed by Pro- fessor Abbe was made up in the local office of that Company by the manager at Cincinnati. In the meantime very great interest was being taken in the same direction by Professor I. A. Lapham, of Milwaukee, Wisconsin, and it was perhaps Professor Lapham who per- sonally interested a prominent member of Congress from Wisconsin, Hon. H. E. Paine, who finally introduced into Congress the bill which, on becoming law, created the Weather Service of the United States. In the session of February 9 to April 20, 1870, a joint resolution was passed by Congress, which required the Secretary of War to take meteorological observations at the military stations in the interior of the continent, and at other points in the states and territories of the United States, and to give notice on the northern lakes and on the sea-coast, by magnetic telegraph and marine signals, of the approach and force of storms. At the same session an appropriation of $15,000 was made to carry into effect the foregoing resolution. This work was placed by the Secretary of War in the hands of the Chief Signal-Officer, as it involved questions of signalling and had been recommended by him. At this time General Myer was Chief Signal-Officer, and it was very fortunate for the meteoro- logical service of the States that it was first placed in such energetic hands. UNITED STATES WEATHER BUREAU 41 On March 3, 1871, an appropriation was made for this service in the following terms : " For manufacture, purchase, or repair (of) meteorological and other necessary instruments ; for telegraphing reports ; for expenses of storm-signals announcing probable approach and force of storms ; for instrument shelters ; for hire, furniture, and expense of offices maintained for public use in cities or posts receiving reports ; for maps and bulletins to be displayed in chambers of commerce and boards of trade rooms for books and stationery, and for incidental expenses not otherwise provided for, $102,451." On June 10, 1872, another Appropriation Bill was passed, the preceding being the first appropriation looking to the estab- lishment of a meteorological service throughout the country. The latter bill repeated the former, and after the word storms added the following words, " throughout the United States, for the benefit of commerce and agriculture," and at the end of the clause, in the nature of legislation, was inserted the follow- ing proviso: "Authorising Secretary of War to provide for such stations, reports, and signals as may be found necessary." Annual appropriations were made thereafter for the service, and in terms enlarging its scope, until it was transferred, in 1891, to the Department of Agriculture as the Weather Bureau. The first weather bulletin of the new service was issued November 1, 1870, and the first storm warning a week later. The first weather map appeared on January 1, 1871. This was not the earliest weather map published, because before it there had been those issued under the direction of Professor Abbe, and even before that a series of weather maps had been started elsewhere. In 1861, Admiral FitzRoy inaugu- rated the British system of weather charts and forecasts. On September 16, 1863, Leverrier, at Paris, France, began the publication of a series of weather maps, which have continued without interruption from that day to this. The American series was the third of the twenty or more which are now in progress in various parts of the world. 42 METEOROLOGY General Myer continued in charge of the meteorological service, as Chief Signal-Officer, until his death on August 24, 1880. His administration of this service was characterised by very great energy, which was followed by great success. The forecasts made at that time had, to a larger degree than at any subsequent time until quite recently, the popular approval and confidence. General Myer was a very strong, decisive, and exact executive officer. It was to him that the popular nickname of " Old Probs." was attached, because the official forecasts were then published as " probabilities." Not one of his successors has arrived at so high a grade of general favour as to be endowed with a popular, semi- humorous, and semi-affectionate nickname. After the decease of General Myer, General Wm. B. Hazen became Chief Signal-Officer, the date of his appoint- ment being December, 1880. He died January 16, 1887. His administration of the meteorological service was of a dis- tinctly different character from that of General Myer. It was characterised, not by such administrative energy, but by a notably greater interest in the scientific side of meteoro- logical work. During the administration of General Hazen there was a very large growth and development of scientific meteorology. General Myer always held that scientific work should be left to private enterprise. It was General Hazen's idea that the meteorological service was to be in advance of private enterprise quite as much in the scientific work as in the practical work. It was his belief that the opportunities for scientific study afforded by the general weather service of the United States were far in advance of any opportunity which could be at the command of an individual. General Hazen was succeeded by General (at that time Captain) A. W. Greely. He was senior assistant at the time of General Hazen's decease, and assumed charge until a Chief Signal-Officer was appointed. He was nominated by the President for Chief Signal-Officer and Brigadier-General on February 16, 1887, and was confirmed by the Senate in these UNITED STATES WEATHER BUREAU 43 places March 3, 1887. General Greely had conducted the exploration party of the Signal Service to Lady Franklin Bay in the polar regions, had charge while they remained there, and was one of the few survivors who returned to the United States. General Greely remained in charge of the meteorological service until it was transferred by law from the War Department to the Department of Agriculture on July 1, 1891. He is still in charge of the Signal Service, now relieved of its meteorological duties. The agitation for the transfer from military to civilian hands had existed for a long time, and had been the source of much hard feeling, both in the Signal Service itself and among meteorologists outside of the service. On October 1, 1890, a bill was finally passed which pro- vided that " the civilian duties now performed by the signal corps of the army shall hereafter devolve upon a bureau, to be known as the Weather Bureau, which on and after July 1, 1891, shall be established in and attached to the Department of Agriculture." In accordance with law the service was so transferred, and Mr. Mark W. Harrington, then Professor of Astronomy and Director of the Observatory at the University of Michigan, and founder and editor of the American Meteorological Journal, was placed in charge. By the terms of the transfer the Chief of the Weather Bureau, under the direction of the Secretary of Agriculture, has charge of the forecasting of weather, the issue of storm warnings, the display of weather and flood signals for the benefit of agriculture, commerce, and navigation, the gauging and reporting of rivers, the maintenance and work- ing of sea-coast telegraph lines, and the collection and trans- mission of marine intelligence for the benefit of commerce and navigation, the reporting of temperature and rainfall conditions for the cotton interests, the display of frost and cold-wave signals, the distribution of meteorological informa- tion in the interests of agriculture and commerce, and the taking of such meteorological observations as may be neces- 44 METEOROLOGY sary to establish and record the climatic conditions of the United States, or as are essential for the proper execution of the foregoing duties. The first Appropriation Bill set aside a considerable sum for the distribution of forecasts to farmers. For this reason, and because of the transfer of the Bureau to the Department of Agriculture, especial attention has been given since the transfer to the more extensive and complete distribution of the forecasts to country communities and especially to farmers. The expenditures for the Weather Service of the United States, except in so far as is stated above for the first years, cannot be easily made out from the accounts, until 1882. This is because under the system practised in the War Department, the accounts are not kept separate for individual bureaux, but so far as pay and allowances are concerned are mingled with the accounts of other bureaux. Beginning with 1882 it is practicable to separate them and ascertain the amount actually expended for the meteorological service. These amounts are as follows : Fiscal year ending June 30. Amount Expended. 1882 $988,615-90 1883 993,520-00 1884 . 984,451-30 1885 966,076-44 1886 . 960,812-06 1887 902,042-67 1888 909,410-74 1889 . 853,396-27 1890 . .... . . . 810,622-59 1891 . . . % . ... . 877,659-80 1892 . . , ...:... . 830,783-33 x 1893 . . . ... . , 892,805-20! 1894 . ... . ' . . . 850, 000-00 2 1895 . . . . . . 854,223-00 3 1 Accounts not yet permanently closed. 2 Approximate. (Year not completed at time of writing.) 3 Official estimate submitted to Congress. UNITED STATES WEATHER BUREAU 45 Most of the employees were at first enlisted men in the army, generally of the rank of privates only, with officials of higher rank in the central office at Washington City. Arrangements were soon made by which they could have promotion through the non-commissioned grades, and eventually for admission to the commissioned grades, but the latter was rarely effected. The change in personnel has been slow and conservative, and there are now (1893) many men in the service who have completed their twentieth year in its duties. The enlisted men were protected from partisan aggression by the fact of enlistment. During General Greely's administration, by the Sundry Civil Act, approved October 2, 1888, it was provided that any person performing duty in any capacity as officer, clerk, or otherwise, who has heretofore been paid as an enlisted man, may be continued in such office, clerkship, or employment. This amounted to placing the Signal Service force in the city of Washington under the control of the Departmental Civil Service rules. Under Chief Harrington's administration, on February 1, 1893, by order of the President, the force outside of Wash- ington was brought within the provisions of the Civil Service Act, and the Civil Service protection of the employees of the Bureau was completed, as far as was compatible with the present standing of the law. Professor Abbe was asked by General Myer to join the service at the beginning of its meteorological work, and he has remained in the service continuously from that time to this. Major H. H. C. Dun woody was assigned to the service early in its history and still remains with it. He is the only officer of the army who has remained continuously with the civilian Weather Bureau. Among the better known names of those who have been connected with the meteorological service are Wm. Ferrel (1882-86), deceased, September 18, 1891; Dr. T. C. Mendenhall (1885-87), now Superinten- dent of the U.S. Coast and Geodetic Survey ; I. A. Lapham (1871-72), and Dr. Carl Barus (1892-93). Professor Loomis 46 METEOROLOGY was for many years in close connection with the service, if not actually employed in it. Among those remaining in the service may be mentioned Professor H. A. Hazen (not of kin to General W. B. Hazen), whose service began in 1881, Professor C. H. Marvin (from 1884), and Professor F. H. Bigelow (from October 1891). Early in his administration General Myer attempted to give to his observers a fundamental knowledge of meteorology, and for this purpose established a training school at Arling- ton, across the Potomac from Washington. This was at first called Fort Whipple, later Fort Myer. General Myer's plan was enlarged by General Hazen, until the training school at Fort Myer included both observers and officers, and courses of training, ranging all the way from those of the drill-master in infantry, cavalry, and artillery practice, to courses on physics, electricity, and meteorology by competent professors. This school was closed in 1886 by order of the Secretary of War, and against the protest of General Hazen. It has not been resumed, and the better and more promising state of general education in meteorology makes its existence less necessary. The Bureau endeavours to encourage in a great many ways the increased pursuit of meteorology in schools of all grades, and is meeting with gratifying success. General Hazen made strong endeavours to attract well-educated young men into the service, and had a fair measure of success. Many of these men still remain in the service, while others have become successful teachers. Those who now enter the service must have a good academic education and some knowledge of meteorology to start with. They then have a definite course of practical instruction under the observer in charge. When they reach the grade of Local Forecast Official they are brought to Washington for a brief practical course in weather predictions. Appointment to a professor- ship in the Weather Bureau now involves a competitive test of exacting character, as may be judged from the following quotation from the latest announcement : UNITED STATES WEATHER BUREAU 47 "The examination will be open to all. Success in practical forecasting will count 75 per cent of the examina- tion. An essay upon 'Weather Forecasts, and how to im- prove them' will count 12 J per cent; and an examination on meteorology (text-book, Waldo's Modern Meteorology] will count 12J per cent. " Intending competitors must prepare and forward to the Chief of the Bureau an essay, not exceeding 3000 words, upon 'Weather Forecasts, and how to improve them.' Each essay to be signed by a nom de plume, with true name and address in sealed envelope accompanying. These papers must be in the hands of the Chief of the Bureau by Decem- ber 1, 1893. The judges will be the Chief, the Assistant Chief, and one other. The best ten essays will be selected, and the authors notified to present themselves at this office for competitive test in forecasting, and further examination as to their knowledge of meteorology." Among the most important dates in the history of the meteorological service of the United States are the following : 1871, November 13. First exchange of observations with Canada. This continues to the present and is very helpful in the forecast work of the Bureau. Arrangements are now (1894) in progress for a similar exchange with Mexico. 1872, January. As authorised by the Appropriation Bill for that fiscal year, the service arranged for reports of the stages of rivers, and in the following spring these were utilised in forecasts of floods. 1872, September 3. First balloon ascent of the Signal Service for meteorological purposes. The ascent was made by Samuel A. King, aeronaut, and George C. Schaeffer jr. as meteorological observer. 1873. In the autumn of this year began the report of observations from. the West Indies. 1875, July 1. On this date began the publication of the bulletins and charts of international meteorological obser- 4 8 METEOROLOGY vations. The first was for the date of January 1, 1875. The series was discontinued at the end of 1887, but the monthly and annual summaries continued to 1889. 1876. Stations were established at St. Michael's and St. Paul's in Alaska. 1881-84. The international polar explorations were begun by the United States in 1881. The Lady Franklin Bay party returned in 1884. 1881, April 11. The movement for the assistance of state weather services was initiated by a letter of this date. 1887. In May the first weather crop bulletin was published. 1887. The marine meteorological service was surrendered to the Hydrographic Office of the Navy. 1888. A beginning was made in the installation of auto- matic barographs and thermographs at stations, thus enabling hourly readings to be made of the principal meteorological elements. 1889. The publication of the Bibliography of Meteorology was begun by the appearance of the first part in this year. 1891, July. The system of Local Forecast Officials was first put in operation. 1892, Spring. The special investigation of the Great Lakes was begun. 1892, Summer. The first systematic study of thunder- storms by the meteorological service. 1892, August. The first meeting of the Association of State Weather Services. 1893. Continuous practice work by all forecasters was introduced ; the competitive idea for filling professorships with accomplished forecasters was adopted ; the Flood Section was reorganised and local predictions placed in the hands of local forecast officials ; the first current chart of the Great Lakes was issued ; the first annual volume in the form ful- filling the international requirements was published. CHAPTEE VI HISTORY, ORGANISATION, AND WORK OF THE UNITED STATES WEATHER BUREAU (continued) 2. Organisation THE various bureaux in Washington City are, with one or two exceptions, directly under a member of the Cabinet. The Weather Bureau is under the Secretary of Agriculture. As in the other bureaux, under their proper Secretaries, he can dictate the policy of the Bureau, and can appoint or dismiss any or all employees, with the exception of the Chief of the Bureau, who is appointed by the President and confirmed by the Senate ; and even in this case the wishes of the Secretary would always receive favourable consideration. While the officers of the Bureau may be changed at the will of the Secretary, as a matter of fact such change is infrequent, except in the force of messengers and labourers. The chief officers of the Bureau are continued with little reference to changes of administration, and in the technical observing force such change is practically unknown. The Chief Signal-Officer, while in charge of the meteoro- logical service, was a brigadier -general in the army and received the compensation of that rank, which, with allow- ances, is about $6000. The Chief of the Weather Bureau, since the transfer, receives a salary of $4500, without allow- ances. The Chief Signal-Officer was accustomed to surround E 50 METEOROLOGY himself, for his personal aides, with officers of lower rank, from major to lieutenant, and the persownel and number varied at his desire. The Chief of the Weather Bureau has no such force of employees to draw upon. He was per- mitted by the law of transfer to select four army officers skilled in the work of the service, to tide him over the change from military to civilian regime. As a matter of fact only three such details existed at any one time, and in 1893 the number was reduced to one. Besides that the Chief of the Weather Bureau is surrounded by his staff of civilian professors, four in number, and these positions are quite permanent. Under the Chief of the Weather Bureau, and aiding him in the control of the entire force in Washington, and outside, are the Assistant Chief, the Chief Clerk, the Disbursing Officer, Property Clerk, and four Inspectors. The Assistant Chief is the alter ego of the Chief, and takes his place and duties during the Chief's absence from the city. He is also Chief of the Forecast Division, having direct charge of the most important work performed by the Bureau. In 1893 the Assistant Chief was Major H. H. C. Dunwoody, the detail from the army already referred to. The Chief Clerk has control of the clerical force in Washington, and also of all questions of personnel, under the Chief of the Bureau, outside of Washington. He has also direct charge of the correspondence and of the files. The salary of the Assistant Chief allowed by law is $3000, that of the Chief Clerk $2250. The Disbursing Clerk of the Bureau has charge of all disbursements and receipts of money, and is responsible directly to the Disbursing Officer of the Department of Agriculture, being thus, more directly than any other officer than the Chief of the Bureau, responsible to the Secretary of Agriculture. The Property Clerk has charge of all property of the Bureau, makes arrangements for purchases and sales when 1 UNITED STATES WEATHER BUREAU 51 necessary, receives property, tests its quality, recommends acceptance or rejection, and has it packed and distributed to the various parts of the United States where it is needed. Every item of property in the entire Bureau, from one corner of the United States to the other, is under the direct charge of this officer, and must be accounted for. The system of book-keeping is so complete that should a question arise as to any individual thermometer, for instance, from among the thousands that have been issued for use, by consulting his books it can be ascertained exactly where it is and in whose charge it is, and the receipt for the same can be found. The Inspectors are charged with the duty of personally supervising the stations' outfit, their personnel, and their work. They visit stations at regular intervals to ascertain if the property is in perfect order, if the instruments are in good condition, and if the work is properly performed. They are also sent to stations when it is necessary to have any special inspection performed, for purposes of discipline, or for any other reason. The appropriations for the Bureau are made annually by Congress, and are a part of the appropriations for the Depart- ment of Agriculture. An estimate is carefully made by the officers of the Bureau some months before the session of Con- gress in which the appropriation must be made, and about a year before the appropriation can become available. This estimate is submitted to the Secretary of Agriculture, and after receiving his approval passes to the Committee on Appropriations of the House of Representatives. On receiv- ing their approval it is submitted to Congress. Among the various divisions of the central office at Washington stands foremost the Forecast Division. It has charge of forecasts, of floods, of the telegraphic section, of storm signals, and of the practice which is continuously per- formed by the forecasters. The forecasts are made twice a day, immediately on receipt of the telegraphic reports of 5 2 METEOROLOGY observations from the regular telegraphic stations. As soon as the forecasts are made, the maps are printed in the Bureau office, and the forecasts given to the Associated and United Press Companies, by which they are distributed over the United States. Each forecast contains statements concerning the weather for divisions of the United States, each division being usually a State, or a large part of the State. The fore- cast officials on duty at Washington are kept in constant practice. They generally have the rank of professor. Besides this practice, which occupies a fractional part of the day, each professor is entrusted with other and important duties. One has charge of the instrument-room and of all duties relating to instruments. Another has charge of the Monthly Weather Review, which he edits, and also of a great variety of duties relating to theoretical and scientific meteorology. Another has charge of the collection of statistics concerning tornadoes and other destructive storms. And the fourth is entrusted with the special investigation of the relation of meteorology to magnetism. The telegraph section under this division has a force of operators who receive and send telegrams, and have charge of the various coast telegraph lines belonging to the Weather Bureau. The State Weather Service Division is in charge of the weather-crop work, the thunder-storm work, the distribution of temperature and weather signals, and the snow charts of the Bureau. In general its work is essentially climatological. It is the centre of the State Weather Services scattered over the United States. It receives weekly during the crop season the weather-crop reports from the state centres, and digests them into the Weather-Crop Bulletin, which is im- mediately sent to press, and appears ready for distribution the day the reports are received. It prepares and sends to the state centres, for distribution from those points, the signals for temperature and weather which are intended in general for the agricultural communities ; and in a manner similar to the weather -crop bulletins, during the winter UNITED STATES WEATHER BUREAU 53 season it collects the data for snow charts, and has the charts printed and distributed the day the data are collected. It is also in charge of the special thunder-storm work, this work being done through the machinery of the State Weather Services. The Records Division is entrusted with the care of the records, with the compilation of data of all sorts required for the work of the Bureau or by the general public, and with the publication of reports, more especially the annual reports made for general information. The accumulation of records in charge of this division has, after over twenty years of work, become extremely great, and includes not only the records of the meteorological service which finally ended in the Weather Bureau, but also of that meteorological service which was carried on previously by the Smithsonian Institution, and that also by the Surgeon-General, which to some extent preceded that of the Smithsonian. These records are kept in a fire- proof vault in such form as to be readily accessible. Other private records have been added to these, either by purchase or gift, until the collection forms by far the most complete record of climatological interest to be found in the United States so complete, in fact, that its use is entirely indispensable to any one who wishes to make a competent study of any feature of the weather or climate of the States. In the compilation of data for the general public a great deal of time is spent by the Records Division. All sorts of questions relating to all sorts of features of the weather and climate come constantly to the Bureau. The replies must be made with very great care, so as to be thoroughly authentic. No less important is its duty in checking observations and in detection of errors. The system is so complete that the errors are charged up against the individual observers, and at regular times a statement is issued giving the names of the observers who have been the most free from errors in the preceding interval. To the Instrument Section is entrusted the question of 54 METEOROLOGY purchase and shipment of instruments, their test when received, their condition at stations, their repairs when needed, and the examination of the automatic records as they are received from the various stations. It is also occupied with devising new forms of instruments and new methods of taking observations, and performs a large amount of work in physics, more or less directly connected with this purpose. The Publications Division is in charge of the publishing and mailing of the material of the Weather Bureau. Those matters which are urgent are published in the Bureau office. In addition to that there are a few other publications made by the Bureau office which are not urgent, but are used to fill up the intervals of time on the part of the compositors and pressmen. The most of the other matter issued by the Bureau is published by the Government Printing Office at a fixed price, the same being deducted from the appropriation for the Bureau. In a few special cases the publications of the Bureau are made by joint-resolution of Congress, in which case there is no charge against the Appropriations of the Bureau. The publications made in the Bureau office are the maps of all sorts, the reports requiring immediate distri- bution, and special publications of the same sort. Of the other publications, not matters of so much urgency but actually published in the Bureau, are many of the innumerable " forms " used in the collection and distribution of data, the Monthly Weather Review, which is prepared and printed entirely within the chief office at Washington, and some of the series of printed bulletins in octavo form issued by the Bureau. Also lithographed maps and charts, though not urgent, are usually printed by the Bureau. This division has in its charge a draughting-room, in which maps are pre- pared for printing and other necessary drawings are made ; and the composing-room, in which a considerable number of printers are employed. In this division labour is saved in all possible ways, the most notable one, perhaps, being that of UNITED STATES WEATHER BUREAU 55 the use of a long series of logographs. In the publication of weather tables and forms the same combination of letters and figures frequently recurs, and in this case a single type has been cast to include these letters and figures. In this division is also included the press-room. A large part of the work of the Bureau is lithographed, because the litho- graph makes the cheapest, easiest, and readiest means of publishing urgent data in chartographic form. As a result a force of lithographers is kept in connection with the press-room. Also connected with the division are the folding and stitching room and the mailing-room. The mailing lists are kept with care, in order to economise the publications of the Bureau. Connected with the general office at Washington is also a library, containing a number of books and pamphlets as shown by the accompanying Table, so arranged as to show the annual growth. The library also contains many meteoro- logical and some geographical charts. These books are obtained in considerable part by exchange with other Meteorological Services and with various Govern- ments, in part also by gift, but in large part by actual purchase. The result is a technical and special library of unusual size and value. So large and complete is it, that with the aid of correspondents from all over the world, the librarian has undertaken to publish a bibliography of meteorology. In 1884 the Signal Office began the compilation from the printed literature comprising the books, pamphlets, memoirs, and papers in serial publications of all kinds, relating to meteorology and its applications. In 1887 the number of titles collected and classified was about 50,000. Since this time annual additions have been made, and the references have been brought down to January of 1892. The collection at the end of 1893 comprises about 65,000 titles. From 1889 to 1891 portions of the bibliography were lithographed and milliographed. METEOROLOGY BOOKS AND PAMPHLETS ADDED TO THE LIBRARY OF THE WEATHER BUREAU FROM 1873 to DECEMBER 1, 1893 Fiscal Year ending June 30. No. of Volumes. Gain. No. of Pamphlets. Gain. 1873 2,470 230, 1874 ... ) 542 r 240 1875 3,012^ 470< 1876 1877 1878 1879 3,310J 3,632J 3,82l| 4,149J 298 322 189 328 589J 674J 740J 822 1 119 85 66 82 1880 4,425J 276 889J 67 1881 4,752J 327 958^ 69 I 1827 1882 6,579= 1174 1883 7,753' 963 1884 8,716^ 1027 1885 9,743: I 797 1886 10,540^ I 565 1887 i 9,845^ I 475 1888 10,320^ I 791 1889 11,111% 2500, \ 421 150 1890 2 11,532< 2650^ 1891 12,482< 950 3000J 350 1892 3 12,742J 760 3345 1 545 1893 13,912J 1170 3998J 653 Dec. 1, 1893 14,301^ 389 4640^ 642 1 1260 volumes transferred to the War Department Library. 2 375 volumes discarded. ^ 1 2 pamphlets relatin S to military signalling transferred to UNITED STATES WEATHER BUREAU 57 The following Table gives statistics relating to these portions : When issued. No. of titles. No. in library of Weather Bureau. Percentage in library of Weather Bureau. Number of authors. Part I. Temperature ,, II. Moisture ,, III. Winds ,, IV. Storms 1889 1889 1891 1891 4.400 5^500 2,000 4,300 2100 2435 1100 2380 47 44 55 54 1800 2650 960 1647 Total 16,200 8015 50 In addition to these various sections of the Bureau there is a large mass of correspondence to be cared for. This comes under the direct cognizance of the Chief Clerk, who has a small force of clerks under him to perform this work. The letters received are assigned to the various divisions or individuals most competent to answer them. The replies are drawn up by these divisions or individuals, with the aid of stenographers, and sent to the Chief Clerk for supervision or signature before being mailed. Correspondence, manuscripts, and other papers of importance are passed over to a special official called the File Clerk, who has entire charge of the files of the Bureau, and whose duty it is to speedily find any paper of any date required by any officer, and to return it to its place when it comes back to him. Especial attention is also paid to comments and criticisms of the Bureau from whatever source they come, and to this part of the time of one clerk is devoted. The observers at stations are instructed to send to the central office all comments on the Bureau, and especially criticisms of it. This serves the double purpose of keeping the Bureau in close touch with the popular wants, and of informing the central office of the way in which these wants are filled at the stations. These matters are kept filed in such a way as to be of ready for reference. 5 8 METEOROLOGY There is also connected with the office in Washington a considerable number of labourers and messengers, who perform the various duties of this sort which may arise. They are under a Captain of the Watch. An important duty, and one which takes a considerable number of persons, is the distribu- tion of the weather maps, when printed, to the various public offices and other places in the city of Washington where they are of use. In the scientific and clerical force at Washington 107 persons are employed. In the force relative to the buildings and grounds, including labourers, there are 44 persons, and in the publications force, 32 'persons, making in Washington a total of 183 persons employed, with annual salaries ranging from that of the Chief of the Bureau to that of the lowest labourer, at $300 per annum, and that of charwoman at $240. The principal part of this force is in the clerical grade, receiving salaries which range from $720 to $1800. What precedes relates only to the central office in Washing- ton City. There is also a large number of employees scattered at numerous stations over the entire United States. These stations are : Eegular telegraphic stations, stations in the West Indies (excluding stations in Canada, with which we carry on only an exchange, these stations being controlled by the Canadian Service), river and flood reporting stations, voluntary stations, mountain stations, telegraph line repair stations, , storm signal stations, temperature and weather stations, and stations in the cotton, rice, and sugar regions ; also stations for special reports of thunder-storms and others. The accompanying Table (I.) shows the number of reporting stations of the meteorological service of the United States for the different years since the establishment of this service. In the same Table is the number of State Weather Services, to which reference will be made hereafter. UNITED STATES WEATHER BUREAU 59 TABLE I REPORTING STATIONS OF THE UNITED STATES METEOROLOGICAL SERVICE TELEGRAPH. EH | 1 b 1 YEAR. 3 . | S cc if O> s| "C m If 1 a 3 || .2 ?! 2^ 002 a g JH O M^ s o o ft o 1 ^ 1871 55 1 1 1872 65 7 1 1 1873 71 11 4 39 3 1874 86 15 6 40 332 2 1875 88 15 6 40 1 327 2 1876 ISO 1 15 6 43 1 316 2 1877 123 12 43 1 294 2 1878 146 12 1 51 1 277 2 1879 160 12 1 49 1 25] 2 1880 172 16 1 49 1 245 2 1881 155 17 1 49 2 225 2 1882 152 18 1 50 8 198 2 1883 139 17 1 58 8 253 2 1884 147 12 1 71 13 286 2 1885 160 25 1 96 16 311 2 1886 160 24 1 96 19 307 2 1887 145 23 1 96 19 289 2 1888 145 20 1 92 24 1069 1 1889 145 20 1 85 26 1679 1890 147 20 4 118 28 1924 1891 152 20 4 145 30 2028 1892 148 20 6 177 42 2180 2 1893 145 21 5 209 42 2367 2 1 Includes 38 Military Telegraph Stations. The regular stations of the meteorological service are those from which telegraphic reports are received now twice, formerly thrice, daily. They may consist of a larger or smaller number of officials and employees, varying from one to seven or eight. Where the number is larger it consists of a forecast official in charge, assistant observers under him, those occupied in making the maps, assistants, and messengers. The salaries at these stations range from that of the local forecast official, 60 METEOROLOGY $1500, to that of the messenger, which may be as low as $360 or $300 per year. They have also a regular outfit of instruments, consisting of barometer, thermometer, anemo- meter, and psychrometer at all of the stations, and in addition to these, at more or less of the stations there is a series of self- registering instruments, barograph, thermograph, complete anemograph, pluviograph, sunshine recorder, and possibly others in special cases. They also usually have a small library, consisting of the publications of the Bureau and a few other practical works intended to aid in self-training, and also in making proper and suitable replies to questions addressed to them by the public. At these stations every possible use is made of the telephone and the telegraph, and other means of rapid communication. They are generally located in public buildings, when such buildings exist in the city in which they are placed. In fact, the General Statutes contemplate in the construction of new public buildings the leaving of space for the Weather Bureau, as well as for other Government services which require to be represented in the city in question. These stations publish, in many cases, weather maps by a special process called the milliograph process, a rapid method for turning out fair specimens of map-making, the drawing and printing, folding and mailing, being all done by the employees at the station in question. In fact, many of these larger stations outside of Washington are small central stations, something like that in the city of Washington, and intended for the distribution of meteoro- logical information in their vicinity, as Washington station is intended for the distribution of this information over the United States in general. A list of regular Weather Bureau stations, alphabetically arranged, will be found in Appendix I. Next come the State Weather Services, the centre for these services being generally in the capital of the state. In column 6 of Table I. will be found the number of State Weather Services acting in connection with the general UNITED STATES WEATHER BUREAU 61 meteorological service of the United States. They are usually in close touch with the local state bodies interested in the work of the Bureau. They are frequently called on to make representations before the State Legislatures. They are nearly always in close contact with State Boards of Agricul- ture, and generally some member of the Board is one of the officials of the State Weather Service. These stations are centres for the voluntary observers of the states in which they are placed. From them are distributed the materials and information required by the voluntary observers, and to them are sent by these voluntary observers the meteorological and crop reports which they make at regular intervals. The voluntary stations are furnished with the instruments neces- sary for measuring temperature and precipitation, and are ex- pected also to report on winds and clouds. They take careful observations of the state of the crops, and from week to week this information is telegraphed by them to the central station for the state. It may then be printed by the State Weather Service by counties and distributed in the state ; but in any case it is condensed in a telegram which is sent to the central office at Washington, which telegram is employed in the con- struction of the Weather-Crop Bulletin issued weekly during the crop season by that office. The State Weather Service centre is also the centre for the distribution of weather and temperature signals intended for the information of the country districts, and more especially for farmers. Column 7 of Table I. shows the number of voluntary stations in the United States for each year of the service. It will be observed that the number in 1893 was nearly 2400. It is the aim of the Bureau to distribute these stations with as great uniformity as possible over the entire United States, and although in some regions where there is a special interest in climate, as in the vicinity of Boston, Los Angeles, and San Francisco, the stations are more close together than elsewhere, yet it is the aim of the service to distribute them at distances of from twenty to fifty miles apart only. 62 METEOROLOGY The State Weather Services are due to the need of more detailed information concerning the meteorological elements of importance to crops and their effects on the current con- dition of the crops. Some of the States had independent services of their own, supported out of the State treasuries, as Iowa and Michigan. They had an independent corps of observers, and the endeavour of the national service to extend and complete its work brought it occasionally into conflict with the state services and caused duplication of work. This at an early date gave rise to the idea that the national and state services should join forces for harmonious and economical work. The movement was officially initiated by a circular letter by General Hazen proposing such a union, dated April 11, 1881. The movement at first met with some opposition, but this has been allayed, until now there are forty-two of such services. They cover the entire territory of the United States, except Alaska. The difference between the number of these services (42) and the total number of states and territories (49) is due to the combination of the six New England States into one service and the incor- poration of Delaware into the Maryland Service. The active executive officer of these services, or his first assistant, is always an employee of the Bureau, and the best of these services are aided or supported by funds from the state treasuries. It is probable that such a service will soon be formed in Alaska, so that the entire area of the United States will be covered by this very efficient means of in- forming the public of the effect of the current weather on crops, and of collecting valuable climatological data. The observers of the State Weather Services and the crop correspondents are volunteers and serve without pay. As part compensation for their disinterested services they are furnished with the publications of the Bureau. These observers represent the intelligent interest in the work of the Bureau in their respective localities. They are often professional men, well-informed farmers, men who have UNITED STATES WEATHER BUREAU 63 retired from active business, or such Government employees as postmasters, and it is in a spirit of public enterprise that they perform, often for a long series of years, the work that the Bureau desires. There is no difficulty in finding such observers in the more thickly populated parts of the country, whether old or new, but in the thinly populated districts as the arid regions these stations are yet sparsely scattered. In Alaska the observers are generally missionaries. The materials obtained by these observers are used for state publications, as well as for those of the national service. The whole makes a federal union of weather services, reminding one of the Federal union of the States. The River and Flood Section, under the Forecast Division, has especial charge of the river gauges, which are scattered at frequent intervals up and down the principal rivers, from which reports are received at regular intervals. The number of these stations will be found in column 5 of Table I. The duty of the River and Flood Section is the forecasting of the conditions of the rivers, more especially during the period of flood. This work has heretofore been entirely done in the central office at Washington and was in the hands of one officer, who was alone entitled to make forecasts con- cerning the state of the rivers. It has been found, however, that it is impracticable for a single officer to obtain the intimate familiarity of, and keep in mind the current knowledge of, all the details that are necessary for safe forecasting the entire length of important rivers. There are so many local conditions the width of the river, opportunity for set-back so many conditions that may happen accident- ally, as when a levee breaks, or when at a certain height the river may pour over the banks into an empty space at one side that it has been found necessary to divide up the work among a considerable number of men, who are stationed in the vicinity of the places where the forecasts are to be made. These men become familiar with the details mentioned above, and hence can perform the work more satisfactorily. This 64 METEOROLOGY policy has been introduced since the season of the floods of 1893, and has not yet been sufficiently tested. It is con- fidently expected, however, that its success will be much more considerable than has been experienced heretofore. The cotton, rice, sugar, and other special services are in- tended for the protection of specific crops grown in limited areas. Special reporters are scattered through these regions, who report with special reference to the climatological needs of these individual crops. For the cotton interest this service has been found to be quite successful. There has not been so good an opportunity to test it for the rice and sugar interests. Concerning the relations of the Bureau with the public more in detail, it may be said that the forecasts made at the city of Washington, and at the other stations throughout the United States where such forecasts are made, eighty-three in number, are placed at once in the possession of the news- gathering agencies in that vicinity and distributed throughout the entire region interested. The number of distributing centres annually is given in Table II., on the next page. The forecasts made in the evening at Washington, or else- where, will therefore appear in the morning papers, and the forecasts made from the morning observations will appear in the afternoon and evening papers. Moreover, from Washing- ton and from the various stations where maps are made, the maps are distributed with a free hand to all interests that find them useful. The edition actually employed will be found stated in Table III., on page 66. The rule governing their distribution is, that they shall go only where they will be of general public interest, or to persons who are making a special study of meteorology. The result is, that for each map issued a considerable number of the public probably receive the information that they require. A certain number of newspapers have also reproduced a map on a small scale in their pages for a longer or shorter time. This is done by means of what is UNITED STATES WEATHER BUREAU TABLE II DISTRIBUTING STATIONS, UNITED STATES METEOROLOGICAL SERVICE Year. Storm Signals. Weather and Temperature. Frost. Cotton Region. Stations making forecasts (Weather and Temperature). Station issuing Weather Maps. 1871 1872 19 1873 25 ... 1874 42 ... 1875 43 ... ... 1876 48 ... 1877 44 1878 90 1879 94 ... 1880 102 1881 116 1882 120 ... 144 1883 117 131 1884 110 ... 150 1885 109 21 831 155 1886 116 216 801 147 1887 115 607 788 145 7 1888 118 1055 795 128 37 1889 110 783 840 114 16 30 1890 118 828 812 126 71 37 1891 122 1646 871 121 79 44 1892 124 1888 492 1 125 85 61 1893 156 1613 458 125 83 72 called a " chalk plate," a metal plate with a layer of chalk, on which can be drawn with a needle the outlines of the map. This serves long enough to take a stereotype from, and the stereotype can be inserted in the columns of the newspapers. This particular part of the service depends very much upon opportunities which cannot be influenced by the Bureau largely due to local newspaper rivalry. It may perhaps be said with a fair approximation to truth that there are generally many hundreds of thousands, sometimes millions of copies of the daily weather maps made in the United 1 Reduction due to discontinuance of frost warnings addressed to Operator. F 66 METEOROLOGY OS 61 I l co 05 O(N^T-l- r-l O ft O O C<1 CO CO Oi K3 CO 1> CO O O ^ SO O CM t^OSOiOOOCOOiO lO?ot^c005OrH East Bloomfield N.E. of Nashville Gallatin 24 18 26? Aurora King Carnival Buffalo June 19, 1877 Gallatin Near Lebanon 13? J J Aug. 30, 1877 Sept. 12, 1881 - Oct. 13, 14, 1881 Philadelphia Minneapolis, Minn. Chicago Centreville, N.J. Cow pasture Flambeau R.,Wis. 5 430 Great North-west }> Jan. 19, 1885 Philadelphia Manhattan, N.J. 60 Eagle Eyrie March 13, 1885 j j Union, Pa. 16 )> March 27, 1885 Newby, N.J. 70 jj April 16, 1885 Williamstown, N.J 23 J J June 24, 1886 Providence, R.I. Voluntown, Ct. City of Boston June 25, 1886 W. Greenwich, R.I. Providence 55 j ) June 17, 1887 St. Lewis Hoffman, Ills. 55 "The World" Aug. 13, 1887 Oct. 27, 1892 Philadelphia Fort Myer, Va. Philadelphia Suitland, Md. 40 9 Great North-west Carlotta There have been some studies of spectroscopy and its relations to practical meteorology. Professor Upton, under General Hazen, made a special study of the spectroscope, which was printed as a bulletin. Later, Dr. Jewell, under the direction of Professor Rowland of Johns Hopkins Uni- versity, has made an elaborate study of some new aspects of this problem, which give promise of success in its practical application to forecasting. A paper on this subject is nearly ready for printing. The subject of the radiation from gases is another of the problems in which the Bureau has interested competent students outside of its ranks. In climatology the work of the Bureau has been very extensive. A series of climatological monographs has been published on the Western States, on the arid regions, and on the States occupying the great plains. The Monthly Weather Review, which has appeared continuously for so many years, has a number of contributions on the subject UNITED STATES WEATHER BUREAU o; BALLOON ASCENTS. 91 Size. Men. Height. Temperature. Time. Observer. Aeronaut. Start. Top. Start. Land- ing. o 20,000 2 5,560 59? 40 17-4 18-21 G. C. Schaeffer, Jr. S. A. King. 25,000 2 5,040 59 47 15-00 17-25 A. C. Ford. 92,000 2 6,580 87 67 17-3 19-18 3? J5 92,000 2 16,200? \ ] 7-58 . 12-2 3 9 )) 92,000 7 8,600 86 59 14-59 17-30 F. M. M. Beall. 90,000 7 2,600 74? 18-43 20-00? Winslow Upton. 90,000 2 8,640 43? 17-25? 14-15 G. Hass Hagan. (13th) (14th) 25,000 2 4,800 23 13 16-12 19-5 "W. H. Hammon. 25,000 2 4,350 24 23 13-38 16-12 ? ? ? 9 25,000 2 6,217 51 35 12-27 14-19 >? 3 > 25,000 9 4,310 50 33 12-31 14-20 5 j 5 j 40,000 4 1,000 H A. Hazen. James Allen. 40,000 2 9,780 61 48 7-50 9-39 > 5} ?> 160,000 4 15,410 92 36 17-26 20-17 3 ? J Alfred Moore. 90,000 7 6,940 75 52 16-38 20-36 ) )) S. A. King. 9,000 2 9,400 43 13 15-33 17-5 ) )) John Ellis. of climatology. Eainfall has also occupied the attention of the Bureau to a very great degree. The rainfall maps are published regularly in the Monthly Weather JZeview, and many special studies have been printed in atlas form, or otherwise. A special climatological section was organised and continued during the fiscal year of 1893, and was devoted to the subject of precipitation over the entire United States. The material is now in process of preparation for printing, and will be the most complete contribution ever made on this subject. Hourly observations, since they began, have been the subject of several discussions. One relates more especially to the corrections which must be made in temperature observations taken at any hour of the day to get the true mean temperature of the day. Another relates to the diurnal variations of the barometer, and the conclusions which can be drawn from them. The work of the meteoro- logical service has been in some sense brought together 9 2 METEOROLOGY and condensed in General Greely's American Weather. The more interesting meteorological and climatological features of the United States discovered up to that time are mentioned therein. There have also been some climatological studies of individual regions. A party was stationed for some time in Death Valley, California, and the results of their work have been published in a climatological bulletin on that subject. Continuous meteorological work has been carried on over the Great Lakes for two years. The results so far have been the publication of a map of wrecks by meteorological causes on the Great Lakes, and of a preliminary current map of the same Lakes. The climatology of Chicago was compiled, and has been recently printed. In the matter of forecasts and their verification, a great deal has been printed, both in the annual reports of the Chief Signal - Officer and in other publications. The subject of cold waves has received an especial amount of attention. As to the meteorological side of agricultural science, the studies of the service have been numerous, but have been for the most part made under the Weather Bureau rather than under the Signal Service. Two bulletins on the relations of soil to climate, from competent students of the subject, have been printed, and a paper on the variations of ground water and its relation to meteorology makes another bulletin. A paper has been also printed on the climatology of the cotton plant, and one on the tobacco plant is in process of preparation. A large bulletin has been compiled on the relation of climate to crops, and is ready for publication. It embraces the more important studies that have been made on this subject up to the date of its publication. It may be well to add that the official experiments in rain- making which attracted so much attention for a year [or more, were not made by the Weather Bureau, nor in any way under its direction. The Bureau offered to send experts with the rain-making expeditions, but the expert charged with the duty of conducting them did not see fit to avail himself UNITED STATES WEATHER BUREAU 93 of this offer. The movement in favour of these experiments originated in certain enthusiasts not connected with the Government service, and was brought directly to Congress without the intermediation of the executive departments. The appropriations were made, and the Secretary of Agri- culture, without solicitation on his part, was charged with the duty of expending them for the purpose intended. He called to his aid Mr. Dyrenforth, a patent attorney, who conducted the experiments, and reported on the results. Mr. Dyrenforth reported a considerable measure of success, but unfortunately in no case did the report of unbiased experts agree with his. In the first summer's work he had with him an accomplished meteorologist, but this gentleman's report was adverse to him, and he was discarded. In the second summer it was only those experts who got knowledge of his intentions in time, and without his invitation, who were able to report and this generally under unfavourable conditions and again their reports were unfavourable. Not only did Mr. Dyrenforth not settle the matter experimentally, but he did not leave sufficient evidence in its favour to justify a conservative administration of public affairs in trying again the experiment of making rain by explosions. CHAPTEE IX AIR TEMPERATURE AND ITS MEASUREMENT Temperature the most important Meteorological Factor Meaning of the word Thermometer History of the Instrument Fahrenheit's Scale Why Mercury was selected as the Medium for measuring Tempera- ture Celsius' Scale The Centigrade Thermometer Reaumur's Scale Relations of the three Scales to one another Rules for reducing Readings of one Scale to those of another Melting Point of Ice Freezing Point of Water Boiling Point of Water varies with Atmo- spheric Pressure and with Altitude Definition of a Thermometer Steps in the Construction of this Instrument. No other meteorological factor exercises a more potent in- fluence over the animal and vegetable kingdoms than does temperature. Hence we may fitly commence our study of the weather with an account of the instrument which is in daily use for the purpose of measuring the ivarmth of the air, namely, the thermometer (Gk. #e/>p7, heat; pcrpov, a measure). The principle of the instrument is that it measures temper- ature by the expansion of bodies. The thermometer is supposed to have been invented by Sarictorio, of Padua, in 1590; but the history of the instru- ment is involved in obscurity until 1714, when Fahrenheit, of Dantzic, constructed the thermometer which bears his name. He used mercury instead of spirit of wine, and introduced a scale graduated from the melting point of ice to the boiling point of water at sea-level, and under ordinary conditions of temperature and pressure. Fahrenheit arrived at his zero AIR TEMPERATURE AND ITS MEASUREMENT 95 point in a curious way. He is believed to have held that for all practical purposes the lowest temperature likely to call for registration was that reached in an Icelandic winter. It is, however, more likely that his zero was fixed by experi- ment with a freezing mixture of snow and chloride of sodium (common salt) or of snow and chloride of ammonium (sal ammoniac). Counting then from zero, Fahrenheit made the melting point of ice 32 and the boiling point of water 212, thus dividing the distance between these two crucial points into 180, or half the number of degrees in a circle (360). This scale is commonly used throughout the British Dominions and in the United States of America. Its advantages are that, under ordinary circumstances, the minus sign ( ) is not required, while the smallness of the degrees permits very accurate measurements of temperature, Mercury was selected as the medium for indicating tem- perature, because (1) of its equal expansion at different temperatures equal increments of bulk corresponding to equal increments of temperature ; (2) of its low freezing point (-37-9 F.), its high boiling point (675T F.), its high conductivity of heat, and its low specific heat, or " the amount of heat required to raise 1 Ib. of mercury one degree, in terms of that necessary to raise 1 Ib. of water one degree " (R. H. Scott). For the recording of temperatures below its freezing point, mercury is replaced by spirit, which medium is also used because of its transparency in the construction of the minimum thermometer in common use. In this instru- ment a small and light float of glass or enamel, called the "index," is immersed in the spirit. Hence it is necessary that the spirit should be transparent in order that the exact position of the index may be observed. In 1742, Celsius, a professor in the University of Upsala, in Sweden, divided the scale of the mercurial thermometer between the melting point of ice and the boiling point of water into 100. According to this scale, therefore, the 96 METEOROLOGY melting point of ice is zero, and the boiling point of water is 100 at an atmospherical pressure of 760 millimetres (29'922 inches) in the latitude of Paris. Hence the name Centigrade, by which this thermometer is usually known. It is extensively used on the continent of Europe, and, indeed, by scientific men of all nations. A third thermometer scale is that introduced about 1731 by Reaumur, a French physicist, who was born at La Roch- elle in 1683. According to this scale there are only 80 between zero, the melting point of ice, and the boiling point of water. The Reaumur scale is still used in Russia and parts of Germany, but the Celsius or Centigrade thermometer scale is rapidly superseding it in those countries. The following figures, copied from Chambers' s Encyclopedia (Art. " Thermometer "), will render the relations of these three scales easily understood : Fahrenheit 32 77 122 212 Reaumur . . 20 40 80 Centigrade . . 25 50 100 To reduce a reading taken by one scale to the correspond- ing reading taken by another becomes quite easy, if we remember the primary relations of the scales to one another, namely, 80 R. = 100 C. = 180 F. ; or in the most simple form : 4 R. = 5 C. = 9 F. The only element of difficulty is the management of 32 F., which corresponds to the zero of the other scales. It is quite clear that when we are re- ducing Fahrenheit to Centigrade or Reaumur, we must take away, or subtract, 32 from the result ; whereas in reducing either of the other scales to Fahrenheit, we must add 32 to the result. The following proportional statements may be of use. 1. For Fahrenheit's thermometer : F. : R. : : 180 : 80, i. e. 9 : 4. Therefore F. = ^- + 32 F. : C. : : 180 : 100, i.e. 9 : 5. Therefore F. =^5i + 32 5 AIR TEMPERATURE AND ITS MEASUREMENT 97 2. For Celsius' thermometer : C. : F. : : 100 : 180, i.e. 5 : 9. Therefore C. = ( F '- 32 ) x5 y tTJ C. : R. : : 100 : 80, i.e. 5 : 4. Therefore 0.=^ 3. For Reaumur's thermometer : K. : F. : : 80 : 180, i.e. 4 : 9. Therefore R> = ( F - ~ 32 ) x 4 y R. : C. : : 80 : 100, i.e. 4 : 5. Therefore R. = ~ 5 In speaking of the fixed points on the thermometer scale it will be observed that the expressions used have been " the melting point of ice " and " the boiling point of water." The melting point of ice is exactly 32 F., and of both the Centigrade and Reaumur scales. The phrase is used in preference to the "freezing point of water," because water, if perfectly still, may be chilled several degrees below 32 F. without freezing. Under such circumstances, if it is sud- denly agitated, it will congeal instantly. Again, water which holds a salt in solution has a freezing point considerably below 32 F. Only distilled water is admissible in these delicate experiments. The boiling point of water is a still more variable quantity than the freezing point, and hence the term must be qualified by the addition of the words "at mean sea-level, the baro- meter standing at 29*905 inches in the latitude of London." The French standard of pressure already referred to is 760 mm. in the latitude of Paris. Ebullition takes place the moment the tension of the vapour of water equals the atmospheric pressure. It is evident that the lower the pressure, the more easily will vapour escape from heated water, giving rise to the phenomenon known as " ebullition " or boiling. The converse is equally true, and under a pres- sure of fifty atmospheres the boiling point of water is raised to 510 F. In fact, water can be heated to almost any degree H 98 METEOROLOGY without boiling, provided it is subjected to a sufficient pressure. As a matter of fact, the boiling point falls one degree of Fahrenheit's scale for every 0*589 inch of barometric fall at moderate heights. Accordingly, should the barometer fall to 27'549 inches at sea-level and this actually occurred on January 24, 1884, and more recently on December 8, 1886 the boiling point would be 208 instead of 212. A still more striking variation in the boiling point occurs at great elevations. For instance, at Quito, in Mexico, 9000 feet above the sea, water boils at 194 ; on the summit of Mont Blanc (15,781 feet) at 183 F. ; and on Gaurisankar, or Mount Everest, the highest peak of the Himalayas (29,002 feet), it would boil at 158 F., were it possible to make the experiment. Advantage has actually been taken of this dependence of the boiling point on elevation to roughly measure the heights of mountains. Mr. R. Strachan, F.K. Met. Soc., suggests a simple rule for ascertaining the relative elevation of two stations. It is to multiply by 9 the difference in barometrical readings between them, taken in hundredths of an inch. The result gives the difference in feet between the stations. This rule depends on the prin- ciple that the difference of height corresponding to a differ- ence in barometrical readings of 0*1 inch is approximately 90 feet. For instance, for the change of level from 200 to 290 feet, the difference in barometrical readings (the sea-level reading being 30 inches) is '101 inch at 40 and -098 inch at 50 (R. H. Scott). In the case of the Centigrade and Reaumur scales, all tem- peratures below the melting point of ice have a minus sign ( - ) prefixed. In the case of the Fahrenheit scale the zero point is 32 below the melting point of ice. The minus sign is, therefore, very seldom required for temperatures occurring in the British Isles. The value - 40 represents the same temperature on the Fahrenheit and Centigrade scales. A thermometer consists of a capillary glass tube of AIR TEMPERATURE AND ITS MEASUREMENT 99 uniform bore, hermetically sealed at one end, and blown at the other into a bulb filled with mercury or spirit. Any given temperature is measured by the amount of expansion undergone by the liquid in the bulb when exposed to that temperature. The liquid moves up the capillary tube from the bulb as temperature rises, and retreats towards the bulb as temperature falls. In all cases, the scale of degrees by which the readings are made should be engraved on the glass tube itself, that is, on the stem. The steps in the construction of a thermometer are four in number: (1) calibrating the tube; (2) filling; (3) "curing"; (4) graduation. (1) Uniformity of calibre is attained in glass-blowing by introducing a bead of mercury, say an inch in length, and noting by measurement if this thread of the liquid metal occupies the same extent of the tube at different points. (2) The thermometer is filled, after blowing a bulb at the bottom, by filling a small funnel at the top with mercury and then expelling the air in part from the bulb by heat. As the bulb cools, a vacuum is formed, to fill up which some of the mercury slips back from the funnel into the bulb. The process is repeated until the bulb is quite filled with mercury. (3) " Curing " is referred to afterwards. The process con- sists in laying the instrument aside for a year or so after filling, so that the glass may assume a permanent shape, or " season," and so obviate the error known as " displacement of zero." (4) Graduation is the marking of the scale on the ther- mometer stem, the fixed points of temperature of melting ice (32 F.), and of boiling water at a pressure of 29'905 inches (212 F.) being duly ascertained by direct experiment in each case. CHAPTEE X THERMOMETERS Standard Thermometers Ordinary Thermometers "Displacement of Zero " Registering Thermometers Phillips's Maximum Thermo- meter Negretti and Zambra's Maxinram Thermometer Rutherford's Minimum Thermometer Objections to Spirit Thermometers Six's combined Maximum and Minimum Thermometer Casella's mer- curial Minimum Thermometer Exposure of Instruments Steven- son's Thermometer Stand and Screen The " Wall Screen " Method of Reading the Instruments The Sling Thermometer (Thermometre fronde) Self-recording Thermometers, or Thermographs Electrical and Photographic Thermographs Radiation Thermometers Mean Temperature Average Mean Temperature. THE thermometers used in meteorological observatories are : standard thermometers, ordinary thermometers, registering thermometers, self-recording thermometers, and radiation thermometers. 1. A standard thermometer is made with every precaution to secure accuracy. It is not intended for daily use, but only for testing from time to time the correctness of the ordinary thermometers with which observations are made. The scale should not be cut on the stem for several years after the carefully selected tube and bulb have been filled. This delay is necessary in order to guard against the defect called the " displacement of zero," by which a thermometer is made to read too high. This defect arises from the gradual contrac- tion of the bulb which results from the slowness with which fused glass returns to its original density. Of course, as the THERMOME TERS 101 bulb contracts, it holds less mercury," which is forced into the tube to a higher level than the temperature warrants. Except for use in extremely cold climates, a standard thermometer should be made with mercury, because of its uniform rate of expansion. Its scale should range from far below zero to the boiling point of water. 2. Ordinary thermometers should be constructed of mer- cury. They should be scaled from - 40 to 110 or 120. In the British Isles a range from - 15 or - 10 to 100 is ample. In every case, an ordinary thermometer should carry with it a certificate of verification at Kew Observatory, or other recognised scientific insti- tution. At least once a year each instrument should be tested for the " displacement of zero," by being plunged into a mass of melting snow or ice. The Kew Observatory Thermometer (Fig. 1) is an excellent instrument, particularly adapted for taking reliable observations at sea, as it is proof against the corrosive action of salt water and of damp, and is protected by a copper case. This thermometer is 12 inches long, with the degrees etched on the stem and the figures indelibly burned on the porcelain scale, which ranges from to 120 Fahr. It is made after the Meteoro- logical Office and Admiralty pattern, and carries with it a certificate of verification at Kew Observatory. 3. Registering thermometers are so constructed as to enable us to read off from them the highest or lowest temperature to which they have been exposed in a given length of time usually twenty-four hours. As it is too inconvenient to read these instruments at the close of a civil day that is, at midnight they are by arrangement read at the latest observing hour of the day, or 9 P.M. in the British 102 METEOROLOGY Isles. The thermometer which is used for registering the highest or maximal temperature of the day of twenty-four hours is called a "maximum thermometer." Similarly, that which registers the lowest or minimal temperature is called a " minimum thermometer." In both maximum and minimum thermometers the contrivance by means of which we are able to read the extremes of temperature is called the " index." Maximum thermometers are of two kinds, called after their designers, Phillips's and Negretti and Zambra's. L..CASELLA LONDON I ! I I I I I ,1 T I .111.! I l I i I I I I I I I I ! I I 10 I I 10 I 20 I 30 I 40 I 50 I 60 170 I 80 I 90 I I FIG. 2. A, Phillips's, and B, Negretti and Zambra's Maximum Thermometers. In the instrument invented by Professor Phillips, F.R.S., of Oxford (Fig. 2, A), the index is really a small fragment of the column of mercury separated from the main body by a tiny bubble of air. When the column expands, this fragment is pushed before it, but when the column contracts, it remains behind in the capillary tube, so marking the point of highest temperature. This delicate arrangement is apt to get out of order, the air bubble being displaced and so permitting the index to coalesce with the main thread or column of mercury, thus converting the instrument into an ordinary thermometer. Negretti and Zambra devised a plan which is as ingenious as it is simple (Fig. 2, B). The mercurial thread in the thermo- meter tube forms itself the index in this way : the bore of the THERMOMETERS 103 tube is bent at a sharp angle and so much reduced in calibre close to the bulb that, while the expansion of the mercury in the bulb is quite capable of forcing the liquid past the con- striction into the tube, on cooling, the portion of mercury which has so passed into the tube breaks off from the main body and remains in the tube to register the highest point to which it had reached. In fact, when contracting, cohesion fails, and the mercury in the bulb is unable to draw back the portion in the tube, a separation taking place where the tube is both bent and constricted. The maximum thermometer should be suspended in the thermometer stand or Stevenson screen horizontally, or almost horizontally. The bulb end should be gently and slightly depressed before reading, so as to secure that the index is in apposition with the constriction in the tube at the near or bulb end. Great care should be taken to handle the instru- ments as little as possible while reading them; otherwise erroneous records may result. These thermometers are both set in the same way. They are taken in the hand and swung briskly, bulb downwards ; or else they may be lightly tapped, bulb downwards, on the wooden ledge of the stand. In this way the instrument becomes for the time being an ordinary thermometer, showing the existing temperature of the surrounding air. Minimum thermometers are also of two kinds Casella's, a mercurial thermometer; Rutherford's, a spirit thermometer. The former is a beautiful instrument, and especially adapted for use in tropical climates, where the intense heat by day causes spirit to volatilise quickly. The latter is the minimum thermometer, which is in almost universal use at our home and colonial stations (Fig. 3). In its construction a small metallic index is immersed in the spirit. Before using, this index is allowed to run down to the end of the column of liquid by sloping the thermometer with the bulb uppermost. In this way the thermometer is " set." It should then be placed in a nearly horizontal position in the screen, any slight inclination io 4 METEOROLOGY being towards the bulb, so as to facilitate the backward movement of the index when the spirit contracts with a falling temperature. When this happens, the index is drawn back with the spirit by the force of capillary attraction. On a rise of temperature taking place, the spirit expands and flows past the index, which remains behind to mark the lowest or minimum temperature. A spirit thermometer such as that just described should be carefully watched and periodically compared with a FIG. 3. Rutherford's Minimum Thermometer, filled with pure alcohol for ordinary registration, engine-divided on the stem. reliable mercurial thermometer, for some of the spirit is apt to volatilise and afterwards to condense in the distal or further end of the tube, causing the instrument to read too low by two, three, or even more degrees. Such an accident is easily remedied by swinging the ther- mometer backwards and forwards, bulb downwards. It may, however, be necessary to cautiously heat the distal end of the tube in the flame, or near the flame, of a spirit lamp. This causes the condensed spirit again to volatilise. If the instrument is then cooled in the position bulb downwards, the freshly condensed spirit will gradually trickle downwards and join the main body of the liquid. Spirit thermometers are by no means as sensitive as mer- curial ones, mercury having a much lower specific heat and a much higher conductivity than alcohol. Hence, it is becoming usual to make the bulb of the spirit thermometer of such a shape forked or cylindrical that as large a surface of the spirit as possible shall be exposed to the action of the air. THERMOMETERS Mention should be made of the registering thermometers which were devised in the eighteenth century, but which are now discarded as useless for scientific purposes. In 1757, Lord Charles Cavendish, a Vice-President of the Eoyal Society, read a paper before the Society on a maximum thermometer and a minimum thermometer which he had designed. This paper will be found in the 50th volume of the Philosophical Transactions (p. 300). His instruments suggested to Mr. James Six, a quarter of a century later (in 1782), the idea of an improved registering thermometer, which has ever since been known as " Six's Ther- mometer" (Fig. 4). It combines in one instrument a maximum and a minimum thermometer. It consists of a long tube bent parallel to itself in the centre like a siphon, and terminating at each extremity in a -bulb, one of which bulbs is larger than the other. The bend of the tube is filled with a plug of mercury. The remainder of the tube and both bulbs are filled with spirit, but in the smaller bulb there is also a bubble of highly compressed air. A small needle index of steel, with a capillary filament attached to it, floats on each end of the plug of mercury. As temperature rises, the spirit in the larger bulb expands, and pushes the mer- curial plug with one of the indexes in front of it into the distal tube. When temperature falls, the spirit of course con- tracts, and the elasticity of the compressed air bubble in the distal bulb causes the mercury to pass back after the retreating spirit, the distal index remaining behind to mark the point of highest tem- perature. But as the mercury passes back into the proximal FIG. 4. Six's Thermometer. io6 METEOROLOGY tube, it pushes the other index before it towards the larger bulb until temperature ceases to fall. When this happens, the proximal index remains to mark the lowest temperature. The indexes, being of steel, can be "set" for a new observation by means of a magnet in this way by attraction they are drawn back to the surface of each end of the plug of mercury. Since a mercurial minimum thermometer is much better suited for use in hot climates than a spirit thermometer, it may be well to describe Casella's ingenious instrument at some length. Mercury is the only fluid employed in its make. The bulb and column are of the same size as in the standard maximum thermometer, and cold is thus registered under precisely the same conditions as heat. No steel or other index is employed. In this thermometer advantage is taken of a curious property of mercury, which tends to adhere to glass in vacuo. Various experiments were undertaken by Mr. Louis P. Casella, F.K. Met. Soc., for the purpose of discovering some means by which in a mercurial thermometer the mercury itself might be detained at the point of lowest temperature, and so serve as a self-registering minimum thermometer. It occurred to him that the adhesive property of mercury for glass in VGWMO together with the fact that, where two tubes are united to one bulb, this fluid will rise by expansion in the larger, and recede by contraction in the smaller, tube might enable him to attain his object. The result was the invention of probably the first instrument known to register past indications without having or forming any separate index. The general form and arrangement of this ingenious instru- ment are shown in the accompanying illustrations (Fig. 5), of which the upper drawing represents a full-sized section. A tube with large bore is made to come off from the upper side of the thermometer stem a short distance in front of the bulb. At the distal end of this large-bored tube a flat glass diaphragm is formed by the abrupt junction of a small pear-shaped ch of THERMO ME TERS 107 chamber (ab), the inlet to which at b is larger than the bore of the indicating tube, or stem of the thermometer. The thermometer being set, as temperature falls, the mercury in the bulb contracts and withdraws the fluid in the stem only. When the minimum temperature is reached, the mercurial column in the stem marks it, not only at the moment, but after- wards also. For as soon as the mer- cury in the bulb begins to expand FIG. 5. Casella's Mercurial Minimum Thermometer. with a rise of temperature, it finds an easier passage into the tube of larger bore, and through it into the pear-shaped chamber beyond. In the tube of smaller bore, that is, the stem of the thermometer, adhesion or capillary attraction holds the mercury and prevents its recession from the point indicating the lowest temperature that had been reached since the instrument was set. To set the thermometer, it should be placed in a horizontal position, with the back plate suspended on a nail, and the lower part supported on a hook. The bulb end may now be gently raised or lowered, causing the mercury to flow slowly until the bent part is full and the chamber (ab) is quite empty. At this point the flow of mercury in the long stem of the thermometer is arrested, and indicates the exact temperature of the bulb, that is, of the air, at the time. When out of use, or after transit, it may happen that, on raising the bulb, the mercury will not at first flow out from the small chamber (ab). In such a case, however, a slight tap with the hand on the opposite end of the instrument, with the bulb uppermost, will readily cause it to do so. io8 METEOROLOGY The maximum and minimum thermometers, the ordinary or dry-bulb thermometer, and the wet-bulb thermometer, by which the temperature of evaporation is shown, as will be FIG. 6. Stevenson's Thermometer Stand. afterwards described, 1 should all be suspended in a suitable screen or thermometer stand. Thermometers should be protected from the direct or reflected rays of the sun, but at the same time should be freely 1 See p. 191. THERMOMETERS 109 exposed to the open air. These ends are best attained by placing them in a thermometer stand, such as the louvre- boarded box designed by Mr. Thomas Stevenson, C.E., of Edinburgh hence called the Stevenson stand or screen (Fig. 6). The pattern of this screen, which has been approved by the Royal Meteorological Society, is described in the Quarterly Journal of the Society (vol. x. p. 92, 1884). The screen is a double louvred box, its interior being 18 inches long, 11 inches wide, and 15 inches high. It has a double roof, the upper one projecting 2 inches beyond the body of the screen on all sides, and sloping from front to back. The front is hinged as a door and opens downwards. The thermo- meters are suspended on uprights near the middle of the screen, which should be painted white within and without, the finishing coat consisting of white paint and copal varnish. The screen should be mounted on four stout posts over short grass and freely exposed. It should not stand in the shade or within 7 feet of any wall, particularly of one with a southern aspect. The door of the screen should open towards the north. In towns, at many telegraphic stations, and on board ship, a " wall screen " must take the place of the freely exposed Stevenson stand just described. It consists of a covered case, with louvred wings, fixed on a wall facing north, at the height of 4 feet from the ground, by means of large holdfasts. The Royal Meteorological Society recommends that the observations with the thermometers just described should be made as follows : Having opened the screen, the dry and wet bulb thermometers are to be read first, so that they may not be affected by the nearness of the observer. The maxi- mum thermometer is to be read next, by noting the point at which the end of the column of mercury is lying. The mini- mum thermometer is read last, by noting the position of the end of the index furthest from the bulb. The surface of the column of spirit shows the temperature at the time of observa- tion. A second reading of all the thermometers should be IIO METEOROLOGY taken to guard against any mistake in the first entry. The maximum and minimum thermometers should then be set. When set, the end of the mercury in the maximum and the end of the index farthest from the bulb in the minimum should indicate the same temperature as the dry bulb. The door of the screen should finally be closed, after fresh water has been poured over the wet-bulb thermometer. Sling thermometer. Under the name of thermometre fronde (sling thermometer) the French meteorologist M. Arago, in 1830, devised a method of measuring air tempera- ture by means of a thermometer attached to a string and allowed to swing rapidly round for the space of half a minute or so. By this method the use of a screen is dispensed with, and even in full sunshine a close approximation to the true air temperature in the shade may be obtained. This method is, of course, applicable only for isolated observations. 4. Self-recording thermometers, or thermographs, are so arranged as to record their own readings, independently of the observer, either at frequent intervals in the case of the electrical thermograph, or continuously, as in the photo- graphic thermograph. In the electrical thermographs designed by Dr. Theorell, of Upsala, and Professor F. van Rysselberghe, of Ostend, the thermometer tube is open at the upper end, and a wire is introduced into it which, by a clock-work mechanism long before devised by Sir Charles Wheatstone, is caused to descend at regular intervals until it touches the surface of the mercury. At the moment of contact an electric current is generated, which causes a needle to prick a paper, on which the thermometer scale is marked, at the point corre- sponding to the height of the mercurial column at the time. The wire is then raised mechanically and contact is broken (R. H. Scott). In Sir Charles Wheatstone's thermograph the mercury became oxidised by the electric spark produced at the moment that the dip separated from the mercury (etincelle de rupture) ; but this inconvenience has been obviated. THERMO ME TERS 1 1 1 A photographic thermograph is in use in the stations of the First Order managed by the Meteorological Council of the Royal Society. In this instrument a bubble of air is introduced into the column of mercury, and this moves up and down with the temperature, the bore of the tube being larger than in Phillips's maximum thermometer. A lamp is placed before the instrument, and a photograph of the space occupied by the air bubble is continuously taken on prepared paper stretched on a drum, which is caused to revolve on its own axis once in forty-eight hours. At Greenwich Observatory a thermograph of a rather different construction is employed. It is made somewhat on the principle of the Kew Barograph. The light is allowed to pass through the thermometer tube above the level of the mercury on to the sensitised paper. We in this way get a continuous photographic tracing which corresponds along its lower edge with the temperature range, the level of the mercury abruptly cutting off the photographic tracing below. 5. Radiation thermometers. The subject of radiation is so important that we shall consider it in the next chapter, and there explain the instruments by which it is measured. Having taken observations with the thermometers already described, we are in a position to ascertain the Mean Temper- ature, or that temperature which has an intermediate value (1) between the several successive hourly temperatures re- corded by a thermograph, or read by an observer every hour throughout an entire solar day of twenty-four hours ; or (2) between the extreme readings recorded in that time ; or (3) between the dry-bulb readings taken twice or thrice daily, at 9 A.M. and 9 P.M., as at all British stations of the Second Order ; or at 7 A.M., 1 P.M., and 9 P.M., as in Russia ; or at 6 A.M., 2 P.M., and 10 P.M., as in Austria. These are the three methods adopted for ascertaining what is known as mean temperature of a day. The mean tempera- ture of a month is obtained by dividing the figure 31, or 30, or 29, or 28, as the case may be, into the sum of that number 112 METEOROLOGY of daily values. The mean temperature of a year is similarly obtained by dividing the figure 12 into the sum of that number of monthly values. The term average mean temperature is properly applied to that temperature which represents the mean of a number of means. For example, in Dublin the mean temperature of May, 1893, was 56*7, but the average mean temperature for May in that city in a long series of years (1865-1892, that is, twenty-eight years) was 52'0. We say then that the mean temperature of May, 1893, was 4*7 above the average. When dealing with diurnal extremes of temperature, a sufficient approximation to the mean temperature is obtained by taking the arithmetical mean of the maximum and minimum thermometer readings, according to the formula Min. + {Max. -Min.} x -5 = M.T. A careful comparison, however, with the results yielded by thermograms, as the tracings taken by the thermograph are called, has suggested the following empirical formula, in which the coefficient (7, a variable quantity from month to month, takes the place of the constant coefficient '5. Min. + {Max. - Min.} x C. = M.T. The annexed Table gives the coefficients for the different months. Months. Coefficient. January "I December J ' 0-520 February November . 0-500 March October . 0-485 April September ... , :_; 0-476 May \ August / 0-470 June \ July j- 0-465 THERMO ME TERS 113 In accordance with this Table, the mean temperature of May, 1893, in Dublin, was not Min. + {Max. -Min.) x "5 but Min. + {Max. - Min.} x -470 Interpolating the actual values we have not 50'6+ {62 7 -50 -6} x -5 = 56 7 but 50 -6 -I- (627 -50-6} x '470 = 56 '3 CHAPTEE XI RADIATION Heat: how transmitted Conduction Convection Radiation Solar Radiation Terrestrial Radiation Effect of Altitude on Temperature : how brought about Radiation Thermometers Black- bulb Thermo- meter in vacua Bright-bulb Thermometer in vacuo Southall's Helio-pyrometer Herschel's Actinometer Pouillet's Pyrheliometer Grass Minimum Thermometers Earth Temperatures Critical Temperature at depth of 4 feet Duration of bright Sunshine Sun- shine Recorders. HEAT is communicated, or transmitted from body to body or from place to place, in at least three different ways by conduction, by convection, and by radiation. Conduction is the transmission of heat through a con- ductor, or a substance or body capable of being a medium for its transmission for example, an iron poker, as contrasted with a stick, which latter is a non-conductor. To quote the American Cyclopaedia : " The communication of heat from one body to another when they are in contact, or through a homogeneous body from particle to particle, constitutes con- duction." As a rule, conductors of heat are also conductors of electricity. In practical meteorology we have illustrations of conduc- tion of heat in the propagation of changes of temperature from the surface of the earth to the successive strata of the subsoil ; and in the alternate heating and chilling of the lowest strata of the air through contact with the ground. RADIATION The subsoil temperature is recorded by means of underground thermometers, such as are figured in the accompanying illustrations (Figs. 7 and 8). Convection is the trans- ference or transmission of heat by means of currents generated in liquids and gases by changes of tem- _ perature and other causes. When a spirit lamp is applied to the bottom of a vessel of water, the heated water at the bottom expands, becomes specifically lighter, and so rises to the surface, carrying with it or conveying the heat it has received. Thus, by convection, heat is diffused at last through the whole mass of the water in the vessel. A similar experiment on a stupendous scale in nature causes hot and cold winds in the atmosphere, as well as vast ocean currents like the Gulf Stream, which conveys heat or warmth to high lati- tudes along the north-western coast of Europe. Convec- tion, like conduction, applies to heat and electricity alike. Radiation is the transmission from a point or surface of rays of heat along divergent lines (Lat. radius, a semi-diameter of a circle ; hence a beam or ray of light proceeding from a bright object along TT ,. IT Underground divergent right lines or radii), not from particle to Thermometer. FIG. 7. Underground Thermometer. FIG. 8. n6 METEOROLOGY particle of the same body (as in conduction), but from one body to another, through air, or vacuum, or space. Radiant heat is, in fact, identical with light, only the wave-lengths of the rays of which it consists are at moderate temperatures longer than those corresponding to red light, and so they do not present the phenomena of light. If, however, the tem- perature of a body is increased, it begins to glow with a dull red light, which passes through shades of yellow, violet, and blue, until an intensely heated body is said to be incandes- cent, which means that it gives off a light as white as that of the sun, and which contains in their proper proportions all the colours of sunlight. Radiant heat, then, spreads along straight lines, diverging in all directions from the source of heat. " Its intensity," says Dr. Alex. Buchan, 1 "is proportioned to the temperature of the source, is inversely as the square of the distance from the source, and is greater according to the degree of inclina- tion of the surface on which the rays fall." Heat is radiated towards the earth from the fixed stars, the planets, the moon, but, above all, the sun. Indeed, for all practical meteorological purposes we may assume that the sun's rays are the only source whence heat reaches the earth's surface. We speak, therefore, of solar radiation alone in con- nection with the receipt of heat by the earth. In a paper on the " Conservation of Solar Energy," read before the Royal Society on March 2, 1882, Dr. C. William Siemens, D.C.L., F.R.S., observed : " The amount of heat radiated from the sun has been approximately computed by the aid of the pyrheliometer of Pouillet, and by the actino- meters of Herschel and others, at 18,000,000 of heat units from every square foot of its surface per hour, or, put popu- larly, as equal to the heat that would be produced by the perfect combustion every thirty-six hours of a mass of coal, of specific gravity = 1*5, as great as that of our earth. 1 Introductory Text-Book of Meteorology, p. 48. William Blackwood and Sons : Edinburgh and London. 1871. RADIATION 117 "If the sun were surrounded by a solid sphere of a radius equal to the mean distance of the sun from the earth (95,000,000 of miles), the whole of this prodigious amount of heat would be intercepted ; but considering that the earth's apparent diameter, as seen from the sun, is only seventeen seconds, the earth can intercept only the 2250 millionth part." In accordance with physical laws no sooner does the earth receive heat from the sun than it begins to radiate it back again into space in all directions. Hence we speak of terres- trial radiation. From what has been stated above in a quotation from Dr. Buchan, it is clear that solar radiation is much less in winter than in summer, owing to increased inclination ; but it is also less in July than in December (taking the earth as a whole), because in the latter month the sun and the earth are some three millions of miles nearer to each other than in the former. According to Mr. E. H. Scott, F.R.S., with the existing value of the eccentricity of the earth's orbit, the amount of heat received in perihelion (the southern summer) is to that received in aphelion (the northern summer) as 1-034 is to 0-967. Solar radiation is also interfered with by clouds, but is not materially affected by the air through which it passes ; nor is it diverted from a straight course by the wind (Buchan). Terrestrial radiation tends to dissipate into space the heat which the earth has received from the sun, and as a conse- quence temperature falls in winter, when the slanting rays of the sun pour down less heat upon the earth's surface. Again, not only the seasonal, but also the diurnal, range of temper- ature, depends on radiation. By day, solar radiation pre- dominates and temperature rises ; by night, solar radiation ceases while terrestrial radiation "continues, and so temper- ature falls. Just as solar radiation is interfered with by clouds, so an overcast sky interrupts terrestrial radiatipn. ii8 METEOROLOGY Hence dew is not deposited on a cloudy night, because the thermometer does not fall below the temperature of saturation, or the dew point. But even an excess of mois- ture in the atmosphere interferes with terrestrial radiation, so that very low temperatures are never felt in damp weather, while severe frosts occur in spring nights, when the air is very dry and the sky is often clear. A covering of snow at one and the same time prevents and facilitates radiation. The explanation of this paradox is that the snow acts like a cloud canopy and interferes with radia- tion from the surface of the ground, which is in this way kept warm. But, further, snow is a bad conductor of heat, and so the warmth is imprisoned beneath it through non- conduction. Hence the surface of the snow becomes intensely cold, for no heat reaches it from below, while it radiates freely into space what heat it does already possess. Dr. Hann 1 calculated that in a vertical column of absolutely dry air the thermometer should fall 1 F. for every 182 feet of altitude, or 1 C. for every 100 metres. In nature, how- ever, there is practically no such thing as absolutely dry air. The moisture in the atmosphere, then, is liable to be con- densed as the temperature falls with increasing altitude. But in the process of condensation latent heat is set free in large quantities with the effect of lessening the rate of cooling in the vertical column of air. Sir John Herschel long ago calculated that the slower rate of cooling amounted to about 1 F. for every 300 feet of vertical height, and this value is generally accepted. It is, however, necessary to explain that sometimes in winter conditions of temperature are actually reversed, and an " upbank thaw," as it is called, may occur on mountains with the arrival on their slopes of a warm air current, while the cold dense air in the valleys and plains below may cause unbroken frosts at lower levels. In estimating the influence of radiation upon climate, it is to be borne in mind that the specific heat of water is much 1 AUgemeine Erdkunde. Third edition. Tempsky : Prague. 1881. RADIATION 119 higher than that of land in the proportion of about four to one. Hence solar radiation heats water much more slowly than it heats dry land, and again water cools in turn much more slowly than dry land does. In these facts we recognise one explanation of the modifying and mollifying effects of the ocean upon climate. Its presence controls temperature, forbidding it to rise quickly in summer or to fall quickly in winter. Of course, by convection also, currents of cool water flow towards warm regions, and cur- rents of warm water towards cold regions. We are now in a position to resume the description of various thermometers employed in meteor- ological observations, which was begun in the preceding chapter. 5. Radiation thermometers. Solar radiation is measured by the black -bulb thermometer in vacuo, an instrument which was first suggested by Sir John Herschel. The Rev. Fenwick W. Stow, M.A., of Aysgarth Vicarage, Bedale, Yorkshire, de- scribes this instrument as follows : The insulated solar maximum thermometer, usually called the black bulb in vacuo, is a sensitive maximum thermometer, having the bulb and a given portion of the stem covered with lamp black, the whole being enclosed in a glass tube from which all air and moisture have been removed, so that the heat of the. sun's rays is thus obtained, apart from the influence of vapour or passing currents of air. The stem near the bulb must be blackened to prevent reduction of temperature in the bulb through conduction, the bright stem chilled by radiation in this way FIG. 9. Solar Radiation Thermo- meter Stand. 120 METEOROLOGY affecting the bulb. This delicate instrument should be placed on a stand 4 feet above the ground, in an open space, with its bulb directed towards the south-east, and free from con- tact with any substance whatever. The Royal Meteorological Society recommends the use, in addition to the black bulb, of a bright-bulb thermometer in vacuo. The readings of this latter instrument will, of course, be lower than those of the black bulb, because the bright bulb will radiate freely the heat which it receives from the sun's rays. Fig. 9 represents these thermometers in situ. The helio-pyrometer, arranged by Mr. T. Southall, of Birmingham, gives extraordinary readings at times (216, 217, and even 231 '5 in July, 1859), and these readings are confirmed by water being caused to boil violently in a small vessel attached to the apparatus. One of Casella's solar radia- tion maximum thermometers, made on Professor Phillips's principle, is fixed on a cushion at the bottom of a box, the sides of which are also cushioned, and a thick piece of plate glass is laid upon the top to prevent currents of air carrying off the heat as well as with the view of preventing the cooling effects of terrestrial radiation. The box is placed in such a position that the sun's rays may fall as nearly as possible perpendicularly on the glass. A change of position to secure this end may be, required twice or three times a day. No doubt a portion of the sun's heat is lost by reflection from the surface of the plate-glass cover, but the amount of the loss can be calculated. Other solar radiation instruments which deserve mention are : Sir John Herschel's actinometer (Gk. a/crts, a ray ; perpov, a measure), Padre Secchi's solar intensity apparatus, and Pouillet's pyrheliometer (Gk. nvp, fire or heat ; irjAios, the sun ; /XCT/OOV, a measure). By means of this last instrument the effect of the sun's heat upon a given area is ascertained by the number of degrees of heat imparted to a given quantity of mercury in five minutes. The Richard system for recording solar heat (actinometer) RADIATION 121 is partly based upon researches made by Professor Violle, and is represented in Fig. 10. Two thermometers, the bulb of FIG. 10. Richards' Actinometer. one of which is bright, while that of the other is a dull black, are protected by glass spheres and record on a single sheet. 122 METEOROLOGY so that the difference of their readings, and also the times of their respective maxima, can be easily seen. When using the black bulb in vacuo, observations should also be made with the ordinary maximum thermometer in the shade. The greatest amount of radiation during the day will then be approximately indicated by subtracting the maximal temperature in the shade from the maximal reading recorded by the solar radiation thermometer. Terrestrial Radiation. The thermometer used for register- ing this meteorological factor is a delicate self -registering spirit minimum thermometer, of Kutherford's construction, which FIG. 11. Casella's Bifurcated Grass Minimum. is enclosed in a glass cylinder for protection. To increase the sensitiveness of the spirit " grass minimum," Mr. Casella designed a thermometer in which the bulb, being extended in a forked form, exposes a greatly increased surface to the air (Fig. 11). In this way the instrument is rendered little, if at all, less sensitive than Mr. Casella's mercurial minimum already described. A thermometer intended to measure terrestrial radiation should be suspended over a piece of smooth lawn grass on wooden props shaped like Y's, at a height of only 3 inches or so, in order to escape the disturbing influence of the wind. The amount of terrestrial radiation is determined by subtracting from the minimal temperature recorded in the thermometer screen the minimum registered on the grass. Should the ground be covered with snow the radiation ther- mometer should be laid upon the surface of the snow. RAD1A TION 123 Where a grass plot is not available the thermometer should be placed on a large black board laid upon the ground. Earth Temperatures. In connection with radiation it is desirable to ascertain the tempera- ture of the soil at fixed depths. This may best be done by using Symons's earth thermometer (Fig. 12). 1 It is a sluggish thermometer mounted on a short weighted stick attached to a strong chain. It is lowered by means of this chain into a long, stout, iron pipe, pointed at the lower end and driven into the earth to any required depth 1 foot, 2 feet, 3 feet, or 4 feet below the surface. The top of the iron pipe or tube is closed by a tight-fitting iron cap. The tube should be driven into the soil below short grass and in a well-exposed situa- tion. Mr. Casella also has designed a self -registering thermometer for immersion to any depth in the earth or wells, where it will record the maximum and minimum temperatures for a required interval of time. From observations at the Calton Hill, Edinburgh, Principal Forbes concludes that the seasonal variations of temperature are reversed at a depth of 24 feet the greatest warmth occurring at that depth about January 4, and the greatest cold about July 13. Below 40 feet there is no annual varia- tion in the temperature of the soil. The temperature of the soil, as shown by the earth ther- " Improved Form of Thermometer for observing Earth Temperature." By G. J. Symous, F.M.S. Quarterly Journal of the Meteorological Society, vol. iii. p. 421, 1877. FIG. 12. Symons's Earth Thermometer. I2 4 METEOROLOGY mometer, has a vital bearing on public health. Systematic observations at the City Meteorological Observatory, 299 Oldham Road, Manchester, have convinced Dr. John Tatham, lately the able Medical Officer of Health for that great city, but now Medical Superintendent of Statistics at Somerset House, that, in accordance with the views of Dr. Edward Ballard, F.R.S., when the earth temperature at the depth of 4 feet from the surface rises to 56 F. infantile diarrhoea may be expected to become epidemic in the city. This " critical temperature," as Dr. Ballard aptly calls it, was reached at a very early date (June 20-21) in 1893, and accordingly the deaths from diarrhoea, which had been 4*7 per week on the average in the six weeks ending June 10, compared with 2'0 per week during the same period in 1892, increased from 11 in the week ending June 17, to 18 in that ending June 24, 58 in that ending July 1, 72 in that ending July 8, 83 in that ending July 15. In 1892, the 4 feet earth temperature did not reach 56 until August. There can be no doubt that the prevalence of cholerine or epidemic summer diarrhoea, the prevalence of enteric fever, and that of cholera are all equally determined by this critical subsoil temperature of 56. Duration of Bright Sunshine. Within the past twelve years striking observations have been made on this point under the auspices of the Meteorological Office, London, and of the Royal Meteorological Society and the Scottish Meteoro- logical Society. It is manifest that the amount of solar radiation will depend on the amount of bright sunshine. A remarkable instance of this occurred in the spring of 1893, when the registered amount of bright sunshine was much in excess of the average, and when solar radiation was so power- ful as to cause a marvellous blossoming not only of the ordinary spring flowers and shrubs, but also of shrubs which rarely flower in ordinary years in the climate of the British Isles. The instruments which are used for recording the dura- RADIATION 125 tion of sunshine are (1) the Campbell -Stokes Burning Recorder; (2) the Whipple-Casella Universal Sunshine Re- corder ; (3) the Jordan Photographic Recorder. The principle of the first two of these instruments is the same. It consists in burning a tracing in a piece of mill-board placed in the focus of rays from a glass sphere, which acts as a lens when exposed to bright sunshine. The burning recorder was originally devised in 1854 by Mr. John F. Campbell, F.G.S., of Islay, and was improved by Professor Sir George G. Stokes, Bart., F.R.S., of Cambridge. 1. The Campbell-Stokes Sunshine Recorder consists of a sphere of glass 4 inches in diameter, supported on a pedestal in a metal zodiacal frame (Fig. 13). It should be fixed in FIG. 13. The Campbell-Stokes Sunshine Recorder. an open position, so that the sun's rays may fall upon it at any time between sunrise and sunset. It must face due south, so that the sun's image shall fall upon the meridian line marked on the inside of the ring supporting the record- ing cards when the sun is itself upon the meridian. The axis of the ring in question must be inclined to the horizon at an angle equal to the latitude of the place, and the instru- 126 METEOROLOGY ment must be level as regards east and west. 1 There are three grooves in the ring which supports the card ; one holds rectangular cards with hour figures printed upon them suit- FIG. 14. The Whipple-Casella Universal Sunshine Recorder able for the equinoxes ; one, long curved cards similarly time-marked for summer; and one, short curved cards for winter. A card being fixed in the proper groove according 1 Quarterly Journal of the Meteorological Society, vol. vi. p. 83, 1880. " Description of the Card Supporter for Sunshine Eecorders adopted at the Meteorological Office." By Professor George Gabriel Stokes, M.A., F.R.S. RADIATION 127 to the season of the year, the sun when shining burns away or chars the surface at the points on which its image falls from moment to moment, and thus a tracing of bright sun- shine is given. A card should be removed daily after sunset, and a new one inserted ready for next day. This apparatus costs 9 : 9s. 2. The Whipple-Casella modification of this instrument FIG. 15. Jordan Photographic Sunshine Recorder. has divided latitude and diurnal circles, so that it can be set for any locality and for any day in the year, thus earning its name of "Universal Sunshine Recorder." It is an expensive instrument, costing .17 ; but, owing to its powers of adjust- ment to time and place, it requires merely a strip of card- board duly hour-marked instead of Sir George G. Stokes's equinoctial and summer and winter cards. (See Fig. 14.) 3. In 1838 an automatic Daylight or Sunlight Recorder was invented and constructed by Mr. T. B. Jordan, who 128 METEOROLOGY wrote and published an account of his invention in the Sixth Annual Report of the Royal Cornwall Polytechnic Society (p. 185). The instrument, however, which now goes by the name of the "Jordan Photographic Sunshine Recorder," was designed in 1885 by Mr. James B. Jordan of the Home Office. 1 Two forms of the Jor- dan Photographic Sun- shine Recorder are in use. The first pattern, represented in Fig. 1 5, brought out in 1885, consists of a cylindrical box, on the inside of which a sheet of sen- sitive cyanotype paper FIG. 16.-Improvecl Jordan Photographic j s carefully placed day Sunshine Recorder. , .. !L _ . . by day. Ihe sunlight is admitted into the box by two small apertures, and acts on the paper chemically, leaving a tracing as the ray travels across it owing to the earth's rotation. The improved pattern (Fig. 16) has two semi -cylindrical boxes, one to hold the forenoon, the other the afternoon record. A slit, through which the beam of sunlight finds entrance, is placed in the centre of the rectangular side of each box, so that the length of the beam within the chamber is the radius of the cylindri- cal surface on which it is projected. The path of the sun- beam, therefore, follows a straight line on the sensitive paper at all seasons. The instrument must be carefully adjusted to the meridian and to the latitude of the place, and must be firmly fixed (Win. Marriott, F.R. Met. Soc.) 1 Quart. Journ. of the Royal Met. Soc., 1886, vol. xii. p. 23. CHAPTEE XII ATMOSPHERIC PRESSUEE The Barometer and its Uses Galileo's Observation Torricelli's Discovery The "Torricellian Vacuum" Principle involved in Nature's abhorrence of a Vacuum Fluids used in constructing a Barometer : Mercury, Water, Glycerine Jordan's Glycerine Barometer Esti- mation of the Height of the Atmosphere Pascal's Experiments Esti- mation of Mountain Altitudes by means of the Barometer Scaling and Lettering of the Barometer The "Weather Glass," or Wheel Barometer, of Robert Hooke History of the Barometer. WE have already seen in Chapter II. that the atmosphere has weight, and can be weighed. The instrument by which this is effected is called the barometer (Gk. /3apos, weight ; fjiTpov, a measure). But it would be most misleading to sup- pose that the only use of the barometer is the mere weighing of the atmosphere. By a careful study of the properties of this marvellous instrument we are enabled to measure the heights of mountains, to ascertain the distribution of atmo- spheric pressure over the earth's surface by sea as well as by land, and at the different seasons of the year ; to under- stand in consequence the prevalent winds at all times and in all places, to trace the ever-shifting distribution of atmo- spheric pressure over vast districts, and finally, to "fore- cast " the weather. This may be done either by a considera- tion of barometrical observations taken at a single station, or by means of telegraphic information as to a number of such observations taken synchronously (or at the same K i 3 o METEOROLOGY moment of time) at many stations scattered over a large area like the west, north-west, and centre of Europe, or the United States of America and Canada. Surely such far-reaching potentialities as those now in- dicated bespeak for so wonderful an instrument our liveliest interest and most attentive study. An observation of Galileo Galilei, of Pisa, the father of experimental science, that water would not rise in a pump more than "eighteen cubits" (diciotto braccia) above the level of a well, led to the discovery of the pressure of the atmosphere by Evangelista Torricelli, his pupil and successor in the Chair of Philosophy and Mathematics at Flor- ence, who also devised the means of measuring that pressure. Torricelli's famous experiment was made in 1643. He was testing Galileo's dictum that "Nature abhors a vacuum" (up to 32 feet, in the case of water), and for convenience employed mercury. By doing so, he found that Nature's abhor- rence of a vacuum varied for different fluids. Torricelli filled a tube, 3 feet long, with mercury, and then inverted it, and plunged its lower end into a basin filled half with mercury and half with water. So long as the lower end of the tube remained below the level of the mercury in the basin, the height of .Fie. 17. Torricelli's Experiment. A TMOSPHERIC PRESSURE 1 3 1 the column of mercury in the tube proved to be about 30 inches, and a vacant space of 6 inches was left at the top of the tube a space which afterwards came to be, and is still, known as the Torricellian vacuum. The moment, however, that the lower end of the tube was raised above the surface of the mercury in the basin into the overlying stratum of water, all the mercury in the tube rushed out, its place being taken by the water, which equally readily rushed in and filled the tube completely. Eea'soning out the matter, the philosopher concluded that some one force existed which was able to support a column of mercury to a height of 30 inches in the tube, but a column of water to a much greater height. This force could be none other than the pressure of the atmosphere on the open surface of the fluids mercury and water in the basin. Thus was the barometer discovered. Torricelli further proved that Nature's abhorrence of a vacuum was represented by a column of fluid inversely pro- portional to its specific gravity. Take the very fluids under consideration water and mercury; the specific gravity of water being 1, that of mercury is 13*594, and we get this inverse proportion : 1 : 13'594 : : 30 inches : x = the required height of a column of water supported by the atmosphere. 13-594 x 30 inches = 407*82 inches =33*99 feet. This principle has been taken advantage of in selecting fluids for the construction of a barometer. Thus mercury is one of the handiest, because, in addition to other recommenda- tions, it requires a tube only 32 inches long in consequence of its high specific gravity. On the other hand, if we could use water in an immense tube 35 or 36 feet in length, the smallest variations in atmospheric pressure could easily be observed. Water, however, is not available because of its high freezing point. Hence we select glycerine, the specific gravity of which is 1*26, and which, while undiluted, does 1 32 METEOROLOGY not freeze at any known terrestrial temperature (a 50 per cent solution freezes at - 31 C., or - 23*8 F.) In practice we find that a column of glycerine 27 or 28 feet high will yield most valuable barometrical indications. The propor- tional statement is : 1*26 : 1 :: 33 '99 feet : x = the required height of the column of glycerine in feet or inches 1-26)33-99(26-976 feet, or 3237 inches. 252 879 756 1230 1134 960 882 780 756 24 The proportional statement between glycerine and mercury works out as follows : 1'26 : 13*594 : : 30 inches : x = the required height of the column of glycerine in inches or feet 1-26)13-594x30, i.e. 407 '82(323-6 inches=3237 inches quamproxime 378 =26-975 feet. 298 252 462 378 840 756 84 Jordan's glycerine barometer, used at the Times Office, London, consists of a gas tube, five-eighths of an inch in diameter and 28 feet in height. As glycerine has a singular affinity for water, the glycerine in the cistern of this gigantic barometer is covered with a layer of paraffin oil. The advantage of the glycerine barometer then is that it magnifies tenfold, as it were, the readings of the mercurial A TMOSPHERIC PRESSURE 133 barometer 323 '7 inches on the scale of the glycerine barometer corresponding to 30 inches on that of the mer- curial barometer. Another interesting application of the principle that the THE TIMES OFFICE, 2 A.M. READINGS OF THE JORDAN BAROMETER (CORRECTED) DURING THE PAST TWENTY-FOUR HOURS. FEBRUARY 2627. 2 4 A.M. P.M. 10 N. 2 4 6 8 10 M.~~2~ O Inches. 320, 319 29-7 -29-6 FIG. 18. heights of columns of liquids or gases are inversely propor- tional to their specific gravities is the attempt to determine the height of the atmosphere. As air is about 10,000 times lighter than mercury, the height of the atmosphere should on this principle be 10,000 times 30 inches, or 300,000 inches, that is 4*7 miles. In Chapter II., however, it has been shown that the density of the aerial column lessens according to its height, and so, as a matter of fact> the height of the atmosphere is vastly greater than 4*7 miles an alti- 134 METEOROLOGY tude which falls short of the highest peak of the Himalayas by 4000 feet. Torricelli's surmises received their full confirmation at the hands of the French philosopher, Blaise Pascal, of Clermont, in Auvergne. In 1647 he was in Paris, when the thought struck him that if the Torricellian theory of atmospheric pressure was correct, the height of the column of mercury supported by the air should be less on the top than at the foot of a mountain. He accordingly wrote to his brother- in-law, Perrier, who lived at Clermont, to request him to ascend the neighbouring Puy de Dome, with the Torricellian tube in his hands. It was not until September 19, 1648, that Perrier was able to carry out the long-projected experi- ment. In the presence of a distinguished company of savants in Clermont he on that day repeatedly performed the Torri- cellian experiment. The party then ascended the mountain, which at a distance of 8 miles rises some 3510 feet above Clermont, and to Perrier's surprise, and ultimately to Pascal's delight, the mercury was found to stand 3*33 inches lower on the summit than at Clermont. On the way down, at Font de 1'Arbre, the column was proved to have an inter- mediate height. Perrier's observations on this memorable day gave 3458 feet for the height of the Puy de Dome above Clermont, and the actual height is now stated to be 3511 feet. The account of this experiment was given by Blaise Pascal himself in a pamphlet published in Paris in 1648, and entitled " Kecit de la grande Experience de I'Equilibre des Liqueurs." 1 During the years 1649-1650 readings of the "Torricellian column," as it was called, were taken daily, and at the same time, by Pascal at Paris, Perrier at Clermont, and Chanut and Descartes at Stockholm, " in order to see if anything could be discovered by confronting them with one another." Mr. 1 Neudrucke von Schriften und Karten uler Meteorologie und Erd- magnetismus. Herausgegeben von Professor Dr. G-. Hellmann. 4to. Berlin : A. Ascher and Co. 1893. A TMOSPHERIC PRESSURE 1 35 Richard Strachan, F.R. Met. Soc., who gives much of the fore- going information in a lecture delivered under the auspices of the Meteorological Society in 1878, 1 observes: "Pascal was thus the pioneer of the synchronous observations upon which modern storm-warnings depend." In 1665 the Hon. Robert Boyle observed the Torricellian column in relation to weather, and gave it a scale and letter- ing. In the same year Robert Hooke invented the "weather glass " or wheel barometer. In the Philosophical Transactions for 1666, p. 153, we read that " Modern Philosophers, to avoid Circumlocutions, call that Instrument, wherein a Cylinder of Quicksilver, of between 28 and 31 inches in Altitude, is kept suspended after the manner of the Torricellian Experiment, a Barometer or Baroscope 2 ... to detect all the minute variations in the Pressure and weight of the air." A very full historical account of the barometer was com- municated to the Royal Meteorological Society on March 17, 1886, by the President, Mr. William Ellis, F.R.A.S., of the Royal Observatory, Greenwich. His Presidential Address will be found in the twelfth volume of the Quarterly Journal of the Royal Meteorological Society (No. 59, July 1886, p. 131). 1 Modern Meteorology, p. 70 et seq. London : Edward Stanford. 1879. 2 Gk. /Sctpoj, weight; (T/COTT^W, I inspect. CHAPTEE XIII THE BAROMETER The Mercurial Barometer Extreme Limits of Atmospheric Pressure at Sea-Level "Torricellian Vacuum" Attached Thermometer Mount- ing of the Mercurial Barometer Two Difficulties in the Construction of this Instrument How they are Surmounted " Error of Capacity " Capacity Correction The Fortin Barometer The Kew Barometer (Adie) The Siphon Barometer (Gay-Lussac) The "Gun Baro- meter" (FitzRoy) The Wheel Barometer, or "Weather Glass" (Hooke) Self -registering Barometers, or "Barographs" King's Mechanical Barograph Ronalds's Photographic Barograph Redier's Mercurial Registering Barometer Wheatstone's Electrical Barograph Transmission of Barometric Indications by Electricity (J. Joly) Substitutes for Mercurial Barometers : the Aneroid Barometer Its Altitude Scale Bourdon's Metallic Barometer Measurement of Heights The "Engineering Aneroid" Sympiesometer Hypso- meter. THE most usual form of barometer is a glass tube, about 34 inches in length, closed at one end and carefully filled with pure mercury, of the specific gravity of 13'594. If necessary, the mercury in the tube should be boiled to expel all air. The tube is then placed vertically with its open end dipping into a cup of mercury called the " cistern." When so placed the mercury falls in the tube at sea-level, to 30 inches, or some point not very much above or below that level, according to the pressure of the atmosphere at the time. In Dublin the extreme limits of pressure recorded since 1860 have been highest, 30'935 inches, on January 10, THE BAROMETER 137 1882; lowest, 27758 inches, on December 8, 1886. But these values by no means represent the extreme range of the barometer. On Thursday, January 5, 1893, the reading at Moscow was 31 '27 inches. On Saturday, January 26, 1884, the barometer fell to 27*332 inches at Ochtertyre, near Crieff, in Perthshire, and on Saturday, February 5, 1870, the reading of 27*33 inches was recorded on board the Cunard steamer Tarifa in the North Atlantic, in lat. 51 N. and long. 24 W. But even these extremes, all reduced to 32 and mean sea-level, have been exceeded. In a communication to Nature, dated January 6, 1887 (vol. xxxv. p. 344), Mr. Blanford states that " the cyclone which on the morning of September 22, 1885, swept over False Point, on the coast of Orissa, gave the lower readings 27*135 inches at the beginning of the central calm, and 27*154 inches half an hour later (both readings reduced to 32 and sea-level)." These readings were made by a verified standard barometer, and are thoroughly authentic. For comparison with English standards a further subtractive correction of *011 inch has to be applied, which would make the lowest reading 27*124 inches. In an interesting paper 1 on "The Storm and Low Barometer of December 8 and 9, 1886," Mr. Charles Harding, F.K. Met. Soc., quotes from Professor Loomis's Contributions to Meteorology, chap, ii., a reading of 31*72 inches, reduced to sea-level, observed at Semipalatinsk, in Western Siberia (lat. 50 24' N., long. 80 13' E.) on December 16, 1877, the reading at Barnaul being 31*63 inches at the same time. This gives a difference from Mr. Blanford's reading, 27*12 inches, of +4*6 inches, which is probably the maximal range of the barometer ever observed at the earth's surface (reduced to sea-level). These extreme readings, although rarely observed, should be provided for in the scaling of a barometer, which should range from 32 inches to 26 inches, or less to allow for altitude. 1 Quart. Journ. of the Royal Met. Soc., vol. xiii. p. 201, 1887. I 3 8 METEOROLOGY The space above the mercury in the barometer tube is still called the " Torricellian vacuum," and, provided the tube has been properly filled, this space should contain nothing except a little of the vapour of mercury. As mercury expands with heat, it is essential that a ther- mometer should be attached to every barometer, in order to show the temperature of the mercurial column. Once this value is ascertained, a suitable correction must be applied to the reading of the barometer so as to reduce it to the fixed or standard temperature of 32 F. The mercurial barometer is best mounted in a brass case, because the coefficient of expansion of brass by heat is well known a matter of great importance, as the tables for correcting readings for temperature are based upon the coefficients of expansion of mercury, glass, and brass. Barometers mounted in wood are of inferior value for scien- tific purposes. The attentive reader will at this point suggest to his own mind two difficulties in the construction of the barometer. One is, how to cover the cistern so as to prevent the escape of the mercury, and so render the instrument portable with- out interfering with atmospheric pressure on the surface of the mercury contained in the cistern. This is effected by constructing the bottom of the cistern of leather, as in the Fortin barometer ; or by covering a small cavity in the roof of the cistern with leather, as in the Kew barometer. Again, it is evident that the level of the mercury in the cistern will change according as the barometer rises or falls. If it rises, there will be more mercury in the tube and less in the cistern, and the level of the mercury in the latter will fall. On the other hand, if the barometer falls, there will be less mercury in the tube and more in the cistern, in which the level of the mercury will in consequence rise. In a word, " the zero of the scale does not always correspond with the level of the mercury in the cistern" (Fred. J. Brockway). As in all cases the height of the barometer is calculated THE BAROMETER '39 from the level of the mercury in the cistern, we must apply a correction for the error arising from the change of level in the cistern the " error of capacity," as it is called. Formerly, tables for applying a "capacity correction" were employed, but they are not now required, owing to the adoption of barometers of the Fortin, Kew, or Siphon patterns. (1) The Fortin barometer has a pliable or flexible base to its cistern. (2) The Kew barometer has a con- tracted scale. (3) The Siphon barometer dispenses with the use of a cistern alto- gether. 1. In the Standard Barometer, com- monly called Fortin's barometer, the start- ing-point, or zero, of the scale is formed by an ivory pin, which must be brought into exact contact with the surface of the mercury in the cistern whenever an observation is made. This is effected by fixing a screw below and in contact with the flexible leather bottom of the cistern. The adjustment is made by means of a thumb-screw, which raises or depresses the flexible leathern base of the cistern until the tip of the ivory pin technically called the fiducial point and its image reflected in the mercury in the cistern appear exactly to touch each other, when viewed through a glazed aperture in the wall of the cistern. In Fig. 20, an ingenious arrangement devised by Mr. Wallis, for facilitating the 140 METEOROLOGY adjustment of the barometer scale, is represented. It can be clamped to any of the barometers constructed on the Fortin principle. 2. In 1853, Mr. P. Adie, of Edinburgh, invented a barometer for use at sea, which is commonly known as the Kew barometer (Fig. 21). It is so called because it was re- commended by the Kew Committee of the British Association for adop- tion by the Government as best suited for marine observations then about to be commenced by the Admiralty and the Board of Trade. Its distinctive features are a brass frame, a contracted tube, having a pipette, a closed FIG. 20.- waiiis's arrangement for c i s tern, and a scale of contracted adjusting the Ivory Point. . . , f . inches. In this, the "Marine Barometer," the tube is of small calibre throughout the greater part of its length in order to lessen the oscillations of the mercury caused by the ship's motion, which are tech- nically known as " pumping." This renders the instrument rather sluggish, but not materially so. The cistern is entirely composed of iron (because brass being an alloy is liable to be acted on by mercury), and only a small aper- ture in its roof is left through which atmospheric pressure is able to exert itself on the contained mercury. This aperture is, as has been said above, covered with leather to prevent the escape of the mercury. In this instrument the " error of capacity " is compensated by contracting the divisions on the scale from above down- wards, in proportion to the relative sizes of the tube and the cistern. In ordinary Kew barometers the diameter of the tube is about 0*25 inch, and that of the cistern about T25 inch. Accordingly, starting from 32 inches^correctly marked THE BAROMETER 141 off from a definite point below, the " inches " of the scale are shortened in the proportion of 0'04 inch for every true inch. Every tube is fitted with an " air-trap," which is a small funnel or pipette inserted somewhere between the range of the column and the neck of the cistern. The pipette was first proposed by Gay- Lussac in order to stop the ascent into the Torricellian vacuum of any air or moisture which may work its way from the cistern into the tube between the glass and the mercury (Fig. 22). The so-called " Gun Barometer " was designed by Admiral Eobert FitzEoy, in 1861, for the naval service. It is a modification of the marine barometer, and is intended to withstand the con- cussion of heavy ordnance. The glass tube is surrounded wherever possible with vulcan- ised india-rubber tubing as packing. This checks the vibration from firing, but does not hold the tube too rigidly. 3. To Gay-Lussac we owe the Siphon barometer, which consists of a bent glass tube of uniform calibre, but with one branch or leg much longer than the other. The longer limb is closed at the end and carefully filled with pure mercury, the shorter limb is quite open, and serves as a cistern. As ^ e mercury falls in the long limb, leaving Kew Baro- the Torricellian vacuum above it, it must rise to an equivalent height in the short limb. The motion in each limb is exactly one-half of what takes place in a Fortin barometer. The atmospheric pressure, or " height of the barometer," is the difference between the two levels, so that two readings must on every occasion be taken FIG. 22. Gay-Lussac Air-Trap. 142 METEOROLOGY one, of the level of the mercury in one limb ; the other, of the level in the other limb. This instrument is the only mercurial barometer suitable for mountain climb- ing, owing to its lightness and portability (Fig. 23). The ordinary wheel barometer, or "weather glass," was invented in 1665-6 by Robert Hooke, Secretary of the Royal Society. It is a siphon barometer. Resting on the mercury in the shorter liinb is a float connected by a silken cord with a light counterpoise at the other side of a fixed pulley, round which the cord is coiled two or three times. A needle indicator attached to the axis of this pulley rotates with the rise and fall of the mercury round a graduated circular dial, on which are also the words : "set fair," "fair," "change," "rain," "much rain," and " stormy." These words are intended to indicate what weather may be expected when the needle points to each part of the dial. Although a popular instrument, the wheel barometer is of no scientific value. Its principle, however, has been applied in the construction of one form of self-registering barometer, or baro- graph (Gk. /Sdpos, iveight ; y/oa(/>co, / write). A pencil is attached to the cord connected with the float, and this pencil is so arranged that it draws a continuous tracing on ruled paper which is moved by clockwork. In a modified barograph of this kind, ruled metallic paper spread on a revolving vertical drum of about 4 inches diameter is pierced at given intervals (usually every hour) by a needle shot out by clockwork, and ingeniously connected by means of a pulley with the mercurial surface in the short limb of a siphon barometer. The drum or cylinder in this baro- FIG. 23. Siphon Barometer. THE BAROMETER 143 graph is made to revolve once a week by means of clock- work. One of the costliest barographs in existence was designed in 1853 by the late Mr. Alfred King, C.E., of Liverpool (Fig. 24). About 130 Ibs. of mercury are employed in the construc- tion of this instrument, and the effects of friction, which are the great drawback in wheel barometers, are entirely overcome by the most sensitive mechanical arrangements. In this in- genious barograph the tube is partly supported by the mercury in the fixed cistern, which, as it rises and falls, raises and depresses the tube. A delicate mechanical contrivance records this change of level continuously on a revolving drum. The barometric column is made to show nearly 6 inches for each inch of the ordinary barometer. This instrument has for many years been in use at the Bidston Observatory, near Liverpool. It is fully described in the late Mr. Hartnup's "Report to the Mersey Docks Board for 1865." Mr. R. H. Scott, F.R.S., speaking of barographs, 1 observes : "The principle of registration generally adopted in this country for the better class of barographs is photographic, not mechanical." The late Sir Francis Ronalds in 1847 designed a photo- graphic barograph, which in a modified form is employed in the First Order Stations of the Meteorological Office, London. The principle of the instrument is described at length in the "Report of the Meteorological Committee of the Royal Society for 1867" (pp. 40-42). "The barometer is of the ordinary pattern and the light is admitted through the Torricellian vacuum, so that the actual height of the mercury itself is photographed without the intervention of any mechanical contrivance" (R. H. Scott). Those who are interested in this subject of barographs will find at p. 412 of the second volume of the Quarterly Journal of the Meteorological Society ', a description of a new barograph invented by M. Louis Redier, which was communicated to 1 Elementary Meteorology, p. 77. Fio. 24. Alfred King's Barograph. THE BAROMETER 145 the Society on March 17, 1875, by Mr. G. J. Symons, F.R.S. The apparatus is so arranged that all the work is done by a powerful clock movement, and the barometer, of the siphon type, has only to direct the action of the clockwork. In 1886, M. Redier, in a pamphlet, described a later form of his mechanical barograph under the title "Nouveau barometre enregistreur a mercure." In it the barometer is at rest. A differential clock train keeps a light horizontal arm in continuous slight vertical oscillation close to the point of a stalk rising from the mercury in the lower branch of a siphon tube. As the arm follows the stalk in all its varia- tions of position, the barometric variations, through a pencil, become continuously recorded. In addition to mechanical contrivances and photography, electricity has been employed in the construction of the barograph. Sir Charles Wheatstone, in the British Associa- tion Report for 1842, suggested the adaptation of electricity for the purpose. He proposed that a platinum wire, controlled by a clock, should make contact at given intervals with the mercury in the tube of a barometer or other instrument, for example the dry and wet-bulb thermometers, so creating an electric current which should determine both the record and the value of the element (W. Ellis). This principle has been since applied in the barographs included in the combined meteorographs of Salleron (1860), Theorell (1869), and Van Rysselberghe (1873), the records being all intermittent. Transmission of Barometric Indications by Electricity. In T882, Mr. J. Joly, of Dublin, described in Nature (vol. xxv. p. 559) a plan for ascertaining the reading of a distant mercurial barometer, connected with the recording station telegraphically. He carries two wires through the head of the barometer tube. One of these of a given diameter is continued downward into the mercury to a point below which the mercury never falls. The continuation of the other is a fine carbon thread, also of a given diameter, carried to the same point and there joined to the wire. The outer ends of L J4 6 METEOROLOGY the wires pass to the recording station, an electric current sent from which traverses both wire and carbon in its passage. The carbon being a substance of high resistance, a very small change in its effective length due to the rise or fall of the mercury in the barometer tube, exposing less or more of the fine carbon thread, will tell on the potential of the return- ing current. This variation of potential would, in Mr. Joly's opinion, be sufficiently marked to enable an observer at the recording station to measure the barometric variations at a station four miles distant, involving eight miles of wire. Before explaining the method of reading the barometer, it may be well to describe some, substitutes for mercurial barometers which have been devised. The Aneroid Barometer is the chief of these. It was invented by M. Vidi, of Paris, in 1843, and patented in the following year. The French patent is dated April 19, 1844. As the name implies it contains no fluid (Gk. a, priv. vrjpos, wet or damp, hence liquid or fluid ; and eiSos, form or shape). For this reason the aneroid is also known as the " holosteric barometer," the word "holosteric" meaning "en- tirely solid " (Gk. 6Aos, whole ; crre/aeos, solid). In this in- genious instrument the pressure of the atmosphere is measured by its effect in altering the shape of a small, hermetically sealed, exhausted metallic box, called the " vacuum chamber." This vacuum chamber is composed of two discs of corrugated German silver soldered together. Its sides are made in con- centric rings, so as to increase their elasticity, and one of them is fastened to the back of the brass case which contains the whole mechanism. A strong spring also fixed to the case is so arranged as to act in opposition to the motion of the vacuum chamber, preventing its sides collapsing when the effect of reduced atmospheric pressure is added to that of extreme exhaustion of the chamber. A lever, composed of iron and brass so as to compensate for changes in temperature, connects the spring, by means of a bent lever at its further end, with a watch-chain which is wound about THE BAROMETER 147 an " arbor " (axle or spindle). An index-hand or pointer, fixed to the arbor in front, is by its revolution caused to rotate backwards and forwards over an experimentally graduated dial, and so is made to mark the variations in atmospheric pressure from time to time. A very slight alteration of the size of the vacuum chamber produces a large deviation of the index-hand, ^^tla. of an inch, causing it to move through 3 inches as marked on the dial. FIG. 26. Aneroid, extra " 'small, in silver case. FIG. 25. Aneroid Barometer. FIG. 27. Field's Engineering Aneriod Barometer. FIG. 28. Extra-sensitive Aneroid Barometer. When pressure increases, the falling in of the corrugated sides of the vacuum chamber will pull upon the lever, which in turn, acting through the second or bent lever, will pull upon the chain, drawing it off the arbor and so causing the pointer to move across the dial towards the right, marking high pressures. 148 METEOROLOGY When pressure decreases, the expansion of the vacuum chamber will allow the compensating spring to push away the lever, which will relax the chain, allowing it to be wound round the arbor by a spiral hair-spring, which will move the arbor and pointer towards the left, marking low pressure. In 1851, Kusk added an altitude scale to the aneroid barometer. A Metallic Barometer, designed by M. Bourdon in 1851, is a modification of the principle of the aneroid. This instrument is described in Besant's Elementary Hydrostatics (Chapter V. "Notes"), and in the Report of the Jury on Philosophical Instruments in the Great Exhibition of 1851, as well as in a lecture by Mr. James Glaisher, F.R.S., on these instruments. It consists of a thin elastic metal tube of elliptic section, in shape a portion of a circle, closed at its ends and exhausted of air. Alterations in the pressure of the atmosphere are indicated by the ends of the tube ap- proaching towards each other when pressure increases, and receding from each other when pressure diminishes. These motions are communicated by gearing work to an index hand traversing a dial plate. No definite explanation of the principle of action of this instrument was offered until the Rev. E. Hill, M.A., Fellow of St. John's College, Cambridge, communicated a paper on the subject to the Meteorological Society on February 21, 1872. The above particulars are taken partly from Mr. Ellis's address, but chiefly from Mr. Hill's communication. His explanation, however, is of too recondite a nature to be reproduced here. Aneroids, while very sensitive, are apt to get out of order owing to defects in construction, rusting, or loss of elasticity in the springs. They are therefore not used at Second Order Stations or for concerted observations, for which accurate mercurial barometers are indispensable. If an aneroid is employed its readings should be frequently compared with those of a reliable mercurial barometer re- duced to 32 F. It is a popular instrument because of its THE BAROMETER 149 convenient size and portability. Besides, it requires no cor- rection for its own temperature. Measurement of Altitudes. An aneroid in good order will show with precision the difference in height between the various stories in a lofty house, the varying gradients in travelling on a railway, and mountain or balloon elevations it may be up to 24,000 feet. One of the chief uses of the aneroid, indeed, is the measurement of altitudes. Owing to the elasticity of the atmosphere, the reduction of pressure does not proceed evenly with altitude, and accordingly special altitude scales have been computed, which are engraved on the dial of the instrument. A correction for the temperature of the air (not for that of the instrument) must always be made, and so in the "Engineering Aneroid," invented by Mr. Rogers Field, B.A., Assoc. Inst. C.E., F.R. Met.Soc., and manufac- tured exclusively by Mr. L. Casella, this cor- rection is taken into account by making the scale adjustable for temperature (Fig. 27). While on this subject, it will be well briefly to describe two other instruments which are used for ascertaining the altitude one a barometer, the other a thermometer the sympiesometer and the hypsometer respectively. The sympiesometer, or "compression measure" (Gk. (n;/z7rie(ris, compression, from cru/zTrte^w, to press or squeeze together ; ptrpov, a measure), was invented by Mr. P. Adie,of Edinburgh (Fig. 29). It is a sensitive but unreliable kind of barometer (using this term in its strict etymological sense), consisting of a glass tube 18 inches in length and three-quarters of an inch in diameter, which Flo 29. terminates in a closed bulb above, and, after a Sympiesometer. sharp bend, in an open cistern below. The pressure of the air, acting through the latter on the surface of a fluid, such as oil or glycerine, in the lower part of the cistern and of the METEOROLOGY tube, forces it upwards so as to compress an elastic gas, such as hydrogen or air, in the upper part of the tube and FIG. 30. Portable Leather Case for holding Casella's Hypsometer. in the bulb. The amount of compression is read off on an adjustable scale, the index of which must be set to the division on the scale corresponding to the temperature indicated by an attached thermometer. The principle of the hypsometer (Gk. v^os, height; /uer/oov, a measure) is based on the fact, already referred to in Chapter IX. p. 97, that the boiling point of water falls according as atmospheric pressure is reduced. The in- strument consists of a vessel for water, with a spirit lamp for heating it, and an enclosed thermometer for showing the temperature of ebullition. In Casella's hypsometer (Fig. 31), a strong, small-bulbed thermometer, divided and figured on the stem, is sheltered from cold when in use by a double telescopic chamber, into which it is introduced to any required depth through a loose piece of india-rubber at the top. When the water boils, the vapour fills the inner chamber and envelops the thermometer, bulb and stem alike, finally descending in the outer chamber and escaping by a pipe outlet. Mr. Casella has constructed a smaller instrument on the same principle, which is much used by Alpine travellers. FIG. 31. Casella's Hypsometer. CHAPTEE XIV BAROMETRICAL READINGS Attached Thermometer Mounting of Barometer Method of taking a Barometrical Observation "Capillary Action" Capillarity The Vernier Graduation of British Barometers Corrections to be applied to Barometrical Readings : Index Error, Capacity, Capillarity, Temperature, Altitude Verification of a Barometer The Catheto- meter Kew Certificate Schumacher's Formula for Reduction of Barometer Readings to 32 Ordnance Datum for Great Britain Ordnance Datum for Ireland Table of Corrections for Altitude con- trolled by existing Air Temperature and Pressure Laplace's Formula for finding the difference in Height between two Places Determina- tion of Mountain Heights by the Barometer. SINCE mercury expands by heat and contracts by cold, it is necessary that every barometer should carry a thermometer closely attached to its metal case, or preferably to its glass tube and cistern. By means of this " attached thermometer " the observer is placed in a position to apply a proper cor- rection to the reading of the barometer with the view of reducing that reading to the fixed standard of temperature, or 32 F. Before any observation is made, the barometer should be mounted in a room with an equable temperature, not near a fireplace or a stove. Its scale should be on a level with the observer's eye 5 feet, or 5 feet 6 inches above the floor. It must hang vertically : " Care should be taken that no readings from a barometer which is not hanging truly vertically should I 5 2 METEOROLOGY ever be recorded." 1 "To facilitate readings, a piece of white paper or of opal glass should be fixed immediately behind the part of the tube at which the readings are taken, and if the barometer be of the Fortin pattern another piece should be placed behind the cistern." 2 This arrangement may be seen well represented in Fig. 19 above (p. 139). The method of taking a barometrical observation is as follows : 1. The attached thermometer should first be read, no matter what kind of barometer is employed. The tempera- ture of the external air (dry -bulb reading) should also be taken. 2. Next, in the case of a Fortin or standard barometer, the mercury in the cistern should be adjusted by turning the screw at the bottom, so that the tip of the ivory pin, or the fiducial point, should barely touch the surface of the mercury. This manipulation is not required, nor is it possible, in the case of the Kew barometer, in which (as has been explained) the scale of shortened inches compensates for the error due to capacity. 3. The barometer tube should, after this, be gently tapped to overcome any tendency to adhesion between the mercury and the glass, and to allow capillary action to assert itself. It is necessary to mention that, in obedience to what is called "capillary action," a liquid like water, capable of wetting a clean glass tube open at both ends, will rise in such a tube above the level of its surface in the vessel containing it, and higher and higher according to the fineness of the bore of the tube. Hence the term "capillary," from the Latin "capilla," a fine hair. Further, the liquid will stand above the general level in the tube where it approaches the sides, so that its upper surface in the tube will be curved 1 Instructions in the Use of Meteorological Instruments, compiled by Robert H. Scott, M.A., F.R.S. Reprinted 1885. 2 Hints to Meteorological Observers. By William Marriott, F.R. Met. Soc. BAROMETRICAL READINGS 153 and concave, owing to capillary attraction. On the other hand, a liquid like mercury, incapable of wetting such a tube, will stand in the tube below the level of its surface in the vessel, and where it approaches the sides of the tube, its level will be below its general level in the tube, so that its upper surface will be curved and convex, owing to capillary repul- sion. This causes the mercury in a barometer always to stand a little lower than the height due to atmospheric pressure, and necessitates a correction for capillarity. Such a correction is less in a barometer in which the mercury has been boiled in the tube than in an unboiled tube, for by the boiling a film of air, which in unboiled tubes adheres to the glass, is expelled. The error is also reduced by widening the bore of the tube ; for example, the depression in a boiled tube of a quarter of an inch in diameter is '02 inch, whereas in a similar tube of half an inch diameter it is only '003 inch. 4. It follows from the foregoing that, in reading a baro- meter, the height should be taken from the very apex of the convexity, or of the meniscus, as it is called (Gk. /X^I/ICTKOS, a crescent, from pjvr;, the moon). This is done by means of the vernier, the two lower edges of which should be brought, by turning the mill-head pinion which moves the rack up or down, to form a tangent with the convex surface of the mer- cury. As Mr. Marriott well remarks : " The front and back edges of the vernier, the top of the mercury, and the eye of the observer must be in the same straight line." But what is the vernier ? It is a short scale named after its inventor, Pierre Vernier, made to slide by means of a rack and pinion along the divisions of a graduated scale, such as that of a barometer, and its divisions are so contrived as to be slightly shorter than those of the barometer scale, which is generally divided into inches, tenths, and half-tenths, or five-hundredths ('05) of an inch. The vernier is made equal in length to twenty-four half-tenth divisions of the barometer scale, and is then itself divided into twenty-five equal parts. From this it follows that each space on the vernier scale falls 154 METEOROLOGY short of a space on the barometer scale by the twenty-fifth part of '05 inch, or TTS x "A" ~ 2 "5^00 = TU~O = "002 inch. Each division of the vernier, therefore, represents a difference . of '002 inch, or one five- hundredth of an inch in pressure, while by interpolating a reading between any two divi- sions of the vernier, we are enabled to read the pressure to - 001 inch, or the one-thous- andth of an inch. In using the vernier, the division on the barometer scale at or below which the lower edge of the vernier stands after setting it should first be read off. In the accompany- ing figure two cases are illustrated. In one (that to the left), the lowest line on the ver- nier scale exactly coin- cides with the division 29 '50 on the barometer The reading is therefore 29-500 inches -&\ n Of c I b - I - H 9 > %> N 4 - - BRA. LONDO 3 > 2 CD 3 > .=> - - 1 - - 2 - I - >J 1 _ - - d u-. ^ h I j - [ :> (Ju [ ae i 3 29 E ; c 29 ase 2. FIG. 32. FIG. 33. Method of reading the Vernier. 20 5 10 scale. 05 oo precisely. In the other (that to the right), the reading on the barome- ter scale gives us 29 "65, but the height of the column of mercury is in reality that BAROMETRICAL READINGS 155 amount in inches plus the vernier indication. On looking up the vernier scale in this case, we find that the second and third divisions above the figure 3 coincide with a division on the barometer scale. It is therefore necessary to add the decimal '035 to the first reading, thus : 29 '65 +'035 = 29-685 inches. In cases where it is hard to say which division of the barometer scale is that Mow the lower edge of the vernier, the reading of the latter will itself point out which division on the barometer scale should be taken. For example, in the left-hand drawing the correct reading is manifestly 29-500 inches, not 29'50 + -050 = 29-550 inches. In fact the value of the vernier reading in the example is zero : 29-50 + 000 = 29-500 inches. It may be well to repeat, what has been already conveyed in different words, that English barometers are usually gradu- ated in the following way : 1. Every long line cut on the barometer scale represents one-tenth of an inch ('100 inch). 2. Every short line cut on the barometer scale represents five-hundredths of an inch ('050 inch). 3. Every long line cut on the vernier scale represents one- hundredth of an inch ('010 inch). 4. Every short line cut on the vernier scale represents two- thousandths of an inch ('002 inch). CORRECTIONS TO BE APPLIED TO HEADINGS OF THE BAROMETER Before barometrical readings taken synchronously at different places by various observers can be compared with each other and used for scientific purposes a number of corrections must be applied to the recorded readings. Many of these corrections have been already mentioned, but all of them must now be referred to and classified according as i 5 6 METEOROLOGY they relate to a given instrument, or are applicable to the readings of any instrument taken under the same conditions. The corrections of the former class are three in number I. Index error. II. Capacity. III. Capillarity. Those of the latter class are two in number IV. Temperature. V. Altitude, or height above sea-level. I. Index Error. This is detected by careful comparison with a recognised standard barometer. It includes all errors in graduation of the scale. The detection of the index error is simple in the case of the Fortin barometer, but compli- cated in that of the Kew barometer. The latter instrument must be tested at every half inch of the scale from 27 to 31 inches, because its inches are less than true inches. To pass through this ordeal it is necessary to use artificial means of increasing or reducing pressure, and so the instrument and the standard have to be placed in an air-tight chamber con- nected with an air pump. The instruments can thus be made to read higher or lower as the air in the chamber is com- pressed or exhausted. Glass windows through which the instruments can be seen are placed in the upper part of the iron air-tight chamber, but of course the verniers cannot be used, as the observer is outside the chamber. To overcome this difficulty an apparatus called a cathetometer (Gk. /catferos, let down ; hence 17 Katferos [sc. ypa/ifnj], a perpendicular line, a perpendicular height ; fj^rpov, a measure, has been devised. This is a vertical scale, on which a vernier and a telescope are made to slide by means of a rack and pinion. The divisions on the scale correspond exactly with those on the tube of the standard barometer. The cathetometer (Figs. 34, 35, and 36) is placed at a distance of 5 or 6 feet from the air-tight chamber. The telescope carries two horizontal wires, one fixed, and the other capable of being moved by a micrometer screw. The FIG. 34. Cathetometer constructed for the Indian Government. FIQ. 35. Cathetometer, as used at Kew Observatory. FIG. 36. Cathetometer, 6i feet in height. BAROMETRICAL READINGS 159 difference between the height of the column of mercury and the nearest division on the scale of the standard can be measured either with the vertical scale and vernier, or with the micrometer wire. The errors detected in this way include not only the index error, but also the correction required for capillarity. II. Capacity. The meaning of this term has been already explained. It will be remembered that the siphon barometer requires no correction for capacity. In the Fortin barometer it is provided for by adjusting the scale to the level of the mercury in the cistern, and in the Kew barometer, by shorten- ing the "inches" cut on the scale. In barometers with closed cisterns and a scale of true inches engraved on the case, there is a certain height of the mercurial column which is correctly measured by the scale. This is called the neutral point, and it should be marked on the scale by the maker, who should also state the ratio of the interior area of the tube to that of the cistern thus : Capacity = -j. From these data the correction for capacity is found by taking a fiftieth part of the difference between the height read off and that of the neutral point, adding the resulting value to the read- ing when the column is higher than the neutral point, sub- tracting it from the reading when it is lower than that point. III. Capillarity. It has been shown above that the effect of capillary action is always to depress the mercury in a barometer. The amount of depression is nearly inversely proportional to the diameter of the tube, and is always greater in unboiled than in boiled tubes. The correction for capillarity is always additive ( + ), and according to the Report of the Committee of the Royal Society on Physics and Meteorology, 1840, it varies from + 0'004 inch in the case of an unboiled, and +0'002 inch in that of a boiled tube of the diameter of 0'60 inch, to +0142 inch in the case of an unboiled, and + 0*070 inch in that of a boiled tube of the diameter of O'lO inch. The certificate of verification of a barometer issued from Kew Observatory includes the three corrections we have been i6o METEOROLOGY considering as applicable to the individual instrument. Here is a copy of the certificate which accompanied the barometer in use at my own station ^of the Second Order : Corrections to the Scale Eeadings of Barometer, 877, by Adie, London. in. At 27 '5 in. At 28-0 in. At 28 -5 iii. At 29-0 in. At 29-5 in. At 30-0 At 30-5 in. At 31 -0 in. + 0-030 in. + 0-026 in. + 0-022 in. + 0-019 in. + 0-015 in. + 0-011 in. + 0-008 in. + 0-004 When the sign of the correction is + , the quantity is to be added to the observed reading ; and when - , to be subtracted from it. The corrections given above include those for Index Error, Capacity, and Capillarity. Pro. B. STEWART, KEW OBSERVATORY, Jan. 7, 1867. ^. ^' BAKER. IV. Temperature. Both the mercurial column and the brass scale of a barometer expand by heat, and so the height of the column varies with temperature. It therefore becomes necessary to reduce all observations to what they would have been at a given temperature, which is taken as a standard. This standard temperature is 32 F. An elaborate table for reducing the readings of barometers mounted in brass frames to 32 F. has been computed from the following formula given by Schumacher : h = reading of the barometer, t = temperature of attached thermometer, m = expansion of mercury for 1 F., taken as '0001001 of its length at 32, s = expansion of the substance of which the scale is made; for brass s, is taken as '00001041 of its length (h) at the standard temperature for the scale, viz. 62 F. BAROMETRICAL READINGS 161 In this Table the sign of the correction changes from + to - at the temperature of 29, as the formula gives nega- tive results for 3 below 32. V. Altitude. Every barometrical observation should be reduced to mean sea-level as a standard, because as the barometer measures the pressure of the atmosphere, the height of its column will vary with that pressure, becoming less as we ascend and leave some of the atmosphere, and, therefore, some of its pressure below us ; and, on the other hand, becoming greater as we descend and leave more of the atmosphere and more of its pressure above us. For Great Britain the mean sea-level at Liverpool has been selected by the Ordnance Survey as their datum, and the altitude of a barometer at any station in England, Scotland, or Wales may be easily determined by reference to the nearest Ordnance Bench Mark. The Ordnance Datum plane for Ireland differs from that for England and Wales by - 7'4 feet that for Ireland being low water spring -tides, while that for England is mean tide-level. In order that observations in the two countries should be exactly comparable, the Meteoro- logical Office, in 1890, issued new tables for use in Ireland in reducing barometrical readings to the Liverpool Ordnance Datum. Any table of corrections for altitude or reduction to mean sea-level must take cognisance of two disturbing elements, the temperature of the air and the actual air pressure at sea- level at the time of observation. The air temperature must be taken from the dry-bulb thermometer, not from the ther- mometer attached to the barometer. Table II. in Appendix I. to Instructions in the Use of Meteorological Instruments, com- piled by direction of the Meteorological Council by Mr. Robert H. Scott, M.A., F.R.S., contains data for reducing to sea-level barometrical observations made at every 10 feet from 10 to 1500 feet above the datum, and at temperatures varying by 10 from - 20 F. to 100 F., that is a range of 120. The Table is given for two pressures at the lower or M 162 METEOROLOGY sea-level station, namely, 30 and 27 inches. For inter- mediate pressures the correction may be obtained by inter- polating proportional parts. For heights exceeding those given in the Table, the value, at the sea-level, of a barometer reading at a station, the height of which is known, may be calculated from the follow- ing formula : ,0 26 S CO., ) ( From a table of common logarithms, the natural number corresponding to log , is found ; or, = n, And h = n h'. In this formula h and li = barometer reduced to 32 F. at the lower and upper stations respectively, t and t' = the temperature of the air at the respective stations, /= elevation of upper station in feet, I = latitude of the place. The above formula is merely an inversion of the well- known formula given by Laplace in his Mecanique Celeste, for finding the difference of elevation between any two places by means of the barometer, which, adapted to Fahrenheit's thermometer and English feet and inches, is 10443430/ In this formula / is the difference of elevation between the two stations, and x is the height of the lower station above the sea-level. In the last factor an approximate value must be used for/. BAROMETRICAL READINGS 163 Not only, then, can we reduce the barometer reading at one level to that at another, the relative heights of the two stations being known, but we can conversely determine the difference in height between two stations if we know the barometrical readings and the temperature at each at the same moment of time. In other words, we can determine the height of a mountain by barometrical readings taken simultaneously on the summit, and at sea-level. CHAPTEE XV BAROMETRICAL FLUCTUATIONS Periodic and Non-periodic Variations in Atmospheric Pressure Regular and Irregular Variations Regular : Diurnal and Annual Irregular : Cyclonic and Anticyclonic Distribution of Diurnal Variations Explanation Dove's Theory Mr. Strachan's Objections Diurnal Range of Pressure Bibliography Annual Variations of Pressure Periodical Anticyclonic and Cyclonic Systems Explanation of their Formation Trade Winds Irregular Variations in Pressure Isobars : Primary and Secondary Shapes Cyclonic and Anticyclonic Secondary or Subsidiary Depression V-shaped Depression Straight Isobars " Wedge " of High Pressure "Col " of High Pressure Cyclonic and Anticyclonic Systems contrasted "Radia- tion Weather" "Intensity" Path of Cyclonic Systems Weather Changes accompanying their Passage "Veering," "Hauling," "Backing" of the Wind Anticyclonic Weather: (1) in Winter; (2) in Summer. ATMOSPHERIC pressure as measured by the barometer is subject to two classes of variations periodic and non-periodic. The first are regular ; the second, irregular. The regular, or periodic, variations are (1) diurnal, (2) annual. The irregular or non-periodic variations are (1) cyclonic, (2) anticyclonic. 1. Diurnal variations are best marked within the tropics, or in the torrid zone. They are less marked in temperate climates, absolutely because their physical cause is there less potent, relatively because the irregular variations in atmo- BAROMETRICAL FLUCTUATIONS 165 .spheric pressure so frequent in higher latitudes tend to mask them. They gradually sink to zero towards the Arctic and Antarctic Circles and the Poles. So regular is the daily rise and fall of the barometer in the tropics that Humboldt said that the time of day might be inferred from it within seventeen minutes. As the earth rotates on its axis day by day, the hemi- sphere facing the sun becomes overheated, the air over it expands, becomes specifically lighter, rises and tends to flow away from the day hemisphere to the night hemisphere. The barometer consequently falls in the hottest part of the day, reaching a minimum about 3 P.M. But a second, though less decided minimum occurs about 3 A.M. This cannot be caused in the same way as the day minimum, for, as a matter of fact, the air is coldest about 3 or 4 A.M. We must there- fore seek elsewhere for an explanation. It is to be found, according to Dove (and his theory received the sanction of Sabine), in the state of tension of aqueous vapour in the early morning. As we shall see when we come to discuss the subject of the moisture of the atmosphere, barometrical pressure is made up of two elements the pressure of dry air, and the pressure of aqueous vapour suspended in the atmosphere. This latter is technically called the elastic force, or tension of aqueous vapour. Now long before 3 A.M. dew has fallen heavily in other words, the aqueous vapour has been condensed by the nightly fall of temperature, and has left the atmosphere in the condition of dried or desic- cated air. In this way the tension of aqueous vapour is largely withdrawn, and the barometer falls. As there are two minima of pressure, so there are two maxima. Of these the first occurs about 10 A.M., the second about 10 P.M. Condensation of the air after a cold night partly accounts for the forenoon wave-crest of pressure. But another potent cause is rapid evaporation and consequently increasing tension of aqueous vapour. The evening maxi- mum is no doubt due to a brisk decrease of temperature, 1 66 METEOROLOGY causing condensation of the atmosphere coupled with the saturated state of the air after the evaporation of the day- time. The vapour tension, or elastic force, in a word attains its maximum. The foregoing theory affords a rational explanation of these interesting diurnal fluctuations of pressure, and it receives support from the fact that at stations far distant from the sea, or with a high mean temperature that is, at places where the diurnal ranges of temperature are least inter- fered with by large evaporating surfaces like the ocean or by moist winds the maximum at 10 P.M., and the minimum at 3 A.M., which are largely due to the condition of the aqueous vapour, are only slightly marked. Dove's theory, however, does not receive universal accept- ance, because (in the words of Mr. R. Strachan, F.R. Met. Soc. 1 ) " the diurnal range of vapour tension does not always and everywhere conform to the simple oscillation." Accord- ing to Mr. Strachan, in the Island of Ascension we still have a double period for the diurnal range of the barometer, but the vapour tension and the dry air pressure of which it is composed both exhibit a double period also. Such cases completely demolish the theory. Mr. Strachan adds: "A hypothesis then remains yet to be framed which shall account for the diurnal range of the barometer in all seasons and places." Be that as it may, the fact remains that every day of twenty-four hours sees two vast waves of high pressure and two equally vast troughs of low pressure sweep round the globe at a speed equal to the revolution of the earth on its axis. It is as if two solar tides, stupendous in extent, with their alternate ebb and flow, were generated in the atmo- sphere by the action of the sun, or, as Inspector -General Robert Lawson suggests, by alternate accelerations and retardations of the motion of the atmosphere revolving with the earth on its axis, caused by the relation which the atmo- 1 " The Barometer and its Uses," Modern Meteorology, p. 89. 1879. BAROMETRICAL FLUCTUATIONS 167 .sphere bears to the orbital motion of the earth as distinguished from its axial motion. The diurnal range of pressure, as the difference between the extreme daily oscillations is called, exceeds one-tenth of an inch within the tropics at Calcutta it is as great as 0'127 inch in January (dry north-east monsoon), but only O093 inch in July (moist south-west monsoon) the average for the whole year being Oil 6 inch. At Plymouth and in Dublin it is about 0'020 inch, or only one-sixth of the tropical value. In St. Petersburg it is O012 inch, and within the Arctic Circle it merges gradually into the annual range, owing to the length of the circumpolar day and night. From observations in Dublin, extending over as many as thirty years, I am prepared to say that the diurnal range of pressure is quite perceptible in anticyclonic weather, especi- ally in spring-time, when the air is dry and the diurnal range of temperature is large. It is doubtless even better marked at an inland station like Parsonstown or Armagh under like circumstances. Observations, carefully analysed by Mr. Francis Campbell Bayard, have shown that it in- creases steadily from north-west towards south-east over Western and Central Europe. The following references to the recent bibliography of diurnal range of pressure may prove of interest. The papers will be found in the Quarterly Journal of the Royal Meteoro- logical Society. \ . On the Diurnal Variations of the Barometer. By John Knox Laughton, M.A., F.R.A.S. (vol. ii. p. 155 read April 15, 1874). 2. The Diurnal Inequalities of the Barometer and Ther- mometer, as illustrated by the observations made at the summit and base of Mount Washington, N.H., during the month of May, 1872. By W. W. Rundell, F.M.S. (vol ii. p. 217 read June 17, 1874). 3. On the Diurnal Variation of the Barometer at Zi-Ka- Sitka, 57 0'. St. Petersburg, 59 56'. Greenwich, 51 28'. Halle, 51 28'. Is * 3 I 1 Geneva, 46 13'. a s 3 w . 1 | Grt. St. Bernard, 45 50'. * * o Toronto, 43 38'. Philadelphia, 39 50'. p '^ I J San Francisco, 37 48'. Calcutta, 22 35' 2-8 Cumana, 10 27'. BAROMETRICAL FLUCTUATIONS 169 Wei (a suburb of Shanghai, 31 15' north latitude), and Mean Atmospheric Pressure and Temperature at Shanghai. By Rev. Augustus M. Columbel, S.J. (vol. ii. p. 232 read June 17, 1874). 4. Suggestions on certain Variations, annual and diurnal, in the Relation of the Barometric Gradient to the Force of the Wind. By the Rev. W. Clement Ley, M.A., F.M.S. (vol. iii. p. 232 read June 21, 1876). 5. On the Diurnal Variation of the Barometer at the Royal Observatory, Greenwich. By William Ellis, F.R.A.S., of the Royal Observatory (vol. iii. p. 467 read June 20, 1877). 6. On a Method of sometimes determining the amount of the Diurnal Variation of the Barometer on any parti- cular day. By the Hon. Ralph Abercromby, F.M.S. (vol. iv. p. 198 read June 19, 1878). 7. The Daily Inequality of the Barometer. By W. W. Rundell, F.M.S. (vol. v. p. 1 read May 15, 1879). 8. Diurnal Variations of the Barometric Pressure in the British Isles. By Frederick Chambers, Meteorological Reporter, Bombay (vol. v. p. 133 read February 19, 1879). See a paper by the same author in the Philosophical Transactions of the Royal Society for 1873" Convection Current Theory." 9. The Diurnal Range of Atmospheric Pressure. By Richard Strachan, F.M.S. (vol. vi. p. 42 read December 17, 1879). 10. Results of Hourly Readings derived from a Redier Barograph at Geldeston, Norfolk, for the four years ending February 1886. By E. T. Dowson, F.R. Met. Soc. (vol. xiii. p. 21 read November 17, 1886). 11. On the Cause of the Diurnal Oscillation of the Baro- meter. By Robert Lawson, LL.D., Inspector-General of Hospitals (vol. xiv. p. 1 read November 16, 1887). 12. The Diurnal Range of the Barometer in Great Britain 1 7 o METEOROLOGY and Ireland, derived from the hourly records of the nine principal Observatories in the Kingdom during the years 1876-80. By Francis Campbell Bayard, LL.M., F.R.Met.Soc. (vol. xv. p. 146 read April 17, 1889). 2. Annual Variations in atmospheric pressure are on a far vaster scale than the daily ranges we have been consider- ing. As typical examples, we may adduce the high-pressure areas observed in January over the central districts of North America (30'20 to 30*30 inches) and over Central Asia (30-30 to 3040 inches and upwards. These anticyclonic systems on a gigantic scale give place in July to equally well-marked low pressure, or cyclonic, systems the mean pressure falling over the central parts of North America to 29 '80 inches and less, or half an inch on the average below the mean pressure of January; and over Central Asia to 29*60 inches and less, or eight-tenths of an inch below the January mean. Again compare the low-pressure areas of January situated over the Pacific south of Alaska (29 '60 inches), and over the Atlantic south of Greenland and Iceland (29-40 inches), with the comparatively high mean pressures for July in these oceanic regions 29*90 to 30'00 inches over the Pacific, 29-80 to 29-90 inches over the Atlantic. Let us seek for an explanation of these phenomena. Over the centre of that vast continent of the Eastern Hemisphere or Old World, which is formed by Europe and Asia,, the air in summer becomes much warmer than that over the Atlantic Ocean to the west, and over the Pacific Ocean to the east. In consequence, the air is rarefied, and the barometer falls over Russia, Siberia, and other inland countries the isobars, or lines of equal barometrical pressure, curving round the area of lowest pressure while it remains comparatively high over the North Atlantic and North Pacific Oceans. In accordance with Buys Ballot's Law, a circulation of air will commence round the barometrical depression thus BAROMETRICAL FLUCTUATIONS 171 formed : an immense cyclone develops, the winds blowing against the hands of a watch, from south-west in India and China (the south-west monsoon) ; from south, south-east, and east in Japan and North-eastern Siberia; from north-east and north in North-western Siberia and Northern Russia ; from north-west and west over the west and south of Europe and South-western Asia. In winter, on the other hand, the air over the central districts of Europe and Asia, rendered dry by the intense heat of summer and its accompanying excess of evaporation, becomes rapidly chilled to an extreme degree. The autumnal snows cover the ground, cutting off terrestrial radiation and causing a still more decided fall of temperature. By this the air is condensed and the barometer rises at a time when a vacuum is forming over the Atlantic and Pacific Oceans owing to the updraught and lateral dispersion of the light warm air which had been resting upon the surface of those oceans, and which may at the time possess a temperature 60 or even 80 higher than that of the air over the interior of the great continent. Owing to the advancing season also, and the consequent general decrease of temperature, the air over these oceans becomes saturated with moisture, frequent rains result, and a further reduction of atmospheric pressure results, caused by the latent heat set free in the formation of rain. In this way, conditions are brought about which arc the reverse of those observed in summer : an immense anti cyclone is formed, the winds circulating round and out from the centre of high pressure in a direction with the hands of a watch, blowing from north-west and north in Japan and China ; from north-east in India (the north-east monsoon) ; from east and south-east in Russia and Southern Europe ; from south-west in the British Isles ; and from west in Northern Russia and Siberia. In Central Russia and Siberia a region of calms will exist near the position of the highest atmospheric pressure. In the winter season the predominant \vinds over Scandinavia are south-easterly, but this apparent 172 METEOROLOGY anomaly is in fact a beautiful fulfilment of the very laws it seems to contradict. We have seen that in winter a barometrical depression exists over the North Atlantic Ocean, particularly over that portion of it which is called the Norwegian Sea. It is this which draws the wind from south- east over Sweden and Norway, in strict agreement with Buys Ballot's Law. A precisely similar state of things, though on a somewhat reduced scale, holds good in the Western Hemisphere, or New World. In summer, a barometrical depression, or cyclonic system, develops over Upper Canada and the Central States of the Great Republic, round which minimum the prevailing winds sweep in a gentle curve against watch-hands. In winter, on the contrary, an area of high atmospheric pressure, or anticyclonic system, develops in the same region, and round its central zone of calms the prevailing winds blow with watch-hands. Hence the prevalent north-west and north winds, which bring to Labrador, Lower Canada, and the Eastern States the rigorous winters of the American Atlantic Seaboard ; although, of course, the setting of a polar current of iceberg-laden water southwards along that seaboard intensifies the rigours of the climate, just as the warm waters of the Gulf Stream in laving the western shores of Europe temper the climate even further north than the Arctic Circle. Pfi equatorial regions, where air temperature and moisture are constants throughout the year, the annual variation in atmospheric pressure is trifling in amount. In the southern hemisphere, however, seasonal changes in pressure again become marked, although they are not so pronounced as in the northern hemisphere where dry land or continent so largely takes the place of ocean. Trade Winds. Leaving out of count the great disturb- ances of pressure from winter to summer and from summer to winter, caused by the rise and fall of temperature over the continents of the Old and New Worlds, we find that a belt BAROMETRICAL FL UCTUA TIONS 173 of comparatively high pressure, from 30'00 to 30*20 inches, encircles the earth at the tropics both north and south of the equator, while over the equator and the immediate vicinity to 10 or 15 north and south, the barometer stands from one-tenth to two-tenths of an inch lower. These areas of high and relatively low pressure oscillate backwards and forwards with the season : in January the northern zone of high pressure approaches the equator, while the corre- sponding southern zone recedes from it. Conversely, in July, the northern zone retreats northwards, while the southern advances towards the equator. In obedience to Buys Ballot's Law, permanent winds blow from these respective areas of high pressure towards the equatorial trough of low pressure, constituting the North-east Trades of the Tropic of Cancer and the South-east Trades of the Tropic of Capricorn. Dr. Alexander Buchan, in a masterly analysis of baro- metrical observations taken at some four hundred stations scattered all over the globe, has ascertained that atmospheric pressure is lowest throughout the year over the Antarctic Ocean, about 29 inches. " In the hemisphere where winter reigns, the greatest pressure lies over the land; the larger the continent, the greater the pressure. In the hemisphere where summer reigns the low pressures are over the land, the high over the oceans." Mr. R. Strachan, whose words we have just quoted, gives the following Table of the most remarkable areas of high and low pressures : Period. Position. Pressure. Inches. Iceland 29-4 December, January, February . 50 K 170 W. 50 N. 100 E. 29-6 30-4 to 40 S. 30-0 June, July, August . / 40 N. 90 E. \ 30 N. 40 W. 29-5 30'2 So far, periodical variations in atmospheric pressure 174 METEOROLOGY have been our theme. We have now to consider those irregular variations which daily, monthly, and yearly occa- sion changes in wind and weather over more or less exten- sive areas of the earth's surface. They are measured or determined by drawing lines of equal barometrical pressure, or isobars, on a map of the area under discussion, which is then called a synoptic weather chart. These isobars are drawn for each tenth of an inch. They tend to assume two primary ,-- '^29-7 Cyclone Wedge / / Cyclone - s /% r i / ' W/ ! / / \ f ,' / V-depressv > FIG. 38. Cyclonic and Anticyclonic Isobars. and five secondary shapes (Fig. 38). If they enclose an area of low pressure, forming a circle or an oval, they are described as cyclonic in shape, from the Gk. /o'/Jh (h being the water pressure in millimetres), and is also marked. The velocity of the wind is, so to speak, weighed. Neither temperature, pressure, moisture, rain, snow, nor hail has any influence upon this anemometer, the action of which is re- garded by the inventor as perfectly satisfactory. The Hage- mann anemometers are manufactured by Nyrop of Copenhagen. H. Direction. Lomonosow (1751) attached to the vane- spindle of his anemometer a vertical wheel, with a tube con- taining mercury running round the greater part of its cir- cumference perhaps 300. As this wheel, bridled by a spring, turned on its axis, a small quantity of the mercury was poured out into a tray beneath, divided into thirty-two radiating compartments ; the compartment in which the mercury was afterwards found indicated the direction of the wind, the quantity of mercury its force. Beaudoux (1777) registered the direction only by fine sand falling into a similarly divided tray. Goddard (1844) used water in the same way. Craveri (1866?) adopted a similar method of registry, corn grains being the weight employed. ANEMOMETRY AND ANEMOMETERS 265 I. Inclination. Various instruments have been designed with the object of showing whether any given current of wind has an upward or downward tendency. Benzenberg (1801) caused the windward end of the direction vane to carry a vertical fork open to the wind ; across this was fixed the axis of a horizontal vane, which showed the inclination of the wind. Cacciatore (1840), Director of the Observatory at Palermo, caused the velocity of the wind to be given by a horizontal fan of four curved sails, which, being segments apparently quadrants of a cylinder, necessarily revolved in one direction. A similar fan on a horizontal axis was fixed in a rectangular frame fastened to an upright spindle, so as always to swing away from the wind and rotate in a plane at right angles to the wind's direction. It could be acted on only by the vertical component of the wind. More modern instruments for observing the inclination are those invented by Professor Hennessy (1856) and Father Dechevrens (1881) of the Observatory of Zi-Ka-Wei, near Shanghai (Sur Vln- dinaison des Vents). In 1886, Mr. Louis Marino Casella, F.K. Met. Soc., de- scribed to the Koyal Meteorological Society an altazimuth anemometer which he had designed and patented. The object of the instrument is to record continuously the vertical angle, as well as the horizontal direction and force of the wind. A full description of his instrument will be found in the Quarterly Journal of the Royal Meteorological Society, vol. xii. No. 60, October 1886, p. 246. The existence of air- currents moving in a direction more or less inclined to the plane of the horizon, or "inclined currents" as Mr. Casella calls them in his paper, being conceded, the necessity for the study of their inclination and velocity is at once apparent. Mr. Casella's anemometer, which includes in its construction the principle of the engineering instrument known as the altazimuth, records continuously on one sheet of paper the pressure, direction, and inclination of the wind, with all its changes, the pressure plate being always maintained truly at 266 METEOROLOGY a right angle to the wind. The apparatus for indicating the direction of the wind consists of a vane (Fig. 61 A) constructed of a pair of diverging blades fixed to a cap, mounted so as to rotate about a vertical axis, the motion of this vane being transmitted by a vertical tubular shaft passing downwards FIG. 61 A. Vane of Casella's Altazimuth Anemometer. through the usual fixed column to the registering mechanism. This tubular shaft, called the " direction tube," is made to operate the styles which record its movements through the medium of pinions and wheels, conveying motion to two discs, which are made to carry pencils in a vertical position and equi-distant ; three styles are used, so that one is always ready to enter on the scale at one side when another leaves it at the other side (Fig. 6 IB). ANEMOMETRY AND ANEMOMETERS 267 The apparatus for indicating the inclination of the wind, that is, its divergence from a horizontal plane, consists of a similar vane (composed of a pair of diverging blades), mounted on a horizontal axis within the direction vane, so balanced as FIG. 6lB. Casella's Altazimuth Anemometer. to assume normally, when no wind is blowing, a position in which its longitudinal axis is horizontal. To ensure this, the vane is brought to a condition of stable equilibrium as closely approximating to that of instability as it is possible to bring it. 268 METEOROLOGY The oscillating motion of this inclination vane is transmitted to its registering mechanism by a tubular connecting rod, jointed to the vane by a pair of links, which move up and down inside the direction tube ; the inclination tube is so connected with the carriage of a style. Thus its longitudinal motion only affects the latter, which thus records upon the scale the oscillations of the vane due to the varying inclina- tion of the wind. The pressure plate is a disc having an area of 1J square feet fixed to a guide rod, fitted to slide between pairs of guide rollers in the frame of the inclination vane, and moving with it. This plate should have had a cone at the back, which was omitted in its construction. In order to prevent the varying positions of the pressure plate affecting the balance of the vane, its motion is constantly and exactly compensated by a movable weight running on rollers, so arranged that the weight moves to a proportionate extent in the opposite direction to the pressure plate, so as to maintain the balance of the vane in all positions of the pressure plate. The motion of the pressure plate is transmitted to the apparatus for measuring the force by means of a chain attached to the guide rod of the plate, and passing down over a pulley through the tubular shaft of the inclination vane. To prevent the weight of this chain affecting the accuracy of the records, it is exactly balanced by a counterpoise hanging in a casing carried by the cap. In this way the pressure plate is kept perpendicular to the direction of the air current, not only in azimuth but also in altitude. The apparatus for measuring the pressure consists of a cistern containing mercury and a displacement plunger immersed therein, connected to a frame of guide rods joined together above and below the mercury cistern, which works up and down against guide wheels mounted around the cistern, the chain being attached to the bottom of the frame. The plunger has a varying ratio of displacement for successive depths of immersion, so that the scale may be open for the ANEMOMETRY AND ANEMOMETERS 269 smaller and compressed for the greater (and less frequent) pressures. In order to check the motion of the plunger and avoid inaccuracy in the indications, due to the momentum of the parts, the lower end of the plunger is provided with a disc fitting more or less closely to the sides of the mercury cistern, so as to prevent the too rapid passage of the mercury from one side of the disc to the other. The frame is connected to the carriage of the marker for registering the motions of the plunger by means of a bell- crank lever. The carriage carrying the recording pencil is mounted to travel upon rollers running upon the oppositely bevilled edges of a horizontal bar and rollers running on the front and back surfaces, by which it is truly guided with the least possible friction. The markers are all metallic, sliding in sockets and pressing by their own weight or by springs on the paper. The scales for the different records are marked upon a single sheet of paper wrapped round a cylinder, rotated at a uniform speed by a clock movement in the usual manner. As a mere anemoscope, Mr. Laughton considers that none of these elaborate contrivances excels the simple little feather vane in daily use on board our men-of-war. It is a tapering tail, 8 inches long by 1^ wide where broadest, made of the softest down or feathers, and tied to the top of a staff by a short thread. Let the wind blow how it will this must stream with it. J. Musical Anemometers have been suggested and designed by Hooke (1667), Athanasius Kircher, Leupold (1724), and Delamanon (1782). These instruments were so constructed as to emit musical sounds when the wind blew upon them. They were scientific toys at the best. K. The Helicoid Anemometer. In the Quarterly Journal of the Eoyal Meteorological Society (vol. xiii. p. 218, 1887), Mr. W. H. Dines, B.A., F.R. Met. Soc., has a paper on a " New Form of Velocity Anemometer," which he read before 270 METEOROLOGY the Society on April 20, 1887. In this instrument an attempt was made to measure the velocity of the wind by the rotation of a small pair of windmill sails, the pitch of the sails being altered automatically, so that their rate may always bear the same ratio to that of the wind. These sails present what is called a "helicoid" surface (Gk. e'Atf, a spiral; and efSos, resemblance), or one which may be rotated about its axis in a FIG. 62. Vane of Richards' Anemo-Cinemograplie. current of air (the axis of course pointing in the direction of the current) without causing any deflection or whirl in the air passing over it. L. The Anemo-Cinemographe. On May 19, 1892, the late Mr. G. M. Whipple, B.Sc., Superintendent of the Kew Observatory, laid before the Royal Meteorological Society the results of a comparison of Richards' Anemo- Cinemographe with the standard Beckley Anemograph at ANEMOMETRY AND ANEMOMETERS 271 the Kew Observatory. 1 This ingenious instrument (Figs. 62, 63, 64, and 65) is a modification of the old Whewell fan, or windmill vane, the change being in the shape of each FIG. 63. Richards' Anemo-Cineinographe. blade of the vane, which is made oval and fitted at an angle of 45 to the axis. The vanes are said by MM. Richards to have been carefully calibrated. The fan is formed by six little wings or vanes of sheet aluminium, 4 inches in diameter, 1 Quart. Journ.ofthe Royal Met. Soc., vol. xviii. p. 257. 1892. 272 METEOROLOGY inclined at 45, riveted on very light steel arms, the diameter of which is so calculated that the vane should make exactly one turn for the passage of a metre of wind (Fig. 62). Its running is always verified by means of a whirling frame fitted up in an experimental room where the air is absolutely calm, and, if necessary, a table of corrections is supplied. The recording part of the apparatus is called the Anemo-Cinemo- FIG. 64. Richards' Anemo-Cinemographe (Second Form). graphe, and, in principle, is as follows : The pen, recording on a movable sheet of paper, is lowered at a constant rate by means of a conical pendulum acting through a train of wheel work, whilst a second train, driven by the fan, is always tending to force it up from the lower edge of the paper ; its position, therefore, is governed by the relative difference in the velocity of the two trains of wheel work, being at zero of the scale when the air is calm, but at other times it records the rate of the fan in metres per second (Fig. 63). Fig. 64 represents another pattern of the same instrument, which, FIG. 65. Anemo-Cinemographe in position. T 274 METEOROLOGY however, does not record the direction of the wind, MM. Richard having constructed another apparatus for that purpose. Another pattern of the anemo-cinemographe (Fig. 65), with endless paper running 1 \ inches a minute, has been made by MM. Richard for the Bureau Central Meteorologique of France for studying the storms on the top of the Eiffel Tower in Paris. Notwithstanding the recommendation of the Vienna Meteorological Congress, Robinson's and Osier's anemometers still hold the field in British observatories at all events. A Robinson anemometer should be well exposed, its machinery should be kept well oiled, and the following particulars should always be furnished with a register of its indications : 1. Length of arm (axis to centre of cup) ; 2. Diameter of cups \ 3. How the registration is effected (mechanically, electrically, or otherwise) ; 4. Name of maker ; 5. Height above the general surface of the ground (Win. Marriott). The mean direction of the wind may be calculated by the formula proposed by Lambert towards the close of the last century. It is given by Mr. Scott as follows : E. - W. + (N.E. +S.E. -S.W. -N.W.) cos 45 ' n * N. - S. + (N.E. +N. W. - S.E. - S.W.) cos 45 In this equation < is the deviation of the mean direction from north round by east. In 1889-90, Mr. W. H. Dines, B.A., F.R. Met. Soc., carried out a series of experiments on the resistance of plates of various forms at oblique incidences to the wind, and com- municated the results to the Royal Society in a valuable paper. 1 Mr. Dines subsequently carried out a series of com- parisons between specified anemometers at the request of the Council of the Royal Meteorological Society, the cost being defrayed by the Meteorological Council. The instruments compared were : 1 "On Wind Pressure on an Inclined Surface," Proc. Roy. Soc., vol. xlviii. pp. 233-257, Velocity Instruments. A NEMO ME TRY AND A NEMO ME TEJKS 275 1. Kew Pattern Robinson Anemometer. 2. Self-adjusting Helicoid Anemometer (Dines). This instrument is described as stated above in the Quarterly Journal of the Royal Meteorological Society, vol. xiii. p. 218, 1887. Small Air Meter. Pressure f 4. Foot Circular Pressure Rate. Instruments, t 5. A special modification of the Tube Anemometer. The conclusions arrived at are given in a note appended to the Report of the Meteorological Council to the Royal Society for the year ending March 31, 1892 (pp. 23, 24). They are to the effect that, with proper precautions, the tube anemometer a combination of Lind's and Hagemann's instruments will form a most useful and convenient instrument, that the relation between pressure and velocity for a foot circular plate, and at ordinary barometrical pressure, is P='003 V 2 ; and that the factor of the Kew Pattern Robinson Anemometer is practically constant for all velocities except very low ones, that its value must lie between 2 '20 and 2 - 00, and that there is a very great probability that it is within 2J per cent of 2 '10. Mr. Dines read his paper on " Anemometer Comparisons " before the Royal Meteorological Society, on April 20, 1892. It is fully illustrated, and was published in the Quarterly Journal of the Society (vol. xviii. No. 83, July 1892, p. 165). At pages 254 and 255, illustrations are given of an enlarged anemometer or anemograph, constructed by Mr. L. Casella for harbours and public observatories. In this arrangement (Fig. 54) windmill fans are added tojthe wind vane, causing the mean direction of the wind to be accurately indicated by means of a revolving cylinder (Fig. 55) to which paper is attached. The direction as well as velocity is continuously shown for every minute of time by means of a clock, which forms part of the instrument. The exposed part of this anemometer may be placed at any height, whilst the registering part is kept in a room or other covered place for observation. CHAPTEE XXII ATMOSPHERIC ELECTRICITY Identity of Atmospheric Electricity and that obtained from an Electric Machine The Electroscope The Nature of Electricity Atmospheric Electrical Phenomena Electrical Density, Force, and Potential Use of the Electroscope The Collector The Electrometer Coulomb's Torsion Balance The Electrophorus The Replenisher The Diurnal and Annual March of Atmospheric Electricity Its Distribution Thunderstorms : Professor Mohn's Classification Cyclonic and Heat Thunderstorms Geographical Distribution of Thunderstorms. THE identity of atmospheric electricity with that obtained from an electrical machine, foreshadowed by Dr. Wall in 1708, was finally proved by Benjamin Franklin in June 1752. His classical experiment of obtaining electricity from the clouds by flying a kite need not be referred to in detail. In a letter to Mr. Peter Collinson, F.R.S., dated Philadelphia, October 1752, Franklin describes his electric kite. 1 Suffice it to say that the experiment is a dangerous one. In June 1753 M. de Romas in France repeated it, using a fine wire 550 feet long instead of a string, with the result that he ob- tained flashes 9 or 10 feet in length, which were accompanied by a loud report. On one occasion De Romas was struck down, but not killed, by such a charge. In August of the same year, Professor Richmann, of Petersburg, lost his life during a thunderstorm. He approached the end of the con- ducting wire, when a ball of fire apparently leaped to his head, killing him on the spot. 1 Philosophical Transactions, vol. xcv. p. 565. 1752. ATMOSPHERIC ELECTRICITY 277 Lemonnier proved, by means of insulated metal rods, that the atmosphere is charged with electricity even in fine weather. Volta and De Saussure, subsequently, each constructed an instrument called an electroscope^ for collecting atmospheric electricity and demonstrating its effects. In De Saussure's electroscope the electricity conducted from a rod to two little pith balls suspended by fine wires in a glass case caused them to diverge from one another. Alessandro Volta of Pavia substituted two blades of straw an inch long for the pith balls. The most delicate of all electroscopes is that which is called the "gold-leaf electroscope," Before, however, I attempt to describe the chief electrical phenomena in nature, it will be desirable to draw attention to certain rudimentary facts relating to the nature of electri- city. These are admirably summarised in the following sen- tences taken from an address on " Atmospheric Electricity," delivered before the Royal Meteorological Society on March 21, 1888, by the late Dr. W. Marcet, F.R.S., then President of the Society. 1 " Like heat" says Dr. Marcet, " electricity is the manifesta- tion of a peculiar condition of a body, and bodies are said to be electrified when after having been rubbed, or placed in communication with an electrified object, they exercise an attraction or repulsion, more or less great, upon other light bodies. On inquiring into this phenomenon, it is found that the electricity developed by friction from different substances, such as glass on one hand, and sealing-wax on the other, is not identical, and that there are consequently two electricities varying from each other in some of their characters. It may be said in a general way : " 1. That there is an attraction between an electrified body and another not electrified. " 2. That there is an attraction between two bodies electrified, the one by a rubbed glass rod, the other by a rubbed stick of sealing-wax, or of some resinous substance. 1 Quart. Journ. of the Royal Met. Soc., vol. xiv. p. 197. 1888. 278 METEOROLOGY "3. That there is a repulsion between two bodies, both of which are electrified either by glass or by sealing-wax. " To put these laws of nature more clearly, electricities of different kinds attract each other, and electricities of the same kind repel each other. " In order to distinguish between the two kinds of elec- tricity, the one is called vitreous electricity, from its being generated from glass, and is known as positive, while the other is called resinous electricity, and is known as negative" Atmospheric electricity may, from time to time, reveal its presence by very unequivocal phenomena, of which the chief are (1) Thunder and lightning, (2) hailstones, (3) aurora the aurora borealis being peculiar to the northern hemi- sphere, the aurora australis to the southern. % But apart from these manifestations, observations should be made upon the electricity existing in the air under ordinary circumstances, so as to determine, firstly, whether it is positive or negative ; secondly, what is its intensity or tension. In a Keport on Atmospheric Electricity, drawn up at the request of the Permanent Committee of the First Interna- tional Meteorological Congress at Vienna, and published by the Meteorological Council in 1878, Professor J. D. Everett, M.A., Queen's College, Belfast, observes that in discus- sions relating to the electrical condition of the air at a specified point, three things must be carefully distin- guished : electrical density, electrical force, and electrical potential. (a) The electrical density at a point in the air is the quantity of electricity, per unit volume, with which the air at the point is charged. (6) The electrical force at a point is the force with which a unit of positive electricity would be acted on, if brought to the point without altering, by its inductive action, the previously existing distribution. , (c) The electrical potential at a point is the work which A TMOSPHERIC ELECTRICITY 279 would be done by electrical force upon a unit of positive electricity passing from the point to the earth, the movement of this unit being supposed not to disturb the pre-existing distribution. In Mr. K. H. Scott's Instructions in the Use of Meteorolo- gical Instruments * concise information as to the ap- paratus used in researches on atmospheric electricity will be found. From that source chiefly the following is culled : 1. The electroscope is in- tended to show the nature or kind of electricity pre- sent in the air. By far the most sensitive instru- ment for this purpose is the gold-leaf electroscope, in which electricity col- lected from the neighbour- ing atmosphere is made to act through a metal rod, called a conductor, upon two delicate gold leaves suspended at the end of the rod and applied closely to each other. The leaves, When brought under the FIG. 66. -Gold-leaf Electroscope, influence of the same kind of electricity, will diverge or repel each other. As very little electricity can be observed near the ground, the conductor should be placed in contact with the air at some height above the earth's surface, by means of a 2. Collector. This may be a metallic arrow tied to one 1 Reprinted, p. 60 et seq., 1885. London : E. Stanford, 280 METEOROLOGY end of a conducting string and then shot upwards into the air. The electroscope will be found electrified as the arrow mounts. A gilded fishing-rod may be substituted as a conductor, its lower end being insulated, that is, surrounded by a non-con- ductor such as caoutchouc. Volta's collector is a flame burning at a height, either in a lantern hung to a mast and connected with the electroscope by a wire, or in the form of a slow burning match attached to the top of a long metal rod. The electricity of the air, in the neighbourhood of the flame, by its inductive action on the conductor, causes electricity of the opposite kind to accumulate at the upper extremity, whence it is constantly carried off by the connection currents in the flame, leaving the conductor charged with electricity of the same kind and potential as the air. It is necessary again to explain that the term potential, as applied to electricity, means the energy of an electrical charge measured by its power to do work ; it is electro-motive force. "When one body is charged with electricity to a higher potential than another, electricity tends to pass off, so as to equalise the potential on the two bodies " (R. H. Scott). Mr. Scott says that " when we speak of the motion of the electricity from one body to another, we say that this is effected owing to difference of potential." He adds : " Differ- ence of electric potentials may very well be termed ' difference of electric heights.' " The water-dropping collector, invented by Sir William Thomson, now Lord Kelvin, Professor of Natural Philosophy in the University of Glasgow, is on the same principle as Volta's method. A copper can is placed on an insulating support, either of ebonite with its surface thinly coated with paraffin, or of glass surrounded with pumice-stone impregnated with sulphuric acid. From the can a small pipe projects far into the air and terminates in a fine jet. The can being filled with water, and the tap which opens into the jet being turned on, a small stream of water is allowed to flow out guttatim, ATMOSPHERIC ELECTRICITY 281 in drops. In half a minute the can is found to be electrified to the same extent and in the same way as the air at the point of the tube. This collector cannot be employed in frost, for the water freezes in the jet. At such times the use of a slow burning match, made of blotting paper, steeped in a solution of nitrate of lead, dried and rolled, is recommended by Lord Kelvin. Since electrical density is greater on projecting surfaces and less on hollow surfaces than on planes, the collector should not be near trees, or houses, or within a closed space. 3. Electrometer. This is an instrument which is intended to measure the amount of electricity, or the electric intensity, tension, or potential. The earliest electrometer was Coulomb's Torsion Balance, by means of which one of the principal laws of electricity was discovered that two electrified bodies, whose size is very small in comparison with their distance apart, attract or repel each other with a force proportional to the inverse square of the distance which separates them. Lord Kelvin has designed two kinds of electrometer (1) the Quadrant Electrometer for observatory use ; (2) the Portable Electrometer. In the Quadrant, or modified Divided-Ring, Electrometer, a needle of thin sheet aluminium, cut so as to resemble in form a figure " 8 " with the hollows filled in, and carrying above it a small light mirror weighing only a fraction of a grain, is suspended from its centre by two fine silk threads, the distance between which can be varied at will. The needle swings horizontally inside a shallow cylindrical brass box, which is cut into four equal segments or quadrants, each insulated separately by glass supports, but connected alternately by thin wires. Each pair of quadrants is also connected to a stiff wire passing through the case of the instruments, to form the two electrodes, or terminals, for the attachment of the collecting and earth wires. The base of the electrometer contains a Leyden jar, partially 282 METEOROLOGY filled with strong sulphuric acid, and a platinum wire, hung from the lower surface of the needle, is made to dip into the acid. A lamp and a divided scale are placed about a yard in front of the instrument, and the light shining through an aperture in the frame of the scale is reflected by the mirror on the scale, where the position of the image of a wire stretched across the hole can be accurately observed. In order to make use of this electrometer the needle must be charged with electricity from a small electrophorus or electricity-bearer, brought into contact with a wire (charging electrode) dipping into the sulphuric acid at the bottom of the Leyden jar. One of the electrodes connected with the segments is then joined by a wire to the water -dropping collector; the other is placed in communication with the earth through a wire attached to a gas-pipe, or similar con- ductor. The needle will then be deflected towards either one side or the other, according as the electricity of the atmosphere is of the nature to repel or attract it, and the extent of repulsion, as measured on the scale, is proportional to the amount of difference of potential between the atmo- spheric and terrestrial electricities. To secure that the needle shall remain fully charged, an auxiliary apparatus for the generation of electricity, termed a replenisher, is fixed inside the case, by turning which the charge can be restored to its original potential. This is indicated by a small gauge consisting of a light lever, made of thin aluminium, and fixed to the top of the instrument. One end of this gauge carries an index which moves in front of a small scale. The other end is flattened into a plate of about a square centimetre in area, which is ' repelled by another plate, similarly electrified, fixed to the top of the instrument, and in metallic connection with the sulphuric acid of the Leyden jar, so as to be charged to the same potential as the indicating needle. The position of the index being therefore once determined, it is easy, by giving a few ATMOSPHERIC ELECTRICITY 283 turns to the replenisher, at least once daily, to bring the potential of the charge of the instrument up to its original value. The scale value of each electrometer must be experi- mentally determined by means of a galvanic battery of con- stant intensity such as DanielPs. Knowing the electro-motive force of the cell employed in the battery, the indications of FIG. 67. Thomson's Portable Electrometer. the electrometer scale may be converted into terms of the absolute unit of electro-motive force or "volts." If the electrometer is used as a self-recording instrument, a drum carrying photographic paper, and maintained in rota- tion at a uniform rate by a chain of clock-work, is substituted for the divided scale, and the aperture is reduced so as to form a mere dot of light on the cylinder. Thomson's quadrant electrometer was used, in conjunc- tion with a water -dropping collector, for photographically recording atmospheric electricity at the Kew Observatory from 1874 to 1885. It has been in use at the Koyal Observatory, Greenwich, since 1877. The instrument was 284 METEOROLOGY described, with illustrations, in the British Association Report for 1867, p. 489. In Lord Kelvin's Portable Electrometer (Fig. 67) the electricity is collected by means of a burning fuse at the extremity of a vertical wire. An illustrated description of it is given in the British Association Report for 1867, p. 501. Another electrometer which is highly recommended is Peltier's. It was used for more than thirty years by the late M. Quetelet at Brussels, and for upwards of twenty years at Utrecht. This electrometer is described in the Annuaire Meteorologiqiie de France, 1850, p. 181, and in the British Association Report, 1849 (Transactions of Sections, p. 11). Quetelet, according to Mr. R. H. Scott, drew the following conclusions from five years' observations with the electro- meter at Brussels : 1. The diurnal march of electricity, at a constant height above the ground, exhibits two maxima and two minima. 2. The maxima and minima of electrical tension precede by about an hour those of barometric pressure (see Chap. XV. p. 165 above). The maxima occur when temperature is either rising or falling most rapidly 8 A.M. and 9 P.M. in summer, 10 A.M. and 6 P.M. in winter. The day minimum corresponds with the period of maximal temperature and minimal humidity. The epoch of the night minimum has not been satisfactorily determined, but was referred by De Saussure and Schubler to shortly before daybreak. 3. The annual march of electricity presents one maximum in summer (June) and one minimum in winter (January). In nature the atmosphere, whether clear or cloudy, always shows an electric reaction it is in a state of electric tension, which increases remarkably with altitude, as has been observed by De Saussure, Erman, Quetelet, Lord Kelvin, and the Hon. Ralph Abercromby (by the last named on the Peak of Teneriffe in July and August 1878). Under a clear sky atmospheric electricity is nearly always positive. According to Peltier, land is always negative in its electrical character, A TMOSPHERIC ELECTRICITY 285 while Becquerel observed that sea-water is always positive. M. De la Rive holds that the positive electricity of the air is derived mainly from the sea. In these facts we gain a clue to the origin of thunderstorms and other atmospheric phenomena of an electric character. Professor Henry Mohn, Director of the Norske Meteoro- logiske Institut, Christiania, has classified thunderstorms into two groups Heat Thunderstorms and Cyclonic Thunderstorms. The former type belongs to summer and to hot climates the latter to winter and to insular climates. Cyclonic Thunderstorms are so called because they accom- pany deep atmospheric depressions such as traverse the North Atlantic Ocean and the north-western sea -board of Europe, especially in winter. Scarcely a gale of wind of any extreme intensity occurs without attendant electrical pheno- mena. Occasional flashes of sheet-lightning light up the sky over wide areas during the passage of a winter storm, and here and there sharp hail and sleet squalls, with a few vivid flashes and loud peals of thunder, are experienced. While these cyclonic thunderstorms are not so violent they are quite as dangerous as summer thunderstorms, if not more so, because in them the clouds drift at a lower level, so that the lightning is more likely to strike the ground. Heat Thunderstorms are especially associated with sudden and extreme alterations in atmospheric temperature. Perhaps a cool night has been followed by a blazing sun and a light south or south-east wind. Vast quantities of aqueous vapour rise in the atmosphere as the result of rapid evaporation. Massive cumuli form, the upper edges of which become more and more dense, and appear snowy white as the sun shines upon them. These clouds are probably surcharged with positive electricity. Then a light surface current of air arises and blows towards the approaching cumuli, while an angry- looking lurid cloud stratum, negatively electrified, forms in the lower strata of the air, and is seen constantly to change its shape and density. Presently, the top of the piled-up 286 METEOROLOGY cumuli spreads out into a dense cirriform sheet, and at once thunder is heard, and rain or hail begins to fall in great quantities. With flash after flash, peal upon peal, the storm momentarily gathers strength and increases in violence. The hail and rain fall intermittently in drenching showers, and the whole sky becomes overcast, while the wind either falls light, dies down to a calm, or shifts perhaps to the opposite point of the compass in a fierce squall. The rain then lightens, the thunder and lightning become less frequent and more distant, and gradually the sky clears and the air feels cool and fresh temperature perhaps being 10, 15, or even 20 lower than before the storm. From a careful analysis of the thunderstorms of 1888 and 1889 over the south and east of England, undertaken at the instance of the Council of the Royal Meteorological Society, Mr. William Marriott, F.E.Met.Soc., arrives at the con- clusion that thunderstorm formations are small atmospheric whirls in all respects like ordinary cyclones. The whirl, which is most probably confined to a stratum of air at only a short distance from the earth's surface, not more than 4000 to 6000 feet, may vary from one mile to ten miles or more in diameter. Thunderstorms usually occur when the isobars show large areas of ill-defined low pressure containing several shallow minima, called " thunderstorm depressions," or when there is a "lane" or "trough" of low pressure between adjoining areas of relatively high pressure. Mist and fog often precede thunderstorms, which may accompany high barometer readings as well as low. On May 21, 1888, there was a thunderstorm with the barometer at 30 '40 ins. On March 26, 1888, there was one with pressure below 29'00 ins. When isobars are drawn for hundredths, instead of ' tenths, of an inch, a number of small but distinct areas of low pressure, or cyclones, with regular wind circulation, may generally be recognised in thundery weather. These "thunderstorm depressions" often circulate round a large but shallow area of low barometer, forming secondary or A TMOSPHERIC ELECTRICITY 287 subsidiary depressions to it as a primary. At other times they travel perhaps for hundreds of miles along a direct line, the rate of progression being sometimes as much as 50 miles an hour. On May 18-19, 1888, a storm passed across England from Christchurch, Hants (8.15 P.M.), to Edinburgh (4 A.M.) and Cupar-Fife (4.5 A.M.) Similarly, on June 2, 1889, a storm travelled northwards from Wiltshire (3 A.M.) to Edinburgh (10.44 A.M.), and probably to Kirkwall, in the Orkneys (3.37 P.M.), a distance of 550 miles, at a uniform rate of 5Q miles an hour. Thunderstorms often break out over the same line of country on consecutive days, and nothing is more curious than to watch thunder-clouds springing into existence in the sky time after time above a particular place or district, as if there was a direct electrical attraction between the earth just there and the superincumbent atmosphere for the time being, and this no doubt is in reality the case. Heat thunderstorms show a diurnal and an annual periodicity. In tropical climates their periodicity is best marked. According to Arago, Jamaica is peculiarly liable to thunderstorms. From November to April the day breaks cloudless, but between 11 A.M. and 1 P.M. the mountains of Port Koyal become covered with towering thunder-clouds. At the last-named hour rain falls in torrents, lightning flashes in all directions, and the crash of thunder is incessant and deafening. But the storm is quickly spent, and a brilliant evening follows. It is an old observation of Caldcleugh 1 that at Rio Janeiro it was customary to state in invitations whether the guests were to assemble before or after the thunderstorm, which was practically a daily episode. Europe presents examples of both types of thunderstorms. On the Continent and in England heat thunderstorms are prevalent in the summer months, the result being that the rainfall of July and August is particularly heavy, if not 1 Quoted by Daniell in his Meteorological Essays and Observations (First Edition, p. 335. 1823). 288 METEOROLOGY incessant. In Ireland, Scotland, and Norway heat thunder- storms are less frequent, while cyclonic thunderstorms are apt to occur, especially in the south-east quadrant of deep winter depressions. In Iceland thunderstorms are almost unknown in summer, whereas they frequently occur in winter. Dr. Alexander Buchan, in papers contributed to the Scottish Meteorological Society on the " Meteorology of Iceland " 1 and on the " Rainfall of Scotland," 2 says, that during the twenty- five years, 1846-1870, thirty-one thunderstorms occurred at Stykkisholm, Iceland, in January, seventeen in February, eight in March, six in April, two in May, none in June, two in July, none in August, five in September, five in October, fourteen in November, and twenty-five in December. In the second of _ the two papers mentioned, Dr. Buchan shows that the thunderstorms of the north-west of Scotland belong to the cyclonic type, while those of the south-east are heat thunderstorms for the most part. England is celebrated for its thunderstorms in summer. They are far more severe and far more frequent than those felt in Ireland. This is due to the fact that the southerly winds in front of the depressions, which the thunderstorms accompany, have crossed the sea in the case of Ireland, but land that is France in the case of England. The contrast of temperature between the south wind in front and the north wind behind the centre or trough of low pressure is, therefore, much greater for England than it is for Ireland. Again, the heated air rising over France is negatively electrified, while the warm sea-air in Ireland is positively electrified. There is, then, -attraction between the negative electricity of the ascending current and the positive electricity of the atmosphere over England, whereas over Ireland both the atmosphere and the ascending current of warm moist air are positively electrified. Hence there is no attraction, and 1 Journal of the Scottish Meteorological Society, New Series, vol. ii. p. 289. 1869. 2 Loc. cit., vol. iii. p. 251. 1873. ATMOSPHERIC ELECTRICITY 289 no electrical energy is evoked. At the same time it is true that negative electricity forecasts rain, of which there is no lack in Ireland. On the other hand, as Mr. Scott points out, a sudden development of positive electricity in wet weather is a certain sign of the sky clearing. CHAPTEE XXIII ATMOSPHERIC ELECTRICITY (continued) Lightning Thunder Varieties of Lightning : zigzag, or forked ; diffused, or sheet ; globular, or ball lightning Fulgurites Eapidity of Light- ning St. Elmo's Fire Hail The Aurora : how caused ; its height ; its colour Ozone Lightning-conductors. Lightning, according to Professor Balfour Stewart, owes its brilliancy to the generation of heat along the path of the electric discharge so intense as to render the various con- stituents of the air momentarily incandescent. This genera- tion of heat is due to the resistance of non-conductors in the air to the discharge which takes place when clouds charged with different electricities approach each other. Thunder is the noise, or atmospheric vibrations, produced in the first place by the tremendous expansion due to the heat of the lightning flash, and then by an in-rush of air to fill up the vacuum so caused. Its prolonged reverberations are merely an acoustic phenomenon an echo on a stupendous scale. When thunder is heard close at hand, it sounds first like a volley of musketry, because a separate report accom- panies each zigzag movement on the part of the flash which is pursuing its uneven, if rapid, paths through masses of air of different conducting powers moist air being a better conductor than dry air. As sound travels infinitely less quickly than light, a flash which is a mile away will be seen about five seconds before the thunder is heard. ATMOSPHERIC ELECTRICITY 291 Lightning may be described as of three kinds (1) zigzag or forked lightning ; (2) diffused, summer, or sheet lightning ; (3) globular or ball lightning. Forked lightning does not occur in nature as drawn by artists. We know this, thanks to Mr. James Nasmyth's observations communicated to the British Association in 1856. His statements have been amply confirmed by sixty photo- graphic reproductions of lightning flashes, received by the Thunderstorm Committee of the Royal Meteorological Society in 1888, in reply to a circular sent out in June 1887. The first report of that Committee, drawn up by the Hon. Ralph Abercromby, F.R.Met.Soc., goes to show that lightning assumes certain typical forms 1. Stream lightning; 2. Luminous lightning ; 3. Ramified lightning ; 4. Meandering lightning; 5. Beaded or chapletted lightning; 6. Ribbon lightning. Flashes of forked lightning take place horizontally or vertically between oppositely electrified clouds ; or vertically between the negative earth and a positive cloud. The latter is very dangerous. But even when a living object is not in the direct path of the discharge, and so killed by the effect of the electricity on the nervous centres or muscular system, death may ensue by induction from what is called the " return shock." Suppose two clouds of opposite electricities are hovering over the earth at no great elevation. They will induce opposite electricities to their own in objects on the ground beneath them. A discharge now takes place between the clouds, establishing electrical equilibrium so far as they are concerned. When this takes place the induced electricity in objects on the ground disappears, causing such a nervous shock to living beings as to deprive them of life. When a telephone gong sounds during a thunderstorm, it indicates that a return-shock is taking place. Summer or sheet lightning is the diffused flash of light which illuminates the horizon or the distant clouds when a thunderstorm is raging at a great distance from the observer, 292 METEOROLOGY perhaps 100 or even 150 miles away far beyond the limits (15, or at the most 20, miles) at which thunder is audible. Globular or ball lightning "fire-balls" is more per- sistent than forked lightning, remaining visible for several seconds, or even as long as three minute^ as happened at Milan in 1841 (Arago). It shows itself as a luminous sphere or ball of fire, in diameter varying from a few inches to two or three feet, which moves slowly and at last bursts with a loud report like a bomb-shell. Mr. Scott, in his Elementary Meteorology r , adduces several instances of this rare form of lightning. The destructive effects of lightning are twofold ; mechanical and combustible. If a flash of lightning strikes a sandy soil, it fuses or vitrifies the silicious particles into afulminary tube or fulgurite (Lat. fulgur, flashing lightning). The Ger- man term is very expressive Blitzrohren, lightning tubes. As a matter of fact, there is no such thing as a thunderbolt. In a paper on "The Non-existence of Thunderbolts," con- tributed by Mr. G. J. Symons, F.R.S., to the Royal Meteoro- logical Society on March 21, 1888, 1 the author effectually disposes of this myth. The lightning flash moves with inconceivable velocity. Sir Charles Wheatstone, by means of a rapidly revolving mirror, showed that the duration of a spark, O'l inch in length, in air at ordinary atmospheric pressure was about grinnr f a second. He also ascertained that its velocity along the insulated wire with which he experimented, was nearly 290,000 miles in a second that is, half as great again as the velocity of light, 186,000 miles in a second. Mr. R. H. Scott acknowledges, in his Elementary Meteoro- logy? his indebtedness to Mr. De la Rue for the following calculation of the potential necessary to produce a flash of lightning a mile in length. By his and Dr. Miiller's experi- ments (Philosophical Transactions, vol. clxix. p. 118) with 1 Quarterly Journ. of the Royal Met. Sue., vol. xiv. p. 208. 1888. 2 Page 180. ATMOSPHERIC ELECTRICITY 293 his magnificent battery, the striking distance, between points, when 11,000 cells were used, the potential of each being 1'06 "volts," was 0'62 inch. This striking distance varies with the square of the number of cells employed. Then, as 1 mile = 63,360 inches, we have ^T X 11 ' 000 = 3,516,480 cells, as the amount requisite to produce such a flash. St. Elmo's Fire is the Castor and Pollux of the ancients. It is an induction phenomenon, and occurs when an electrified cloud approaches a prominent or pointed obstacle like the mast of a vessel, a flagstaff, a tree-top, or a lightning-con- ductor. The electricity of the cloud and of the earth com- bine, not in a flash of lightning, but more slowly and con- tinuously, so that a flame seems to rise from the projecting point. Caesar noticed it after a hailstorm, and described it in the words : " Eadem nocte legionis quintse cacumina sua sponte arserunt." The phenomenon, according to Mr. Scott, is of the nature of the " brush " discharge of the electrical machine. It has received many names, such as " St. Elmo's Fire " and " Comozants " a corruption of " corposants " from the Latin corpus sanctum (Italian, corpo santo). Displays of St. Elmo's Fire, accompanied by hissing or loud crackling sounds, are not uncommon at great elevations : on the top of Ben Nevis, in the Alps, and at the high-level observatory on the summit of Pike's Peak in Colorado, United States of America. Pike's Peak, 14,151 feet above the sea, was until lately the highest observatory in the world. Hail is intimately related to atmospheric electricity, as was stated in Chapter XX. (p. 240). Professor Colladon, of Geneva, considers that the heavy rainfall preceding a hailstorm causes a strong downward current of air ; which induces by suction a partial vacuum in the upper clouds. At once a fall of tem- perature takes place accompanied by a sudden reduction of vapour tension, which causes a condensation of moisture into frozen particles. These are alternately attracted and repelled by intensely electrified masses of cloud, thus increasing 2 9 4 METEOROLOGY rapidly in size until they become so heavy that they fall to the ground as hailstones. M. Colladon supposes that the cloud masses become strongly electrified through the agency of lateral up-draughts of air caused by the central down-draught from the shower of rain preceding the hailstorm. It will be observed that this theory closely agrees with the views of Volta as to the formation of hail referred to in Chapter XX. (p. 240). Aurora. This electrical phenomenon is rarely seen in low altitudes or at the equator. It is a luminous appearance in the northern sky (Aurora borealis), or in the southern sky (Aurora australis), and assumes most frequently the aspect of an arch of light above a " dark segment " of sky at right angles to the magnetic meridian, from which arch bright rays or luminous pillars called streamers shoot up with a wavy, quivering motion towards the magnetic zenith. According to Professor Loomis, in America the aurora borealis appears most frequently between latitude 50 and latitude 62 north. In Europe and Asia the auroral region is situated farther north than in America the region of maximal frequency lying between the parallels of 66 and 75. In fact, the aurora is seen oftenest within an oval zone surrounding the north pole, the central line of which zone crosses the meridian of Washington in latitude 56 north, and that of Petersburg in latitude 71 north. Auroral displays are therefore more common in America than in Europe. The shape of this auroral zone bears some resemblance to the line of equal magnetic dip as well as to a " magnetic parallel " that is, a " line everywhere perpendicular to a magnetic meridian." Pro- fessor Loomis thinks it probable that an auroral display round the north magnetic pole of the earth is uniformly attended by a simultaneous display round the south magnetic pole. That a connection exists between the aurora and terrestrial mag- netism is proved by the extreme agitation of the magnetic needle during an auroral display. In a note, further, on the relation between sun-spots and weather, 1 Mr. R H. Scott 1 Elementary Meteorology, Appendix V. p. 392. 1883. ATMOSPHERIC ELECTRICITY 295 points out that modern observations show that the appear- ance of an unusually large spot on the sun's surface is almost invariably accompanied by a " magnetic storm " felt simul- taneously in all parts of the globe. When such magnetic storms occur, brilliant displays of the aurora usually take place the aurora thus also exhibiting a periodicity allied to that of sun-spots, which show epochs of greatest frequency every ten or eleven years. The last epoch of this kind fell in 1891-92. In his presidential address on " Atmospheric Electricity," delivered at the meeting of the Royal Meteorological Society, March 21, 1888, 1 Dr. W. Marcet, F.R.S., says that the aurora borealis is now generally considered to be due to positive electricity from the sea between the tropics being carried into the upper atmospheric regions, and thence wafted to the poles by the higher aerial currents. In the vicinity of the poles it descends towards the earth and meets the terrestrial negative electricity in a rarefied atmosphere. Luminous discharges then take place, their brightness being increased by the presence of masses of ice-particles in the atmosphere. These phenomena occur in the neighbourhood of what are called the North and South Magnetic Poles, and from this circumstance assume the form of bands so peculiar to these aurorae. Researches carried out by Messrs. De la Rue and Miiller 2 go to prove that the aurora may appear at any height between a few thousand feet and 80 to 100 miles. Experiments made with M. De la Rue's battery of 11,000 cells established the interesting fact, that the colour of the discharge with the same potential varied with, and was apparently determined by, the tenuity of the gas or air. The authors give the following table of the pressures at which they actually ob- tained discharges, represented by the corresponding calculated heights, and also of the tints at each height : 1 Quarterly Journ. of the Royal Met. Soc., vol. xiv. p. 207. 1888. 2 Proceedings of the Royal Society, vol. xxx. p. 332. 2 9 6 METEOROLOGY Height in Miles. Tint. Height in Miles. Tint. 81-47 Pale and faint 27-42 Carmine 37-67 33-96 32-87 Maximal brilliancy Pale salmon Salmon coloured 17-86 12-42 11-58 Full' red 30-86 > The roseate and salmon-coloured tints are always near the positive source of the electric or magnetic current. The discharge at the negative terminal, in air, is always of a violet hue, and, accordingly, this tint in the aurora indicates the proximity of the negative source. Ozone. In 1785, the Dutch physicist, Van Marum, while passing a succession of electric sparks from a powerful electric machine through a tube containing oxygen, was attracted by a peculiar odour which developed in the oxygen, and which he attributed to the "electric matter," calling it accordingly the "smell of electricity." In 1840, M. Schonbein, of Basle, named the substance which gave rise to this odour ozone, from the Greek ofw, / have a smell. His views as to the exact nature of ozone passed through several phases. At first he thought it was an element analogous to the halogen group chlorine, bromine, iodine, and fluorine. Then he con- sidered the possibility of its being a constituent of nitrogen, or a higher oxide of hydrogen, because he found that it could also be produced by the action of phosphorus on moist air (1845). Lastly, in 1852, he came to the conclusion, with other observers, De la Rive and Marignac, that ozone is really oxygen in an allotropic state, just as diamond is an allotropic form of carbon, coke or charcoal being the usual forms of this latter element. This view was fully confirmed by the experimental researches of Dr. Andrews in 1856. He proved that ozone was really an allotropic form of oxygen, and that it was ATMOSPHERIC ELECTRICITY 297 identical in its nature by whatever process it was prepared. Andrews also demonstrated that ozone can be turned back into oxygen by exposing it to high temperatures (300 C.). 1 In 1858, Schonbein started a new and plausible hypothesis. He announced that ordinary oxygen was a neutral combina- tion of two oppositely electrical and therefore very active bodies. One of these was ozone, or negative oxygen, which was formed during the electrisation of oxygen or air, the electrolysis of water, the slow oxidation of phosphorus, and the decomposition of most metallic peroxides. He called those substances which evolved this kind of oxygen ozonides. The other, the positive oxygen, or antozone, he failed to isolate, but he assumed its existence in a certain class of peroxides, which he called antozonides, examples being the peroxides of hydrogen and barium in particular, also the so-called ozonised turpentine, cod-liver oil, and ether. This ingenious theory was demolished in 1863 by Sir B. C. Brodie. In 1860, Andrews and Tait presented a very important communication on the subject of ozone to the Royal Society. These observers, in the first place, confirmed a previously known fact that only a small proportion of oxygen (one- twelfth, at most) can be converted into ozone by the electric discharge. But they also found that a constant and con- siderable diminution of volume accompanied the change one hundred volumes of oxygen, subjected to the silent discharge, contracting to ninety-two volumes. Hence ozone must be denser than oxygen. Nor was this all, for when mercury, or some other oxidisable substance, was introduced into the ozonised oxygen, and the ozone entirely absorbed, the residual oxygen had precisely the same volume as it had before the removal of the ozone, so that the density of ozone appeared to be absolutely infinite. Dr. Odling suggested that the formation of ozone might really consist in the condensation of another atom of oxygen into each diatomic or dyad molecule of ordinary oxygen. 1 Philosophical Transactions of the Royal Society, 1856. 298 METEOROLOGY The chemical formula for free oxygen being O 2 , that for ozone would, therefore, be O 3 ; and the density of ozone would be one-half greater than that of oxygen. When 100 volumes of oxygen were reduced by ozonisation to ninety-two, it might be supposed that eight volumes of oxygen combined with sixteen volumes of oxygen to produce sixteen volumes of ozone. The reaction might be represented in this way 80 2 + 160 2 = 160 3 The absorption of the ozone by the mercury, or iodine, etc., might really be only the removal of the third atom of oxygen, which would, of course, leave the volume unaltered 80 3 + 8Hg = 80 2 + 8HgO The same view would account for the mutual reduction which ozone and hydrogen dioxide exercise upon one another, and in fact for all known reactions of ozone 3 + H 2 2 ( = Hydrogen dioxide) = 20 2 + H 2 ( = water) This beautiful hypothesis received a remarkable experi- mental verification at the hands of M. Soret, 1 who succeeded in finding a body, oil of turpentine, which, instead of removing only one atom of oxygen from each molecule of ozone, as most substances do, absorbs the entire molecule, the whole three atoms of oxygen. To take our previous illustra- tion, if the ninety-two volumes of ozonised oxygen were treated with oil of turpentine, a dense white cloud would appear, the ozone would vanish, but instead of the volume remaining the same it would contract to seventy-six volumes, the 16O 3 having been removed bodily instead of being merely reduced to 16O 2 . 2 A great number of experiments have given the atomic weight of ozone as twenty-four, and consequently its molecular weight as forty-eight. This is just the weight of three atoms of oxygen, namely, 16x3 = 48, of which, accordingly, it is now universally believed to be composed. 1 Comptes Rendus, November 27, 1865. 2 Medical Times and Gazette, October 5, 1867, pp. 383, 384. ATMOSPHERIC ELECTRICITY 299 Mr. Francis E. Twemlow, F.M.S., in a paper on ozone, read before the British Meteorological Society on March 17, 1875, indicates the chief points of difference between ozone and ordinary oxygen : 1. It liberates iodine from iodide of potassium. 2. It oxidises rapidly the precious metals. 3. It destroys vegetable colours. 4. It possesses a remarkable smell, like weak chlorine, whilst oxygen is odourless. Ozone bleaches most vegetable colours, and is a strong oxidiser, and so it may be called "nascent" or "active oxygen." The latter fact probably affords the real clue to the supposed connection between an absence of ozone in the atmosphere and outbreaks of cholera, dysentery, and other like diseases. Their relation attracted the attention of M. Quetelet in Belgium, and of M. Andrand, of Paris, in the cholera outbreak of 1849. According to Glaisher, Moffat, Hunt, and others, the occurrence of cholera and choleraic diarrhoea is coincident with an absence or diminution of ozone, and their departure with a return of ozone (C. B. Fox, M.D. 1 ). On the other hand, Dr. Moffat remarks that " the prevalence of influenza, and the spread of catarrhal affections, are in- variably connected with an excess of ozone in the atmo- sphere." The fact is that ozone irritates the mucous mem- branes, and so has been credited with producing epidemic catarrh. Although colourless in its gaseous form, ozone appears as a blue fluid when liquefied by cold and pressure. It decom- poses a solution of iodide of potassium and sets free iodine, which gives a blue coloration when brought into contact with a solution of starch. Ozone test-papers are strips of blotting paper or filter paper steeped in a solution of potassic iodide and starch. When moistened and exposed to an ozone-laden atmosphere such test-papers turn blue. Testing for ozone is even still in an unsatisfactory state. 1 Ozone and Antozone. London : J. and A. Churchill. 1873. 300 METEOROLOGY Any other oxidising agent in the air, such as hydrogen dioxide or nitric acid, will reduce the potassic iodide, so that the test given above is open to serious fallacy. Dr. Cornelius B. Fox, who has paid particular attention to this question in his work on Ozone and Antozone, 1 says that, if we wish to ascertain the amount of ozone present in the air to the exclusion of the other air purifiers, we employ a paper which is alone acted upon by ozone, such as the iodised litmus paper. With this test, we do not take any notice of the amount of iodine set free, but we observe the amount of potash formed by the union of the ozone with the potassium. Potash, being an alkali, has the property of turning red litmus blue. The greater or less conversion of the red litmus into blue shows a greater or less quantity of ozone in the air. Lightning-conductors. B. Franklin devised the lightning- rod, or lightning-conductor, which is now universally adopted. The principle of the lightning-conductor (Fr. paratonnerre ; German, Blitzableiter) is that electricity selects the better of two conducting passages, and that when it has got a sufficient conducting passage it is disarmed of all destructive energy. A lightning-conductor is a metallic rod, usually of galvanised iron or of copper, which terminates above in one or more sharp points, and below in moist earth or in a sufficient ex- panse of water. This metallic rod, when placed on a building or on the mast of a vessel, protects it by affording a ready passage to the electricity of the earth into the atmo- sphere, so establishing electric equilibrium gradually and silently. As Mr. R. H. Scott well observes, 2 " The action depends on what is called the 'power of points.' The elec- tricity on a sphere is uniformly distributed over the surface ; on an oval figure it tends to accumulate at the ends. On a cylinder this tendency is more strongly developed, and, when the cylinder becomes a fine wire, the tension is so great at 1 Page 168. London : J. and A. Churchill. 1873. 2 Elementary Meteorology, p. 183. ATMOSPHERIC ELECTRICITY 301 the end that the electricity soon forces its way into the sur- rounding air and escapes." AJightning-conductor consists of three parts : the pointed rod, overtopping the building; the conductor, or part con- necting the top with the ground ; and the part in the ground. In a very able paper, entitled " Kemarks on some Practical Points connected with the Construction of Lightning-con- ductors," 1 the late Dr. Robert J. Mann, F.K.A.S., President of the British Meteorological Society, laid down the indis- pensable conditions for an efficient lightning-conductor as follows : 1. The lightning-conductor must be made of good conducting material metallically continuous from summit to base, and of a dimension that is sufficient for the ready and free conveyance of the largest discharge that can possibly have to pass through it. 2. It must have ample earth- contacts, and these contacts must be examined frequently, to prove that they are not getting gradually impaired through the operation of chemical and electrical erosion. 3. It must terminate above in well-formed and well-arranged points, which are fixed and distributed with some definite regard to the size, form, and plan of the building. 4. There must be no part of the building, whether it be of metal or of less readily conducting material, which comes near to the limiting surface of a conical space, having the highest point of the conductor for its apex, and having a base twice as wide as the lightning-conductor is high, without having a point pro- jecting out some little distance beyond, and made part of the general conducting line of the lightning-rod by a communica- tion with it beneath. 5. There must be no mass of con- ducting metal, and, above all things, no gas-pipe connected with the main, within striking distance of the lightning-rod, lest at any time either the points or the earth-contacts shall have been so far deranged or impaired as to leave it possible for discharges of high tension, instead of continuous streams 1 Quarterly Journal of the Meteorological Society, vol. ii. p. 417. 1875. 302 METEOROLOGY of low tension, to pass through the rod, and to be diverted from it into such undesigned routes of escape. The Meteorological Society about twelve years ago organ- ised a conference of delegates from various scientific and professional societies to examine into the whole question of lightning-conductors, and the conference drew up a code of rules l very much on the lines indicated by Dr. Mann in his paper published in 1875. The chief matters to be attended to are these : (1) The point of the upper terminal should not be sharp, not sharper than a cone of which the height is equal to the radius of its base. All points should be platin- ised, gilded, or nickel-plated so as to resist oxidation. (2) Rods should be taken down the side of the building most exposed to rain, and held firmly but not too tightly by hold- fasts, so as to allow of contraction and expansion by changes of temperature. (3) The rod should consist of copper, weighing not less than 6 oz. per foot-run, and the conduc- tivity of which is not less than 90 per cent of that of pure copper, either in the form of tape or rope of stout wires, no individual wire being less than No. 12 B.W.G. Iron may be used, but should not weigh less than 2J Ibs. per foot-run. (4) Although electricity of high tension will jump across bad joints, they diminish the efficacy of the conductor ; therefore, every joint, besides being well cleaned, screwed, scarfed, or riveted, should be thoroughly soldered. (5) Iron rods should be painted, whether galvanised or not. (6) The rod should not be bent abruptly round sharp corners. (7) As far as practicable, it is desirable that the conductor should be con- nected to extensive masses of metal, such as hot-water pipes, both inside and outside the building ; but it should be kept away from all soft metal pipes, and from internal gas-pipes of every kind. (8) It is essential that the lower extremity of the conductor should be buried in permanently damp soil, hence proximity to rain-water pipes and to drains is desirable. 1 Report of the Lightning-rod Conference, 16 pp. London : Spon. 1882. ATMOSPHERIC ELECTRICITY 303 It is a very good plan to make the conductor bifurcate close below the surface of the ground, and adopt two of the following methods for securing the escape of the lightning into the earth : A strip of copper tape may be led from the bottom of the rod to the nearest gas or water main, not merely to a lead pipe, and be soldered to it ; or a tape may be soldered to a sheet of copper 3 feet x 3 feet and -^ inch thick, buried in permanently wet earth, and surrounded by cinders or coke ; or many yards of the tape may be laid in a trench filled with coke, taking care that the surfaces of copper are, as in the previous cases, not less than 18 square feet. Where iron is used for the rod, a galvanised iron plate of similar dimensions should be employed. A lightning-conductor should be kept away from a gas-pipe, because if there was any defect in the connections, an electric discharge of high potential might cause a spark and so ignite the gas. PART III. CLIMATE AND WEATHER CHAPTEE XXIV CLIMATE Meaning of the term ' ' Climate " Definition of Climate Accumulated Temperature Effect of Temperature on the Animal and Vegetable Kingdoms Principal Factors of Climate : Latitude, Altitude, Eelative Distribution of Land and Water Presence of Ocean Currents. IN his work on Elementary Meteorology, Mr. E. H. Scott, F.E.S., observes x that the old division of the world by Par- menides 2 into five zones a central torrid zone, northern and southern temperate and frigid zones has been found to be quite inadequate as a representation of the climatology of the globe. In its original and stricter etymological sense the word climate (Gk. /cAt/m, a slope or inclination) was applied to one of a series of regions or zones of the earth running parallel to the equator, from which the earth's surface was supposed to slope to the poles, hence the Latin rendering of K\ifjM t inclinatio ccdi. According to this view, put forward by Claudius Ptolemy, the author of the Ptolemaic System of [the Universe (A.D. 120- 149), climate was determined solely by latitude, and one 1 At p. 338. 2 Of Elis. Flourished circ. B.C. 430. CLIMATE 305 climate differed from another only as regards the relative length of the midsummer day and the relative altitude of the noon- tide sun. As a matter of fact, latitude is only one, and that, as we shall see presently, by no means the most important, factor in the determination of climate. We may define climate as the condition of a country, district, or place, in relation to certain meteorological elements notably, air temperature, atmospheric pressure and wind, atmospheric moisture and electricity viewed more par- ticularly in their effects upon animal or vegetable life. It is these effects which determine the distribution of a fauna or of a flora in a given region of the globe. Mr. Scott points out a that the distribution of the plants of most importance to mankind, such as the cereals, depends chiefly on the summer temperature, while the distribution of animals is more depend- ent on the winter temperature. For example, the district of Manitoba in Canada yields magnificent crops of wheat, although its winter temperature often falls far below zero of the Fahrenheit scale. Maize again succeeds well in extreme climates, the summer season alone being sufficient for its whole life. On the other hand, plants, which, so to speak, are alive throughout the year, like the fuchsia, the laurel, or even the hawthorn, would perish in the bitter winter of the "Canadian North-West," although they flourish in climates like that of the British Islands, where the fate of a wheat crop trembles yearly in the balance and where maize entirely fails to grow. In connection with this topic, attention has already been drawn to the estimation of accumulated temperature, or warmth available for agricultural purposes, expressed in " day- degrees " (see Chapter IV. p. 33). It may be well to repeat that a "day-degree" signifies 1 continued for twenty-four hours, or any other number of degrees, for an inversely pro- portional number of hours, the term " accumulated tempera- ture " indicating the combined amount and duration of an 1 Elementary Meteorology, p. 338. 1883. X 306 METEOROLOGY excess or defect of temperature above or below 42 F. for the period named. Animals can bear a greater range of temperature than plants, and so their territorial distribution is more extensive than that of individual members of the vegetable kingdom. As Mr. Scott puts it: " The distribution of animals is more dependent on the winter temperature." Climate depends chiefly on (1) distance from the equator, or latitude ; (2) physical configuration of the surface ; (3) elevation ; (4) nearness, or otherwise, of oceans ; (5) prevailing winds. In his Lumleian Lectures on "Aero-therapeutics in Lung Disease," delivered before the Royal College of Phy- sicians of London in 1893, Dr. C. Theodore Williams, M.A., F.R.C.P., gives the principal factors of climate as follows : 1. Latitude Naturally the greatest influence as describ- ing the position of the sun towards the earth in a certain region, and thus determining the length and intensity of sunshine. 2. Altitude By which the effects of latitude may be to some extent neutralised, for even in the Tropics, at a height of 16,000 feet, snow and ice may exist, the temperature falling in ascending mountains 1F. for every 300 feet. 3. Relative Distribution of Land and Water, and especially the presence of vast tracts of either desert or ocean ; the former accentuating extremes of temperature, and the latter tempering them. 4. Presence of Ocean Currents, flowing from higher or lower latitudes (as the case may be) and thereby qualifying the climate. 5. Proximity of Mountain Ranges, and their influence on the shelter from wind and on the rainfall. 6. Soil. Its permeability or impermeability to moisture. 7. Vegetation. 8. Rainfall. Its amount and annual distribution. 9. Prevailing Winds. CLIMATE 307 It will be necessary to consider these factors in more detail. I. Latitude. A writer in Chambers's Encyclopaedia (Art. " Climate "), says : " The effect of the sun's rays is greatest where they fall perpendicularly on the surface of the earth, and diminishes as their obliquity increases ; the surface which receives any given amount of the sun's rays increasing with their increased obliquity, as a'b' is greater than ab in the annexed figure, whilst at the same time the oblique rays being subjected to the influence of a greater number of particles of the atmosphere, as c'd is longer than ca, a greater amount of their heat is absorbed before they reach the surface of the FIG. 68. Diagram illustrating the effect of the perpendicular and the oblique falling of the Sun's rays. earth at all. The greater or smaller extent of surface receiv- ing a certain amount of heat, also makes important differences to arise from exposure by slope towards the equator or to. wards the nearer pole." II. Altitude. It has been already shown (see Chapter XI. p. 118) that temperature decreases with increasing" elevation above sea-level, and that this is largely due to diminishing density of the atmosphere, as well as to reduced humidity. Even on the equator perpetual snow and ice are found above a certain height, and Quito, the capital of Ecuador, at an altitude of 9451 feet, enjoys an eternal spring, the mean temperature of the whole year and of every season being steady at 60 F. The fall of temperature with altitude is not, however, uniform at all places having the same latitude. A striking example of this is met with in the climates of the Himalayan 3 o8 METEOROLOGY mountains. On the southern slopes of this vast and gigantic chain the snow-line, or limit of perpetual snow, is depressed by the precipitation of large quantities of snow and rain from the moisture-laden south-west winds coming from the Indian Ocean. These winds, first chilled and deprived of their moisture, but afterwards warmed by the latent heat set free in the condensation of their vapour into rain or snow, cross the summits of the Himalayan peaks and descend towards the plains of Central Asia. Being abnormally dry, these winds have an immense capacity for both heat and moisture, and so, as they descend, they become rapidly still warmer and at the same time lick up both water and snow. The result is that the snow-line on the northern slopes of the Himalayas is at least 4000 feet higher than it is on the southern side. Strictly analogous phenomena attend the passage of a south wind across the Alps, where it is called the Fohn ; of a south-east wind across the mountainous in- terior of Greenland, bringing with it comparative warmth to North Greenland and Smith Sound ; and of a " nor'-wester " across the mountains of New Zealand. This last wind, taking its origin in the South Pacific, deposits its moisture on the western slopes of the New Zealand Alps, and appears in the Province of Canterbury on the east coast as a very dry, and often a hot, wind unaccompanied with rain. It is said that, when the Fohn is blowing in the Alps, the snow melts with marvellous rapidity, so that it is popularly called the " snow-devourer " (Schneefresser). 1 III. Relative Distribution of Land and Water. In our study of climate we may lay down as aphorisms First That hot air is lighter than cold air. Secondly. That the rapidity with which the processes of heating and cooling of air goes on is in direct proportion to the amount of aqueous vapour contained in that air dry air 1 For a full account of the Swiss Fohn wind, see a paper by Dr. Wild, now the Director of the Imperial Observatory at St. Petersburg, Ueber Fohn und Eiszeit (Bern, 1868). CLIMATE 309 becoming heated or cooled more rapidly and more completely than moist air, other conditions being alike. Thirdly. That, consequently, the air over large areas of land, being drier, becomes more rapidly heated in summer and more rapidly cooled in winter than air which is in contact with extensive water-surfaces ; and, Fourthly. That the radiation-heating power of dry land is greater than that of water, as also the radiation-cooling power of dry land is greater than that of water. This group of facts is of paramount importance in climat- ology. The effect of them upon the climate of the great continent of Europe and Asia has already been described in Chapter XV. (see p. 171, above). We can, indeed, form but little idea of the enormous changes of temperature which take place in Central and Northern Asia between the seasons of summer and winter. But that these changes are sufficient to produce the great variation in barometrical pressure on which depends the varying wind-system of the continents of Europe and Asia in those seasons may be easily shown by a comparison of the range of temperature between July and January in an insular T climate like our own, and at Yakutsk in Siberia, which is situated close to the centre of lowest and highest barometrical pressures in those months respectively. At Dublin the mean temperature of July is about 60 F., of January about 40 F. a range of only 20. The corre- sponding mean temperatures at Yakutsk are 66 F. and - 45 F. respectively a range of 1 1 1 . For weeks in summer the thermometer ranges between 80 and 90 at this place, while in winter it may descend 90 below the freezing-point of water. Well does Humboldt observe : 2 1 The term Insular and Continental, as applied to climate, usually signify merely that it is characterised by a small or by an extreme range of temperature respectively, Avithout any reference to the geographical position of a place as regards the seaboard. 2 Kosmos, vol. i. p. 352. 3 io METEOROLOGY 11 The inhabitants of the countries where such continental climates prevail seemed doomed, like the unfortunates in Dante's Purgatory A soffrir tormenti caldi e geli." Or, as Milton has so admirably expressed it From beds of raging fire to starve in ice. The capability of man to endure variations and extremes of temperature," says Dr. Theodore Williams, "has been proved to be very great, for General Greely states that at Fort Conger, U.S.A., in February 1882, he experienced the low temperature of - 6 6 '2 F., and at another time, in the Maricopa Desert, Arizona, he saw noted the air temperature of 114 F., while the metal of his aneroid beside him as he rode assumed a temperature of 144 F." The lowest temperature recorded is - 90 F. at Wercho- jansk, in Siberia, lat. 67 '5 N. This cold station lies in the valley of the river Jana, 330 feet above sea-level. At Poplar River, Montana, North America, the thermometer fell to - 63'1 in January 1885. The principal laws of distribution of annual range of temperature, given by Professor Supan in a paper which appeared in the first volume of Kettler's Zeitschrift fur Wissenschaftliche Geographic, are thus summarised by Mr. R. H. Scott : 1. The annual range of temperature increases from the equator towards the poles, and from the coast towards the interior of a continent. It is greatest 100 F. and upwards in Siberia near Yakutsk. It is least under 20 F. over almost the whole of the sea surface of the globe, in South America and South Africa. 2. The regions of extreme range in the northern hemi- sphere coincide approximately with the districts of lowest temperature in winter. On the whole the range curves in their course resemble the isotherms of January. CLIMATE 3" 3. The range is greater in the northern than in the southern hemisphere. 4. In the middle and higher latitudes of both hemispheres, with the exception of Greenland and Patagonia, the western coasts have a less range than the eastern. 5. In the interior of the continents the range, in moun- tainous districts, diminishes with the height above the sea. Probably nowhere is the influence of the ocean in restrict- ing the annual range of temperature more marked than off the extreme south-west coast of England. In the Scilly Isles the mean temperature of the sea surface ranges between 49 in February and 61 in August, that is through 12. The mean temperature of the air is 46*3 in January and 61 '5 in July a mean annual range of only 15*2. At Yarmouth, also on the sea, but not in the ocean, the temperature of the sea surface ranges from 37 in January to 61 in July the mean temperature of the air being 37*9 and 62'5 in the months named a mean annual range of 24*6. Perhaps no single element has a greater influence upon climate than the presence of water. Its specific heat l is only one-fourth that of dry land, and consequently it absorbs heat more slowly, stores up a larger amount of it, and parts with it less rapidly. Again, owing to condensation by cold, surface water, when exposed to low temperatures, sinks, and its place is taken by the deeper strata of warmer liquid. Equilibrium of temperature is thus maintained in the neigh- bourhood of extensive areas of water. In warm weather evaporation from water surfaces tends to cool the super- incumbent air, to increase the humidity, and to fill the atmosphere with clouds. Hence the equable, cloudy, and moist climates of seaboards. Extensive lakes produce similar effects on a smaller scale. Thus, in the Canadian winter, what may be a storm of rain on the shores of Lake Superior is often a fall of snow a few miles inland. When districts of 1 The specific heat of a substance is the number of units of heat required to raise the temperature of one pound of it by one degree. 3 I2 METEOROLOGY country are partly covered with water, the climate is rendered damp and cool, for the evaporation from wet earth is much greater than that from an uniform water-surface. Under such circumstances there is reason to believe that a climate would even be improved by changing marshes or morasses into sheets of open water. Like good results often follow the carrying out of effective drainage works, and there can be no doubt that, if the water-shed of the Shannon, for example, were properly drained, the climate of the whole of Ireland would be made drier and warmer. This is a subject of vital importance to all who have agricultural interests in view, as well as to those who have regard to the public health. In estimating the effect of water surfaces upon climate it should not be forgotten that, while fresh water attains its maximal density at 39*2 F., and freezes at 32'0, sea water continues to contract until it is chilled down to 26 '2 F., and does not freeze above 28'4. The result is that sea water in the open will not begin to freeze on its surface until all its depths have been cooled nearly to freezing-point, whereas fresh water needs to be chilled throughout only to 39*2 before ice commences to form on its surface. Turning to the question of the influence of dry land on climate, we find that the configuration of the surface of the ground exercises an effect second only to that of the amount of land. Thus, as a rule, when the surface slopes away from the sun, the rigour of the climate is intensified. We have especially striking examples of this in North Germany, Siberia, and those parts of North America which slope towards Hudson's Bay. The converse is also true, and is exemplified in the excessive summer-heat of Northern Italy, India, and Southern China. Most winter resorts are situated on grounds having a southern, or solar, aspect. But leaving out of con- sideration the direct influence of the presence or absence of the sun's rays, the cooling of the air by terrestrial radiation is found to affect localities in very different degrees. Where the surface is uniformly level, as in the case of plains or CLIMATE 313 table-lands, radiation proceeds uniformly, and the whole district is equally chilled. If the air is calm, and the sky clear, radiation goes on rapidly, and the temperature falls. Should the sky be clouded and the air in motion, radiation is uniformly checked. The alternations in either case are similar over the whole area of level ground. In hilly or mountainous districts radiation acts as before, but the air which is cooled becomes specifically heavier, and immediately commences to flow down the mountain sides. As it is replaced by warmer air, the temperature remains compara- tively high and uniform. On high ground, also, the atmo- sphere is seldom calm, so that radiation is usually more or less checked, and warmth is maintained. Valleys, however, experience extreme variations of temperature for two reasons first, because the cooled air flows down into them from the surrounding high grounds; and, secondly, because, being so much shut in, they are fully exposed in the absence of wind to the influence of radiation. Great diurnal range of temperature is a marked feature in inland districts, particularly when the earth's surface is desert or scantily dotted with vegetation. Thus at Mooltan (where the thermometer ranges yearly from 29 to 126 F. in the shade), Kawal Pindi, and other stations in the Punjab, the daily range in April and November may amount to 40. The same state of things is observed in Egypt, on the steppes of Southern Kussia, and on the prairies of North America. IV. Ocean Currents of either warm or cold water modify climate in a remarkable degree. They are named according to the direction towards which they flow. The ameliorating effects of the presence of a vast water-surface are intensified in the case of oceans, such as the Pacific and Atlantic, in winter time, by the setting towards the Arctic and Antarctic regions of immense surface-currents of warm water. The best known of these currents is the Gulf Stream, which flows north-eastward along the western coasts of Europe, and to 3 i 4 METEOROLOGY which we are so largely indebted for our wonderfully mild British winters. "Its climatic effect," says Mr. J. Knox Laughton, M.A., l " when stated in measures of heat, is stupendous it is the very poetry and romance of arithmetic." He proceeds to show that the heat brought by the Gulf Stream into the North Atlantic has been fairly estimated as not less than one- fifth of the whole heat possessed by the surface-water of that division of the ocean. Assuming, with Sir John Herschel, that the temperature of space is 239 below zero, and taking the existing temperature of the North Atlantic as 56 above zero, we find that the heat which it actually has corresponds to a temperature of 295 (namely, 239 + 56), the fifth part of which is 59. If, then, the fifth part of its heat that is, the heat derived from the Gulf Stream were taken away from it, the surface water of the North Atlantic would have an average temperature of - 3 F., or 35 below the freezing- point of fresh water. 2 It is roughly estimated that about five billions of cubic feet of water are hourly poured through the Straits of Florida into the North Atlantic. This water has at the time an average temperature of not less than 65, but after performing a circuit in the North Atlantic, it returns to the Tropics as an undercurrent with an average temperature not above 40. It has imparted to the air over the North Atlantic the heat corresponding to a difference in temperature amounting to 25. Now, the British standard measure of heat the thermal unit is the quantity of heat required to raise the temperature of 1 Ib. of water by 1, while a cubic foot of water weighs about 64 Ibs. With these data, we find that the heat thrown out by the Gulf Stream every hour into the air of the North Atlantic is 25 x 64 x 5,000,000,000,000 thermal units. But every thermal unit, according to the "Law of Equiva- 1 In a lecture on " Air Temperature : its Distribution and Range " (Modern Meteorology, 1879. Edward Stanford. 1879). 2 Croll's Climate and Time, p. 35, et seq. CLIMATE 31$ lence," experimentally established by Dr. Joule, of Man- chester, is capable of lifting a weight of 772 Ibs. through a height of 1 foot. Consequently the heat hourly dispersed from the water of the Gulf Stream, if stored up and ap- plied as power, would be capable of lifting each hour, 772 x 25 x 64 x 5,000,000,000,000 Ibs., through a height of 1 f ot that is, of doing the work of steam-engines having an aggregate horse-power of 3,119,000,000,000 a power equal to that of nearly 400,000,000 ships such as our largest ironclads. 1 Ocean currents of cold water also exist. Of these the most notable, probably, is that which flows out of Baffin's Bay down the eastern shores of North America, and which is known as the American Arctic Current. Its cooling influence is felt as far south as Cape Cod in latitude 42. An Arctic current of far less magnitude flows into the North Pacific through Behring's Straits. It chills the air over Kamchatka and Japan in the summer season, throwing the isotherms over the North Pacific into remarkable loops in July, when the current is strongest owing to the melting of the polar ice. In the southern hemisphere, the oceanic polar currents form a still more striking feature in the physical geography. According to Sir F. Evans, 2 all the surface water between the Antarctic Circle and the parallel of 45 S. seems to drift northwards and eastwards, causing the isotherms on the western coasts of America, Africa, and Australia to dip down towards the equator (R. H. Scott). The best known of these currents is the Peruvian, or Humboldt's current, which washes the west coast of South America. Mr. Scott also points out that the influence of the Antarctic Atlantic current on the west coast of Africa is such that the temperature of the sea near Cape Town is sometimes 20 lower than in the corresponding latitude on the eastern side of the continent. 1 Croll's Climate and Time, p. 25. 2 Brit. Assoc. Report, 1876, p. 175. CHAPTEE XXV CLIMATE (continued) Proximity of Mountain Ranges, Soil, Vegetation, Rainfall, Prevailing Winds Cold Winds: East Wind of Spring in the British Tsles, Mistral, Tramontana, Nortes of Gulf of Mexico, Pamperas, Tormentos, Etesian Winds Hot Winds : Scirocco, Solano, Leveche, Harmattan, Khamseen, Simoom, Hot Wind of Australia, Fohn, Leste. V. Proximity of Mountain Ranges. Dr. Alex. Buchan, in his Introductory Text-Boole of Meteorology, * says that, apart from diverting the winds from their course, the chief effect which mountain ranges have upon the temperature (and so upon climate) is to drain the winds which cross them of their moisture. Colder winters and hotter summers in places to the leeward, as compared with places to the wind- ward, are thus caused ; for the protecting screen of aqueous vapour is partially removed by condensation, and so the country to the leeward of a mountain-chain becomes more fully exposed to both solar and terrestrial radiation. For the same reason the rainfall is lessened in such sheltered localities, although it is of course proportionately increased on the windward side of the mountains. Dr. Theodore Williams cites the extraordinarily dry climate of Colorado, which lies under the lee of the Rocky Mountains, as an instance of the influence of a mountain-range on climate. Nearer home we have similar examples on a much smaller scale in several parts of the British Isles : heavy and continuous rainfalls in the 1 William Blackwood and Sons : Edinburgh and London. 1871. P. 73. CLIMATE 317 mountainous districts of Kerry, Cumberland, and the west of Scotland, contrasting with comparatively dry climates in Dublin, the Lowlands of Scotland, and the coasts of the Moray Firth, Nairnshire, and the Carse of Sutherland. These last-named districts, according to Mr. Scott, owe their good fortune mainly to the fact of their lying on the leeside of an extensive mountain district. As regards Dublin, a range of mountains lies a few miles south of the city, with summits varying in height from 1000 to more than 2500 feet. This mountain-chain intercepts the vapour-laden winds at all points between south-south-east and south-west. In con- sequence, the rainfall is diminished and the sky is com- paratively cleared during the continuance of the southerly and south-westerly winds which so frequently prevail. Dublin and its neighbourhood are the only part of Ireland where the annual rainfall falls short of 30 inches it is about 28 inches and this depends on the geographical situation of the city on the east coast and to the leeward of high lands, grouped into mountains to the south-east, south, and south-west, whereby the rain-bearing winds are drained of their super- abundant moisture before they reach the valley of the Liffey and the plains lying north of that river. VI. Soil. With regard to the absorbing power of heat possessed by soils, Schiibler has arranged them in the following order, 1 100 being assumed as the standard : Sand with some lime, 100 ; pure sand, 9 5 '6 ; light clay, 76'9 ; gypsum, 73'2 ; heavy clay, 7 I'll ; clayey earth, 684 ; pure clay, 66 '7; fine chalk, 61 '8; humus, 2 49. This list shows the high absorbing power of the sands, and the comparative coldness of the clays and humus. The absorbing 1 Parkes's Manual of Hygiene, p. 312. 4th edition. 2 Humus is the organic matter of the soil, which is made up of the products of the decomposition of vegetable substances. These products may be arranged in three classes : (1) those soluble in water crenic, apocrenic, and ulmic acids ; (2) those soluble in alkaline solutions, but not in pure water humic and geic acids ; (3) those insoluble humin and ulmin, 318 METEOROLOGY power of water possessed by soils varies in a similar manner sands retain but little water, clays about 10 to 20 times as much as sands, and humus double as much again. Clays and humus are comparatively unsuitable as sites for building, owing to their characters of coldness and dampness. In some diseases they are very injurious, as, for example, in phthisis, rheumatism, and catarrh. If damp soils be exposed to a high temperature, they may give rise to malaria, owing to decomposition of the organic matter mixed with them. Marshy soils, alluvial soils, old estuaries, and deltas, contain much organic matter, and should be regarded with suspicion. Peaty soils also are largely composed of organic matter, but they are not malarious, owing probably to the preservative properties of peat. Granite, metamorphic and trap rocks, clay-slate, chalk, sandstone, gravels, and the pure sands, are healthy, and suited for building sites. The limestone and magnesian limestone rocks and mixed sands are only moder- ately healthy. 1 Three diseases, leaving ague out of the question, appear to be intimately connected with the presence of water in the soil. In 1862, Dr. Bowditch, of Boston, U.S., drew attention to the relations between the prevalence of phthisis and the amount of sub-soil water. His researches were amply confirmed by Dr. (now Sir George) Buchanan, 2 who discovered that the death-rate from phthisis in various towns in England was greatly reduced in consequence of efficient drainage and removal of the sub-soil water. Some years ago Pettenkofer of Munich advanced the doctrine of the ultimate dependence of enteric fever and of cholera on the varying level of the sub-soil or "under-ground" water, 3 the most dangerous period, according to him, being that of the sinking of the water after a previous rise. 1 Gf. Parkes, Loc. dt. p. 314, seq. 2 Ninth and Tenth Reports of the Medical Officer of Health to the Privy Council. 3 Zeitschrift fur Biologic, 1868. CLIMA TE 319 The high absorbent power of loose sandy soils is doubtless due to the presence of large quantities of imprisoned air, which converts the sand into a bad conductor of heat. Hence such soils heat readily in summer and cool readily in winter near the surface ; but these extremes of temperature do not penetrate to any depth. Dr. Williams states 1 that the sandy soil in an Arabian or Egyptian desert may be heated to 120, 140, or even 200 F., and when the particles of this hot sand are carried through the air by the terrible simoom, the shade temperature may rise to 125. On the other hand, Dr. Buchan says, in his Introductory Text-Book of Meteorology? that in Scotland, for a period of nine years, the temperature at three inches below the surface fell to 26*5 in loose sandy soils, but at a depth of 12 inches the freezing-point was only once observed. In clay soils, at 3 inches the lowest tempera- ture recorded was 28, whilst at 12 inches the temperature often fell to freezing, and even at 22 inches 32 was once more recorded. VII. Vegetation. A district covered with a luxuriant growth of plants and forest trees has a comparatively uniform and temperate climate. By day the heat is lessened, because the vegetation intercepts a large proportion of the sun's rays, which would otherwise heat the earth's surface ; also, because the evaporation from leaves and grasses renders heat latent, and so keeps the atmosphere cool. By night radiation from the surface of the ground is checked, and so the fall of tem- perature is diminished. Forests control evaporation and in- crease the humidity of the air ; they are also said to increase the rainfall, but this seems not to be satisfactorily established. As moist air prevents excessive heat in summer and excessive cold in winter, forests are thus seen to be of use in mitigating extremes of climate. In winter they afford shelter from storms, and in tropical climates the spread of malaria is pre- vented by the interposition of trees. The third number of Petermann's Mittheilungen for 1885 1 Lumleian Lectures, 1893. 2 P. 46. 1871. 320 METEOROLOGY contained an article of exceptional interest by Herr A.Wojei- kof on the influence of forests on climate. The first step to- wards a scientific investigation of this subject was taken when the Bavarian forest meteorological stations were established, and when Prussia, Alsace-Lorraine, France, Switzerland, and Italy followed the example. As a general rule it may be laid down that during the warmer season 1, the tempera- tures of the earth and air are lower in the forests than in contiguous woodless places ; 2, their variations are less ; 3, the relative humidity is greater. The influence of forests in diminishing evaporation from water and the soil is so great that it cannot be accounted for solely by the lower temperature of the warm months, the greater humidity, or even by the shade the protection from the wind afforded by the trees is regarded by Herr Wojeikof as more important than all these factors together in reducing the amount of evaporation. With respect to the influence of forests on rainfall and snow- fall, there is as yet only a single series of observations sup- plying comparative statistics, and extending over a sufficiently long period, namely, six years. These were taken in the neighbourhood of Nancy by the pupils of the School of Forestry of that city, under the direction of M. Mathieu, sub-director of the school. These observations, reported in PolyUUion, 1882, prove that 1. Forests increase the quantity of meteoric waters which fall on the ground, and thus favour the growth of springs and of underground waters. 2. In a forest region the ground receives under cover of the trees as much water as, or more than, the uncovered ground of regions with little or no wood. 3. The cover of the trees of a forest diminishes to a large degree the evaporation of the water received by the ground, and thus contributes to the maintenance of the moisture of the latter, and to the regular flow of springs. 4. The temperature in a forest is much less variable than in the open, although, on the whole, it may be a little lower ; CLIMATE 321 but the minima are there constantly higher, and the maxima constantly lower, than in regions not covered with wood. M. Fautrat, when sub-inspector of forests at Senlis, made observations on forestial meteorology during four years. These, although conducted on a different method, fully cor- roborate those of M. Mathieu in several respects. He adds the following interesting remarks : Rain falls most abundantly over forests with trees in full leaf ; the humidity of the air is much higher over masses of Pinus sylvestris than over masses of leaved species; and the leafage and branches of leaved trees intercept one-third, and those of resinous trees one-half, of the rain water, which afterwards returns to the atmosphere by evaporation. Herr Wojeikof endeavoured to ascertain the influence of forests on the climatic conditions of their neighbourhood in the western parts of the Old World, between the 38th and 52nd degrees N. latitude, the places selected being in all cases in the open. Thus, for the 52nd degree, eight stations were taken between Valentia Island, in Ireland, on the west, and the Kirghiz steppe on the east ; for the 50th, Guernsey on the west, Semipalatinsk on the east, and ten other inter- vening stations ; and so on for each two degrees of latitude to 38 N. The general result of the observations at fifty stations in six different degrees of latitude is that in Western Europe and Asia large forests have a great effect upon the temperature of places near them : the normal increase of tem- perature as we travel eastward from the Atlantic Ocean to the interior of the continent is not merely interrupted by the influence of forests, but places far removed from the coast through that influence enjoy a cooler summer than those actually on the sea. A striking example of this is Bosnia, where the summer is 4*5 to 8*1 F. cooler than in Herzegovina. Bosnia, separated by lofty mountain ranges from the sea, has extensive forests ; Herzegovina, on the contrary, is almost disafforested. Even on the Island of Lissa, where under the full influence of the Adriatic Sea the summer should be cooler, Y 322 METEOROLOGY the temperature is more than 1'8 higher than it is in Bosnia. In Portugal, which is poor in forests, the temperature rises very rapidly towards the interior during the almost rainless summer. The heat is still greater in stony Attica, notwith- standing the proximity of the sea. On the eastern shore of the Caspian, owing to the desert of sand and stone, the summer temperature is extremely high, whereas at Lenkoran, on the western shore of that vast inland sea, a cool though dry summer is enjoyed. In the great Lenkoran forest vegetation is more luxuriant than in any part of Europe, for a tangled mass of climbing plants encircles the trees so that it is always humid in the forest. Yet here the rain curve is a sub-tropical one very little rain falling in the summer, but large quantities in autumn and winter. The water is stored up in the forest, so maintaining evaporation during the summer droughts. " To sum up : forests exercise an influence on climate which does not cease on their borders, but extends over a larger or smaller adjacent region according to the size, kind, and position of forest. Hence, man by afforestation and dis- afforestation can modify the climate around him ; but it is an extreme position to hold that by afforestation the waste places of the earth can be made fertile. There are places in- capable of being afforested, which would not give the neces- sary nourishment to trees" (Nature, vol. xxxii., 1885, p. 115). VIII. Rainfall. The influence of precipitation on climate has already been discussed at some length in Chapter XX. (p. 246). In estimating it, regard should be had to the monthly, seasonal, and yearly rainfall, both in individual periods and on an average of many such periods. Equally important is it to know the number of " rainy days " (or days upon which '005 inch of rain or upwards is measured in the gauge) which may be expected to occur in each month, season, or year. Nor should the probability of heavy or torrential rainfalls 1 inch or upwards be left out of account. It is evident that a moderate rainfall spread over many days throughout the year may constitute a wet climate, while a still heavier rainfall CLIMATE 323 restricted to a given season may characterise what is essentially a dry climate. Another element worthy of consideration is the frequency of thunderstorms, so often attended by torrential rains, and of hailstorms. Lastly, the seasonal limits of snowfall should be investigated and the average and extreme duration of an unbroken snow-covering. This last topic has been ably handled by Dr. Alexander Wojeikof in a paper on the " Influence of Accumulations of Snow on Climate," which was read before the Royal Meteoro- logical Society, June 17, 1885. 1 It has already been shown in these pages that snow is a bad conductor of heat, and accordingly protects the underlying soil from excessive cold. But much depends on the structure of the snow. If it con- sists of loosely piled small feathery flakes, which entangle air in large quantity, it is a thoroughly bad conductor, and so affords most protection. If, however, by alternate thawing and freezing, it solidifies into ice assuming the form called in Germany Fim, and in France neve it is a far better con- ductor of heat, and the underlying soil will quickly freeze. Again, the air over a snow-covered surface will become extremely cold first, because the snow cuts off from it the warmth of the ground, and secondly, because dry feathery snow is a good radiator of heat. Then, as there is but little dust in the air over a snow-covered country upon which the solar rays can act, these rays will be unable by themselves to thaw a deep snow covering. How then is it that the winter snow does melt in the northern parts of Europe, Asia, and North America ? Dr. Wojeikof has no doubt that the thaw is first caused by winds from warmer quarters, or from open oceans. These warm winds cause the upper layer of snow to melt. After it has been frozen again it is changed to neve, that is, to a condition in which it is somewhat diathermanous to solar heat and radiates heat much less freely. Once this happens, the melting of snow goes on much more easily. To a small ex- tent the melting of the snow may be helped by dust brought 1 Quart. Journ. of the Royal Met. Soc., vol. xi. 1885, page 299. 324 METEOROLOGY by the winds from continental areas already free from snow. No doubt a great quantity of heat is expended on the melting of the snow, and so the warm winds are chilled and lose their power of thawing the snow. But near the border of the snow-covered country, when the snow has mostly melted there, the surface of the ground can be heated by the sun and thus become a source of heat for the country lying still further to the northward. The melting of the snow is progressive from say February to June in the northern hemisphere. It begins close to seas which do not freeze, and continental areas which are not permanently covered with snow even in mid- winter. Thence it proceeds intermittently by leaps and bounds until all the lowlands of our hemisphere are freed from their snow-covering. In high southern latitudes no such happy event takes place. Sir James Ross proved that the mean temperature of the shores of the Antarctic Continent, or the islands bound together by glacier ice and so appearing like a continent, is much below the freezing-point even at midsummer. There is no notable melting of snow, and what is melted is very soon replaced by fresh snow. The geographical position explains this. The shores of the Antarctic Continent are washed by an ocean, the surface water of which has a temperature below freezing-point to about 62 S. even in summer, while they lie at a distance of 20 or more from any land area of lower latitude which could supply warmth for the thawing of their eternal snows. We see, then, that the sun's rays are of themselves unable to raise the temperature above freezing- point, notwithstanding the nearness of the sun to the earth in the summer of the southern hemisphere. In our northern hemisphere the melting of the snow keeps the temperature for a long time near freezing-point. This is the reason why April is so much colder than October in Central European Russia, in Canada, and in the more northern of the United States. Hence also the keenness of the easterly winds of spring in Western Europe, even in the British Islands. CLIMATE 325 It is clear that at the period when the mean temperature begins to rise above the freezing-point, very much depends on the store of cold existing in the vicinity in the form of snow and ice. The larger it is, the slower and more irregular will be the rise of temperature. At the beginning of winter, also, a heavy fall of snow over a large continental area intensifies and gives a permanency to succeeding cold. IX. Prevalent Winds. The division of winds into (1) Permanent, (2) Periodic, and (3) Variable, must now be considered. Dr. Theodore Williams well observes that, beyond the use of winds for propelling vessels and machinery, they serve a distinctly hygienic object in dispersing noxious exhalations whether animal or vegetable, in permitting free evaporation and thus preventing accumulation of moisture, and maintaining the circulation of the air which is necessary for the purification of the atmosphere. Their influence upon climate is indisputable. They raise or lower temperature, increase or diminish humidity, cause or prevent rainfall, interrupt sunshine by bringing up clouds, or clear the sky when descending from the higher strata of the atmosphere. Apart from the permanent aerial currents, which are called the "Trade winds," and which blow within the Tropics as a north-east wind north of the equator and as a south-east wind south of the equator ; and from the periodic aerial currents, of which the most striking examples are the Indian monsoons the south-west monsoon (of summer) and the north-east monsoon of winter), there are certain local winds, which prevail occasion- ally and produce very decided effects upon both animal and vegetable life. This class may be subdivided into occasional cold winds, prevalent in winter and spring, and occasional warm winds, prevalent in summer and autumn. On this subject Mr. W. Marriott, Secretary of the Royal Meteorological Society, made a valuable communication at one of the Con- ferences on " Meteorology in Relation to Health," held at the International Health Exhibition in London in July 1884. 326 METEOROLOGY The cold winds are : 1. The East Wind of the British spring, which is dry, cold, and keenly penetrating. Sir Arthur Mitchell, in a "Note on the Weather of 1867 and on some effects of East Wind," 1 shows how much a cold dry wind must chill the surface of the body by conduction, and also by evaporation. He adds : " The quantity of heat which our bodies lose in this way is far from insignificant, and the loss cannot be sustained without involving extensive and important physiological actions, and without influencing the state of health. In feeble and delicate constitutions the resources of nature prove insufficient to meet the demand made on them, and a condition of disease then ensues." 2. The Mistral is a violent north-west wind, dry, cold, and parching, which sweeps the shores of the Gulf of Lyons, drying up and withering vegetation, and predisposing to pleurisy and pneumonia in the inhabitants of Provence. Writing of it, Mr. Scott quotes the old couplet : Le Parlement, le Mistral, et la Durance Sont les trois fleaux de la Provence. 3. The Tramontana is a searching northerly blast, which is felt along the eastern shores of the Adriatic. A similar furious northerly wind is known in Trieste and Dalmatia as the Bora. 4. The Nortes (Northers) of the Gulf of Mexico have a pernicious influence upon health and vegetation. Mr. R. Russell, in his North America : its Agriculture and Climate, states that in Southern Texas, in January 1855, with a Norther, temperature fell from 81 to 18 in forty-one hours. 5. The Pampero is a dry, cold, south-west wind, which pre- vails on the coast of Brazil, blowing with great force across the pampas, or plains, of the river Plate. In the Argentine Republic similar winds are called Tormentos. 6. The Etesian Winds of south-eastern Europe blow across 1 Journal of the Scottish Meteorological Society, vol ii. p 80. CLIMA TE 327 the Mediterranean towards North Africa, apparently to supply the place of the heated air which rises from the Sahara and other African deserts. The chief hot winds are : 1. The Scirocco a hot south-east wind blowing from the immense deserts of Northern Africa. It is a dry wind on the African coast, but blows in Italy and Sicily as a hot, moist wind, from the oppressiveness of which there is no escape. Mr. Marriott states the case well when he says : " though not fatal to human life, it is deadly to human temper." In Sicily, during its continuance, the thermometer sometimes rises to 110 in the shade. 2. The Solano is the scirocco of Spain. It is a very hot, dry, and dusty south-east wind, most deleterious to health and to temper, hence the Spanish proverb : "Ask no favour during the Solano." So also is the Leveche or hot south-west wind of the Iberian Peninsula. 3. The Harmattan of the west coast of Africa is a hot easterly wind, laden with dust and sand from the Sahara. It prevails in December, January, and February. 4. The Khamseen, or Khamsin, is the hot wind from the desert in Egypt. It is so called, not because it lasts for fifty days, but because it is liable to occur during the fifty days following Easter. It blows from S. or S.S.E., the more easterly variety being the most disagreeable. It usually blows for three days, but may last for seven days at a time. The number of Khamseen days in any one year would seem to vary from four to twenty. During its prevalence the air becomes extremely dry, and is filled with fine sand in a highly electrified condition (F. M. Sandwith 1 ). 5. The dreaded and deadly Simoom of the deserts of Arabia, Kutchee, and Upper Scinde, is really a circular storm, or tornado in fact, a whirlwind which lasts only ten minutes or thereabouts. 1 Egypt as a Health Resort, p. 32. London : Kegan Paul, Trencli and Co. 1889. 328 METEOROLOGY 6. The Hot Wind of Australia, locally known as a " Brick- fielder," blows from the north. It is most severe in the months of November, December, and January. In Sydney it may send the thermometer up to 100, once it rose to 106*9, but in Central Australia the heat is even more in- tense, Captain Sturt having reported a shade temperature of 131 on January 21, 1845. Dr. Hann, in his Handbuch der Klimatologie, p. 639, quotes an observation of Dr. Neumann, formerly Director of the Melbourne Flagstaff Observatory, respecting the hot wind of January 21-22, 1860, that "the apples were literally roasted on the trees, where the north wind had set in." This north wind is dis- placed by a sudden south wind which is called a "burster," and its effect is to reduce temperature with marvellous rapidity. 7. The Fohn, or warm, dry wind of the valleys in the north-east of Switzerland, has already been described. 8. The Leste is a very dry and parching wind, sometimes very hot, which blows over Madeira from E.N.E. or E.S.E., taking its origin in the Sahara. Its dry ness is remarkable, for it traverses 300 miles of sea before it reaches the island. There is no doubt that, among the elements which make up climate, temperature holds the foremost place. Accord- ingly, the accompanying Chart (C) will both interest and instruct the reader. It was prepared by Mr. Wm. Marriott, F. R. Met. Soc., some years ago by direction of the Council of the Royal Meteorological Society. The mean annual tem- perature in the shade in degrees Fahrenheit of certain places in various parts of the world is shown on the thermometer scale to the left-hand side, while the highest and lowest shade temperatures, observed at the specified places in all parts of the world, are drawn on the right-hand scale. When the highest temperatures are indicated the names of the places are printed in SMALL CAPITALS ; when the lowest temperatures, in italics. CHART C. ioo c 90' 50 40 3 2 20 3 10 Ferozepore. Madras. Calcutta. Havana. Hong-Kong. Cairo. Algiers. Jerusalem. Rome. Constantinople. San Francisco. Pekin. London. Berlin. Leipzig. Toronto. St. Petersburg. Hammerfest. Freezing Point. Godhaab. Nertschinsk. Fort Churchill (Hudson Bay). Fort Enterprise. Yakutsh. Winter Island. Fort Hope. Boothia Felix. Melville. Island. MURZOUK. COOPER'S CREEK. BAGDAD. CAIRO. I ADELAIDE. SYDNEY. I JERUSALEM. GREENWICH. Moscow. FALKLAND ISLANDS. Barbadoes. Singapore. Lima. Bombay. Senegal. Quito. Adelaide. Athens. Jerusalem. Constantinople. Leh (Ladakh). Greenwich. Blackadder (Ber- wick). Toronto. Chicago. Winnipeg. Montreal. Melville Island. Bogoslowsk. Barnaul. Fort Reliance. Semipalatinsk. Werchojansk. CHAPTEE XXVI THE CLIMATE OF THE BEITISH ISLANDS Division of Climates into (1) Insular, or Moderate ; (2) Continental, or Excessive Distribution of Atmospheric Pressure according to Season Climate of the British Isles : (1) in Summer ; (2) in Winter Isothermals in July and January Features in the Climatology of Great Britain and Ireland Sea Temperatures : in Winter and in Summer Air Temperatures : arrangement of the Isotherms in January, April, July, and October Atmospheric Pressure: its Monthly Distribution Equinoctial Gales. WITHIN the limits of a small book it would be impossible to do full justice to the great theme of climatology. It must suffice to repeat that, because water by its presence not less than by its motion so profoundly modifies climatic conditions, climates are, by universal consent, divided into insular or moderate, and continental or excessive. Of the former, the climate of the British Islands affords a typical example ; of the latter, the climate of Siberia may be taken as a type. But an insular climate is by no means confined to islands (Lat. insulce). The western shores of all continents enjoy moderate climates which are fully entitled to be described as " insular." On the other hand, the interior of continents and their eastern shores are exposed to extremes of heat and cold, which equally justify the appellation " continental," applied to their climate. The great changes which take place in the distribution of atmospheric pressure over the immense continent of Europe and Asia and the adjoining oceans the North Atlantic and 330 METEOROLOGY the North Pacific have been already described and explained (Chapter XV., p. 170). It was there shown that, in summer, a vast cyclonic system develops over Europe and Asia, the wind blowing against the hands of a watch in accordance with Buys Ballot's Law, round an area of low barometer formed over the heated inland regions, from S.W. in India and China (the south-west monsoon) ; from S., S.E., and E. in Japan, and North-eastern Siberia ; from N.E. and N. in North-western Siberia ; from N.W. and W. over most of Southern Europe and South-western Asia. In winter, on the contrary, over the ice-bound, snow- covered, boundless Eurasian plain an immense anticyclone is formed, the winds circulating round and out from the centre of high pressure in a direction with the hands of a watch, blowing from N.W. and N. in Japan and China ; from N.E. in India (the north-east monsoon) ; from E. and S.E. in Russia and Southern Europe ; from S.W. in the British Isles ; and from W. in Northern Russia and Siberia. These considerations facilitate an explanation of the climate of the British Isles (1) in summer and (2) in winter. It will easily be seen how the summer continental de- pression influences the climate of the British Isles. Air is drawn from W. and N.W. over these countries, and as this air blows over the surface of a wide ocean, and from high latitudes, it is cool and moist. Do not these two words describe our summer 1 These ocean winds prevail chiefly on the W. and N.W. shores of Ireland and Scotland, which have thus the rainiest and the coolest summer, while this season is warmer and drier as we go eastward and southward, to the south-eastern counties of England. This is well illus- trated in Dr. Buchan's Chart l of the Isothermals 2 of the British Isles in July. It is not necessary to consider at length the influence of 1 "The Mean Temperature of the British Islands." By Alexander Buclian. Journal of the Scottish Meteorological Society, vol. vi. New Series. No. 64, p. 22. 1882. 2 Gk. iaos = equal, and 6ep/*ri = warmth. THE CLIMATE OF THE BRITISH ISLANDS 331 the winter-system of barometrical pressure on our climate. During the earlier winter months a great stream of warm, very moist air, as a rule, flows north-eastward and northward over these islands round the Atlantic depression, the centre of which lies near Iceland. But this stream does not flow evenly. Along its eastern edge it is in continual conflict with the cold anticyclonic air, which is travelling westward from Russia and Siberia, and immense volumes of the latter are frequently rushing in to supply the place of those volumes of the warm air which, owing to their low density, have presumably risen from the earth's surface towards the higher strata of the atmosphere. This conflict between two such opposite currents of air causes our storms, and those violent and rapid alternations of temperature which are so prejudicial to health in the winter months. The reason for the occurrence of these alternations of temperature will be explained when we remember that most of these gales, or bourrasques as they have been termed, are cyclonic in character, and that they generally cross the British Isles from S.W. to N.E., less frequently from W. to E., and still less frequently from N.W. to S.E. The southerly winds which blow over the country in front of the centre of the storms are warm and moist, while the northerly winds, which prevail over those districts already reached and passed by the centre, are cold, and after a time dry. No better examples of this can be given than the remarkable gales of December 8 and 9, 1872, and of February 2, 1873. In front of the former, temperature rose generally to about 50 over the south of Ireland, most part of England, and all of France ; while it fell almost to the freezing-point over those districts a few hours later when the centre had passed. The second gale referred to was accompanied by a range of 18 (Fahrenheit) over the whole of France. Mr. Scott says 1 that a great contrast of tempera- 1 Weather Charts and Storm Warnings, p. 134. London : Henry S. King. 1876. 332 METEOROLOGY ture between adjacent stations or, so to speak, a great " thermometric gradient," being an indication of serious atmospheric disturbance, is the precursor or concomitant of a serious storm. He quotes, as an example, the gale of November 14, 1875, which followed hard upon a difference of 36 in temperature at 8 A.M. of the previous between Scilly (57) and Wick (21). The effect of the warm Atlantic air-current on the Isothermals of the British Isles is well represented in Dr. Buchan's Chart for January. 1 Anticyclonic wind-systems sometimes prevail over Western Europe, but much less frequently than cyclonic systems. They cause dry, often cold weather, and are much more persistent than cyclones. Anticyclones are better marked, as a rule, in winter than in summer, and historical "hard frosts" in the British Isles are almost invariably connected with one of these systems. The great frost of 1890-91, which lasted in the south-east of England, almost without interruption, from November 25, 1890, to January 22, 1891, was connected with the presence of a large area of high barometric pressure which maintained a nearly permanent position over Central Europe. The in- coming disturbances from the Atlantic could not effect a passage into Europe, but were fended off by the European anticyclone, their centres being kept well out in the Atlantic. Ireland and Scotland came from time to time under the warming influence of these Atlantic depressions or cyclones, and so the frost was neither severe nor continuous in those countries but England was not affected by them, and so the cold held in its intensity particularly in the eastern, south- eastern, and midland parts of the country. Mr. Charles Harding, F.R. Met. Soc., in a paper 2 on this historical frost, 1 " The Mean Temperature of the British Islands." By Alexander Buchan. Journal of the Scottish Meteorological Society, vol. vi. New Series. No. 64, p. 22. 1882. 2 Read before the Royal Meteorological Society on February 18, 1891. THE CLIMATE OF THE BRITISH ISLANDS 333 states that the very dry character of the weather over England during the frost was also attributable to the fact that the European anticyclone embraced the southern portion of the kingdom, and although on two or three occasions there were some rather heavy falls of snow, the aggregate fall of snow and rain was but trifling in comparison with the average. In the chapter on " Weather " in his Elementary Meteor- ology (p. 360), Mr. R. H. Scott well observes : " The weather we experience in Western Europe is dis- tinctly related to these areas of depression and anticyclones, to the rate at which they respectively travel over the earth's surface, and to the distance which intervenes between their respective centres. As in a system of either kind we may meet with winds from any point of the compass, which will have different qualities as to temperature, humidity, etc., according as they belong to one or the other, we see the great importance of the consideration, first pointed out by W. Koppen, 1 and subsequently by Captain Toynbee, 2 that the climatic character of a wind depends on its origin, i.e. on its belonging to a depression or to an anticyclone." He adds : " Anticyclones are generally more or less stationary, but depressions move over the earth's surface, usually from west to east in these latitudes, their paths as they advance, though chiefly ruled by the distribution of pressure, being liable to modification by the irregularities of the surface over which they pass ; and their effects, as to the amount of cloud and rain to which they give rise, being influenced by the same causes. A south-west wind, for instance, may blow over a flat country with a clear sky, but as soon as the air reaches a hill-side and is forced to ascend, the moisture it contains is condensed, clouds are formed, and rain is frequently the result." One of the most recent contributions to the climatology of 1 Repertorium fur Meteorologie, vol. iv. 1875. 2 The Meteorology of the North Atlantic during August 1873, p. 97. London. 1878. 334 METEOROLOGY England and Ireland is a paper by Mr. Francis Campbell Bayard, F.R. Met. Soc., which was read before the Royal Meteorological Society on June 15, 1892. 1 The author care- fully analysed the observations taken during the ten years, 1881-90, at nineteen Second Order Stations (sixteen in Eng- land and three in Ireland), and at thirty-three Climatological Stations (thirty-two in England and one in the Channel Islands), and he arrives at the following general conclu- sions : (1) With respect to mean temperature, the sea-coast stations are warm in winter and cool in summer, whilst the inland stations are cold in winter and hot in summer. (2) The mean maximum temperature occurs at all stations in July or August, while the mean minimum temperature takes place mostly in December or January, except at Llan- dudno and the south-western sea-coast stations, where it is later, taking place in February or March. (3) Relative humidity is lowest at the sea-coast stations and highest at the inland ones. (4) The south-western district seems most cloudy in winter, spring, and autumn, and the southern district the least cloudy in the summer months ; and the sea- coast stations are, as a rule, less cloudy than the inland ones. (5) Rainfall is smallest in April, and, as a rule, greatest in November, and it increases as we travel from east to west. Mr. Bayard's paper, it is true, does not include Scotland in its scope, and three stations in Ireland Londonderry, Dublin, and Killarney are far too few to serve as a basis for climatological conclusions. Nevertheless the foregoing sentences epitomise the facts relating to the climate of the British Islands at large. This is shown by referring to a series of elaborate communications on the subject, which Dr. Alexander Buchan has from time to time since 1862 laid before the Scottish Meteorological Society. 1 See the Quarterly Journal of the Society, New Series, vol. xviii. No. 84, p. 213. THE CLIMATE OF THE BRITISH ISLANDS 335 I. SEA TEMPERATURES In an article on the " Temperature of the British Islands," l Dr. Buchan says that a very cursory examination of the British isothermals is enough to show the powerful influence of the sea in modifying their course in the different months of the year. Hence the temperature of the sea which washes our shores is a question of the first importance in investi- gating the climate of these islands. Observations on sea temperature have been made at different points round the Scottish coasts since 1855, when the Scottish Meteorological Society was founded, and more recently in Faro and Iceland. During the three years, July 1879 to June 1882, observa- tions, from which maps of the sea temperature all round the British Isles have been constructed in the Meteorological Office, London, were taken at certain coastguard stations, lighthouses, and lightships to the number of forty-nine. The results have been embodied in a Meteorological Atlas of the British Isles, published by the authority of the Meteoro- logical Council in 1883. The mean temperatures of the sea surface vary as follows : January . Highest, 49 Cleggan, Co. Galway ; Scilly, Truro, Pen- zarice. Lowest, 37 Yarmouth, Berwick. February . Highest, 49 Scilly, Seven-Stones L.V., 2 Cornwall. Lowest, 37 Burntisland, Fifeshire. March . . Highest, 51 Cleggan, Co. Galway. Lowest, 40 Dunrobin, Holkharn, Leman and Ower, L.V. April . . . Highest, 51 Cleggan, Valentia Island, Scilly. Lowest, 42 Berwick, Leman and Ower L.V., Norfolk. May . . . Highest, 56 Valentia Island. Lowest, 45 Berwick. June . . . Highest, 58 West Coast of Ireland, Bristol Channel, Padstow, Cornwall ; Yarmouth. Lowest, 49 North Unst, Shetland ; Berwick. 1 Journal of the Scottish Met. Soc., New Series, vol. iii. p. 102. 2 L.V. = Light-vessel. 336 METEOROLOGY July . . . Highest, 62 Bristol Channel. Lowest, 52 North Unst, Wick, Berwick. August . . Highest, 64 The Owers L.V., off the Sussex coast. Lowest, 52 North Unst, East Yell, Shetland. September. Highest, 62 Cleggan, County Galway ; Dover. Lowest, 52 Shetland. October . . Highest, 59 Mouth of the Thames. Lowest, 47 Fraserburgh. November . Highest, 54 Truro. Lowest, 44 Fraserburgh, Berwick. December . Highest, 52 St. Agnes' Head, Cornwall. Lowest, 39 Berwick. WINTEK. According to Dr. Buchan, the high temperature of the northern islands in winter is one of the best illustra- tions which could be adduced of the powerful influence of the ocean on climate. The conserving influence of the sea on the temperature is also seen, though in a less degree, in the open- ings of the Irish Sea and the English Channel. The isother- mals indicative of the mildest British climate in winter are seen enveloping Ireland in January. The west and north coasts of Wales share in the genial influence of this offshoot from the warm waters of the Atlantic. The mildest winter climate of Great Britain, however, is found in the peninsula of Devon and Cornwall, which is not only further south, but is also more completely enveloped by the ocean than any other part of the British Islands. A rapid lowering of tem- perature takes place from the Land's End eastwards to Kent, because the English Channel is comparatively shallow, is near the colder continent, and is connected with the colder North Sea. SUMMEE. Owing to the great preponderance of sea over land in the vicinity of the Hebrides, the Orkneys, and the Shetlands, the temperature of these islands is remarkably reduced in summer. A tendency to northing in the summer winds is also mentioned by Dr. Buchan as another cause for the marked diminution of summer heat in the northern parts of Great Britain. The Irish Sea and the English Channel THE CLIMATE OF THE BRITISH ISLANDS 337 moderate the heat of summer along their coasts. Con- versely, warmth is relatively greatest in those parts of the British Islands which are most removed from the direct and indirect influence of the sea. Hence there is a curving northwards of the isothermals of June, July, and August through the central parts of Great Britain, from the Thames Valley northwards to the Moray Firth. The patch of highest mean temperature corresponds closely with the Thames Valley, and is most marked in the immediate vicinity of London. In a paper on the climate of Dublin, written in 1886, I showed how beneficial was the influence of the Irish Sea upon that city, not only in winter and spring, when it softened and warmed by some 5 the keen, dry, searching easterly winds of those seasons, but also in summer. In calm, clear weather in summer time, no sooner has the sun mounted high in the heavens than a cool, refreshing sea breeze a typical " inbat," l as the modern Greeks call it sets in towards the land, so that extreme or oppressive heat is rarely experienced. Indeed, an oppressive atmosphere happens only when a damp, warm, south-west wind is blow- ing, with a more or less clouded sky. On July 16, 1876, the thermometer no doubt did rise in the Irish capital to 87 '2, but this was altogether a phenomenal occurrence. Tempera- tures above 80 in the screen in Dublin nearly always coin- cide with winds off the land, from some point between south and west, and a clear or only slightly clouded sky. On August 15, 1893, the maximum in Dublin was 79 '8 (practically 80), with a calm atmosphere or light variable sea breezes, but with the clouds coming from south-west. Speaking on his paper on the " Mean Temperature of the British Islands," at a meeting of the Scottish Meteorological Society on July 20, 1881, Dr. Buchan laid special stress on the influence of the Atlantic and other surrounding seas upon the temperature. He pointed out the great influence of the Irish Sea in affecting the course of the isothermals, and also 1 Evidently a derivative from 338 METEOROLOGY that of the Atlantic, particularly off the north-west of Scot- land. In winter the temperature of St. Kilda is as high as that of Penzance, and the temperature at Cape Wrath as high as that of the Isle of Wight. Taking the British Islands as a whole, the mean annual temperature in the west, 52 '0, was represented about the same latitude in the east by a mean annual temperature of 51'Q in other words, the west was one degree warmer than the east. II. AIR TEMPERATURES Observations upon this element fully justify Dr. Buchan's dogmatic statement that "the climate of the British Islands is eminently insular ; that is, it is not subject to great ex- tremes of heat and cold, but is remarkably equable throughout the year being much milder in winter and cooler in summer than in continental regions in the same latitudes." We now possess two series of Temperature Charts, which may be accepted as conclusive evidence of the distribution, annually and monthly, of air temperature throughout the British Isles. The first series was drawn up by Dr. Buchan for the Scottish Meteorological Society originally in 1871. It was based upon observations extending over thirteen years, beginning with January 1857, and terminating with December 1869. These observations on the daily maxima and minima were taken at seventy-six stations in Scotland, sixty- seven in England, twelve in Ireland, and fifteen in adjoining countries on the Continent. The mean temperatures used by Dr. Buchan are the arithmetical means of the daily maximal and minimal thermometer readings. The means so obtained were increased by an addition at the rate of 1 for every 300 feet of elevation above the sea, and were set down, so cor- rected, in their proper position on thirteen charts, from which the isothermals for each of the twelve months and for the year were drawn. Dr. Buchan's paper will be found in volume iii. (New Series) of the Journal of the Scottish Meteorological Society, 1873, p. 102. THE CLIMATE OF THE BRITISH ISLANDS 339 Of this paper a still more elaborate communication on the " Mean Temperature of the British Islands," based on twenty- four years' observations ending with 1880, and laid before the Scottish Meteorological Society by Dr. Buchan in 1882, 1 may be regarded as a revision. The observations embrace a period nearly double the length of that of the earlier paper, and were taken at 24 places in Ireland, 132 in Scotland, and 138 in England. Thus, the series of Plates illustrating this second paper and giving the isothermal lines for each month, may be accepted as faithfully representing the temperature of the British Islands in exceedingly close agreement with the true mean annual temperature of this portion of the globe " a datum," says Dr. Buchan, "of no small importance in many inquiries which deal with the physics of the atmosphere and underground temperature." The second series of Temperature Charts was drawn up by the authority of the Meteorological Council and published, in 1883, in the Meteorological Atlas of the British Isles. It is based upon observations of the maximum and minimum thermometers made daily during the twenty years, 1861- 1880 inclusive, at seventy-five stations thirty-one in Scotland and the adjacent islands, thirty -five in England (including the Channel Islands), and nine in Ireland. The mean tempera- tures given in the Atlas, which consists of twelve monthly maps and one yearly map, are the arithmetical means of the daily maxima and minima. They have been reduced to their sea-level value by the addition of a correction at the rate of one degree Fahrenheit for each 300 feet of vertical elevation. The isotherms in these maps are drawn for each degree, the value for each being inserted in large figures at one of the extremities of each line. An analysis of these maps shows that the isotherm of 46, representing the mean temperature of the whole year, skirts the north coasts of the Hebrides and Scotland. The mean 1 Journal of the Scottish Meteorological Society. New Series. Vol. vi. p. 22. 1882. 340 METEOROLOGY temperature then increases as we travel southwards, with many interesting local irregularities, until the isotherm of 52 is found off the extreme south-west of Ireland, whence it passes east-south-eastward by the Land's End in Cornwall to Jersey. The mean annual temperature in the Scilly Islands is 53*1. Scotland lies between the isotherms of 46 (45-8) and 48 (48'3); England between those of 47 and 52; Ireland between those of 48 (48'4) and 51 (51*3). Taking the first month of each quarter of the year, we obtain the following results : January. The area of greatest cold is represented in Scotland by the isotherm of 36, which embraces Aberdeen- shire. Temperature increases from that low value to 40 all along the extreme western coast of Argyllshire and the Hebrides to the Shetlands. In England the area of greatest cold is represented by the isotherm of 37, which covers parts of Norfolk, Lincolnshire, Huntingdonshire, and Cambridge- shire near the Wash. The isotherm of 42 passes southwards down the west coast of Wales across the borders of Devon and Cornwall ; that of 45 just touches the Land's End, while that of 46 (46 '3) passes through the Scilly Islands. In Ire- land the isotherm of 40 embraces an oval-shaped area on what may be called the " lee side " of the island, extending from the western, or inland, half of Antrim southwards to the Counties Kilkenny and Carlow. The isotherm of 41 passes through Dublin south-westwards to Fermoy, and then in a curve towards north-west and north to the extreme north of the island near Lough Swilly. On the other hand, the isotherm of 45 sweeps southwards down the extreme western coast from Achill Island to Valentia. April. This is a transitional month the characteristic winter isotherms are now giving place to those equally char- acteristic of summer. In Scotland the isotherms run from north-west to south-east, with local interruptions 44 crosses Caithness ; 47 Wigtonshire and Dumfries. In England the isotherms run in the same direction 46 skirting the north THE CLIMATE OF THE BRITISH ISLANDS 341 east coast, and 50 showing itself near London north of the Thames, and also over Devonshire and Cornwall. In Ireland temperature is very uniform, ranging from 47 in the extreme north to 49 in Kerry and Cork. July. The summer distribution of temperature is now seeii to full advantage inland districts being warmest, and coast districts coolest. In Scotland, the isotherm of 55 sweeps in a convex curve round the north-west and north coasts from the Hebrides to the Orkneys. The almost cir- cular isotherm of 59 covers the centre of Scotland, including Perthshire, Lanarkshire, and the Lothians. The English coasts vary from 59 in Northumberland to 62 along the shores of the English Channel and Suffolk. Inland, 63 covers the Midlands and 64 is found surrounding and especially to the north of London. In Ireland, 58 skirts the north, and 59 the west coast, while 61 embraces a large area extending from the southern shores of Lough Neagh to Cork. Tipperary and North Cork, with Kilkenny and Carlow, enjoy a mean temperature of 62. The great central plain extending from Galway to Dublin is somewhat cooler 60'4 to 6O8 ; this is doubtless due to the immense quantity of water with which the Bog of Allen and other less extensive peat bogs are charged, as well as to the number of lakes in the centre and west of the country. October. In this month, the winter distribution of tem- perature begins to appear in the drawing of the isothermal lines. Scotland varies from below 47 in the north, north- east, and south-east, to 49 in the south-west ; England from 48'4 in Durham to 53'6 at Penzance, and 55'1 in the Scilly Islands ; Ireland from 49 in a large oval in the north and centre to 52 in the south-east, and 53 off the promon- taries of Kerry and south-west Cork. With respect to the reduction of mean temperature to sea- level in these charts, I may remind the reader that, while it is expedient from a scientific point of view that such a reduc- tion should be made, it is quite unnecessary, nay, even mis- 342 METEOROLOGY leading, from either an agricultural or a medical standpoint. We want to know what are the actual climatic conditions under which both plants and animals live. Further, in a suggestive address on " The Relations of the Official Weather Services to Sanitary Science," delivered before the American Public Health Association at their recent Conference at Mexico, Mr. Mark W. Harrington, the very able Superintendent of the Weather Bureau of the United States Government, very wisely and properly pointed out that the meteorological data required by sanitarians and physicians may not be furnished in a form suitable for their purpose. For instance, among the temperature data for health resorts especially physi- cians particularly want to know the extreme range as well as the mean. Two places may have the same mean temperature, say 45 F., but they may be as far apart as the Poles in their availability for invalids. One place may have an occa- sional range of 40 F. within a few hours, the other may not have an absolute annual range of that amount. " For hygienic purposes," says the writer of the Address, "the details of temperature are of interest, as on a sunny day the temperature may differ greatly in short distances, depending on the exposure and the character of the surround- ings. The meteorologist has defined the air temperature as that of the free air at about the height of a man, the ther- mometer being protected from all radiation. With such a definition the temperature data which could be prepared from observations now taken, and which might be of use to sani- tarians, would appear to be as follows : " The mean temperatures of the hours, months, seasons, and year. " The mean maxima and minima for the months, seasons, and year. " The absolute maxima and minima for the same. " The mean and absolute amplitudes for the same. " Inter diurnal variability (i.e. the mean change in mean daily temperatures). THE CLIMATE OF THE BRITISH ISLANDS 343 " List of sharp changes of temperature of short duration. " Frequency of freezing days (mean temperature less than 32 F.) ; of frost days (minimum temperature below 32 F.); of hot days (maximum temperature 86 F.), and of very hot days (maximum temperature 95 F.) " Mean and absolute dates of the last and first frosts. " Mean and absolute duration of freezing, of hot, and of very hot weather. " Means of temperature of evaporation (i.e. of the wet-bulb thermometer). " This is an element which has not been discussed, so far as known to the writer, though Lieutenant Glassford sug- gested it to him some time ago. Its significance lies in the fact that it would approximately represent the temperature of the person in hot weather. It would help to distinguish between the distressing moist heat of some stations and the more endurable dry heat of others. It should probably be given in means, and associated with the corresponding air temperature (or temperature by the dry-bulb thermometer). It could be given thus : Air ,. Temperature of Temperature. Evaporation. 70 to 80 75 80 to 90 85 90 to 100 95 100 or more ... ...* " A similar table for low temperatures might be of use, as it is thought that dry, cold weather is less hard to endure than wet, cold weather." III. ATMOSPHERIC PRESSURE The " Meteorological Atlas of the British Isles " includes also thirteen maps, showing the distribution of mean baro- metrical pressure over the United Kingdom for each month and for the whole year, during the twenty years, 1861 to 1880. For purposes of comparison the readings have in all cases been reduced to their mean sea-level value ; but I agree 344 METEOROLOGY with the Superintendent of the Weather Bureau of the United States Government in thinking that, from a hygienic or medical point of view, it is the pressure to which an individual is actually exposed and not that felt at sea-level, perhaps 1000 feet below him, which is required in investiga- tions as to the influence of climate on health. This view was evidently shared by the compilers of the Atlas, for each map bears the following " Note " : " The approximate mean pressure for the month (or year) may be found by subtracting from the pressure indicated by the nearest isobars a correction obtained as follows : from 1'21 inch, subtract for each 10 above zero, Fahr., 0'025 inch ; the residue will be the correction for an elevation of 1000 feet, and the correction for the actual elevation will be proportional to this." The barometrical maps, moreover, are full of interest, particularly as the monthly distribution of pressure gives a clue to the direction and force of the predominant winds. In this series of maps, isobars have been drawn for the even hundredths of an inch of pressure so as to correspond as nearly as practicable with the actual observations recorded. The readings are reduced to sea-level and also to 32 F. In the chart for the whole year, and indeed in that for every one of the twelve months, the isobars have a cyclonic or concave trend in the north, but an anticyclonic or convex trend over the south of the United Kingdom. This at once explains the more settled weather of the south contrasted with the less settled weather of the north. Next, we observe that throughout the year pressure is on the average lower in the north than in the south. The differences in pressure are not uniform, however, throughout the year. Thus, in January, the isobar of 29-66 inches runs across the extreme north of Scotland from south-west to north-east, that of 29 '98 inches crosses Kent in a north-easterly direction. Here we have a difference of pressure amounting to '32 of an inch and gradients for south-westerly winds over the whole kingdom. This dif- THE CLIMATE OF THE BRITISH ISLANDS 345 ference steadily diminishes from January through February (when it is '22 inch 2974 and 29'98 inches), March ('14 inch 2976 and 29'90 inches), April ('10 inch 29'86 and 29-96 inches), to May, when it is only '08 inch 2 9 '91 inches and 2 9 '9 9 inches. It will at once occur to the reader that this equalisation of pressure means the dying out of the strong south-west winds of winter and the interspersing of a large proportion of easterly winds with the predominant westerly winds of our latitudes. Once May has passed a gradual revert- ing to the winter type of distribution may be noticed, thus : June N. 29-88 inches ; S. 30 '01 inches difference, '13 inch. July N. 29-84 ,, S. 30'00 ,, , '16 inch. August N. 29-82 ,, S. 29 '98 September!^. 29 '76 ,, S. 29 '96 October N. 2972 S. 29 '92 November N. 2974 ,, S. 29 '94 December N. 29 70 S. 29 -98 16 inch. 20 inch. 20 inch. 20 inch. 28 inch. I have compared these values with those given in Dr. Buchan's charts of the isobars, showing in inches the mean atmospheric pressure of the British Isles, monthly and yearly, on an average of twenty-four years, ending with 1880, and I find a remark- able agreement between the two sets of observations. In November, however, the mean pressure for the north of Scot- land is given by Dr. Buchan as 29 '78 inches. A necessary consequence of the changes in the monthly distribution of atmospheric pressure above indicated is, that January is the stormiest month in the British Isles. A care- ful analysis of the reports of storms received at the Meteoro- logical Office, London, for the fourteen years, 1870-1883, has led Mr. R. H. Scott to the conclusion that there is no strongly- marked storm -maximum at either equinox in September storm prevalence is increasing from a marked minimum in June and July ; in March it is decreasing from a sharply- defined maximum in January. Equinoctial gales, as such, are non-existent. 1 1 Quarterly Journ. of the Roy. Met. Soc., vol. x. , 1884, p. 236. CHAPTER XXVII THE CLIMATE OF THE BRITISH ISLANDS (continued) Distribution of Kainfall in the British Islands Regions of heaviest Rain- fall How determined : Prevalent Winds, Exposure to these Winds, Mountains Regions of least Rainfall Geological Formation Its Local Influence on Temperature Permanent Elevation of Surface Pebble Beds, Sands, and Sandstones Clays and Shales Limestones Crystalline Rocks, whether Slates or Schists Climatological Tables for Dublin. IV. RAINFALL IT is well said by Dr. A. Buchan, in his third paper on the Climate of the British Islands, 1 that, as regards these islands, the greatest differences in local climates arise from differences in the rainfall. Thus, on comparing the climate of Skye with that of the southern coasts of the Moray Firth, their mean temperatures in no month differ so much as 2'0, and for several months of the year they are nearly identical. But the annual rainfall of Skye rises to, and in many places exceeds, 100 inches, whereas at Culloden it only amounts to 26-17 inches, and at Burghead to 25-23 inches. This difference in the rainfall, with the clear skies and strong sun- shine which accompany it, renders the south shore of the Moray Firth one of the finest grain-producing districts of Scotland. It is this aspect of the rainfall which gives it so prominent a place in the climatology of a country. 1 Journal of the Scottish Meteorological Society. New Series. Vol. vii. page 131. 1886. THE CLIMATE OF THE BRITISH ISLANDS 347 Dr. Buchan's article on "The Annual Rainfall of the British Islands " is based on observations of the rainfall made at 547 stations in Scotland, 1080 in England and Wales, and 213 in Ireland; in all, 1840. The period selected for discussion was the twenty-four years extending from 1860 to 1883, inclusive. Dr. Buchan handsomely acknowledges his obligations to Mr. Symons's British Rainfall, a publication which rendered such an inquiry possible. The observed, but in some instances calculated, twenty-four years' averages were transferred to a map of the British Islands, which was then coloured with six different tints these shadings showing the districts where the mean annual rainfall did not exceed 25 inches (pale pink), was from 25 to 30 inches (red), from 30 to 40 inches (dark red), from 40 to 60 inches (pale blue), from 60 to 80 inches (blue), and, lastly, above 80 inches (dark blue). In the "Meteorological Atlas of the British Isles" (1883), the rainfall for the whole year is shown on a map by lines drawn, for each 5 or 10 inches of rain, from place to place having the same annual rainfall. This rainfall map was constructed by plotting on a large scale all the mean annual rainfall values for the fifteen years, 1866 to 1880, inclusive, which are given in " Rainfall Tables of the British Isles," compiled from the records of 366 stations by Mr. G. J. Symons, F.R.S., and published by the authority of the Meteorological Council in 1883. The rainfall indicated by the lines upon a map, drawn on a reduced scale for the Atlas, is the mean quantity actually observed at these selected stations. It is not claimed that this map is perfect, for a much larger amount of rain is known to fall in some places, notably on mountain slopes, of which no such record exists as to admit of its being shown on so small a map. In Ireland and the west of Scotland this defect is aggravated by the fact that for large areas no record whatever is obtainable for the period embraced in the inquiry. It should be mentioned that the Rainfall Tables, prepared by Mr. Symons at the request of the Meteorological Council, 348 METEOROLOGY are illustrated by three coloured maps of England, Scotland, and Ireland, respectively, which exhibit, not only the geogra- phical position of the 366 stations furnishing the rainfall records, but also the area in square miles of the river catch- ment basins in which these stations are severally situated. The key to the distribution of rainfall in the British Islands is " the direction of the rain-bearing winds in their relation to the physical configuration of the surface " (Buchan). The regions of heaviest rainfall, 60 to 80 inches or up- wards, are : Skye and the Western Highlands, the Lake District in Cumberland and Westmoreland, the mountainous district in North Wales, the mountainous district in the south-east of Wales, Dartmoor in Devonshire, the Highlands of West Galway, and the neighbourhood of Killarney and the Macgillicuddy Keeks in Kerry. This distribution of heavy rainfall is determined by 1, prevalent south-west winds, blowing vapour-laden from the Atlantic Ocean ; 2, the exposure to these winds of mountains, or high tablelands like Dartmoor, with valleys opening to the westward or south- westward. On the mountain slopes the warm, moist air is condensed into mist, cloud, and rain. Over the south of Scotland the rainfall is not excessive, because the rain-bearing south-westerly winds have been partially dried in their passage across Ireland before they reach the district in question. The absolutely largest annual rainfalls are in Scotland, at Glencroe, Argyllshire, at an elevation of 520 feet (128 : 50 inches); in England, at the Stye, Cumberland, 1077 feet (185'96 inches so far as yet observed, the heaviest rainfall anywhere in the British Islands), at Seathwaite, Cum- berland, 422 feet (143 '21 inches in the twenty-four years, 1860-83; 139'29 inches in the fifteen years, 1866-80); in Wales, at Beddgelert, Carnarvonshire, 264 feet (116'90 inches), Rhiwbrifdir, Merionethshire, 1100 feet (102*56 inches), Ty-Draw-Treherbert, Glamorgan, 735 feet (96'18 inches) ; in Ireland, at Kylemore, County Galway, 105 feet (89 '40 inches), THE CLIMA TE OF THE BRITISH ISLANDS 349 at Foffany, County Down, 920 feet (72'26 inches), and at Der- reen, Kenmare, County Kerry, 74 feet (69*40 inches). A rainfall of 40 inches a year, or upwards, occurs over about a fourth part of the surface of England and Wales, about half of that of Ireland, and considerably more than half of that of Scotland (Buchan). Nowhere along the whole east coast of Great Britain, or for some distance inland, does the average yearly rainfall reach 40 inches. On the east coast of Ireland, however, the rainfall rises to, or exceeds, 40 inches in the mountainous districts of Wicklow, Down (the Mourne Mountains), and Antrim. On the other hand, the annual rainfall is well below 30 inches in Dublin and its vicinity, for reasons which have been already explained. Wherever mountains or "downs" run east and west, a heavy rainfall is propagated eastwards along their southern face, while the precipitation is diminished to the northward of the barrier which they oppose to the rain-bearing south- west winds. Thus the mountains of Sutherland, the Gram- pians, the Cheviots, the Pennine range, and the Downs of the south of England, all cause an extension eastward of a heavier rainfall along their southern slopes, but a diminution in the rainfall to the northward and north-eastward. Pre- cisely the same thing on a smaller scale is found in connection with the Pentland Hills, near Edinburgh, the Mourne Moun- tains in the County Down, and the Dublin and Wicklow mountains. Leith (28'00 inches), Edinburgh (28'31 inches), Donaghadee (31 '08 inches), and Dublin (28'36 inches), all owe their comparatively small precipitation to their geogra- phical position north-east of the mountain ranges mentioned. The rainfall at ^Belfast (Queen's College) is, on the average, 34*73 inches. This is no doubt due to the proximity of Divis and other mountains north-west of the city, heavy rains fall- ing with south-east winds, which impinge upon those hills. "The influence of the breakdown of the watershed of Scotland between the Firths of Forth and Clyde," writes Dr. Buchan, " is strikingly manifested in the overspreading 350 METEOROLOGY of western parts of Perthshire, Stirlingshire, and Dumbar- tonshire, with a truly western rainfall as regards amount, and the direction of the winds with which it falls ; and in the extension eastwards, through Kinross-shire, of a rainfall of fully 40 inches, which occurs nowhere else over com- paratively level plains so far to the east of the watershed separating the western and eastern districts." Turning to the regions of least rainfall, we find a large area in England extending from the Humber to the estuary of the Thames (exclusive of the higher grounds of Lincoln and Norfolk), over which the annual precipitation falls short of 25 inches. In Cambridgeshire it is generally about 23 inches, except at Wisbech Observatory, where it rises to 26J inches. The smallest rainfall of all is at Shoeburyness, in Essex, where the average for the eighteen years, 1866-83, was only 21*42 inches. In 1874, only 14*20 inches fell at this station. On the higher grounds of Lincoln and Norfolk the rainfall exceeds 25 inches, because the precipitation with easterly winds is increased. Similarly the rainfall of the Yorkshire Wolds is made to exceed that of neighbouring districts. A small patch in the valley of the Thames, from Kew to Marlow, in Bucks, has an annual fall of less than 25 inches Kew Observatory, twenty-four years, 1860-83 = 25*26 inches ; fifteen years, 1866-80 = 24*67 inches. Between the valley of the Thames and the Humber, the rainfall nowhere reaches 30 inches, except near the Chiltern Hills (Buchan). In Scotland, the annual rainfall falls short of 30 inches in the north-eastern part of Caithness, round the Moray Firth from Tain to the mouth of the Spey, along the east coast from Peterhead in Aberdeenshire to Burntisland in Fifeshire, the low ground of Midlothian and East Lothian, and lower Tweeddale from Kelso to Berwick. The absolutely smallest rainfalls are observed on the very shores of the Moray Firth from Tarbet-Ness to Burghead (25 to 28 inches), the extreme north-east of East Lothian (25 to 29 inches), and the lower Tweed from Coldstream to Jedburgh (26^ inches). THE CLIMATE OF THE BRITISH ISLANDS 351 The only part of Ireland where the rainfall falls short of 30 inches is Dublin and its vicinity (about 28 inches). The reason for this diminished rainfall has been given above. V. GEOLOGICAL FORMATION In a lecture on "The Physical Influences which affect the British Climate," delivered early in the year 1893, in the Public Health Department of King's College, London, 1 Mr. H. G. Seeley, F.R.S., Professor of Geography in the College, aptly observes : " The areas of heavier rainfall are the regions of higher land, and a rainfall map in England closely approximates in its broad features to a geological map." Allusion has already been made to the marked in- fluence on rainfall exercised by the high lands and mountain chains in various parts of the United Kingdom. The con- figuration of the coast line and the distribution of high and low ground govern the rainfall to a remarkable extent, and in this way control climate generally. As Mr. Seeley remarks : " Next to the situation of our islands upon the earth's surface, the most important element in climate is the geological structure, and contour of the surface of the country." In his lecture, Professor Seeley shows that there are two ways in which the geological structure affects climate : first, it has a local influence on temperature ; secondly, it is a main element in modifying the relative durability of the rock material which determines the elevation of the surface. It would be foreign to the purpose of this book to enter into details as to the geology of the British Islands. Suffice it to say, with Professor Seeley, that the chief geological formations which have a bearing upon climate may be classed as (1) pebble beds, sands, and sandstones ; (2) clays and shales ; (3) limestones. There are, in addition, certain altered con- ditions of these simple forms, in which a more or less crys- talline texture is developed, which may be the micro-crystalline texture of slate or the micro-crystalline texture of schist. 1 See The Journal of State Medicine, vol. i. No. 4, p. 165. April 1893. 352 METEOROLOGY 1. The pebble beds, sands, and sandstones are commonly warmer and drier than other rocks. Their dryness is due to the existence of porous interspaces between the quartz grains of which these rock forms are so largely composed. This wholesome property may be interfered with by the presence of a cement which will bind the grains of quartz together, or of a bed of clay, which will render the sand im- pervious. The dryness, or otherwise, of sand strata will also largely depend on the angle at which they are inclined ; horizontal strata are naturally less dry than inclined strata. The warmth of a sandy soil is probably due to its dryness as well as to the low specific heat of the quartz. 2. The particles of which clays are composed are extremely small and consist chiefly of silicate of alumina. Some clay soils contain as much as 40 per cent of alumina, but the usual proportion is much smaller. In Scotland, clay soils are found chiefly on the coal-measures, the boulder clay, and as alluvium in the valleys. The last named is the richest form of clay and is known as carse clay. In the north of England, the aluminous shales of the coal-measures yield soils in their properties very like those in Scotland. England also abounds in clay soils derived from other geological forma- tions such as London clay, plastic, weald, gault, and blue lias qlay. An astonishing quantity of water may be held in a clay soil, which has an almost boundless affinity for moisture. In a warm, dry summer, wide and deep chasms open in clay owing to the evaporation of the water it contains. Clay land is looked upon as cold a condition attributed by Professor Seeley, theoretically, to the small size of its constituent particles and the way in which they are divided from each other by films of water. Through evaporation from a clay soil, the superincumbent atmosphere is rendered moist and cool. Precisely the same effect is produced in Ireland by the water-soaked morasses or bogs, which, according to Sir Robert Kane, M.D., cover 2,830,000 acres, or about one-seventh part of the entire surface of the island. The Bog of Allen stretches THE CLIMATE OF THE BRITISH ISLANDS 353 in a vast plain across the centre of the island, having a summit elevation of 280 feet. Its apparent influence on the mean temperature has been alluded to above. Owing to the retentive and non-porous nature of clay soils, there is no deep filtration and underground storage of water. The superficial strata become water-logged, and the imprisoned waters are very liable to contamination with surface impurities or with the products of chemical decomposition in the clay itself. 3. The third great group of water-formed rocks is the limestones (Seeley). The carboniferous limestone covers an im- mense area in the Pennine chain and North of England, and forms the base of nearly all our coalfields. It also underlies the peat-moss bogs of Central Ireland. The oolitic limestones, more or less continuous, and chalk, stretch from the York- shire coast to the South of England, forming parallel ridges of hills. Being soluble under flowing waters charged with carbonic acid gas, the surface is always deeply scored with valleys (Seeley). The ancient and oolitic limestones are not as absorbent as the newer chalk, which is very pervious to water. As water percolates through these limestones, it becomes highly charged with lime salts. According to Professor Seeley, limestones always give up a good deal of vapour under sunshine, and have a warm steamy atmosphere above them in summer, which is in marked contrast to the bracing air of sandstones with silicious or calcareous cements. When there is even a thin bed of clay on the summit of a limestone ridge, such as forms the insoluble residue left by atmospheric denudation, the climatic conditions are changed. 4. The crystalline rocks, whether slates or schists, usually occur in elevated country in the west of England, in Scotland, and in parts of Ireland. They are remarkable, chiefly, for their impervious and almost insoluble character. " The high and irregular ground which they form, like their western position, causes them to have a great effect in radiating heat, and therefore in producing winds which descend from the mountainous regions, and in condensing rain." 2 A 354 METEOROLOGY Professor Seeley, in the paper from which I have so largely quoted, observes with justice that " the influence of the soil upon climate is complicated by the effects of the climate in transporting and forming the superficial soil." The foregoing sketch of the Climate of the British Islands may fitly conclude with two Climatological Tables for the City of Dublin, lat. 53 20' N., long. 6 75' W., altitude 50 feet. The materials for these Tables were obtained in the pre- paration of an Abstract of Observations taken by me in the City of Dublin during the twenty-three years, 1865-1887, inclusive. This abstract was compiled at the request of M. L. Cruls, the Director of the Imperial Observatory at Eio de Janeiro, Brazil, who undertook to prepare a Dictionnaire Climatologigue Universel, the publication of which was to be under the care and at the expense of the Observatory of Brazil. TABLE I. TEMPERATUKE, HUMIDITY, CLOUD, RAIN, AND WIND TEMPERATURE If o BAIN WIND 'o | ll Wg & +* c "o >> O jj P Mean Mean .I>.OOO^tl COCOCO^OOOrH i-H -# CO XO * - S g^ t^O [w&'o g Illlllflflll 364 METEOROLOGY houses situated on the low and swampy banks of the Neva, belonging to an indigent labouring population ; and, indeed, it is not strange that low-lying and over-crowded cellars, beneath which the soil has scarcely stiffened, with a favour- able and confined oven-temperature, should foster the contagion and occasion a constant, though tardy, propagation of the disease. Whether conditions of this kind held at Breslau I am unable to say but, in any case, it is certain that violent epidemics during severe winter-frost very rarely, if, indeed, ever, occur." Professor Faye goes on to say that, while the epidemic (of 1853) was at its worst at Christiania, the atmosphere was steadily warm and the air, in addition, clear and very still. This continued for about three weeks, during which the daily numbers of cases, which were then at the highest, scarcely varied. At this point of time the middle of September the air was set in motion by a strong and stormy north-west wind, and, remarkably enough, the number of cases fell, next day, to about one-half. Similarly, at Bergen, during the epidemic of 1848-49, a strong and cold north-easterly gale, supervening on a lengthened period of milder temperature, caused a considerable fall in the number of cholera cases. In the epidemic of 1866 the acme of mortality from cholera was reached in Dublin about the middle of October, the weather of the preceding week having been continuously calm, cloudy, foggy, damp, with a very high barometer, and a great deficiency of ozone (the latter showing a mean value of only 10 per cent at the Ordnance Survey Office, Phoenix Park). The decrease in the mortality was consequent on a freshening breeze and a change of wind from N.E. to S.W., a diminution of barometrical pressure, a moderate and continued rainfall, a rise in ozone to 70 per cent, and a gradually falling temperature. The coincidence of a high barometer with a great development of cholera has often been remarked, but striking exceptions are also on record. ACUTE INFECTIVE DISEASES 365 Keeping in view the fact that heavy rain and a strong breeze are most valuable detergents and disinfectants, it seems probable that the calm weather consequent on slight baro- metrical gradients, so common in anticyclonic, or high-pressure systems, has more influence than the mere height of the barometer itself. In December, the epidemic died out rapidly, and no death occurred later than the 29th of that month, on which day it is most interesting to note the intense frost of January 1867, was ushered in by a fall of temperature amounting to 15 in a few hours. The meteorological conditions just alluded to, and the influence of season, are to be classed among the predisposing causes of cholera. Its exciting cause is, of course, the intro- duction into the human system, and particularly the intestinal canal, of the specific virus or contagium of the disease, the comma-bacillus of Koch. I thoroughly agree with Mr. Ernest Hart when he says with uncompromising dogmatism : " We may lay aside all pedantry and mystery-talk of 'epidemic constitution,' 'pandemic waves,' 'telluric influences,' 'cholera blasts,' 'cholera clouds,' 'blue mists,' and the like terms of art with which an amiable class of meteorologists have delighted to cloak ignorance. Asiatic cholera is a filth disease, which is carried by dirty people to dirty places" " Cholera," he adds, " does not travel by air waves or blasts. We can drink cholera and eat cholera, but we cannot f catch ' cholera in the sense in which we catch measles, scarlatina, or whooping-cough. Cholera is carried by men in their clothing and their secretions along the lines of human inter- course. Earlier epidemics of Asiatic cholera took three years to reach us, by caravan and fitful travel, from its Asian home. It comes now not as a pedestrian or a horseman, but by locomotive and fast steamboat." I believe that cholera is taken precisely as enteric fever is taken, that is, it is most usually swallowed in water, less commonly in milk, or it is eaten in solid food, or exceptionally it is inhaled and swallowed with the saliva when a liquid medium containing its virus has 366 METEOROLOGY evaporated, leaving that virus to be air-borne for a short distance. Further, I am satisfied that cholera, like cholera nostras or cholerine, and enteric fever also, becomes much more virulent when the subsoil temperature reaches the critical point of 56 F. at four feet below the surface. As this occurs most readily when the level of the subsoil water (German, Grundwasser) is low, the significance of Pettenkof er's theory is at once evident. That veteran sanitarian says in one of his later papers : 1 " The fluctuations in the level of the subsoil water have a meaning for aetiology, only because they are traced back to those primary influences by which air and water are made to share, in varying proportion, the possession of the pores of an impregnated soil. Beyond that they have no significance." 3. Diarrhoeal Diseases These propositions may be laid down : 1. In summer and autumn the tendency to sickness and death is chiefly connected with the digestive organs diarrhoea, dysentery, and simple cholera or cholerine (cholera nostras), being the affections which are especially prevalent and fatal during these seasons. 2. In summer and autumn a rise of mean temperature above the average increases the number of cases of, and the mortality from, the diseases named. 3. On the other hand, a cool rainy summer and autumn controls their prevalence and fatality. 4. Diarrhceal diseases become epidemic when the subsoil temperature at a depth of 4 feet below the surface permanently reaches 56 F. ; this may therefore be called the " critical temperature." 1 Zeitschrift fur Biologie, Heft, vi p. 527. 1870. Quoted by Hirsch, loc. cit. vol. i. p. 466. ACUTE INFECTIVE DISEASES 367 In Table III. facts are given, which support the first of these propositions. Reference to the last two columns of the Table will convince the reader that yearly towards the end of July or the beginning of August, on an average, diarrhoeal diseases, and particularly cholerine, assume epidemic proportions in singular obedience to a law of periodicity and with all the suddenness of an explosion. Of every 100 deaths from diarrhoeal diseases taking place annually, only 3 occur in the four weeks ending June 17, only 3'9 in the four weeks ending July 15. Then the percentage runs up to 9*7 in the next four weeks (ending August 12), and to no less than 23 '0 in the period ending September 9. In the eight weeks ending October 7, 45-5 of every 100 deaths from diarrhceal diseases take place. Similarly in the case of simple cholera, of 100 deaths occurring in the whole year, only 0'6 takes place in the four weeks ending May 20 ; whereas in the period ending September 9, 26 4 9 take place, and in that ending October 7, no less than 2 8 '8 5 5 '7 per cent of the annual mortality in eight weeks. From the curves of mortality given by Dr. A. Buchan and Sir Arthur Mitchell in their paper on " The Influence of Weather on Mortality from Different Diseases and at Different Ages" (Journal of the Scottish Meteorological Society, vol. iv. p. 187), it would appear that the diarrhceal and choleraic death-rates rise in London to a yearly maximum about three weeks earlier than in Dublin. This is doubtless due to the earlier rise of the subsoil temperature at 4 feet to 56 in London than in Dublin. In support of the second and third propositions we have only to refer to the Reports of the Registrar -General for Ireland for the years 1868 and 1887 (warm, dry years), and for the cold, wet year 1879. The facts may best be thrown into a short tabular statement as follows : 368 METEOROLOGY TABLE II. 1 3 & 1868 1877 1879 SI "* tl .so Q 'o 6 a A II EH ^5 3 fi 1 If * 11 5 a 5 o ^3 I. II. III. IV. 447 55-5 607 45-3 39 22 289 77 i 11 419 53-1 59-3 43-3 31 27 331. 71 2 11 1 39'3 497 56'4 43-8 39 38 54 54 ] 1 516 427 12 494 460 14 47'3 185 2 In his Weekly Return of Births and Deaths in Dublin for August 22, 1868, the Registrar-General for Ireland wrote : "The number of deaths from diarrhoea registered during the week amounted to forty-nine, showing an increase of twenty- three on the number registered during the week preceding, and being thirty-five more than the average deaths from this disease in the corresponding week of the four previous years." In his return for the corresponding week in 1879, the Registrar- General observed : " Owing chiefly to the low mortality from diarrhoea, the number of deaths from zymotic diseases is con- siderably under the average for the thirty-fourth week of the last ten years." As a matter of fact, only one death from diarrhoea was registered in the whole Dublin Registration Dis- trict in this week, and the largest number of deaths from the disease registered in any week during 1879 was eight in the week ending Saturday, September 13. A more striking con- trast can hardly be imagined than that between the epidemic prevalence of diarrhoea in the warm season of 1868 and its absence in the cool summer of 1879. The year 1868 may be cited as an example of an unusually warm year. There was an almost complete absence of frost, and during ten out of the twelve months the mean tempera- ture was above the average : the excess varying from O5 in ACUTE INFECTIVE DISEASES 369 January to 3 '8 in March the warmest March within the twenty years now under discussion. October and November were cold the deficit of temperature amounting to 2*0 and 1'1 respectively. Notwithstanding this, the mean tempera- ture of the whole year was 51 '6, compared with an average of 49 '8 (excess = 1'8). A remarkable drought prevailed from the last week in April to the 10th of August, when a tropical rainfall occurred. During this period of nearly three and a half months, only 2*797 inches of rain fell in the city. On six occasions during the summer of this year the ther- mometer rose to 80 in the shade in Dublin the highest readings of all being 86 ? on July 15 and 85 on July 21. On August 1 the maximum was 82, and even as late as September 6 the high reading of 77 was noted. In marked contrast to 1868, and as an instance of a cold year, 1879 stands out in bold relief. The annual mean temperature was only 47 '3 that is, 2 '5 below the average (49 - 8). Every month was colder than usual the deficit of mean temperature ranging from 6*1 in January, 3*6 in April, 3'5 in July, and 3'4 in December, to 0'3 in October and 0*5 in November. Curiously enough, these last-named months were relatively the coldest in the warm year, 1868. There was a singular absence of summer heat in July and August ; in each of these months the shade temperature ex- ceeded 70 on one day only in Dublin, and on nine days in July it did not reach 60. The low temperature was accom- panied with to some extent depended upon a continuous rather than a heavy rainfall. During the six months ending September 30, rain fell on 125 out of 183 days that is to say, on two out of every three days. The amount of cloud during this cold, damp, sunless year, was 7 '5 per cent over the average. The cold weather, which persisted almost throughout 1879, set in first on October 21, 1878. This period of low temperature had probably not been paralleled for intensity and duration within the present century. In their classical paper already quoted, Dr, Buchan and 2 B C^Ji (i Ii 1- 1 IT !C- J>- 00 00 s$ 's M ACUTE INFECTIVE DISEASES 371 Sir Arthur Mitchell speak of " the close and direct relations which the progress of mortality from these (diarrhoeal) diseases bears to temperature. This relation is seen in the startling suddenness with which they shoot up during the hottest weeks of the year, and the suddenness, equally startling, with which they fall on the advent of colder weather." The authors point out that the death-rate curves for diarrhoea and cholerine rise and fall about a month earlier than do those for dysentery and epidemic cholera. The annual phases of the former diseases are, in other words, about a month earlier than those of the latter. For all four diseases, the curves are reproduced in all their essential features from year to year. In very hot summers the numbers of deaths are enormously increased, and in cold summers, such as 1860, the deaths from bowel complaints are correspondingly few. The fourth proposition was first advanced by Dr. Edward Ballard, in his elaborate Keport to the Local Government Board for England upon the causation of the annual mortality from " Diarrhoea," which is observed principally in the summer season of the year. 1 That a high atmospheric temperature conduces to a high diarrhceal mortality, and a low atmospheric temperature to a low diarrhoeal mortality, Dr. Ballard admits : it is, he says, "an established fact which no one can dispute." But his inquiry showed that the influence thus exerted is not a direct influence, except in so far as it affects also infant mortality from all causes. Rainfall again exerts an influence on diarrhoea, but apparently' not equally in all periods of the diarrhoeal season. The diarrhoeal mortality is greater in dry, less in wet seasons. But here again the influence exerted is not direct (e.g. by a washing of the atmosphere, so to speak), but indirect, namely, by its effect mainly in preventing the rise and (probably to a less extent) in hastening the fall of the temperature of the earth. 1 Supplement in Continuation of tfie Report of the Medical Officer for 1887. Seventeenth Annual Report of the Local Government Board, 1887-88. London : Eyre and Spottiswoode, 1889. Quarto, p. 1 et. seq. 372 METEOROLOGY Wind and comparative calm affect the diarrhoeal mortality. Other things being equal, calm in the diarrhoeal season promotes it, and high winds tend to lessen it. But soil and the temperature of the soil are far more, important predisposing causes of diarrhoeal diseases. Their prevalence and fatality is low in dwelling-houses built on a foundation of solid rock. Deep and wide and frequent fissur- ing of the rock in a town, or superficial alternations of rock with looser material, modify this immunity. On the other hand, a loose soil, more or less freely permeable by water and by air, is a soil on which diarrhoeal mortality is apt to be high. Of all natural soils, sand and surface mould to a considerable depth are "the most diarrhoeal." Gravel varies in its relation to diarrhoeal mortality according to its texture : fine, sand - like gravel predisposes to diarrhoeal prevalence ; coarse, rock-like gravel is more wholesome. Clay soils do not in themselves favour diarrhoea. A soil which is a mixture of clay, sand and stones (commonly called a "marl"), is apparently favourable or unfavourable to diarrhoeal mortality in proportion as it is loose and permeable on the one hand, or plastic on the other. The presence of much organic matter in the soil renders it distinctly more conducive to high diarrhoeal mortality than it otherwise would be. Hence dwellings built upon made ground, the refuse of towns, or the site of market -gardens, are unwholesome. And, of course, a sewage -soaked subsoil is most unwholesome and dangerous. Excessive wetness and complete dryness of the subsoil appear to be alike unfavourable to diarrhoea. Habitual dampness, which is not sufficient to preclude the free admission of air to the interstices of the subsoil, favours diarrhoeal prevalence. Dr. Ballard, however, considers that the Temperature of the Soil is a far more effective element in the causation than any of the meteorological factors just mentioned. He con- structed for London and many other towns in the kingdom a large number of charts, showing week by week for many ACUTE INFECTIVE DISEASES 373 years the earth-temperature at a depth of 1 foot from the surface and at a depth of 4 feet also, each chart showing in addition the diarrhoeal mortality of the corresponding weeks. The general result shown by these charts is as follows : a. The summer rise of diarrhoeal mortality does not commence until the mean temperature recorded by the 4-foot earth thermometer has attained somewhere about 56 Fahr., no matter what may have been the temperature previously attained by the atmosphere or recorded by the 1-foot earth thermometer. /?. The maximal diarrhoeal mortality of the year is usually observed in the week in M 7 hich the temperature recorded by the 4-foot earth thermometer attains its mean weekly maximum. y. The decline of the diarrhoeal mortality coincides with the decline of the temperature recorded by the 4-foot earth thermometer, which temperature declines very much more slowly than the atmospheric temperature, or than that recorded by the 1-foot earth thermometer. The epidemic mortality may in consequence continue (although declining) long after the last - mentioned temperatures have fallen greatly, and may extend some way into the fourth quarter of the year. 8. The atmospheric temperature and that of the more superficial layers of the soil exert little, if any, influence on the prevalence of diarrhoea until the temperature recorded by the 4-foot earth thermometer has risen to 56 F. Then their influence is apparent, but it is a subsidiary one, not- withstanding the statement made by Dr. August Hirsch that the summer diarrhoea of children makes its appearance as an epidemic only in those districts whose average temperature for the day in the warm season is rather more than 15 C. (59 F.) 1 It is interesting to notice that in an excellent article on " Cholera Infantum," which appeared in the Medical Annual for 1893, Dr. E. Meinert, of Dresden, entirely adopts Dr. 1 Handbook of Geographical and Historical Pathology, vol. iii. p. 379. New Sycl. Soc. 1886. 374 METEOROLOGY Bal lard's views as to the meteorological aetiology of this disease, while he also expresses his entire concurrence with Dr. Ballard's statement that density of buildings, whether dwelling-houses or other, upon area quite apart from density of population upon area promotes diarrhoeal mortality to a remarkable degree, particularly because crowding together of buildings of whatever sort restricts and offers an impedi- ment to the free circulation of air. Dr. Edward W. Hope, the Assistant Medical Officer of Health for Liverpool, has investigated the influence of the mode of feeding of young infants upon the prevalence and fatality of diarrhoea, and arrives at the following conclusions : * 1. Infants fed solely from the breast are remarkably exempt from fatal diarrhoea. 2. Infants fed in whatever way with artificial food to the exclusion of breast milk are those who suffer most heavily from fatal diarrhoea. 3. Children fed partially at the breast, and partially with other kinds of food, suffer to a considerable extent from fatal cUarrhcea, but very much less than those who are brought up altogether by hand. 4. As regards the use of "the bottle," it is decidedly more dangerous than artificial feeding without the bottle. In relation to this part of the subject, Dr. Ballard's ob- servations go to show that the circumstances of food-keeping, of its exposure to telluric emanations (e.g. in underground cellars), or to emanations from accumulations of domestic filth, etc. (e.g. when kept in pantries, etc., to which such emanations have more or less free access), tends to render it liable to produce diarrhoea, especially where the storing place of food is dark, and not exposed to currents of air. Dr. Ballard believes that a working hypothesis, or pro- visional explanation, that would best accord with the whole 1 Cf. Dr. Ballard's Report, p. 6. ACUTE INFECTIVE DISEASES 375 evidence in his possession bearing on the production of epidemic diarrhoea, may be stated as follows : 1. The essential cause of diarrhoea resides ordinarily in the superficial layers of the earth, where it is intimately associated with the life processes of some micro-organism not yet detected, captured, or isolated. 2. The vital manifestations of such organism are dependent, among other things, perhaps principally, upon conditions of season and on the presence of dead organic matter which is its pabulum. 3. On occasion, such micro-organism is capable of getting abroad from its primary habitat, the earth, and having become air-borne obtains opportunity for fastening on non- living organic material, and of using such organic material both as nidus and as pabulum in undergoing various phases of its life-history. 4. In food, inside of as well as outside of the human body, such micro-organism finds, especially at certain seasons, nidus and pabulum convenient for its development, multiplication, or evolution. 5. From food, as also from the contained organic matter of particular soils, such micro-organism can manufacture, by the chemical changes wrought therein through certain of its life processes, a substance which is a virulent chemical poison. 6. This chemical substance is, in the human body, the material cause of epidemic diarrhoea. To the foregoing, we have only to add Dr. Meinert's words : " The poison, or a combination of poisons, appears to work upon the medulla oblongata, for there lies the centre for intestinal secretion, vomitings, convulsions, respiratory and vaso-motor phenomena." CHAPTER XXIX ACUTE INFECTIVE DISEASES (continued) Influence on the Prevalence of Enteric Fever of (1) Season ; (2) Tempera- ture and Moisture ; (3) Soil and Underground Water Outbreaks at Terlingand in Trinity College, Dublin Seasonal Mortality from Enteric Fever in Dublin for Twenty Years, 1872-91 Typhus Fever a Disease of Winter and Spring Influence of Season, Overcrowding, Defective Ventilation, Temperature and Atmospheric Moisture Seasonal Mortality from Typhus in Dublin for Twenty Years, 1872-91 Seasonal Prevalence of (1) Smallpox ; (2) Measles; (3) Scarlatina. 4. Enteric Fever IT is fitting that this disease should be taken next in order after cholera and the diarrhoeal diseases, to which it presents so many points of analogy. Season. Enteric fever is most prevalent in autumn and early winter, hence the names by which it is often described in America "Autumnal" or "Fall Fever" (Austin Flint). The exciting cause of the disease seems to be called into action only, as Murchison says, "by the protracted heat of summer and autumn, while it required the protracted cold of winter and spring to impair its activity or to destroy it." An examination of the Returns of the Registrar-General for Ireland shows that enteric fever exhibits, as the summer rolls by, a decided tendency to increase in Dublin at an earlier period than typhus. This is, no doubt, partly due to the fact that the secondary phenomena of enteric fever are ACUTE INFECTIVE DISEASES 377 generally developed in connection with the digestive system, acute and infective diseases of which system increase towards autumn. The accompanying diagram (1) is reproduced from the Annual Summary of z | the Registrar - General 1 for England for 1890: **$ fcSafs^s^^ Temperature and Moisture. Hot, dry, calm summers increase ; the prevalence of en- teric fever, which is less frequent in cold, wet, stormy seasons. Warm, damp weather, however, predisposes to the dis- ease. Floods occurring in badly drained locali- ties may impregnate * o^ sources of drinking- d water with the germs of enteric fever, and so lead to its outbreak. Soil and Under- ground Water. l Pro- fessor von Pettenkofer 1 An excellent resume of various papers on this sub- ject in the Zeitschrift fur Biologic will be found in the Ugeskrift for Lccger, Copenhagen, January 30, 1869. A translation by my father, Dr. W. D. Moore, ap- peared in the DuU. Journ. of Med. Science, vol. xlvii. p. 497, May 1869. 378 METEOROLOGY and Professor Buhl, of Munich, have shown that when the subsoil water in that city (as measured by the depth of water in the surface wells) is falling the number of cases of enteric fever increases ; when the water level is rising the number of cases diminishes. Liebermeister and Buchanan suppose that these observations simply illustrate the mode in which the disease is communicated by means of drinking-water. When the subsoil water is low any noxious matters in it accumulate and acquire a greater virulence. In the case of an outbreak of enteric fever at Terling, Essex, in December 1867, Dr. Thome Thome, then an Inspector, and now Medical Officer, of the Local Government Board of England, found that the disease had broken out with great severity precisely when the wells were high. 1 Two or three years after the introduction of the Vartry Water Supply into Dublin, in 1868, a serious local outbreak of enteric fever took place in Trinity College, Dublin. It was confined to the resident water-drinkers in the College. An inquiry was instituted into the cause of the outbreak, the Rev. Dr. Haughton, F.R.S., Fellow of Trinity College, Dr. Apjohn, F.R.S., then Professor of Chemistry in the University of Dublin, and Mr. Dowling, then Professor of Engineering in the University, being appointed to act as Commissioners by the Rev. H. Lloyd, D.D., at the time Provost of Trinity College. It was found that, owing to high tides in the river Liffey, and the accumulation of water in the subsoil, in conse- quence partly of the disuse of the pumps after the intro- duction of the Vartry water, and partly of the leakage of the Vartry water itself from defective house-drains, the foul sub- soil water had overflowed into and contaminated the well within the College precincts from which the drinking-water in use in the College was drawn. Ever since that time the level of the subsoil water in Trinity College has been kept low by steam pumping, at a cost of about 300 per annum, with the result that within the past twenty-three years no 1 Tenth Report of the Medical Officer of the Privy Council, p. 51. 1868. ACUTE INFECTIVE DISEASES 379 indigenous outbreak of enteric fever has occurred amongst the residents in the College. TABLE IV. Showing the Total number of Deaths from Enteric Fever in the Dublin Registration District in each of Thirteen Four-weekly Periods in the Twenty Years, 1872-91 ; the Average yearly number of Deaths from this Fever in the Decennial" Periods, 1872-81, and 1882-91, respectively ; and the Percentage of the Total Mortality from the same Fever in each of the said Periods. I. Jan. 1 to Jan. 28 16-1 ]2'1 282 8-6 II. Jan. 29 ,, Feb. 25 16-0 10-8 268 8-2 III. Feb. 26 Mar. 25 14-6 13-2 278 8'5 IV. Mar. 26 Ap. 22 12-3 9'8 221 6-8 V. Ap. 23 ,, May 20 14-3 9-6 239 7'3 VI. May 21 June 17 87 8-1 168 5-1 VII. June 18 ,, July 15 10-6 8-6 192 5-9 VIII. July 16 ,, Aug. 12 9"2 7'9 171 5'3 IX. Aug. 13 Sept. 9 107 10-1 208 6-3 X. Sept. 10 ,, Oct. 7 14-3 10-9 252 7-7 XI. Oct. 8 Nov. 4 127 197 324 9-9 XII. Nov. 5 Dec. 2 15-9 17-1 330 10-1 XIII. 1 Dec. 3 ,, Dec. 30 13-8 20-2 340 10-3 Totals 1692 1581 3273 100-0 1 The thirteenth period included five weeks in 18T3 (no deaths), 1879 (2 deaths), 1884 (7 deaths), and 1S90 (7 deaths). These 16 deaths raise the periodic averages from 13-6 to 13'8 in 1872-81, and from 18-8 to 20'2 in 1882-91, and the yearly averages from 169-0 to 169'2 in 1872-81, and from 156'7 to 158'1 in 1882-91. The preceding Table supplies information as to the seasonal mortality from enteric fever in Dublin in the twenty years ending 1891. The facts are drawn from the Reports of the Registrar-General for Ireland. In this Table the year is divided into thirteen periods of four weeks each. In each of 380 METEOROLOGY the years 1873, 1879, 1884, and 1^0, fifty-three weeks are included in order to bring the Registrar-General's statistics into agreement with the calendar. In these four additional weeks, sixteen deaths from enteric fever were registered, thus raising by so many the number of deaths recorded in the thirteenth four-week period in the Table. The Table shows that, allowance being made for a three weeks' illness before death and registration occur, enteric fever increases in prevalence and fatality towards the end of July, that is, with a rise of the subsoil temperature at 4 feet to and above the critical point of 56 F. Its epidemic character becomes pronounced in September and continues until the close of February, after which the disease becomes less frequent and deadly, reaching its spring minimum at the beginning of May. From this time to the end of June is also the period of its annual minimum, while its annual maximum takes place about the middle of November. These results agree remarkably with the curve for typhoid fever for all ages and both sexes given by Buchan and Mitchell. 1 This is a well-marked curve (they say) resembling the curve for scarlatina in showing the maximal death-rate in October and November, but differing from it in the duration and phases of the minimal period. Scarlatina falls below its average in the beginning of January, typhoid fever not till the last week of February; scarlet fever has its absolute minimum period from the middle of March to the middle of May, typhoid fever from the middle of May to the end of June ; scarlet fever begins steadily to rise in the second week of May, typhoid not till the beginning of July, when the heat of summer has fairly set in. 5. Typhus Fever Typhus is essentially a disease of winter and spring that is, of the colder seasons of the year. Among the predisposing 1 "The Influence of Weather on Mortality," Journ. of the Scot. Met. Soc. t vol. iv. p. 197. ACUTE INFECTIVE DISEASES 381 causes of this fever, season and atmospheric temperature are commonly included. Season. During twenty-three years, January and March were the months in which the number of admissions of typhus patients to the London Fever Hospital reached a maximum the minimum falling in September, August, and July. This distribution was from time to time disturbed by an epidemic, outbreaks of typhus commencing and advancing irrespective of season. An examination of the Registrar-General's (Ireland) Returns of deaths from typhus in Dublin, undertaken many years ago, led me to the conclusion that the death-rate from typhus attains its maximum in January and its minimum in September. The reason for this is not far to seek. Typhus is often intimately related to overcrowding, and affections of the respiratory organs are among its most frequent complica- tions. Hence we should expect to meet with it, especially in the colder seasons of the year. Murchison points out that typhus does not always become more prevalent with the commencement of cold weather, nor does it decline im- mediately on the advent of summer. He correctly infers from this that the increase of typhus in winter and spring is not so much due to the direct effect of cold as to the continued overcrowding and defective ventilation of the dwellings of the poor in cold weather. The accompanying Table gives the facts relating to the deaths from typhus in Dublin during the twenty years, 1872-91, inclusive. Apart from our present inquiry, one gratifying circum- stance stands prominently out from the figures in the foregoing Table, and that is, the fact that typhus fever is practically dying out in Dublin. The number of deaths from the disease fell nearly 50 per cent to 507 from 996 in the second decennium discussed in the Table. An analysis of the Table proves that the mortality from typhus reaches a minimum in the ninth and tenth periods August 13 to October 7 ; while the minimal death-rate from 382 METEOROLOGY enteric fever has already occurred in the eighth period July 1 6 to August 1 2 ; this fever exhibiting, as the summer rolls by, a decided tendency to increase at an earlier period than typhus. The highest percentage death-rates from typhus are met with in the seasons of winter, spring, and early summer 10 '4 per cent of the fatal cases being registered in the second period (January 29 to February 25), and lO'O per cent in the fifth period (April 23 to May 20). TABLE V. Showing the Total number of Deaths from Typhus Fever in the Dublin ^Registration District in each of Thirteen Four- Weekly Periods in the Twenty Years, 1872-91 ; the Average yearly number of Deaths from this Fever in the Decennial Periods, 1872-81 and 1882-91 respectively ; and the Percentage of the Total Mortality from the same Fever in each of the said Periods. H . M 2 83 'S ,3 S w" 3.3,4 X5 0! . * ofle Four-week Periods. Corresponding Periods in Calendar. ^tsP |g| l|l Pi T~ !** |l| I. Jan. 1 to Jan. 28 9-0 4'3 133 8-8 II. Jan. 29 Fefc. 25 . 9-8 5'9 157 10-4 III. Feb. 26 ,, Mar. 25 . 8-1 5'4 135 9-0 IV. V. Mar. 26 Ap. 22 . Ap. 23 May 20 . 7'4 10-2 5'8 4'8 132 150 8-8 10-0 VI. May 21 , June 17 . 9-6 31 127 8-4 VII. June 18 , July 15 . 7-1 41 112 7-5 VIII. July 16 , Aug. 12 . 7'5 2'4 99 6-6 IX. Aug. 13 , Sept. 9 . 4-6 3-4 80 5-3 X. Sept. 10 , Oct. 7 . 51 2-0 71 4-7 XI. Oct. 8 , Nov. 4 . 6-0 3-4 94 6-3 XII. Nov. 5 , Dec. 2 . 6-9 2-9 98 6-5 XIII. 1 Dec. 3 Dec. 30 . 8-3 3-2 115 77 Totals . 996 507 1503 100-0 1 The thirteenth period included five weeks in 1873 (4 deaths), 1879 (3 deaths), 1884 (no deaths), and 1890 (no deaths). These 7 deaths raise the periodic average from 7'6 to 8-3, and the yearly average from 98'9 to 99-6, in 1872-81. ACUTE INFECTIVE DISEASES 383 According to Buchan and Mitchell, 1 the curve for typhus is above the average from January to the beginning of May, and with the exception of the hot season of July and beginning of August, it is below the average from the middle of May to the end of September. It seems probable that the curve has two maxima, the larger in the early months of the year, and the smaller in the height of summer. Buchan's and Mitchell's typhus curve is based on only six years' returns of mortality 1869-74. Temperature and Moisture in the Atmosphere do not seem to have any marked predisposing influence on typhus, notwithstanding the opinion advanced by Dr. T. W. Grim- shaw, now Kegistrar-General for Ireland, in 1866, 2 that a warm moist state of the atmosphere seemed to favour an increase of typhus, whereas dryness with cold had a contrary influence. Murchison was unable to trace any such connection, but points out that exposure to cold and wet, if long con- tinued, depresses the nervous system and so favours the onset of typhus. 6. Smallpox Turning now to the principal eruptive fevers, we find that, although the incidence of smallpox is apparently in- dependent of climate, yet the season of the year has a marked influence upon the prevalence of the disease. Nearly all writers are agreed that, while outbreaks of smallpox may occur at all seasons, they mostly begin towards the end of autumn and in the early spring, or in the cold season. In a word, smallpox is essentially a disease of winter and spring. In the British Islands, and Western Europe generally, for example, the monthly number of cases is high from November onwards ; but from May a rapid decline in the prevalence of 1 Loc. cit. p. 197. 2 " On Atmospheric Conditions influencing the Prevalence of Typhus," Dubl. Quar. Journ. of Med. Science, May 1866. 384 METEOROLOGY I a the disease takes place, the least number of cases being observed in September. The accompanying diagram (2) is copied from the Annual ^ Summary of Births and I Deaths of the Registrar- S, s. General for England for 1890. It shows the weekly departure from the average weekly number of deaths from smallpox (17) in Lon- don in the fifty years, 1841-90 inclusive : In this diagram the thick horizontal line represents the mean P weekly mortality from smallpox in London, on i ^ the supposition that 1 the mortality is spread equally over the ; fifty-two weeks of the year the fifty - third week, when it occurs, being ignored. The HI [ 1 1 [ | | | | | | * curved line represents the amount per cent by which the average mortality in each week i differs from this mean. When the percentage k?tl i ~t , '9 it V4- * for any week is above the mean, the amount |r < |- of the percentage excess is marked above the horizontal line representing the mean ; and when the ACUTE INFECTIVE DISEASES 385 percentage is below the mean it is marked below the line. It must be remembered that the data on which the curve is formed are the deaths registered in each week, not the deaths which occurred in the week, and that the registration is usually a few days after the death ; and, secondly, that the curve relates to deaths that is, the final termination of the attack of illness, and not its commencement. So that, in estimating the effect of season in generating smallpox, allow- ance must be made for the average duration of this disease when fatal that is, eleven or twelve days. It is, moreover, pos- sible that the curve of mortality may, for another reason, not accurately represent the curve of prevalence. For it may be that an attack of smallpox is more likely to terminate fatally if it occurs at one season for example, midwinter than if it occurs at another, such as midsummer. The diagram shows that at the beginning of February, and in the second half of May, the weekly number of deaths was 35 per cent in excess of the average weekly number of 17 deaths represented by the mean line, whereas at the end of September there was a deficit of 43 per cent in the weekly number of deaths as compared with the same average weekly number over the whole year. In Dublin, during the autumn of 1871, the prevalence of, and mortality from, smallpox increased with a fall of mean temperature below 50, and the greatest severity of the epidemic was experienced in the first half of the following April, shortly after a period of intense cold for the time of year. With the rise of mean temperature to between 55 and 60 in the middle of June, the epidemic declined rapidly. Abundant rainfalls seemed to be followed by remissions in the severity of the epidemic, and the converse was also true. 1 Buchan and Mitchell say that the curve for smallpox is 1 Manual of Public Health for Ireland 1875, p. 298. Dublin : Fannin and Co. See also Buchan's and Mitchell's Paper in the Journal of the Scottish Meteorological Society, 1874. 2 c 386 METEOROLOGY one of the simplest of the curves, showing that the mortality from the disease is above the average from Christmas till the end of June, the maximum falling in the last week of May, and the minimum in the last week of September. From statistics as to the prevalence of the disease in Sweden, by months, in the years 1862-69 inclusive, 1 it appears that the greatest prevalence of smallpox is observed in May, the cases in that month being 13*7 per cent of the total cases occurring in the year ; while the least prevalence is observed in September, when only 3*9 per cent of all the cases in the year occur. From November the monthly number of cases is high, but from May a rapid decline in the prevalence of the disease takes place. When due allowance has been made for difference of climate, these results agree very closely with the observations which have been recorded in this country on the relation of small- pox to season. Dr. Edward Ballard, 2 writing of the epidemic of 1871, observed : " There is some reason for believing that the variations of the epidemic (of smallpox), from week to week, are influenced to a certain extent by atmospheric conditions and more especially by variation in temperature." He then quoted a series of remarkable coincidences between the fluctuations of mean temperature and those of the small- pox mortality in London during the winter of 1870-71. In the number of the Medical Times and Gazette for May 13, 1871, he wrote: "The epidemic has now lasted a good six months. It may be regarded as assuming a distinctly epidemic form in November, shortly after the mean temperature of the air had fallen decidedly below 50. In the progress of the seasons 1 These statistics were compiled from exhaustive annual reports by the late Dr. Wistrand, as to the morbidity of Sweden, and are the direct fruit of an admirable system of disease-registration, which has been in operation for many years in Sweden, and also in the other Scandinavian countries. 2 Medical Times and Gazette, March 11, 1871. ACUTE INFECTIVE DISEASES 387 we have now arrived at a time when this mean temperature is again reached. The mean temperature of the last three weeks, as recorded at Greenwich, has been 50, 50 '7, and 49*7. It is customary about the second week in May for some check in the consecutive weekly rises of temperature to take place, but after this, in the ordinary or average progress of events, the steady rise towards the summer temperature may be expected to set in, and with it there is at least a hope that the epidemic will begin to fade." A week later, the same writer said : "The sudden fall of deaths in London from smallpox which occurred last week, namely, from 288 to 232, occurring about three weeks after the mean temperature of 50 was reached, appears to be confirmatory of the favourable hopes we expressed last week, that the epidemic had, for this season, arrived at its climax." And so it had, for although the decline was occasionally interrupted, the virulence of the epidemic was broken in May, in accurate fulfilment of the anticipations which had been grounded on a consideration of the influence of tempera- ture on its progress. 7. Measles Seasonal Prevalence. Although, like smallpox, apparently independent of climate for it is met with alike amidst Arctic snows, in temperate latitudes, and under the tropical sun measles prevails especially in the spring and autumnal quarters, of the year. An analysis of the weekly returns of deaths from measles in the Dublin Registration District, published by the Registrar-General for Ireland, long since led me to the con- clusion that a mean temperature above 58*6 was not favour- able to the spread of this disease, and that a mean temperature below 42*0 was equally inimical to its prevalence. 1 These results are in strict accord with those arrived at by Dr. 1 Manual of Public Health for Ireland. 1875. Pp. 300, 301. 388 METEOROLOGY Edward Ballard, who says 1 that the only condition concerned in the arrest of the spread of measles in summer is the rise of ^ the temperature of the 1 . :: H .- >" g | 1 air o H & . & ^ .. _r, | 60 f above a mean of F., while towards winter a fall below 42 F. also distinctly tends to check the disease. The accompanying diagram (3), copied from the Annual Summary of Births, Deaths, and Causes of Death in London and other great towns for 1890, by the Registrar - General for England, is based upon the weekly returns of deaths from measles in London for the fifty years, 1841-90 inclu- sive. In it, the mean line represents an aver- age weekly number of 34 deaths from the dis- ease under discussion, and the weekly curve shows a double maxi- mum and a double minimum, the larger maximum falling in November, December, and January, with an extreme excess of 50 1 Eleventh Report of the Medical Officer of the Privy Council 1868. No. 3. Pp. 54-62. ACUTE INFECTIVE DISEASES 389 per cent in the fourth week of December, and the smaller in May and June, with an extreme excess of 25 per cent in the first week of June. The larger minimum falls in August, September, and October, extreme deficit being 45 per cent below average in the last week of September, and the smaller minimum in February and March extreme deficit, 30 per cent below average in the third week of February. According to Buchan and Mitchell, who examined the London death-rates for the thirty years beginning with 1845 and ending with 1874, the measles curve is remarkable in showing a double maximum and minimum during the year, the larger maximum occurring in November, December, and January, and the smaller in May and June ; the larger minimum in August, September, and October, and the smaller in February and March. The most rapid fluctuation takes place in the fall observed from Christmas to the middle of February, the weekly deaths falling from 50 per cent in excess of the average to 30 per cent below it that is, from 51 to 24, the average weekly number of deaths throughout the year being 34. This curve is one of the steadiest from year to year, both the December and the June maxima being well marked in nearly every one of the thirty years analysed by Buchan and Mitchell, and the yearly minima also being well marked. In Table VI. is contained an analysis of the deaths from measles registered in the Dublin Registration District during the twenty years ending 1891, with the corresponding figures for scarlet fever. The two annual maxima and minima for measles are shown in the last column, but it will be observed that, as compared with London, the Dublin spring minimum is feebly marked, while the incidence of the autumnal minimum is later. Similarly, the summer maximum falls later in Dublin than it does in London. It is instructive to compare the figures for measles with those for scarlet fever. It will be seen that these diseases are correlative, measles being very much in evidence when 03 oo < > co <= . cS i-H C5 r I " ** CT 8 GO b- > aT 03 1-3 Cj 03 H ! 3 8 2 g S si a^? r3 S oj O > b> (Mi-l(Ni-li-lrHrHrHr-l i^-ICpOO^.-^^O5^3O>OOii^HO5 :l 3 -2 S.--C s b'C p O) ppin ACUTE INFECTIVE DISEASES 391 scarlet fever is infrequent, and the latter disease attaining its autumnal maximum when the prevalence of measles is only beginning to increase. 8. Scarlatina or Scarlet Fever Climatic Influences do not play a prominent part in deter- mining the geographical distribution of this disease, for although the tropical and sub -tropical regions of Asia and Africa have so far almost entirely escaped scarlet fever, yet it has often prevailed epidemically in the tropical countries of South America ; and, on the other hand, in certain cold or temperate climates scarlet fever is among the rarest of There is, however, evidence that season does influence its prevalence. "Scarlatina" observes the Kegistrar-General of England, 1 "discovers a uniform, well-marked tendency to increase in the last six months, and attain its maximum in the December quarter, the earlier half of the following year witnessing a decrease." In Dublin, also, the disease is almost invariably most prevalent and fatal in the fourth quarter of the year. From an analysis of the weekly death-rate from scarlatina in Dublin it would seem that this fever shows a tendency to increase when the mean temperature rises much above 50, while a fall of mean temperature below this point in autumn checks the further rise of the mortality. 2 In this city, scarlet fever is most fatal in the forty-sixth week of the year (middle of November) and least fatal in the twenty -fourth week (middle of June). Dr. Edward Ballard draws inferences which confirm these results. 3 The "Annual Summary of Births, Deaths, and Causes of Deaths," of the Registrar-General of 1 Twenty - eighth Annual Report of Births, Deaths, and Marriages, p. 38. 2 Manual of Public Health for Ireland. 1875. Pp. 303, 304. 3 Eleventh Report of 'the Medical Officer of the Privy Council. 1868. No. 3. Pp. 54-62. 392 METEOROLOGY England, for 1890, is illustrated by the annexed diagram (4.), CO ^ oo *& g M 3 T ^ ^ Q ^ s$ showing the weekly mortality curve for scarlet fever in London on an average of thirty years (1861-90). The curve consists ACUTE INFECTIVE DISEASES 393 of a single wave, which rises to its crest (60 per cent above the mean line, which represents an average weekly number of 44 deaths) in October and November, while the trough extends from February to August. It is a suggestive fact that the corresponding curves for diphtheria and enteric fever are also single wave curves, closely resembling each other and that of scarlatina in rising to a crest in October and November, and showing a trough from February to August. From their analysis of the deaths from scarlet fever registered in London in the thirty years, 1845-1874, Buchan and Mitchell conclude that this disease has its maximum from the beginning of September to the end of the year, and its minimum from February to July. The period of the highest death-rate is from the beginning of October to the end of November, being nearly 60 per cent above the average, and the lowest in March, April, and May, when it is about 33 per cent below its average. In each of the thirty years the deaths increased at the time of mean maximum, and in all except four of the years the increase was considerable. During ten of the years a high death-rate was continued on into the year immediately following, but in every year the deaths became fewer, and diminished steadily, if not rapidly. CHAPTER XXX THE SEASONAL PREVALENCE OF PNEUMONIC FEVER 1 IN April 1875, Dr. T. W. Grimshaw, now Registrar-General for Ireland, and I, read before the Medical Society of the King and Queen's College of Physicians a paper on what we ventured to call " Pythogenic Pneumonia." This paper, which was published in the number of the Dublin Journal of Medical Science for May 1875, 2 was based upon observations of pneumonia in Steevens' and Cork Street Hospitals, Dublin, during the summer of 1874 when an epidemic of the disease prevailed in the Irish capital as well as upon an analysis of the statistics of death from bronchitis and pneumonia registered in Dublin during nine years ending with 1873. In the same communication, the meteorological and epidemic con. ditions of 1874 were discussed, and our researches seemed to warrant us in drawing the following conclusions : 1. That the bibliography of pneumonia indicates the existence of a form of the disease which arises under miasmatic influences, and is contagious. 2. That this view is supported by the relation which exists between this form of pneumonia and certain zymotic affections notably, enteric fever and cholera and by the resemblance between it and epizootic pleuro-pneumonia. 1 Reprinted, by permission, from the Transactions of the Ninth Session of the International Medical Congress. Vol. v. p. 45. Washington, D.C., U.S. A. 1887. 2 Vol. lix. No. 41. Third Series. Page 399. SEASONAL PREVALENCE OF PNEUMONIC FEVER 395 3. That its aetiology justifies us in regarding the disease as a zymotic affection and in naming it "pythogenic pneumonia." 4. That pythogenic pneumonia presents peculiar clinical features which enable us to distinguish it from ordinary pneumonia. 5. That much of the pneumonia which prevailed in Dublin during 1874 was of this pythogenic character. 6. That whereas ordinary pneumonia is specially prevalent during a continuance of cold, dry weather, with high winds and extreme variations in temperature, pythogenic pneumonia reaches its maximum during tolerably warm weather, accom- panied with a dry air, deficient rainfall, hot sun and rapid evaporation. The years which have elapsed since the publication of this paper on " Pythogenic Pneumonia " have been fruitful in the literature of the subject to an unprecedented degree. Among the many monographs on pneumonia which have of late appeared, perhaps the most valuable are that by the late Dr. August Hirsch, Professor of Medicine in the University of Berlin, on the Geographical and Historical Pathology of the Disease, 1 and that by the late Dr. C. Friedlander, of Berlin, on the " Micrococci of Pneumonia." 2 Hirsch, after pointing out that pneumonia, even in its narrowest acceptation of fibrinous- or so-called croupous pneumonia, is an anatomical term that includes several inflammatory processes differing from one another in their aetiology, goes on to observe that the prevalence of the malady depends very decidedly upon certain influences of season and weather. He gives an elaborate Table of percentages of pneumonic prevalence in the several months at a large number of places in Europe and America. According to this Table, 1 Handbook of Geographical and Historical Pathology, Vol. iii. Translated from the Second German Edition, by Charles Creighton, M.D. London : The New Sydenham Society. 1886. 2 Fortschritte der Medicin. Band 1, Heft 22, Nov. 22, 1883. Trans- lated for the New Sydenham Society. By Edgar Thurston. 1886. 396 METEOROLOGY the largest number of cases falls in the months from February to May ; the smallest number in the period from July to September. Taking the average for all the places mentioned in the Table, it appears that 347 per cent of the patients were attacked in spring (March to May inclusive) ; 29*0 in winter (December to February); 18 '3 in autumn (September to November); and 18 '0 in summer (June to August). The combined percentage for winter and spring is 6 3 '7 ; that for summer and autumn is 3 6 '3. If the number of cases in summer be taken as 1, then autumn has T02, winter 1'6, and spring 1 '9. Nearly all the recorded epidemics of pneumonia have occurred in winter and spring. From the foregoing considerations, Hirsch confidently concludes that the origin of the malady is dependent on weather influences proper to winter and spring, and more particularly on sudden changes of temperature and considerable fluctuations in the proportion of moisture in the air. He holds that any exceptionally large number of cases of " inflammation of the lungs " at the other seasons, more especially in summer, has coincided with the prevalence of the same meteorological conditions phenomenally at that season. " But that conclusion," he goes on to say, " is still further borne out by the fact that in those northern regions (Russia, Sweden, Denmark, Germany, England, the North of France, and the Northern States of the American Union) where the most sudden and severe changes of temperature fall in spring, the largest number of cases is met with in spring also ; while in the warmer and sub-tropical countries (Italy, islands of the Mediterranean, Spain and Portugal, Greece, Algiers, Southern States of the Union, Chili and Peru), which are subject to those meteorological influences, for the most part, in winter, it is winter that represents the proper season of pneumonia. And that applies not merely to sporadic cases, but, in part at least, to epidemic outbreaks of the malady as well. One other fact deserves to be noticed here, namely, that those tracts of country, especially in the tropics, which are highly SEASONAL PREVALENCE OF PNEUMONIC FEVER 397 favoured in their climate or in the steadiness of the temper- ature from day to day (Egypt, many parts of India, including Bengal and the plain of Burrnah, California, etc.), are subject to pneumonia to a comparatively slight extent." In the paper on "Pythogenic Pneumonia," by Dr. Grimshaw and myself, will be found a Table, compiled from the returns of the Registrar-General for Ireland, which shows the number of deaths from bronchitis and pneumonia registered in the Dublin Registration District in each quarter of the nine years, 1865-1873 inclusive. According to that Table, of every 100 deaths from bronchitis, 44 on the average occurred in the first quarter of the year, 22 in the second, only 10 in the third, and 24 in the fourth quarter. Thus, the mortality from bronchitis was twice as great in the first as it was in the second quarter, and more than four times greater in the first than in the third quarter. Very different were the facts as to pneumonia : of every 100 deaths from this disease, 32 on the average occurred in the first quarter, 27 in the second, 16 in the third, and 25 in the fourth quarter. The mortality from pneumonia was only one-fifth greater in the first than in the second quarter, and only twice as great in the first as in the third quarter. The extreme winter fatality of bronchitis and its low summer fatality were equally wanting in the case of pneumonia. A careful analysis of the weekly returns of the Registrars- General of England and Ireland for ten years ending with 1885, and of the same returns for the year 1886, brings out a similar remarkable contrast between bronchitis and pneu- monia, as to the time of year when these diseases are respec- tively most prevalent and fatal in London and Dublin. Table I. contains the figures relating to pneumonia ; Table II. those relating to bronchitis. Each Table sets forth the weekly average number of deaths in London and in Dublin from pneumonia and bronchitis respectively, in the ten years, 1876-85, as well as the actual weekly number of deaths from these diseases in the year 1886. COXQCOOlOOrHOT-IOi-ICNt^ s COiO^rHCO^Ci-HOrH t-l^-OOOOr-li ICNl(OrHr-li 1 ! IXOOO^-^OOCO -^ kO COCICOOOOt^rHCOOSOrHOOrH Or Ol 02 O 1 t--COO5Ort<(7-lOOOi lO PHQ- sl rH t^m IO Oi J>* rH rH 4 oo METEOROLOGY In Tables III. and IV. these numerical results are thrown into curves. It will be observed that the statistics for London and for Dublin agree to a remarkable extent. In both cities bron- Tat) e 1 1 1 . Shou/iny tfie average weeMy number of'Dea&us from Bronchitis, and from, Pneumonia, in, the Decade 1876-1885 in Ike Dublin Rs,Qrhsfratt/?n District ^_ chitis falls to a very low ebb in the third, or summer, quarter of the year (July to September inclusive), when only 12 per cent of the deaths annually caused by this disease take place in Dublin, and only 11 per cent in London. In the last, or fourth, quarter (October to December inclusive), the per- SEASONAL PREVALENCE OF PNEUMONIC FEVER 401 centage of deaths from bronchitis rises to 27 in Dublin and to 30 in London. The maximal mortality occurs in the first quarter (January to March inclusive), when it is 38 per cent Table IV Showing the arercujeu>ee/cl.y Dcafhs from. Bronchitis. an