Ti.7-7 MR. WALKEK'S PATENT COMPASS. ^W-^Sr^ yy ^v^z^g; THE MAGNETISM OE SHIPS, Clje Hariiter's Compass; BEING A RUDIMENTARY EXPOSITION OF THE INDUCED MAGNETISM OF IRON IN SEA-GOING VESSELS, AND ITS ACTION ON THE COMPASS, IN DIFFERENT LATITUDES, AND UNDER DIVERSIFIED CIRCUMSTANCES. BY WILLIAM WALKER, COMMANBEB R.N. KXAMINEK IN NAVIGATION AND SEAMANSHIP, ETC LONDON: PIPER BROTHERS & CO., PATERNOSTER ROW CHARLES WILSON, (LATE WILSON & NORIE,) LEADENHALL STEEET. 1853. Vb PREFACE. WHEN a person writes anything in the shape of a book for publication, he is expected to give a reason why he writes it, and for what purpose. In order to satisfy this expectation, I have to inform the reader that, although cheap elementary and popular treatises on almost every subject relating to mechanical, manual, or practical industry can be obtained in a condensed form, and at small cost, one looks in vain for a sound, practical, elementary, and popular work on the induced Magnetism of Ships, and the influence it exerts on the Mariner's Compass. A work of this kind, if written in language familiar to seamen, and^ yet sufficiently comprehensive in detail, would necessarily tend to expand the mind of a young navigator. Parliamentary enquiries have shewn, that in every week there are at least ten British merchant ships wrecked, stranded, or seriously damaged; and b M277805 VI PREFACE. ships are constantly running in one direction on a com- pass course, whilst the persons in charge firmly believe they are going in another. Ships shape compass courses towards rocks and shoals, by orders from the navigators, who think they are perfectly safe in shaping such courses. It is in this way that much property and many lives are lost, and no small amount of pro- fessional reputation called in question. The writer, therefore, thinks that, if more were known about the Magnetism of Ships and the Mariner's Compass, fewer misfortunes would befall the shipping of this and of other countries. Being sensitively alive to nautical matters, and impressed with an opinion that sufficient skill and precaution were not employed at sea, he wrote, some ten or twelve years ago, several articles on these subjects for the periodicals of the day ; such as the Naval and Military Gazette, the Shipping Gazette, and Nautical Magazine ; and some of his remarks have been quoted, or have found places in works of a more permanent character. Having acquired much personal experience at sea, as a navigator in the mercantile marine, as a master in the Royal Navy, as an officer in the Royal Dockyards, having had the superintendence of all kinds of ships PREFACE. Vll and compasses, and in swinging them for compass deviations, it was considered, that by bringing to- gether some of his previously published remarks, notes, hints, and practical experiments, to form a small volume, it might prove of some practical utility to young seamen aspiring to rise in their profession. Seamen, however, are generally persons with a very limited amount of what is usually called education. They are generally persons from the humble walks of life, who have been sent to sea at a very early age, in order to be inured to a life of danger, toil, and pri- vation. The young sailor has a vast amount of knowledge to acquire before he can be justly entitled to the designation and rating of A.B. or able seaman. An able seaman, properly so called, has all his practical duties at his fingers' ends, and knows exactly what to do, and how to do it ; but his limited amount of education disqualifies him for graphically describing the principles of his art to another. However gifted he may be in the actual practice of his calling, or in the intuitive performance of seamanship, he cannot communicate, in writing, to others the knowledge which he acquired by experience alone. Hence it has resulted, that almost all our practical and scientific works, on Navigation and the Management of Ships, Vlll PREFACE. have been produced, compiled, or methodically ar- ranged, by landsmen; who, however gifted in other respects, would be found unfit to put into practice at sea the principles and precepts they undertake to propound in books. The young sailor's earliest years are passed in watching or working on deck or aloft by day, and in making and shortening sail or looking out at night. The place appropriated for his relaxation or repose, in the forecastle of a merchant ship, is often dark, ill-ven- tilated, and unsuitable for a place of study or mental improvement. A sea boy, then, has but slender means for acquiring the necessary amount of education which every person should possess who may be entrusted with even the charge of a watch in a ship at sea ; and yet it is to these boys that we must necessarily look for all the officers of our extensive and valuable mer- cantile marine. Iron now enters largely into the formation of a ship's hull, rigging, or cargo. The compass, often the helms- man's only guide, can seldom be placed beyond the influence of the induced magnetic action of the iron in a vessel. Coast navigation, in dark nights, or in foggy weather, has become most hazardous. Sensible of these facts, PREFACE. IX the Lords Commissioners of the Admiralty now require, that young officers, under examination for navigation at the Royal Naval College, shall possess a certain amount of knowledge relative to a ship's local attraction. The Board of Trade, under authority of the Mer- cantile Marine Act, inserts, in the examination sheets for masters and mates in the mercantile marine, certain nautical problems for candidates to solve ; and among other things they are required to understand, and to " explain the meaning of deviation of the compass, and the method of determining and correcting it." I have however found, that it is instruction, rather than examination, that these candidates require. How can it be otherwise, when not one in twenty of those who undertake to teach navigation know anything more of the action of iron on a ship's compass, than that it is liable to attract and derange the free action of the needle ? Navigation is now taught in an instructive and efficient manner by able teachers, in the nautical branch of the Upper School, in the Royal Hospital at Greenwich. I was recently fortunate in being present at a half-yearly examination of the boys, and was much gratified by finding that they are now regularly taught the known principles of a ship's attraction on X PREFACE. its compass under different conditions, principles which a few years ago I ventured to submit to the Admiralty and Compass Committee, and which, at that time were deemed rather visionary, or of small practical importance. These principles are, however, now regarded by government as a necessary part of a navigator's education, and he is legally required to obtain this knowledge where he best can find it. Having now shewn why this unpretending volume was put together for what purpose it was intended that a necessity exists for an elementary treatise of the kind, in order that seamen might better understand the subject, it is now set adrift, to be picked up by such persons as may require practical information on the Magnetism of Ships, and the Mariner's Compass. W. W. Plymouth, June, 1853. CONTENTS. 1. The directive power of the loadstone has contributed to improve our knowledge of the form and magnitude of the earth; and enabled the mariner to safely advance into unknown seas. The mariner's compass, a principal agent in promoting seamanship, and improving Naval architecture. The Author's attention drawn to the magnetic action of a ship he commanded on S.E. coast of Africa, in 1817, where the compass deviations were of an opposite character to those found in England. He suggests to the " Navy Board " that iron masts, intended to be put into a frigate, would act as magnets upon the compass; and recommends that such effects should be watched. Mariner's compass held in great veneration by seamen. An elementary work on the compass and the induced magnetism of ships regarded as of practical utility to young navigators 1 2. The antient Phoenicians, Greeks, and Romans had coasting vessels only; and were probably without steering compasses for more than 1000 years. The Chinese aware of the directive properties of the magnet, and use it for directing the route of armies 4477 years ago 5 3. Chinese fortune-tellers used magnetised needles in their vocation. Chinese dictionary of the fourth century mentions magnetised needles being used in ships, to direct their course. A Chinese squadron sails from the Eiver of Pekin to the Persian Gulf, about the year 1291, as recorded by the old traveller Marco Polo. De- scription of a modern Chinese compass its unfitness for use among European seamen as a steering instrument 7 4. Introduction of primitive compass into Europe probably due to the Arabs early mention made of it in use among the Saracens during the time of the Crusades. An Arabian MS. of 1242 gives a curious description of a sea compass, and how it was made and used when wanted in thick or dark weather. Some reasons as- signed why the early compass should remain so long in a rude and unimproved state 9 5. After termination of the Crusades, mercantile ships in the Medi- terranean left to freely pursue their trade. Flavius Giogo, of Amalphi, invents or impro ves the compass. Position of Amalphi. Xll CONTENTS. Sec. Page. Probable improvement made in the compass at Amalphi 11 6. Comparative merits of Chinese compass and that improved by the Amalphian. Naval improvements resulting from adoption of a more perfect steering instrument subsequent to 1302. Discoveries of new countries in Atlantic. Columbus discovers America. Portuguese sail to India round Cape of Good Hope 13 7. Vasco de Gama,the Portuguese, greatly surprised to find a pilot on East Coast of Africa well skilled in the use of a compass, quadrant, and sea chart. Indian compass inferior to those in De Gama's ships. Reflections on the slow progress made in naval science in these times ; resulting mainly through intolerance, ignorance , and superstition. Confidence of seamen in their compass 15 8. Columbus, in his first voyage in 1492, first observes the variation by the compass of his ship no longer pointing to the north pole star. Alarm of his people in consequence thereof. On Colum- bus's return to Spain, his report of the compass having varied in its direction with regard to the heavens not credited 16 9. The " variation " still discredited in a work on navigation published in 1545, fifty-three years after Columbus discovers and reports it. It is ultimately received as a thing completely established in 1556. An Englishman first discovers the dip of the magnetic needle and how 17 10. Variation at London in 1622 decreasing. Azimuths and azimuth compasses invented. Dampier observes discrepancies in varia- tion in 1680 19 11. Captain Cook finds that, with ship's head in opposite directions, the variation observed is not the same two ships steering same compass course not found to go parallel. Magnetism of ship's iron observed to cause compass deviations. Captain Flinders the result of his observations and opinions, on his return from the South Seas 20 12. Captain Scoresby his observations made in Arctic regions on the mariner's compass. Great value of results obtained by intelligent practical navigators 22 13. Consequences of introducing much iron in ships. Professor Barlow makes experiments on iron with compasses proposes a plan to correct them. Barlow's views of induced magnetism considered. Ships difficult to be navigated by reason of their local attraction. Many wrecked by steering wrong courses 24 14. Compasses often made for sale, rather than for practical utility. How improved, and by whom 26 15. Short notice of the magnet the name it has received in different countries, and the uses to which it has been applied 26 16. The directive property of the magnet has led to very important results. Antients aware of impropriety of much iron being in- troduced in ships. Sinbad the sailor. Property of magnet to CONTENTS. Xlll Sec. Page, magnetise hard steel. Opinions from time to time entertained of terrestrial magnetism. Keflections thereon 29 17. Facts relating to magnetism established by Barlow, Harris, and others. Professor Whewell's remarks on the vast utility of the mariner's compass 31 18. Principles or properties of the magnet, which have been found to be correct by actual experiments 33 19. Remarks and observations on the dip of the magnetic needle, with a table of dips in different parts of the world at the pre- sent time 38 20. All substances more or less susceptible of magnetism by induc- tion. How seamen may make experiments on magnetism by their own compasses and the iron in their vessels. Remarks on iron and wooden vessels. Ship-builders and seamen should acquire a knowledge of practical magnetism 40 21 Induced magnetism in iron its properties dependent on quality, hardness, position, &c. Experimental proofs given of variable changes in position of the induced poles of iron. Interesting experiments may be made with a shot 42 22. How the dip of the needle may be easily found without a dipping needle 46 23. How a small portion of iron, placed near a magnet, may become magnetic by that magnet. How a large mass of iron will be- come magnetic by induction from the earth, and not by a small permanent magnet 47 24. Experiments proving that although an iron rod or bar may be so placed as not to derange the free action of a compass, it may yet act as a magnetic conductor to large masses at a distance from a compass importance of this principle. Stowage of iron cargoes. Iron tanks, ballast, and shot in ships' holds. Oscillation of a ship's compass-card when steering a course in a high sea remedies usually applied inefficient 48 25. Demonstration, by diagrams, of the compass oscillation, as arising from a ship's guns or transverse masses of iron 50 26. Vibrating motion of compasses greatest when powerful compass- needles were mounted in wooden bowls. Copper and brass bowls substituted for wooden. Weak needles in copper bowls require " twiddling lines." Compass oscillation, caused by change of induced magnetic polarity of ship's iron, mistaken for mechanical action. Experiments made for Author by Mr. Hoffmeister, in H.M. ships Cornwallis and Jupiter, in opposite hemispheres 53 27. How change of magnetic polarity in a ship's iron may be experi- mentally shewn to cause compass oscillation 54 28. Polarity of iron not permanent. Necessity of knowing how iron acts on a compass, before a person can cure compass errors or XIV CONTENTS. Sec. Page, oscillations. Professor Barlow's views inconsistent with ob- served facts and why his plan of compass correction partially failed 55 29. Plan proposed by Mr. Airy for correcting iron ship's com- passes. His rules of great practical local utility. Variable quantities cannot everywhere be cancelled by constant quan- tities. Tables of variable compass deviations in H.M. ships Erebus and Terror in the two opposite magnetic hemispheres. These tables confirm the Author's arguments and observations. 56 30. Comparative disturbance of iron and wood-built ships upon their steering compasses 60 31. How derangement of a ship's compass may be detected at sea. What precautions may be taken when doubts exist about the correctness of compass courses. In what direction ships are likely to deviate from the course steered 61 3*2 The former section more fully considered (53 33 Mass of metals generally before and below compass, and conse- quences resulting therefrom. Errors of compass on given courses not the same when ship upright and afterwards inclined. Upright error may be doubled when ship is inclined one way, and cancelled when she is inclined the other way. How H.M.S. Vanguard deviated from her compass course 64 34. Wooden ships generally deviate from their intended course in one particular direction. Steam mail packet Columbia wrecked on Seal Island by shaping a course directly for it. " Committee on Shipwrecks," enquire into cause of loss of the Columbia. Evidence given by master and others, that Columbiahad always deviated to the southward of her intended course across the Bay of Fundy. An assumed current out of the bay Magnets afterwards applied to correct compass. Current assumed to continue out of the Bay of Fundy. " Parliamentary Committees" not best tribunals for detecting nautical blunders. If magnets had not been applied, in all probability the Columbia would have cleared Seal Island, and continued to record the imagi- nary current out of the Bay of Fundy. Ships running up or down English Channel generally deviate towards France. The Irish steam packets, in the coasting trade, do not steer opposite courses in going up and down the English Channel. The mail steamer Solway lost near Corunna 67 35. Reliance East India-man lost on coast of France, in 1842. A huge iron tank in her after hold. How the iron tank would affect her compass. Iron tanks act magnetically on compasses as solids of the same linear dimensions. H.M. ships Nymphe, Pallas, and L'Aimable lost in Frith of Firth 71 36 Six wooden mail steam-ships lost or stranded by deviation to the southward of their intended courses in northern hemisphere, CONTENTS. XV Sec. Page, aud how. Curious adventure of a merchant vessel having a cargo of machinery. The master's care and prudence. The owner's doubts about the master's propriety of conduct 74 37 Inductive magnetism of ships, of same kind as the magnetic hemisphere of ship's position. Southern hemisphere less en- cumbered with ships and shoals than northern hemisphere. Ships in southern seas generally deviate from their course in an opposite direction to that in the northern hemisphere. Loss of Thetis on Cape Frio 76 38 Old captains, in East India trade the result of their experience in crossing the trade winds in the Atlantic. H.M.S. Malabar's deviations in same direction as the Thetis. Azimuth on weather side of poop not the same as on lee side. Practical rules published by Admiralty not infallible. Deviation tables shewing changes in position of resultant points in different parts of the world. Kules for reconstructing deviation tables at sea noticed. Mariners should depend upon actual observa- tions, rather than on empirical rules. Variation and deviation always involved in amplitude and azimuth observations 78 30 Effects on ship's reckoning produced by a cargo of wrought iron tanks. Wreck of H.M.S Apollo and part of her convoy. H.M.S. Royal William, 120 guns, in company with two wooden steamers and a brig how these ships' compasses differed, and the con- sequences thereof. A steam-ship going astray by her compass deviation, and towing a Dockyard lighter astern. Good chro- nometers, charts, instruments, and logarithmic tables for facilitating calculations, have contiibuted to make seamen careless in keeping their dead reckoning. Longitude by chro- nometer liable to be erroneous when latitude by dead reckoning incorrect an instance given 84 40 Makers of mariners' compasses should understand the rudi- mentary principles of magnetism. The compass should be tested and taken care of. Essential properties of a good compass. How a seaman may test the magnetism of his compass without much scientific knowledge. Some remarks relative to the preservation or deterioration of the magnetic powers of compass-needles on shipboard 89 41 Compass oscillations detrimental to good steerage. Good helms- men can appreciate the value of a good compass. By adding weights the compass becomes slow of motion, and helmsmen must then be guided as much by the waves and clouds ahead, as by his compass in the binnacle 91 42 Copper and brass-boxed compasses have almost entirely super- seded the wooden-bowled compasses, and why. Screening influence of metal in proximity to needle. Government atten- tion drawn to compasses for H M. ships. Compass supervision XVI CONTENTS. Sec. Page, necessary and advantageous for the public service. Care should be taken of steering machinery. Kemarks on old system of making compasses for sale and for securing a price for repair and " retouching" them 92 43. Seafaring men not so well informed about magnetism as they should be. How Mr. Norman discovered the dip of the com- pass-needle and by what means he preserved horizontality in the card. A generally received opinion shewn to be erroneous when applied to the mariner's compass. Demonstrations given, that the centre of gravity of a freely suspended magnetic needle is not in the same vertical line with its pivot of support or point of suspension, unless it be at the magnetic equator of the world, where there is no dip of the needle. Compass oscilla- tions arise from the compass-needle being mounted in a position of permanent instability. Force of a body in motion considered 96 44. Opinions relative to the force of moving bodies discussed. Effects produced proportional to causes. Quantity of matter and velocity of motion combined. Force of moving body, simply as the quantity of matter into the velocity of motion. Eeasoning applied to a compass-needle when mounted on a vertical point 100 45. Ordinary construction of mariners' compasses very imperfect and inconvenient in high latitudes why so. Various plans tried to cure compass instability. Experiments tried by order of the Admiralty with compasses of different kinds in a steamer in the North Sea their comparative merits stated by Captain Johnson, R.N., superintendent of the compass department. Remarks thereon 102 46. Great magnetic power and small weight of a compass-card, of ordinary construction, incompatible with efficiency in a steering instrument at sea. Some sized needles better than any other, for ordinary use. Observations relative to the fixing and adjustment of the compass-needle, card, cap, &c., before the needle is magnetised 105 47. Great importance of a correct rectification of a compass-needle to its card and cap. Mode of mounting compass-box and bowls. Double suspension, by means of a cone carrying a vertical pivot to receive card and cap such cone being jewelled, and traversing upon the ordinary vertical pivot in the compass-bowl 108 48. Conditions of foregoing arrangement examined. Brass cone not magnetic acts as additional gimbals, promotes free action, acts as friction roller to card, &c. When compass dis- turbed, the double suspension reduces vibrations and retards oscillations. Admiralty Compass Committee for improving CONTENTS. XV11 compass. Some notice of improved standard compasses by committee. Their great power and great cost. Utility of these instruments in smooth water or on shore. Their inutility in stormy weather at sea. Compass Committee had not inves- tigated the principles of their compasses 109 49. The Author ordered to fit two compasses on his plan for trial at the Admiralty, in 1843. He takes one of the Committee cards with 8 needles, and he magnetises it for adjustment employ- ing Mr. Dent, the chronometer maker. Comparative results obtained. Experiments on steering compasses their power augmented test. Results involved with time 112 50. Friction question discussed. A powerful binnacle compass in an iron ship unsteady, by reason of ship's magnetism passing from side to side, when course near magnetic meridian. This difficulty obviated by abstracting half the magnetism from the needle. Needle less sensitive to local magnetic impressions, as the ship rolls from side to side 114 51. Needles made by Author for comparison of magnetic power. Result as reported by officers of Devonport Dockyard. The superiority of Mr. Walker's needles, due to mode of tem- pering, and uniform hardness. Comparative magnetic power of a single compass-needle, compared with the magnetic power of eight or twelve needles of Strasburg steel, mounted on single compass-cards, when the weights to be carried in either case are involved. Results obtained by experiment, on board the Recruit, at sea, and reported accordingly. Com- pass steadiness and efficiency not dependent on additional friction 116 52. Further consideration of induced magnetism. Preceding dia- grams referred to for elucidation. How iron ships act so powerfully to derange their compasses. Quantity of iron em- ployed in the construction of a first-rate sailing man-of-war, without steam machinery 119 53. Compass deviations in ships. The resultant of the magnetic forces exerted on the compass by the various articles of iron on board. Small masses at short distances act as powerfully on a compass as large masses of iron at long distances sub- ject one of complexity and bewilderment to seamen. Further reasoning on these difficult subjects 121 54. A wooden model, with iron guns, made by the Author, in 1843, and results exhibited at Admiralty and Trinity House. Author ordered to cany out an experiment in a three-deck ship. He is not altogether a free agent in carrying out this experiment. Details of experiment made in the St. Vincent, 120 guns 123 55. In what way the St. Vincent was swung her deviations ascer- tained by different compasses why these deviations were XV111 CONTENTS. Sec. Page, small. Weights required to heel St. Vincent 8 degrees. Com- passes on poop, binnacles, bread -room, and orlop deck. Magnetic disturbance by tanks in opposite direction to mag- netic action of guns upon a compass . 126 56. Magnetism of a ship's guns and tanks have a tendency to balance each other. Inequality of compass deviations, arising from the ship's inclination. Experiment not fully carried out. Compass disturbance in ships of the line proba- bly augmented by landing guns 128 57. H.M.S. Thunderer lands her guns, and proceeds to Mauritius, supplied with a powerful compass, with compound needles. A discussion on the magnetic condition of this ship, and her standard compass. Eeported working of the Thunderers standard compass. The Author called on to explain why the compass was unsteady. Compasses of the Asia, 84 guns. Caledonia's binnacle compass 132 58. An iron sloop-of-war in difficulties, by reason of compass errors, when inclined under sail. The Author obtains permission to make experiments on the Recruit, similar to those made in the St. Vincent. Eesults obtained, with remarks thereon. Danger of trusting to the permanency of the points of the compass upon which the errors are = 0. Recruit's points of no devia- tion different under four conditions at Plymouth. Birkenhead steam-ship. Official report of experiment to the Commander- in-Chief 135 59. Old seamen distrustful of matters relating to their compass. Plans proposed for its improvement. Hand-books for guidance of navigators, with tables. " Practical Eules " for ascertaining compass deviations. Remarks on these rules. Soudan, iron steamer. Magnets for correcting iron steamers' compasses not always of practical utility 141 60. If one compass be corrected by magnets, and another similar compass have a deviation table, then by assuming the one to be correct and by applying the deviation error to the other, the same result might be obtained in working the day's work, and great errors might arise in the reckoning. Peculiarity in iron screw steamers. Magnets convenient for counteracting great deviations. Courses should be shaped by a standard compass. Mutual action of compasses on each other when too close. Variation distinguished from deviation 143 61 Inconstancy of the magnetism in ships. Observations on com- pass correction by iron alone. Compasses sometimes made correct by accident. A case of the removal of ohjectional iron increases compass errors 33 per cent. Barlow's correct- ing plate. Theory entertained by him inadmissible J47 62 Terrestrial gravitation greatest at earth's surface. Induced mag- CONTENTS. XIX netic poles of an iron sphere greatest at its surface. How the induced poles of an iron disc may change positions. Action of iron sphere in H.M.S. Rea-uit. Local attraction of iron at magnetic equator. Iron transoms in ships how they may act on compass. Iron bolts or bars may be made to act as magnets by position. Such iron properly placed would per- form the office of correcting magnets, and vary their forces as the ship changed her geographical position, or turned round 151 63 Subject of compass correction further discussed. Pendulous apparatus inconvenient Fixed bars more efficient. Iron ships with iron beams. Vertical pillars in iron ships might be turned to account. How errors arising from the ship in- clining may be corrected. Better correct a compass by intention than leave the matter to accident. Deviation com- pass indicates positions of no error, or errors when they arise by the ship's inclination or change of latitude 44 Navigators are called upon to understand the nature of a ship's local attraction, and to correct their reckonings. Practical hints thrown out for their benefit. Influence of similar com- passes on each other. Binnacle compass should be amid- ships, and why. Lubber's point how it should be placed. Cabin funnels and stoves to be guarded against, if of iron. Binnacle compass unfit for taking bearings or azimuths. A standard compass how it should be placed, and for what purpose. Standard should be of the deviation kind, and why. Azimuths and amplitudes worked out at sea, give the variation and deviation combined. The result obtained. A correction for the course at the time of the observation. Although a ship's magnetism changes in passing from one magnetic hemisphere to another, some time is required for its full development. Compass errors arising from magnetic action of iron best cured by agency of metals of similar kinds. Prudence and precaution requisite to guard against accidents. How made available. What should be done when ships anchor in roadsteads. How the errors of the compass may be tabulated with a little care and attention, as the ship swings by wind or tides. Checks to compass errors obtained at sea by means of astronomical bearings. Sights for chro- nometer may be made available for several useful purposes in navigation. Compasses in wooden steamers. Compasses in iron sailing vessels deviation very great close aft. Iron steam ships how the machinery acts on compass, and how their steering gear may be most advantageously placed. Experience and common sense may determine where compass should be fixed 159 XX CONTENTS. APPENDIX. Page. A. Plate and specification of Author's Patent Mariner's Compass for indicating its errors and deviations 167 B. Letter from Mr. Airy to Editor of Nautical Magazine relative to the permanent magnetism of ships, and his mode of correcting it by magnets ; with his remarks on the Author's statements on the same subject. The Author's reply (through the same channel). The subject of compass correction by magnets further discussed. A quotation! Table of compass deviation of one of the iron steamers for the Niger expedition, in England and at Sierra Leone, are examples of Mr. Airy's correction. Other examples of ditto in royal mail steamers, in England and at the Cape of Good Hope. Scientific bodies rather too tenacious of pre-conceived opinions 173 C. Copy of an official report of the standard compass of H.M.S. Thunderer, on a voyage to the Mauritius and back to England, proving that compass-needles with great power, when mounted in the ordinary way, are very unsteady in stormy weather with a high sea 185 D. Official report of results obtained in extensive magnetic experi- ments made on board H.M.S. St. Vincent, 120 guns 188 E. Correspondence relating to the loss of the iron screw steamer Tribune 191 F. Eemarks and correspondence relative to Messrs. Dent and Preston, and their compass patents 192 G. Official reports on the Author's improved binnacle and boat's compasses 195 H. Interesting correspondence relating to artificial magnets and compass-needles 198 I. Eeplies to criticisms on Author's observations in Nautical Magazine, 1844 201 K. Authority of the Lords Commissioners of the Admiralty to make magnetic experiments with H.M. sloop Recruit an iron-built sailing vessel 206 ON THE MAGNETISM OF SHIPS, AND THE MAEINEK'S COMPASS. 1. IF our knowledge of the properties of the loadstone had still been confined to its power of at- tracting iron, we must have remained ignorant of the form and magnitude of the earth, of its proportions of land and water, and of the various races of men and other animals that inhabit it. The mariner must still have continued to row or sail slowly along the land during fine weather, with a fair wind and a clear sky ; and if he ventured beyond the ordinary limits of his knowledge, it would behove him to look out for a place where he might " beach his boat," or secure her in some sheltered creek, before darkness or foggy weather should overtake him. But the magnetic pro- perties of the loadstone were ordained for a useful purpose ; for although the discoveries of these pro- perties were made but slowly, and even yet remain covered, as it were, by a semi-transparent veil, their practical utility has been very great. The introduction of the mariner's compass, even in its primitive and rude state, brought about a complete B 2 THE MAGNETISM OF SHIPS, revolution in the theory and practice of naval archi- tecture and seamanship. It extended our geographical knowledge, and opened a social and commercial in- tercourse between different nations of the earth. By it countries previously unknown to Europeans were discovered and colonized ; and the ends of the earth were actually joined together by circumnavigation. In February, 1817, the author was appointed to command H.M. Store Ship Despatch, to be employed in the conveyance of naval timber from a forest on the south-east coast of Africa, to the Dock-yard at Simon's Town, Cape of Good Hope. In this vessel he made fourteen trips round Cape Aguilhas,* a name, by-the- by, given to this Cape by the Portuguese, on account of the magnetic derangement of their compass needles in its locality when Vasco de Gama sailed round it. The Despatch had about 40 tons of cast iron ballast, chain cables, two guns, and wrought-iron diagonal riders in her hold. In coasting along in an east and west direction, the vessel was constantly getting to the northward of her reckoning, or farther to the northward than she should get. On applying a pocket compass to the spindle of the capstan or other article of iron, it was evident the local magnetism of the vessel had changed since she left our northern latitude. On returning to England, the ship was again found to deviate from her reckoning in a southerly direction, and not in a northerly, as was found to be the case at the Cape of Good Hope. In the meantime iron tanks were introduced, chain cables and iron knees, and a Mr. Bill took out a patent * " Aguilhas " in Portuguese signifies Needles. AND THE MARINER'S COMPASS. 3 for iron bowsprits and lower masts for men-of-war. The Lords Commissioners of the Admiralty ordered H.M.S. Phceton to be supplied with an iron bowsprit and a mainmast on Mr. Bill's plan. I wrote a letter to Admiral Sir Byam Martin, then Comptroller of the Navy, stating the result of my experiments on the mag- netism of ships ; and I gave an opinion that the heels of iron lower -masts and bowsprits would repel the north point of the compass needle, and I recommended that the effect should be observed in the Phceton. I received the following reply to my communication : "Buckland, Ashburton, October 19th, 1822. " SIR, I am much obliged to you for the judicious observa- tions contained in your letter of the 4th instant, in reference to the effect of iron masts on the compass; and you may be assured that a matter of so much moment will be carefully watched. " I am, Sir, " Your very humble Servant, " Mr. W. Walker." T. B. MARTIN." It appears, by an able article in the first volume of Papers on Naval Architecture, p. 100,^ that Admiral Martin requested Professor Barlow to report upon the effect that iron masts would produce on the Phceton's compasses ; that on swinging the ship her greatest de- viation did not exceed 1 40' an error unusually small in a frigate of her class; that the heel of the iron mainmast repelled the north point of the compass * Observations on the effect produced by Iron Masts, &c. on the Com- pass Needle ; with an Account of an Experiment made to ascertain the Local Attraction of H.M.S. Phceton (Capt. Start), fitted with an Iron Main- mast and Bowsprit. By Mr. J. Bennett, Naval Architect. B2 THE MAGNETISM OF SHIPS, needle nearly as much as the whole of the guns, tanks, iron ballast, and shot of the vessel attracted the north point; and that when the iron mast was ultimately taken out and laid upon the ground in Portsmouth Dock-yard, its head and heel retained a portion of the magnetic polarity it had acquired in its vertical position when "stepped" in the ship. The statements I had made to Sir T. B. Martin, in my letter of the 4th of October, 1822, had consequently been verified. Practical magnetism has already done much for the increase, as well as comfort, of mankind. It has stimulated scientific research, and done more for the advancement of the physical sciences than any other branch of human knowledge. The mariner's compass is held in veneration by a thorough sailor. In a dark and cloudy night, or during a thick fog, he steals softly aft under pretence of putting something to rights ; but his real object is to take a glance at the compass, to see how the ship's " head lies." Seamen know but little of the doctrines of magnetism, but they know full well that the compass is their only guide at sea, and that by it the ship's course is shaped. If a sailor discover an iron nail, or a marline spike, left by some "lubber" near the binnacle, he slily consigns it to "Davy Jones' locker,"* without any qualms of conscience, for he knows intui- tively that iron has no business there. To those seamen who are navigators, it is considered that a brief essay on the mariner's compass will not only be amusing, but really useful in their profession. It is my opinion, as an experienced seaman, that if * That is, he throws it overboard. AND THE MARINER S COMPASS. 5 more were known by navigators of practical magnetism, (as for example, how the stowage of a ship's cargo, or the arrangement of the iron within a ship, might affect her compass, )fetver ships would be lost; for all those ships that actually run on shore with a fair wind, when steering a compass course intended to lead them clear of all danger, are without doubt wrecked through want of skill in the navigator. It is, therefore, my intention to present the reader with, a condensed account of the mariner's compass, and of the very slow progress that practical magnetism has made ; and how this knowledge has been applied to purposes on land, as well as at sea. I shall give a short notice of the theoretical views that have been, from time to time, entertained by philosophers of magnetism ; and shall conclude by shewing the prac- tical application of what is really known of the prin- ciples of local attraction, and in what way a ship's reckoning is liable to be influenced by the local mag- netism of a ship and her contents. 2. Although the Greeks, Egyptians, Phoenicians, Carthagenians, and Romans, had ships fitted for coast navigation, and generally capable of entering shallow waters, or of being hauled on shore, they have left us no historical record of anything like a compass being used in their vessels. They knew so very little of the magnet and its properties, that their priests had not attempted to impose the mysteries of magnetism on the credulity of the people. All they knew of the magnet was, that iron was attracted by it. From the days of Homer to the time of the Crusades, in the 12th century, there are good grounds for believing THE MAGNETISM OF SHIPS, that the magnet was not in any way applied to pur- poses of navigation in Europe. The Chinese are, without doubt, a very ancient people ; and although I am not disposed to believe all that has been translated to us from their history, yet a good deal of the Chinese history has been con- nected with the history of the Heavens ; and, therefore, verified to a certain extent. The Reverend Pere Gaubil examined the records of thirty-six eclipses of the sun in the Chinese history, and found only two doubtful and two false. The author of the Histoire Universelle, in speaking of China, says, "La Boussole, ainsi que la Poudre a tirer etoit pour eux une simple curiosite." And in another place, " La Boussole, qu'ils connoissoient, ne servoit pas a son veritable usage de guider la route des Vaisseaux ; ils ne navi- goient que pres des cotes." We are informed by Du Halde, who was a missionary in China, and who wrote a history of China from data he procured from Chinese books, that about the year 2634 B.C. the Emperor Hoang-ti, being at war, an instrument was invented, which, being placed in a car, pointed to the south, and enabled the imperial army to direct its march, and surprise the enemy during a thick fog. If this statement be correct, it affords evidence of the Chinese making use of the directive power of the magnet 4487 years ago. The same author informs us, that 2883 years ago an embassy reached China from Cochin; that the ambassadors had experienced great difficulty in finding their way to the imperial court; but on taking their final audience, Du Halde says, " Tcheou-kong gave them an instrument, of which AND THE MAKINEH's COMPASS. 7 one end pointed to the north and the other to the south, that they may find their way home with less embarrassment than they had experienced in their route to his dominions. The instrument was then called Tchi-nan, and this is the name which the Chinese now give to the mariner's compass." 3. In a Chinese Dictionary, compiled about the end of the fourth century, there is the following passage : " They had then ships which directed their course to the south by the magnetized needle."* " The fortune-tellers rub the point of a needle with the stone of love, for rendering it proper to indicate the south." These extracts shew that the Chinese made use of magnetism for land and sea voyages, at a very early period of their history. The old Venetian traveller, Marco Polo, whilst in the service of Koublia Khan, obtained the command of a Chinese fleet of fourteen ships, each ship having four masts and nine sails. This fleet was prepared to convey a Chinese Princess to the Persian Gulf; it sailed from the river of Pekin early in the year 1291, and was eighteen months in making a passage to the Persian Gulf. Some of these junks had crews of 250 men. This expedition is mentioned, in order to show that, in former times, the Chinese undertook longer sea voyages than they now undertake. It is extremely probable that the compass was in common use in the fleet referred to, although no mention is made of it in Marco Polo's Narrative. * The sea coast of China generally runs in a north and south direction, and the monsoons prevail on the coast. The south end of a Chinese compass needle is generally coloured red. 8 THE MAGNETISM OF SHIPS, The compass of the modern Chinese is probably nearly similar to those used two thousand years ago. One in the writer's possession may be thus described : A very small steel bar, about an inch in length and of the diameter of a sewing-needle, poised with great correctness, and strapped to the top of a small copper hemispherical cup, which serves as a socket to receive the point of a vertical steel pivot, (the point of a small needle,) rising from the centre of a circular hole in the wooden compass-box. The bottom of this circular hole is covered with a thin film of silver or zinc, upon which a meridian line is drawn as a diameter. There is a round hole in the centre of the metallic circle, large enough to allow the needle to traverse freely above it, but small enough to prevent the lower part of the copper hemispherical cup from rising above the pivot of support, so as to endanger the " unshipping of the needle." The circular hole and needle are covered by glass, held in its place by a circular wooden ring. The south end of the needle is coloured red. The compass-box is of boxwood, four inches in diameter and three-quarters of an inch in depth ; on its upper surface are drawn seven concentric circles, that nearest to the needle is divided into eight equal parts ; circles Nos. 2, 3, and 4 have each 24 divisions, No. 5 has 72 equal parts, circle No. 6 appears to have 48 divisions, and the outer one is divided into 72 equal parts ; the whole are inscribed with Chinese characters, in black or red ink. The machine is varnished and neatly finished. It is used in China by land and sea voyagers, by surveyors, architects, jugglers, &c. Now, a compass of this kind is by no means conve- AND THE MARINER'S COMPASS. 9 nient to steer by ; because, if such, a compass be placed in a binnacle, with its meridian line or one of its symbolical characters towards the ship's head, then all the other points or characters being painted on the box would necessarily remain in a constant position with reference to the ship's course ; in fact, the point or character might with equal propriety be drawn upon the ship's deck, as upon a box fixed in the binnacle. It would be impossible for a European helmsman to steer a ship by a Chinese compass. 4. The introduction of the mariner's compass into Europe was probably due to the Arabs, during the Holy Wars of the Crusaders on the eastern shores of the Mediterranean. The Italians, French, Norwegians, and even the English, have endeavoured to claim this most useful instrument as an invention of their own ; but it appears to me, that such claims cannot be sustained. The earliest mention made of the compass in Europe is to be found in some old poetry, written by a certain Guyot de Provins, about the end of the 12th century, and preserved in MS. in the Royal Library of France. Cardinal de Titri, a native of France, who had been engaged in the Crusades, and was appointed Bishop of Jerusalem, wrote an Oriental history, wherein he described the compass as being in familiar use among the Saracens, on the coast of Syria, although a novelty to himself.* There is, in the Royal Library of Paris, an Arabian MS., written in 1242, by Ba'ilac Kibdjaki, wherein the sea compass of this early period is described.! " We * British Annual, 1837. + Letter of M. Humbolt, translated by Klaproth. 10 THE MAGNETISM OF SHIPS, have to notice amongst the properties of the magnet, that the captains who navigate the Syrian seas, when the night is so dark as to conceal from view the stars which might direct their course, according to the position of the four cardinal points, take a basin full of water, which they shelter from the wind by placing it in the interior of the vessel ; they then drive a needle into a wooden peg, or a corn stalk, so as to form the shape of a cross, and throw it into the basin of water prepared for the purpose, on the surface of which it floats. They afterwards take a loadstone of sufficient size to fill the palm of the hand, or even smaller, bring it to the surface of the water, give to their hands a rotary motion towards the right, so that the needle turns on the water's surface ; they next suddenly and quickly withdraw their hands, when the two points of the needle face the north and south. They have given me ocular demonstration of this process during our sea voyage from Syria to Alexandria, in the year 640 (or A.D. 1242)." Here, then, we have a clear description of the primitive European compass, and how magnetism was communicated to the needle, stuck into a reed of straw, and made to float in a bowl of water. In those times, the Saracens had possession of the sea coasts ; but still the mariners of Syria and Egypt had to manage their navigation under the government of their Mahomedan conquerors. Their manner of com- municating magnetism to a needle made to float on water, " so as to form the shape of a cross," as des- cribed by the Arab in the above quotation, is worthy of notice. There is magic as well as superstition in it. AND THE MARINER'S COMPASS. 11 During a period of 200 years (from 1100 to 1300), the western world was convulsed with wars of no ordinary kind. From the farthest limits of the east, the Turks and Tartars had extended their conquests towards the west, overturned all the old governments and civil institutions ; and whilst the infidels were propagating their religious opinions by the sword, the Pope had established the Inquisition. Under such cir- cumstances, need we be surprised, that arts declined, and science slumbered, and that we hear little or nothing of a machine which, however rude or mys- terious, was in use for directing the course of ships. In consequence of the vast number of Crusaders that precipitated themselves on Palestine, all the ma- ritime ports of any note engaged their vessels, either as transports for the pilgrims and the troops ; or else their ships were employed as traders, to supply the armies with provisions and stores. The Venetians, Genoese, and the people of Amalphi, rose in wealth and power, by their profitable employment during the war between their Christian brethren and the Mahome- dans of Western Asia. A bitter hatred, heightened by religious fanaticism, was kept up between them ; and, generally speaking, it would have been unsafe for a Christian sailor to adopt and openly use a Saracen compass. 5. The Holy Wars or Crusades terminated about the year 1291, leaving the mercantile navies of the Mediterranean to follow their commercial occupation. About the year 1302, one Flavius Giogo, a native of Amalphi, is said to have invented the mariner's com- pass. " Seven miles to the west of Salerno, and thirty 12 THE MAGNETISM OF SHIPS, to the south of Naples, the obscure town of Amalphi displayed the power and rewards of industry. The land, however fertile, was of narrow extent ; but the sea was accessible and open. The inhabitants first assumed the office of supplying the Western world with the manufactures and productions of the East, and this useful traffic was the source of their opulence and freedom. The government was popular under the administration of a Duke, and the supremacy of the Greek Emperor. Fifty thousand citizens were numbered within the walls of Amalphi ; nor was any city more abundantly provided with gold, silver, and the objects of precious luxury. The mariners who swarmed in her port excelled in the theory and practice of navigation and astronomy, and the discovery of the compass, which has opened the globe, is due to their ingenuity or good fortune. Their trade was extended to the coasts, or at least to the commodities of Africa, Arabia, and India."* Here we have a free, rich, and enterprising mercantile and maritime people (and without an Inquisition), who, if they did not actually invent, were likely to greatly improve the compass, upon which the prosperity of their little territory so mainly depended. We have seen that the ancient compass of the Chinese, and that used by the Saracens, was altogether unfit for general purposes at sea. Any intelligent, shrewd captain, unshackled by authority, and not having the terrors of "the Holy Office" before his eyes, would soon hit upon a method to improve the compass. The man of Amalphi no doubt did improve the mariner's compass, * Decline and Fall of the Eoman Empire, vol. iv. p. 72. AND THE MARINER'S COMPASS. 13 by simply introducing a needle large enough to carry a card having the cardinal and other points painted on it. Such a compass would differ from the more ancient one in this all-important property, of indicating at once the direction of the ship's keel, and the bearing of all external objects. 6. The compass of China, as has already been seen (3), had its points painted on the box, which would turn along with the ship, the little magnetic needle being the only part about it that preserved its position, with reference to external objects in the heavens or on the ocean ; but a compass such as we now use, or such as I believe was introduced by the Amalphian captain, having a card of the cardinal and intermediate points, borne up and traversing on a pivot, but held in a permanent position by the directive force of the magnetic needle to which the card is attached, would form an instrument of superior practical utility. The ship, compass-box, pivot, and every part of the apparatus is moveable under the needle and its attached card, which card remains in a constant position with reference to the magnetic meridian. So great and yet so trifling an improvement would secure to Flavio Giogo the honour due to original genius. The prac- tical utility of such an instrument would force itself upon the public, and the successful application would soon secure its adoption by practical navigators, al- though many of the old superstitious coasters might continue to doubt its directive powers, and fear to speculate on magnetic doctrines, or even venture to use the new instrument, without free permission from the directors of their consciences. 14 THE MAGNETISM OF SHIPS, From this time ships began to be improved in form and magnitude; the oar was laid aside for the sail; vessels were no longer fit for being beached; they required to carry provisions and water for longer pas- sages ; instead of coasting along shore, they shaped a direct course ; sea charts had to be drawn ; navigation began to assume something like a scientific appearance ; and people became better acquainted with each other, and ascertained what they might advantageously ex- change. A competition in maritime adventure arose in Europe, greatly to the advancement of geographical and hydrographical knowledge. We have already noticed, that from the time of Homer to the end of the 13th century, discoveries had been few and far between ; but when the magnetic needle had been so far improved and fitted to a compass that could be safely used at sea, we find Europeans making most rapid advances in all the sciences. Adventurers, in- stead of confining themselves to the shores of the then known world, advanced without fear into unexplored seas, in search of new countries. In 1378, the Venetians discovered Greenland. The Normans discovered the Canary Islands in 1405. The Portuguese discovered the Madeiras in 1420, and they sailed to the Coast of Guinea in 1482. In the year 1489, the brother of Columbus brought maps and sea- charts to England; and in the year 1492 Columbus himself discovered America; and five years afterwards, Vasco de Gama, a Portuguese, sailed round the Cape of Good Hope, and entered the "Indian Ocean. Here the Portuguese found a great number of ships, a well regulated trade on the coast of Arabia, Africa, and AND THE MARINERS COMPASS. 15 India, as well as within the Red Sea and Persian Gulf. 7. When Vasco de Gama reached Melinda, he applied to the King for a pilot to conduct him to Calicut, on the coast of Malabar. He obtained as a pilot a native of Guzerat, and the Portuguese shewed this man an astrolabe, but he paid small attention to it. They were greatly surprised to find this pilot well skilled in the use of the compass, the quad- rant, and geographical charts ; but the compasses in use in the Indian seas were found to be inferior to those in the Portuguese fleet. Hence we may infer, that the Indians, sailing with their periodical winds, had little need of great nicety in the construction of their compass. They were content with a very imper- fect instrument, as the Chinese continue to be up to the present time ; the arts and sciences were probably on the decline ; whereas Providence had decreed, that the barbarians of Europe should emerge out of igno- rance, explore the world and its wonders, shew its connections with the solar system, explain the phe- nomena of nature, and prove that the whole is the work of a bountiful Creator. During a period of 180 years that the mariner's compass had been in use among the Christians of the 14th and 15th centuries, its character had been slowly but surely established, notwithstanding the intolerant and superstitious spirit of the times. Every thing likely to expand the faculties of the human mind, or appearing above the comprehension of the vulgar, was represented as profane or abominable, and dealt with accordingly. Men of superior abilities in their pursuits, 16 THE MAGNETISM OF SHIPS, instead of being patronized, were either actually per- secuted, or else met with no encouragemet in advancing the progress of useful knowledge. It was dangerous for men to meddle with doctrines or opinions of any kind, not sanctioned, received, or approved by the clergy ; and this intolerant spirit extended to a much later period than I refer to ; as witness the persecution of poor Galileo, who was thrown into the dungeons of the Inquisition at Rome, in the year 1633, for having ventured to assert that the earth was round, and turned daily on its own axis ! It was under such unfavourable circumstances as these that maritime discovery, and the art of navigation and seamanship advanced, guided by the compass and a few maps or diagrams of erroneous construction. Experience had taught seamen, that the compass was a faithful guide, that its needle pointed towards the pole star, and that the card which the needle preserved or held in an apparently permanent position, pointed out to them the course they ought to steer in returning from their eommercial, exploratory, or predatory ex- peditions. 8. Christopher Columbus sailed from Spain,* in search of new regions, or in search of a new track to an old continent. Whilst sailing westward with the trade wind, on the 14th September, 1492, he discovered that the north point of the compass-needle no longer pointed towards the polar star. A deviation of this kind would take place but slowly, as the ships changed their geographical positions. The oscillations of the * See his history, and the difficulties he had to surmount before he obtained the means of undertaking his voyage. AND THE MARINER'S COMPASS. 17 compass card on its pivot, whilst running down the " trade," would in a great measure tend to disguise the variation; the depression of the pole star in a more southerly latitude, cloudy weather, and other circum- stances, might combine to prevent even a Columbus from observing the variation of the compass, until its amount would banish all doubt about it. This discovery so alarmed the ship's company that they mutinied, asserting and believing, that they would never be able to return to Spain, since the compass itself began to deceive them ! Columbus had the address to calm their fears, and command their services. But mark what followed. On his return to Spain, his statement, that the compass had varied in its direction, was not believed. The opposition to his correct views, and the mortifica- tion and persecution this great man had experienced, must have taught him the propriety, or rather the expediency, of being silent about magnetic variation, especially as his compasses had, in all probability, resumed their usual direction on the ship's return to Spain. Although other navigators had observed and announced the variation of their compasses, the learned of those times would not admit the fact ; they rather chose to charge seamen with ignorance, and inaccuracy in their observations, than admit errors in the principles established by themselves. 9. Pedro de Medina, at Valladolid, in his Arte de Navigar, published in 1545, denies the variations of the compass ; but the concurring reports of commanders of ships in distant voyages obliged the landsmen, in their closets, to give up the point. Martin Cortez, in a treatise on navigation, printed at Seville before 1556, c 18 THE MAGNETISM OF SHIPS, treats it as a thing completely established."* So here we see that a period of at least sixty years had elapsed, from the time of Columbus' s observing and reporting the variation of the compass, "before the truth of its existence was admitted. About the year 1580, one Robert Norman, an Eng- lishman, and a maker of "Compasses for Mariners," found, that however nicely he balanced his needles before he magnetised them, he was always obliged to counterbalance that end which pointed to the north, by a bit of wax, or other substance, in order to keep the card in a horizontal position. Mr. Norman suspended a steel needle on its centre of gravity, and having touched it with a magnet, it dipped or pointed down- wards, in the plane of the magnetic meridian, but about 72 degrees below the horizontal plane. This property is called the " magnetic dip." Mr. Norman published his discovery; experiments were made in various parts of the world, and it was ascertained that the magnetic needle remains nearly horizontal within the tropics, but that it dips towards the poles in both hemispheres. This property of the magnetic dip, began to shake the confidence of seamen in the sta- bility of their compass : it furnished data for philoso- phical speculation. The magnetic dip did not appear to derange the horizontal direction of the magnetic needle, nor to influence the variation of the compass, and consequently a ship's dead reckoning. It will, however, be seen in the sequel, that the magnetic dip is a very important element in the theory as well as practice of navigation. * Encyclopaedia Britannica; article, Vaiiation. AND THE MARINER'S COMPASS. 19 10. Observations began to be carefully made and recorded of the variations and dip of the needle. In the year 1780, the variation at London was llj east, and in 1622 only 6j, and in twelve years later it had decreased two degrees. These facts were made known to mariners by the publication of " A Discourse Mathe- matical, on the Variation of the Magnetic Needle; by Mr. Henry Gillebrand Gresham, Professor of Astro- nomy." These announcements threw mariners into new alarms and perplexities ; for in those days there were no published tables of amplitudes, or modes by which seamen might compute the sun's azimuth, and thereby find the variation of the compass at sea. Since the year 1580 up to the present time, the varia- tion has been observed to change about 36 degrees towards the west; that is to say, it has changed its direction one-tenth part of a complete circle in Great Britain.* Azimuth compasses were invented for finding the variation, and tables were computed and published for facilitating calculations at sea. Variation charts were drawn, and sea charts had the variation of the compass inserted on them. But navigators had frequent occa- sion to observe, that their observations of the variation made at sea did not agree with previously recorded observations made by others in the same localities ; nor even did their own observations agree among themselves. The celebrated William Dampier, whose voyages and adventures gave an impulse to maritime enterprise, observed (Dampier noted everything worth * It appears, from observations made and recorded in England, that the variation of the compass is decreasing, and also the dip, since 1819. c 2 20 THE MAGNETISM OF SHIPS, notice,) discrepancies of this kind in his observations for the variation, making it either more or less than he knew it should be ; and he says, " These things, I confess, did puzzle me."* This was about the year 1680. About one hundred years later, and during the voyages of Capt. Cook, the same kind of magnetic disturbances were apparent in their observations. Mr. Wales states, that "variations observed with the ship's head in different positions, and even in different parts of her, will materially differ from one another, and much more will observations observed on board dif- ferent ships." 11. The observations made during Cook's voyages would necessarily command attention, and excite ob- servation. In the year 1790, Mr. Downie, a master in the royal navy, when serving in H.M.S. Glory, has remarked, " I am convinced that the quantity and vicinity of iron, in most ships, has an effect in attracting the needle; for it is found by experience, that the needle will not always point in the same direction, when placed in different parts of a ship : also it is very easily found, that two ships, steering the same course by their respective compasses, will not go exactly pa- rallel to each other; yet, when their compasses are on board the same ship, they will agree exactly." When- ever large fleets were assembled to sail under convoy of ships of war, it was usual for the Commodore to intimate by signal the course to be steered by the fleet during the night, and it was usual to find these fleets much dispersed the following morning; the compass courses of the ships composing the fleet having differed * See Nautical Magazine, 1837, p. 247. AND THE MARINER'S COMPASS. 21 considerably among themselves. It was no longer doubted that the iron within a ship exerted an influence upon the compass, but it was not known in what way this influence was exerted. It was then supposed, and it is still believed by many, that iron attracts the com- pass ; that is to say, the north end of the compass needle is attracted by the iron, and hence the term local attraction, applied to the kind of magnetic dis- turbance under consideration. Captain Flinders, R.N., had been employed in sur- veying Australia, and of course had ample opportunity of noticing and noting anomalous observations in mag- netic bearings, and in observations made on board, for the variation of the compass, in the southern as well as in the nothern hemisphere. On his return to England, his observations were communicated to the Admiralty, and their lordships were pleased to direct a series of experiments to be made on the compass, on board one of her Majesty's ships, at Sheerness. The result of these experiments may be briefly stated. 1st. That the compass-bearing of a distant object was different in different parts of the ship. 2nd. That the binnacle compass gave true bearings of a distant object, when the ship's head was north or south. 3rd. That the greatest error in the bearing by com- pass was when the ship's head was east or west. Flinders concluded and correctly that the local attraction in the same ship would be different in different parts of the world, and that it would change with the magnetic dip. Capt. Flinders died in 1814. A paper of his, which 22 THE MAGNETISM OF SHIPS, appeared in the Philosophical Transactions of the Royal Society, upon " the differences in the magnetic needle on board H.M.S. Investigator, arising from an altera- tion in the direction of the ship's head," may inform us of the author's views. 1st. He supposed an attractive power, with different bodies in a ship, capable of affecting the compass, to be collected into something like a centre of gravity, or focal point, and that this point is nearly in the centre of the ship where the iron, shot, &c. are deposited. 2nd. He supposed this point to be endowed with the same kind of attraction as the pole of the hemi- sphere where the ship might be. Consequently, in New Holland, the south end of the needle would be attracted by it, and the north end repelled. 3rd. That the attractive power of this point is suf- ficiently strong, in a ship of war, to interfere with the action of the magnetic poles of a compass placed in the binnacle. 12. Captain Scoresby, who had commanded several ships in the northern whale fishery, and being an intelligent and well-informed man, directed his at- tention to the mariner's compass. His employment in high northern latitudes, where the magnetic dip and magnetic intensity are very great, furnished him with opportunities of making useful observations on magnetism. In his paper " On the anomaly in the variation of the needle," in the Philosophical Trans- actions for 1819,* we have the results of his obser- vations ; viz. * See Servingtou Savery's Paper on Magnetism, in the London Philosophical Transactions for 1730. 23 1st. That all iron on board a ship has a tendency to become magnetical ; the upper ends of the opposite bars being south, and the lower north poles, in the northern hemisphere, and vice versd. 2nd. The combined influence of all the iron is con- centrated in a focus ; the principal south pole of which, being upwards in the northern hemisphere, is situated in general near the middle of the upper deck. 3rd. This focus of attraction, which appears to be a south pole in north dip, attracts the north point of the compass, and produces the deviation in the needle. 4th. This deviation varies with the dip of the needle, the position of the compass, and the direction of the ship's head. It increases and diminishes with the dip, and vanishes at the magnetic equator. It is a maxi- mum when the ship's head is west or east, and it is proportional to the sines of the angles between the direction of the ship's head and the magnetic meridian. 5th. A compass placed on either side of the ship's deck, directly opposite to the focus, gives a correct indication on an east and west course, but is subject to the greatest deviation when the ship's head is north or south. Captains Flinders and Scoresby were both practical and theoretical seamen and navigators, and were en- dowed with a considerable amount of philosophical and mathematical skill. They made careful observa- tions on the action of the ship's iron on their com- passes, and communicated the result of their valuable observations to the public. Had they been less prac- tical, we might never have heard of their opinions of local magnetism ; or, had they been more mathematical 24 THE MAGNETISM OF SHIPS, and theoretical, we might have been favoured with an hypothetical treatise on magnetism, founded on an imaginary base, and supported by mathematical for- mula contrived for the purpose. Investigations of this kind, although of the utmost importance in search- ing for those "laws of nature" that govern our planet, are generally beyond the comprehension of seamen, and tend rather to bewilder than to enlighten their minds. 13. In the meantime, our ships continued to receive additional quantities of iron in their construction and equipment. Iron knees were substituted for wood, iron tanks for wooden casks, iron ballast for shingle, iron bolts for wooden tree-nails, iron cables for rope cables, iron rigging for hempen, and vessels began to be built entirely of iron. The consequence of all this was to render ships more difficult to be navigated, by reason of the local magnetism of the iron they contained. Attention was aroused to the subject, and Professor Barlow took it up, (1) and received the countenance and support of Government in his investigations. Mr. Barlow made and recorded a great number of valuable experiments on the compass ; he advanced a theory of magnetism, which was received with favour; and he proposed a plan for correcting the deviation of the compass, by means of an iron disc, placed near the binnacle, so as to counteract the effect of the greater masses of iron lying forward in the ship, and below the horizontal plane of the compass. This plan, if it did not entirely correct the local attraction in these latitudes, greatly lessened the errors that arose in the reckoning. AND THE MAKINER'S COMPASS. 25 The failure of Mr. Barlow, in his endeavours to correct the compass, arose from his theoretical views of magnetism not being in accordance with experimental facts. He supposed, with Flinders, Scoresby, and others, that there was a central magnetic focus in a vessel, which acted on the compass. He supposed that the magnetism of an article of iron depended upon the position of its centre, and not upon the po- sition of its extremities, with reference to its action on the steering compass ; and he did not believe, " that ever any particular action had been discovered between two pieces of iron." He was not aware of the fact, that any two pieces of iron will act upon each other as magnets, as well as upon an artificial magnet.* After several trials of the correcting plates, in both hemi- speres, they began to be disused, and are now almost entirely laid aside. These magnetical discussions and experimental trials were not followed by that public advantage to Navigation that might have been derived from them. Seamen again relapsed into indifference about their compasses. They began to think that iron, being so largely employed about ships, was really not so dangerous as their forefathers had taught them to regard it. The results have been as might have been expected. Our ships became more difficult to be navi- gated, and the masters less prudent and skilful in keeping their dead reckonings. The number of ship- wrecks have consequently been greatly increased : many sailing vessels shaping a compass course, and running on shore with a fair wind; whilst steam- vessels * See Professor Barlow's valuable work on Megnetic Attraction, second edition, 1824. 26 THE MAGNETISM OF SHIPS, in great numbers have, from errors in their compasses, run on rocks at the full speed and power of their engines, and have been of course destroyed, and many of their people drowned. 14. We have now given a short history of the mariner's compass, and the reader will have noticed, that its improvement and the discoveries of its pro- perties have been made but slowly. Seamen have seldom been allowed to meddle with it, or pass an opinion upon its merit. The importance of the com- pass appears to have transferred it to the care of philo- sophers or ship-chandlers, and many a compass has been made " to sell," and not to steer by. There are a vast number of patent compasses, differing in price and in degree of utility, now in use ; but seamen should bear in mind, that the compass needle, when saturated with magnetism, must necessarily point in the direction of the magnetic meridian, unless it be acted on by some external magnetic force w r ithin the vessel. The compass is influenced by three conside- rations arising from a single cause ; viz., its variation, its dip, its local attraction and repulsion by the ship and her contents. As this would be an inconvenient place to discuss the merits of the patent compasses re- ferred to, the subject will be adverted to in the sequel. 15. Before we treat of the practical application of the known principles of magnetism, and magnetic at- traction and repulsion, it is proper that a short notice should be given of the magnet itself. The loadstone is an ore of iron, and contains as much as 80 or 90 per cent, of the pure metal. It is extensively dissemi- nated over the globe, but is generally found in large AND THE MARINER S COMPASS. 27 masses in those rocks which geologists denominate as primitive. The property of the loadstone for at- tracting iron was well known to the ancients, and in several countries this property procured it the appel- lation of "leading stone, touch stone, stone which attracts iron, the stone of love," &c., names which it still retains. In almost every country where the loadstone is known, it has received a name indicative of some inherent property in that mineral. We here add a list of nations, with the name of the magnet in the language of the country, and its signification.* NATIONS. NAME OF LOADSTONE. SIGNIFICATION. English . . Loadstone Stone that carries a load or weight. French .. Aimant The lover. Spanish . . Iman The stone that attracts iron. Portuguese Iman,Padre de Cevar Ditto. Italian Calamita (?) Greek . . The iron stone . . The stone that attracts iron. Dutch Geyl stein The sight stone. Danish . . Magiieit From Magnus , the shepherd. Swedish . . Segel stein . . Seeing stone, victorious stone, &c. Icelandic Leider stein.. .. The leading stone. Irish Tarrangart . . The drawer. Welch . . Tywysfaen . . The conductor. Hungarian . Mavnit-Ko . . The lore stone. Russian . . Polish Magneit Magnit Kiamen From the Greek shepherd Magnus. The loving stone. Dalmatia Gvozdetegh.. The drawer of nails. Finland Randan-wetarga The attractor of iron. Chinese. . Che Chy. Tohi-nan The stone that directs or conducts. Mandchow . Sele'i-edchen . . . . The master of iron. Japanese . j Thru-chy . . . . 1 Zi-syakf Conducting stone. Stone for rubbing the needle. Thibetan .. Rdho-r-hatlen . . The stone for the steel needle. Tankin in D'anamtcham . . The stone which shews the south. Siamese . . Milik The stone which attracts iron. Birman . . Than-lvik Kyouk .. Ditto. Malayan . . Batu-brani . . The stone of enterprise. Cingalese Kandhoksgaluk The stone which loves. Arabic Hadj arechcheiyatm The devil's or wizards's stone. Persian . . Makuathes . . The magnetic stone. Grecian . . Stone of Heracliu Attractor and repulser of iron, &c. Sanscrit. . Thoumbaka . . The kisser. * From the British Annual, 1837. 28 THE MAGNETISM OF SHIPS, This wonderful stone has, therefore, been eminently distinguished above every other kind of mineral, by names given to it by different nations, which at once convey to our minds a sense of some of its singular properties : thus, we find it called, the stone that carries a load, that loves, that attracts, that points out, that directs, that leads, that conducts, which shews the south, the nail drawer, the master of iron, the attractor and repulsor, the stone for the steel needle, the wizard' s or devil's stone, the stone that loves, the kisser, the stone of enterprise, &c. These names were probably given to the loadstone at very remote periods of antiquity ; that is to say, before the mariner's compass was invented, or before it was known that the loadstone possessed an almost unlimited power of transferring its own virtues to any number of steel bars, without being sensibly weakened in its magnetic intensity. If it had been known to what important uses magnetised steel bars could be applied, as to navigation, to mining, and other important purposes, how many more names might have been added, and every one of them con- veying a new application of its principles. Magnets are now employed in working the electric telegraph. They assist in sending messages from one town to another, with a velocity greater than that with which sound travels. Messages are now correctly sent with as much speed as a cannon ball travels through the air. It is by the agency of artificial electricity and magnetism that man has accomplished this modern wonder. Official despatches are now correctly sent, by telegraph, over-land through air, and even under rivers and the sea itself, with the rapidity of lightning ; and AND THE MARINER'S COMPASS. 29 we wonder what name an Arab would give to such an apparatus ! 16. It does not appear that any of the names, in the list we have collected, conveys any idea of the loadstone's having been applied to navigation; and yet it is to this wonderful mineral, and its transferable magnetic properties to steel, and the practical applica- tion of it to the steering and conducting of ships (when all other resources fail us), that we owe the greater part of our knowledge of the world we inhabit, the ocean we have explored, and the intercourse we keep up with the remotest habitable regions. It was believed that the loadstone fed upon iron ! This was by no means an unreasonable supposition, since natural magnets actually acquire additional magnetic intensity by being kept in contact with iron. It is on this principle that loadstones are armed with soft iron, in order to increase their power. It was seriously believed by the ancients, that if much iron were used in the construction of their ships, magnetic rocks on the sea shore might attract the vessels, and hold them firmly attached. Who has not read the wonderful adventures of Sindab the Sailor, as detailed in the tales of the " Arabian Nights ? " How would Sindab' s historian have managed an iron steam vessel? The moderns, as well as the ancients, have ascribed wonderful physical, as well as moral effects to the magnet ; its properties have been applied by imposters in their systems of astronomy, astrology, divination, prediction of future events, divinity, law, and physic ; and even in this enlightened age, mes- merism is practised and patronised. 30 THE MAGNETISM OF SHIPS, The property of a magnet, in communicating a per- manent magnetism of its own kind to hardened steel, and the directive power of a freely suspended steel magnetic needle arranging itself in a north and south direction, induced a belief that some mysterious agency in the heavens held the compass-needle in the direction of the pole star. It was afterwards considered, that magnetic rocks might abound in the polar regions of the world, and draw the needle in that direction ; and some supposed, that the earth itself contained a great magnet in its central parts. The variation of the magnetic needle proved that these views could not be correct ; because, if the the north star itself had been a magnet, if the rocky regions towards the poles had been formed of loadstones, or, if the earth had held a great magnet in its central part, any of these agencies, if permanently fixed in the heavens, or in the earth, would not have induced a change in the direction of the compass-needle. It is more reasonable to suppose, that magnetism, electricity, and galvanism combine to form a myste- rious agency pervading the world ; for electricity has been known to invert or destroy the magnetism of a ship's compass, and by galvanism, needles may be magnetised. We know comparatively but little of the internal structure of the earth. The cuttings of the engineer, the punctures of the miner, or the scratches of those who dig or quarry its surface, have penetrated but a very small portion of the distance between the surface and centre of the earth. We are, however, certain, that the earth's mean density is greater than that of any rocks known to exist near its surface. THE MAKINER'S COMPASS. 31 Our geological researches enable us to assert, that the globe contains masses of metals, and metalliferous veins, abundantly disseminated among the stratified and crystallized rocks, which form an external crust. There is evidence to shew, that the central parts of the globe possess a higher temperature than its parts near the surface ; that subterranean fires exist in it ; and that the masses of matter composing our planet may be regarded as a galvanic arrangement, its solid parts being connected or covered by an ocean of brine. There are chemical formations, as well as decompo- sitions, constantly going on in it; and the electrical, magnetical, or galvanical currents we witness, may result from the physical structure of our earth. If we adopt this view, of the globe being a galvanic mass, many difficulties in our magnetical speculations may vanish : for example, the changes in the daily variation of the compass, and the great change that has taken place in this variation during the last 260 years, may have arisen from changes in the internal or external temperatures of the earth in its various parts, as in Greenland and elsewhere. 17. It is still believed by many, that iron and steel are the only substances susceptible of magnetism ; whereas every known substance is more or less susceptible of magnetic action. Mr. Barlow found, that the brass box of a very fine compass, with which he had been making experiments, had acquired a permanent magnetism. Sir W. S. Harris, in his paper on the transient magnetic state of which various substances are susceptible,* has given the following * Philosophical Transactions, 1831. 32 THE MAGNETISM OF SHIPS, table of the comparative magnetic inductive suscep- tibility of the following substances : METALS, &c. |ij Z7. n It A TJ^ |i Cast Zinc. 1 Cast Tin. 1 Cast Lead. | Solid Mercury. | = i ^ - Cast Antimonv. Cast Bismutli. 1 3 Marble. 7i 1 Water. I Comparative \ Magnetic [ M 20 16 10 6.9 3.7 2 1 1.8 1 >.:r> 0.87 i.av 0.27 Energy J Sir W. S. Harris found, that by condensing the metals their magnetic energy was increased, and that all substances receive or take up magnetism more rapidly than they part with it. The above conclusions 'were drawn from experiments made on metals subject to the action of artificial magnets vibrating within discs, or rings of the metals, included in the above table. Professor Whewell in his Bridgewater Treatise,* has remarked, "When we consider the vast service which magnetism is to man, by supplying him with the mariner's compass, many persons will require no other proof of this property being introduced into the frame of the world for a worthy purpose. . . Magnetism (he adds) has been discovered in modern times, to have so clear a connection with galvanism, that they may be regarded as different aspects of the same agents ; all the phenomena we can produce with magnetism, we can produce with galvanism. That galvanism exists in the earth, we need no proof. Electricity, which appears to differ from galvanism in the same manner in which a fluid in motion differs from a fluid at rest, appears to be galvanism in equilibrium ; and recently Mr. Fox f found * On the Power, Wisdom, and Goodness of God, p. 113. f Of Falmoutk, in Cornwall, AND THE MARINER'S COMPASS. 33 by experiment, that metalliferous veins, as they lie in the earth, exercise a galvanic influence on each other. Something of this kind might have been expected from masses of metal in contact, if they differ in temperature, or, in other circumstances, are known to produce galvanic currents ; hence we have undoubtedly streams of galvanic influence moving along the earth ; but whether or not such causes as these produce the directive power of the magnetic needle, we cannot here pretend to decide. They can hardly fail to affect it." The opinion here given is from high authority, and I cordially agree with it. The whole of the materials forming a ship are susceptible of magnetism by in- duction from the earth, sea, and atmosphere ; the mechanical construction is such, that the whole fabric of the ship may be in a transient magnetic state ; not only the iron, copper, lead, brass, and the other metals ; but also the wood forming the hull, fastened or covered as it really is with these metals and their oxides. Need we then be surprised, when we find the steering com- pass deviating from the true direction of the magnetic meridian, or vibrating several points on each side of the course, when a vessel rolls from side to side. 18. The following may be stated as magnetic axioms, or principles easily demonstrated to be true by experiment; viz. 1st. The loadstone has two permanent poles, either of which will attract iron or steel, not rendered magnetic. 2nd. The poles of the same name or kind, in different loadstones, repel each other; but poles of an opposite kind attract each other; thus, the north pole of one loadstone will attract the south pole of i) 34 THE MAGNETISM OF SHIPS, another ; but the north poles will repel each other, as well as the south poles. 3rd. The loadstone communicates a permanent mag- netism to steel, and a transient magnetism to soft cast or wrought iron. 4th. The poles of the loadstone communicate mag- netism, by touch, to steel, of an opposite kind to their own. Thus, if the north pole of a magnet touch one end of a steel bar, the end of a bar thus brought in contact will be a south pole, and the other end a north pole. 5th. Steel bars rendered magnetic by the loadstone become themselves magnets, and are capable of ren- dering other bars magnetic. 6th. The attraction and repulsion between magnets, whether natural or artificial, whether transient or permanent, are equal and mutual. 7th. If a magnet be cut into two or more parts, each part will be a perfect magnet, with a north and south pole. But the magnetic force of any magnet will not be so great as the combined magnetic forces of all its parts, after division. 8th. The magnetic attraction or repulsion exerted between two magnets, or between a compass-needle and any piece of iron, is not impeded, diverted, or lessened, by the interposition of any substance whatever (iron exeepted). If, for example, an iron gun were stowed in the bottom of a ship's hold, and a cargo of the most solid materials were stowed above it, the magnetic action of the gun upon the ship's compass in the binnacle would be precisely the same as if nothing had intervened between them. AND THE MARINER'S COMPASS. 35 9th. Hard steel retains magnetism longer than soft metal, and the harder it is made the better for retaining magnetism. 10th. If the north or south poles to two equal and similar magnets be kept in contact, their magnetism will ultimately be destroyed ; but if their opposite or contrary poles be kept in contact, their magnetism will be retained. llth. If a steel bar be delicately poised on its centre of gravity, and then touched by a magnet, the bar or needle will arrange itself in the direction of the magnetic meridian, and in the direction of the magnetic dip. Thus, in England it would point magnetically northward and downward, about 70 degrees from the horizontal plane. The following facts are of much importance to seamen, as they relate to that kind of magnetism which has been named "Inductive," that is, not strictly of a permanent nature, although exerting the same kind of influence upon the mariner's compass as a permanent magnetism would exert. 12th. The earth is magnetic, and gives direction to freely suspended magnetic needles : it has a north and a south pole, and since magnetic poles of opposite names attract each other, the north point of a compass- needle must be a south pole, because it is attracted by the north pole of the earth, and vice versa (2). 13th. If an iron sphere, or any regular or irregular solid of soft iron be imagined as cut by a plane, at right angles to the magnetic meridian, but in the same direction as the magnetic dip, and if this plane be again cut by another imaginary plane passing through the D2 36 THE MAGNETISM OF SHIPS, centre of gravity of the iron, and at right angles to the dip, this last plane will separate the solid into two magnetic hemispheres, where will be found a north and south polarity and a magnetic equator. If the solid be an iron sphere, it will represent a miniature world, with its magnetic poles ; and, if it be of considerable size, will control a small pocket compass when held near it. This magnetism is received from the earth, and will hereafter be more fully explained. 14th. If an iron bar, bolt, or plate, be suspended by a small thread, or by any other means, so that one end shall dip (in England) at an angle of about 60 or 70 degrees from a horizontal level, the iron, although not previously magnetised, will come to rest in the plane of the magnetic meridian; its lower and north ends pointing nearly in the direction of the magnetic dip. 15th. If a piece of soft wrought or cast iron be taken, and held nearly parallel to the piece above mentioned, and if the upper end of one piece be made to approach the lower end of the other piece, an attractive force will be developed; but if the upper ends or the lower ends of the two pieces be brought near together, a repulsion will take place. These pieces, as well as all others, are magnetic by induction from the earth, and will act upon each other, as well as upon a compass-needle, as magnets or magnetised steel bars. 16th. If we place a long bolt or bar of soft iron, in a perfectly horizontal position, and at right angles to the magnetic meridian, or in an east and west direction, and if a delicate compass be placed near the end of the bar, the compass - needle will not be disturbed. AND THE MARINER'S COMPASS. 37 But, if the further end be raised but one degree, the south point of the needle will be attracted. If the further end of the bar be lowered a little, the north point of the needle will be attracted and the south point repelled, in north magnetic latitude, and vice versa in the southern magnetic hemisphere. 17th. If a small delicate and sensitive magnetic needle be allowed to settle in the direction of the magnetic meridian, and if a long straight bolt or bar of soft iron be laid in a north and south direction, with one of its ends near to the centre of the needle, and either east or west from it, the south point of the compass-needle will be attracted by the north end of the iron (in England), and the north point by the south end of the iron. If the end of the iron which is furthest from the magnetic equator, and nearest to the magnetic pole, be raised till the compass-needle returns to the true direction of the magnetic meridian, the axis of the iron will then be at right angles to the direction of the magnetic dip, or in the plane of the magnetic equator; and, by the application of a gunner's quadrant, or any other machine for measuring angles, the angle made by the direction of the axis of the iron, and the vertical, will be found equal to the magnetic dip in all parts of the world ; and the angle made by the iron, from a horizontal level, will be found equal to the complement of the dip. 18th. If we take a freely suspended or delicately poised compass-needle, and allow it to come to rest in the direction of the magnetic meridian, either pole of the needle will attract and be attracted by any part of a small piece of soft iron, such as a small nail ; but if 38 THE MAGNETISM OF SHIPS, a larger piece of iron be used, say an iron bolt, the inductive terrestrial magnetism which the iron receives from the earth will control the compass-needle, and by its position will either attract or repel the needle. When the iron is small, the permanent magnetism of the needle controls it; but when the iron is large in quantity, its inductive magnetism will control the permanent magnetism of the compass-needle. 19th. The induced magnetism with which all articles of iron are saturated is received from the earth, and the polar axis or line supposed to join the two mag- netic poles of any article of iron, not permanently magnetic, is parallel to the direction of the magnetic dip. 20th. Since iron is magnetic by induction from the earth, and polarised by position with reference to the direction of the magnetic dip ; therefore any change in the direction of the ship's head, or any alteration in her angle of inclination, whether in its direction or its amount, will be accompanied by a change in the polarity of the iron contained in the vessel, and by a change in the amount or direction of the local attraction, and its influence on the steering compasses. 19. The magnetic dip is really an element of far more importance to Navigation than has been imagined. The dip changes with the latitude : it is actually a measure of the magnetic intensity of the needle, and an index to the inductive magnetic polarity of the iron within a ship, as well as everywhere else. The dip, then, not only ought to be known and re- corded on our charts, but ships should be furnished with means for finding the dip in long voyages. AND THE MARINER S COMPASS. 39 The following table of dips will be found useful. They are the result of recent observations : A TABLE of the Magnetic Dip at the undermentioned Places; extracted from the best modern authorities : Place or Position. Magnetic Dip of Needle. Place or Position. Magnetic Dip of Needle. Place or Position. Magnetic Dip of Needle. Plymouth 69.12 N Callao 6.14 S. Lat. . . 49. 6 N ) A7 1Q "\T London 69.20 Guayaquil.... 9. 8 N. Lou... 7. OW] O/.lo JN . Rotterdam . . 68.49 Rio Janeiro .. 12.54 S. Lat. . . 40.15 N [ Paris 67.20 St. Catherine 21.40 Lon... 15. OWJ 64.32 Petersburg!! 70. 5 Monte Video 34.51 Lat. . . 35. N ) (+i tj Terceira 68. 6 Valparaiso . . 38. 3 Lon... 15. OWJ Dl. 7 J? Rome 60.24 Conception . 43.15 Lat. . . 23.10 N ) Naples 58.53 Chiloe 48.59 Lon.. 20.45 Wj 53.26 Constantinople. . 56.34 Port Desire . . 5120 Lat. . . 12. 5 N 1 Alexandria . . 43.48 Falkland Is... 53.20 Lon... 26.20 WJ 42.45 Bermuda .... 67.31 R. Santa Cruz 55.16 Lat. .. 1.12N 1 Oft O*7 Montreal .... 76.19 St. Helena . . 18. 1 Lon... 28.44 WJ Z\).Zt jj Halifax 74.45 C. Good Hope 5254 Lat. . . 6.20 S ) New York .... 72.52 Seychelles . . 32. 5 " Lon... 32.40 W) 15.57 Washington . . 71.21 Penang 4.40 Lat. . . 10. 8 S I in *ft Antigua 48.46 Singapore . . 12. 1 Lon... 34.18 WJ iU.OU 55 Jamaica .... 47.19 Borneo 19.48 Lat. . . 13.50 S ) Chagre 32.30 Amboyna .... 21. 9 Lon... 35.1 9 W ]" 1.50 Panama 31.55 Raratongals. 36. 8 Lat. . . 16. S ) 1 O^ Q Bahia 5.24 Macassar Is. . 23.42 Lon... 36.18 WJ 1 . J-J O. Peruambuco . 13. 8 Tahiti 30.17 Lat. . . 24.35 S ) Fernando Po . 0.48 Raratongo .... 36. 8 ., Lon... 44. 5WJ" 17.55 Ascension . . 1.39 Bass' Straits . 69. 8 Lat. . . 27.26 S ) Cocos Islands 24.36 Swan River . . 62.24 Lon... 48.35 WJ 21. 7 Acapulco 39. 3 Sydney 62.49 Lat. . . 34.53 S j 1A Q San Bias .... 45.23 Hobarton 70.40 Lon... 56.13 W[ >x. O )t San Francisco 62.28 Auckland Is. . 73.10 Lat. . . 40.40 S ) Ft. Vancouvre 69.22 terguelens Is. 69.59 Lon... 55.20 Wj 43.15 55 San Diego . . 57. 6 Vlartin's Is. . . 14. 6 Lat. . . 46. S ) -/A i e Sitka 75.51 Bow Island . . 30.15 Lon. . . 60.10 W j oU.lD Manilla 16.27 Vlajombo Is. . 48.18 Lat. ..51.32S ) e^> OQ Hong Kong . . 30. 2 Simon's Bay . . 53. 4 Lon... 58. 7WJ J-t.OO 55 In high latitudes, seamen find that the upper and under sides of articles made of iron greatly affect their compasses ; it is because the dip is also great, and the earth's magnetism greater in high latitudes than near the magnetic equator. But upon the magnetic equator itself, the polarity of iron and its local attraction do not vanish ; the polarity of the iron only coincides with the earth's polarity, but the iron will still continue to 40 THE MAGNETISM OF SHIPS, act on the compass under a new form. If it were possible to sail round the world on a great circle passing over the magnetic poles, the dipping-needle would perform a complete revolution in a vertical plane, and the transient magnetic polarity of the iron in a ship would also perform a revolution along with the dipping-needle. 20. We have seen, that almost every substance experimented on has been found susceptible of a transient magnetic state by induction, and that the earth itself gives out magnetism of its own kind to solids separated from it. Without having recourse to delicate experiments by refined apparatus, the induced magnetism of iron is evident to the senses by the rudest machinery, or even without any apparatus at all. Pieces of wrought or cast iron act on each other, as magnets act on each other, exhibiting all the phenomena of attraction, repulsion, and magnetic conduction; and these metals may be made either to control a magnetic needle, or be controlled by it, in a variety of ways. This view of magnetism has not been entertained by philosophers, because they have never been in pos- session of a sufficient number of experimental facts. We shall, however, put our nautical readers in the way of satisfying themselves on these points, by means of materials used in their ordinary vocation. Iron, when long exposed to the atmosphere, or action of water, gets covered with rust; the outer surface being converted into an oxide of iron, the magnetic properties of the metal undergo a change. When spindles of capstans, weather-cocks of build- ings, &c., remain long in a fixed latitude, and in a fixed AND THE MARINER'S COMPASS. 41 position, the oxidation of the metals and the magnetic action of the earth communicate to the iron something like a permanent polarity; that is to say, a very considerable time must elapse before the iron will part with the magnetism it had acquired in its previous position. Iron vessels furnish an illustration of this kind of magnetism. If an iron vessel's keel be laid down in or near to the direction of the magnetic meridian, the time she may remain on the stocks, and the processes of hammering and clenching of the materials together, will in some measure communicate a magnetism to the vessel of a semi-permanent character. Tor example, if the north end of an iron vessel, when on the stocks, be found to attract the south point of the compass, after the vessel is launched and moored in an east and west direction, it would be proper to moor such a vessel in a direction opposite to that in which she was built and for a long time before the magnetism acquired in building would disappear. Attention is directed to this subject : those who intend to sail iron vessels should see to it. If ships had continued to be built almost entirely of wood, their local attraction would never have been noticed; but the metals now enter largely into the formation of modern ships, and their compasses are proportionally affected. If the compass indicate a wrong course, and we steer by it, we run the ship into danger or actual destruction. It is surely, then, the duty and the interest of all those who have anything to do with ships, to acquire some knowledge of practical magnetism, and more especially of seamen, ^to learn 42 THE MAGNETISM OF SHIPS, the principles of their compass, how they may guard against its errors, and shape a course with more confidence, and less risk, than those can possibly do who may continue to jog on in the old way, preferring to remain ignorant of causes that so materially influence their reckonings, or endanger their lives. 21. We now proceed to give experimental proofs of our fundamental principles of practical magnetism. It is necessary to bear in mind, that all experiments made with iron, in order to exhibit its inductive and changeable magnetic polarity, should be made with iron of uniform quality throughout its mass. Iron that has been re-manufactured is unfit for the purpose, because it may contain pieces of old files, chisels, and fragments of old steel, which might retain magnetism in a permanent form. We should therefore select for our experiments new iron, that has been derived from the ores, and manufactured by a uniform process by machinery ; that is to say, drawn out or rolled, so as to be of uniform density, and of regular form of bolt, bar, or sphere, &c. Experiment 1.* Take a piece of bolt or bar iron, a foot or more in * Sometime before the publication of these papers in the Nautical Magazine, in 1843, and after a conversation I had with Admiral Beaufort, whom I always found ready to listen to scientific or professional subjects, Sir Francis, being at the head of the Compass Committee, invited Col. Sabine and Capt. Johnson to meet me in the Hydrographic Office at the Admiralty. We met accordingly, and in order to refute certain principles generally received, and propounded by Barlow, Airy, and others, I made the experiments here detailed, and gave experimental proof, that one piece of iron will act magnetically upon another of the same kind. These principles were, however, at that time, unfavourably received by the committee. AND THE MARINER'S COMPASS. 43 length, and let it be suspended by a small thread, with Fig. I. an inclination nearly equal to the angle of the magnetic dip. When the twist of the thread (if it have any) has been got rid of, the bar will come to rest in the direction of the magnetic meridian, with its lower end towards the nearest pole B of the earth, and its upper end to- wards the magnetic equator. Take now a second piece of iron, B (Jig. 1,) and Fig. 2. hold it nearly parallel to the sus- pended piece A, with the upper end of B near to the lower end of A, ^ and it will be seen that an attractive \ magnetic force will be exerted. But \ if the upper end of B be brought \ near to the upper end of A, (as in jig. 2,) a magnetic repulsion will take place. In this experiment, it is evident that the magnetic action of the two pieces of iron upon each other, or upon a compass-needle, is in every way similar to the action that takes place between two natural or artificial magnets. The magnetism of the iron is due to its position with reference to the direction of the magnetic dip of the place, and it will be found that, by changing the position of the iron, by turning its ends, the polarity of the iron will be changed in it. Experiment 2. Suspend a bar of soft iron, with an inclination equal, ' 44 THE MAGNETISM OF SHIPS, or nearly equal, to the angle of the magnetic dip, and Fig. 3. allow it to come to rest, (Jig. 3,) then if a cast iron shot or shell be brought near to either end of the bar, an attrac- tion will be manifest; that is to say, whether we bring the upper hemisphere of the shot near to the lower end of the bar, or the lower hemisphere of the shot near to the upper end of the bar, an attraction will take place between them ; because, here we present poles of opposite kinds ; but it is not prac- ticable, in this experiment, conveniently to exhibit a repulsion. A repulsion may, however, be shewn, by bringing the shot near to the upper half of the iron ; but this requires some experience in the operator. Experiment 3. If we take a large shot or shell of new metal, free from rust, and tie a coloured thread round it, so as to Fig. 4. represent a great circle in fact, so as to divide it into two hemispheres, as A B, (Jig. 4,) then let another thread, C D, circumscribe the shot in a direction at right angles to A B, so as again to divide the ball into two hemispheres ; then place the s N sphere upon the horizontal line S N, with the plane of the circumscribing thread C D ver- tically and at right angles to the magnetic meridian N S ; let the shot be rolled backward till C D becomes parallel to the dip, and it will then be found, that the circumscribing thread, A B, has divided the cast iron AND THE MARINER' s COMPASS. 45 ball into two magnetic hemispheres, where C is a north and D a south pole. If now a small and delicate magnetic needle be applied, it will be found that every part of the ball below the thread A B will attract the south point of the needle, and repel the north point ; and all the upper hemisphere will attract the north point of the needle, and repel the south point. Here we have an exemplification of the local attraction of the iron in a vessel. The magnetism developed by the shot, and which it receives from the earth, is precisely of the same kind as that of a gun, a tank, knee, or pig of ballast. The line C D corres- ponds to the magnetic dip of the place, and indicates the position of the magnetic poles of the iron and the shot ; and in fact every other thing has a magnetism in it, and there is no such thing as magnetism without magnetic attraction and magnetic repulsion. Magnetism is compounded of attraction and repulsion. The in- telligent mariner will now begin to see how it is that, in these latitudes, the north point of his compass is drawn forward by the iron in the vessel : he will ob- serve that the upper part attracts the north point; and because all the iron is generally before the compass, and also below it, the north point of the compass-card, which is a south magnetic pole, must necessarily be drawn forward, so long as the nearest parts of the iron in the vessel continue to retain the same kind of mag- netism that the northern parts of the globe retain. Experiment 4. The magnetic dip being of much importance, we now proceed to shew how it may be found, without that 46 THE MAGNETISM OF SHIPS, expensive and not over correct instrument, the dipping needle. 22. Take a small delicate magnetic needle, screened from the action of the wind a good pocket compass answers very well and place it on the ground, or on a table where there -is no iron to affect it. When the needle has settled in the direction of the magnetic meridian, a rod of pure iron, about 3 feet in length, f inch in diameter, and perfectly straight, may then be laid in a north and south direction, with one end within a couple of inches of the compass, and at the same height as its pivot; the rod Avill be found to derange the needle. Let C be a small pocket compass placed on the north and south line S N, and let R be Fig. 5. ^^? an i ron r d pro- vided for the pur- pose of finding the dip of the needle ; the rod R, when laid in the direction of the line S N, its end at S will attract the north point of the compass-needle ; but if the north end of the rod be raised as in the figure, the compass-needle will return to its former position ; and if the north end of the rod be raised still higher, the south point of the compass - needle will approach it. Let, therefore, the north end of the rod be raised, so that its south end will neither attract nor repel the compass - needle, nor disturb it from pointing in the direction of the magnetic meridian, the axis of the rod will then be in the plane of the magnetic equator, and consequently the magnetic dip will be at right angles to it ; that is to say, the angle N S R is equal AND THE MARINER'S COMPASS. 47 to the complement of the dip, and the angle HNS is equal to the dip itself. These angles are easily measured by common apparatus, ivithout a spirit level, or even without the line S N "being a horizontal level. A sector, or a protractor and plumb line applied to the rod R, will shew at once the angle it makes, with a vertical line. A gunner's quadrant would at once measure the angle of the dip. The dip then may be found by the ordinary means possessed by seamen ; namely, by a compass, a bar of iron, and a plumb line. The principle is sound, and the application, to any extent of accuracy, may be readily contrived by in- strument makers who know their business. Experiment 5. 23. In order to shew how iron may, by its inductive property, either control or be controlled by a steering- compass, let a steering-compass be placed anywhere out of the influence of masses of iron. Take any number of pieces of iron of different sizes, from a small nail up to a large spike or bolt. Now, according to Experiments 1, 2, and 3, these nails are each in- ductively magnetic from the earth by position. Take a small nail, and hold it vertically near the north point of the compass, the lower end of the nail being at the same height as the compass-needle ; the nail, instead of repelling, will attract the north point, because the needle converts the nail, for the time being, into an inductive magnet, and controls the earth's inductive magnetism. Change the small nail for a larger, and as you increase the size, you will at last find a certain sized piece of iron that will control the compass 48 THE MAGNETISM OF SHIPS, needle, by the induced magnetism received from the earth. Hence we infer, that although each and every piece of soft malleable iron is magnetic by induction from the earth, yet the quantity of magnetism which the earth imparts to a small nail may be cancelled and controlled by a magnetic compass - needle ; although the natural quantity of magnetism which the earth may communicate to a larger portion of iron may control the compass -needle, and either attract or repel it, according to its position with reference to the direction of the magnetic dip, and the poles of the needle. 24. If an iron rod or bar be placed in an east and west direction from the centre of a compass-needle, and in a horizontal position, it will not disturb the compass, nor will the needle be affected by it ; but a rod or wire so placed will conduct inductive magnetism to the compass. Experiment 6. Let C (Jig. 6,) be a magnetic needle, mounted on a pivot in the box B, and let W E be an iron rod laid in an east and west direc- Fig. 6. tion, and close to the box B ; the needle C will not v be disturbed by the iron rod W E ; but if another piece of iron, V, be held in the direction of the magnetic dip,and brought into contact with the further end of the horizontal iron W E, its magnetism will be conducted to the com- pass-needle C, even if W E be several feet in length. AND THE MARINER'S COMPASS. 49 If the piece V be raised a quarter of an inch from W E, and again be brought in contact, the needle will oscillate ; thereby proving that the iron rod W E is a better conductor of magnetism than atmospheric air. This is an important fact, because great magnetic energy may arise, and influence a steering compass, from the arrangement of the iron in a vessel. If, for example, a merchant vessel have a cargo of iron in her hold, or even iron tanks, steam-boilers, or cylinders, so stowed in the hold, as to be in contact with an iron knee, or iron truss, bolted to the ship's side, and running upwards to the upper deck beams, such a piece of iron, being in contact with large masses of metal in the hold, would conduct or transfer the magnetism from below, and certainly derange the magnetic needle, and cause the compass to indicate a wrong course. It is on this principle of magnetic conduction that separate pieces of iron, when brought into actual contact, act magnetically as a single mass. The water tanks in a ship-of-war, if stowed in actual contact, will act on the compass as if a single tank, of the same size as the aggregate number of small ones in the hold, occupied their places. But if the tanks be kept separate by thin slices of board, then each separate tank, &c., will retain its natural quantity of inductive magnetism, and the place of its poles will change with the motion of the ship. Everybody who has been much at sea, or who has been in the habit of watching the motion of the mariner's compass, must have observed that the com- pass-card does not remain very steady in its bowl during bad weather. When the ship lurches heavily, 50 THE MAGNETISM OF SHIPS, or rolls from side to side, the compass-card oscillates several points from the actual direction of the ship's keel. When a ship is running before the wind, in a high sea, and rolling, perhaps, 15 or 20 on each side of the perpendicular, her compass-card may swim or vibrate a couple of points on each side of the course. To remedy this oscillation of the compass - card, weight is added in the shape of wax, brass bars, &c. ; for it has been considered that this vibration arose from mechanical action. There is, however, no law of mechanics by which this action can be satisfactorily explained.* 25. We have already explained how the poles of a piece of iron are to be found, by means of the mag- netic dip ( 22). Now, the magnetic dip has reference to the earth, and not to a ship and the iron she may contain. We may, for our present purpose, regard the earth as a fixture ; but a ship when afloat and at sea is a moveable body, changing her position and direction, inclining by the force of the wind on her sails, or rolling and pitching about by the action of the waves on her hull. Now, the magnetic dip of the needle, and the consequent magnetic polarity of the iron that a vessel may contain, is always referable to a plumb line, because we measure the dip from a vertical ; con- sequently, the ship and her contents are constantly changing their relative positions to the dip, and also to the magnetic attraction and repulsion which every article of iron that the vessel may contain receives by induction from the earth. Whenever a ship changes * See the specification of my Patent Deviation Compass, in the Appendix (A). 51 her position, or her inclination, a new magnetic force is brought to bear upon the compass ; and when the ship rolls alternately from side to side, equal and op- posite magnetic forces act upon the compass-needle, and cause it to oscillate on each side of the true mag- netic direction of the ship's keel. I beg to call the mariner's special attention to this part of our subject, and to refer him to Jig. 6, in our last experiment, where it is shewn, that an iron bolt laid in an east and west direction, by the side of a compass (as W E), will not affect a compass-needle, even if it be within an inch or two of the compass. Now, let us suppose Fig. 7. that there is a long iron gun on each side of a ship's quarter- deck, and exactly abreast of the bin- nacle. When the ship's head is either north or south, the L - >; * ITT guns will be in an east and west direction, and like W E, (in Jig. 6,) will not derange the compass-needle so long as the ship remains quite upright. But let the vessel be in- clined, as in Jig. 7, the magnetic polarity of the two guns, and also of every bolt, bar, or nail that may be fastened through the sides of the vessel, will have changed its place in the iron. Let the direction of the ship's head be north, and her inclination be to starboard, then the breech of the lee gun B will at- tract the north point of the needle, and its muzzle will attract the south point. E2 52 THE MAGNETISM OF SHIPS, On the other side of the ship, the breech of the weather gun A will attract the south point of the needle, and repel the north point of the compass ; so that the north point of the compass-card will be drawn to leeward by the gun B, and driven to leeward by the gun A; whilst the south point of the needle is drawn to windward by the gun A, and driven to wind- ward by the gun B. If the water be smooth, and the ship's inclination be permanent, this kind of local at- traction will permanently derange the ship's course ; but if she roll from side to side, the compass-card will also vibrate on each side of the course. If a ship's head be north, as before, but her incli- nation to port, by a strong easterly wind, the polarity Fig. 8. of the guns will be in- verted; that is to say, the lee gun A (jig. 8,) will attract the north point of the compass- needle, and draw it to- wards the lee side of the ship ; and the weather gun B will repel the north point, and attract the south end of the compass-needle. In this way, the compass, instead of indicating a course at north, may shew a N. \ E. on a N. b. E. course in smooth water, and in north magnetic dip ; but should the vessel be running before the wind, and rolling heavily from side to side, so that, at every roll of the vessel, the inductive polarity of the iron within her be actually transferred from one side to the other, the compass-card must necessarily go on in an endless oscillation, unless means be devised AND THE MARINER'S COMPASS. 53 to prevent it. The ordinary means resorted to by sea- men is to increase the weight of the card, that is to say, to use a more sluggish instrument. 26. It was owing to the vibratory motion of com- pass-cards mounted in wooden bowls, that copper or brass bowls were substituted for wooden ones. The fact is, that copper or brass is capable of receiving an inductive magnetism from a magnetic needle near it ( 17); and although a brass-bowled compass may not vibrate like a wooden-bowled compass, it is beyond a doubt, that a card mounted in a brass bowl is more sluggish in fine weather.* It is even frequently neces- sary to have small lines attached to what are called heavy compasses, in order that the helmsman or quar- ter-master may agitate the compass, and cause it to traverse in light winds and smooth water. The means, therefore, that have hitherto been adopted by seamen, as well as by makers of ships' compasses, have not been founded on sound principles. These magnetic oscillations were supposed to arise from some principle in mechanics not easily understood. Whereas the vi- bration of the needle arises from a change of place in the magnetic poles of the iron, and other things that enter into the construction of the vessel and her con- tents ; and it will be shewn, hereafter, how these vibrations, and in fact the local attraction generally, may be rectified and got rid of.f * See Table of the magnetic state of which various substances are susceptible ; where it will be apparent, that copper is highly susceptible to transient magnetism, and silver still more so. Copper is to mahogany as 80 to 1, and silver, 154 to 1 ; therefore a silver compass-bowl would be better than one of copper (217). + In the year 1841, H.M.S. Cornwallis was fitted to bear the flag of Vice- 54 THE MAGNETISM OF SHIPS, 27. In order to convince seamen that the oscil- lation of their compass arises principally from magnetic action, the following experiments may be made in any Admiral Sir William Parker, in the East Indies. At my request, Mr. Hoffmeister, the master, kindly undertook to oblige me by noting and recording, for my infonnation, whatever changes might take place in the cast iron guns of the ship, the iron fabric of the vessel, or in a piece of soft square bar iron which I supplied him for the purpose. Mr. Hoffmeister was simply to make the same kind of observations that I had made when in command of H.M.S. Dispatch, on the south-east coast of Africa ; namely, to apply a small compass to the breech and muzzle of the guns, when the ship inclined from side to side ; or to the upper or lower end of a vertical bar of iron, as the ship passed from one hemisphere or latitude to another. The iron bar with which I supplied him he suspended in his own cabin, where a small compass was at hand; and he continued to make observations whilst master of the Cornwallis, and afterwards in the Jupiter, troop ship, to the command of which Sir William Parker had appointed him. The war with China having been terminated, the Jupiter was paid off at Plymouth, and I obtained from Mr. Hoffmeister the recorded obser- vations he had made ; namely, he found 1st. That, in north latitude in the Atlantic, the upper end of his iron bar attracted the north point of his compass, and its lower end the south point. 2ndly. That, when the ship was inclined, the highest end of a gun at- tracted the north point of the compass, and the other end the south point. 3rdly. That, when the ship rolled from side to side, or tacked, the mag- netism of the guns changed from one end to the other. 4thly. That, after crossing the Equator, the magnetism of the vertical piece of iron in his cabin gradually diminished, till between the 10th and llth degrees of south latitude, and 29 47' west longitude, he found the magnetism of the iron to be inverted, its lower end attracting the north, and its upper end the south point of his compass-needle. Mr. H., it seems, had been rather sceptical on the subject; for he made this record in latitude 22 21' S. " No mistake about the attraction ! " 5thly. In latitude 152 / S. longitude 89 E. the upper end of bar attracts the south point of compass: in latitude 3 N. longitude 89 E. the upper end of bar attracts the north point of compass. When on or near the Equator, in 88 E. longitude, the attractions and repulsions were recorded as very uncertain in a calm. It was, however, in this locality, when the ship was on the starboard tack, on a northerly course, that Mr. Hoffmeister observed, that the polarity of the quarter-deck guns, secured in their ports at right angles to the ship's keel, had changed from -f to in one day. AND THE MARINER'S COMPASS. 55 vessel. Place a short plank (in equilibrium) upon anything so that it may rock or roll from side to side like the rolling of a ship ; place a compass upon its middle, the plank being in an east and west direction, and it will be found, that the compass-card will not vibrate, although the plank be moved or heeled from side to side ; place now a large bolt or bar of iron on the plank, on each side of the compass, and it will be seen, that the compass-card will swing or vibrate, if motion be given to the plank. 28. The experiments we have been explaining prove beyond any doubt, that iron is magnetic, that it has magnetic poles, and that these poles are always referable to the direction of the dip of the magnetic needle, and do not remain in a permanent position in the iron. Any attempt that may be made to correct the local attraction of a ship's compass, or the oscillation of the needle in stormy weather, must necessarily fail, unless the operator understand clearly the philosophy of his subject. Professor Barlow failed, because he believed that the central action of all the iron in a ship remained constant in all parts of the world ;* and he did not believe that iron was polarized, as we have shewn it to be. " I am the more anxious to establish this point," says the author, " in consequence of its immediate connection with the method I have proposed for cor- recting the errors of a ship's compass, which has been objected to, on the ground that, according to the theory we have been controverting, the central action of all the iron on board would not remain constant * Barlow on Magnetic Attraction. Second edit. 1842, p. 307. 56 THE MAGNETISM OF SHIPS, under all dips, and in all parts of the world ; but if the hypothesis I have advanced be correct, then the central action of any irregular mass of iron will be in the centre of attraction of its surface, whatever may be the magnetic direction ; and must necessarily remain the same, while the iron and the point from which its action is estimated preserve the same relative situation, as is the case with the iron of a vessel and its com- pass."* Now, although the iron in a ship and her steering compass do actually preserve their relative position within the ship, yet the magnetic energy of the iron and its inductive polarity do not preserve their relative position in the ship, nor to the binnacle ; for we have proved, that the magnetic poles of a piece of iron in a ship are referable to the earth, and not to the ship (12, 25). 29. The Astronomer - Royal published, in the United Service Journal for June, 1840, practical rules or directions, " for correcting the compasses of iron- built vessels." Now, vessels built entirely of malleable iron will hold inductive magnetism, and many of its pillars will, in this country, exhibit a permanent mag- netism. For, independent of that magnetism which an iron vessel may receive in the progress of building, which we have already noticed, the upright bars may conduct upwards ( 24, Jig. 6,) a magnetism from below, or from the boilers, &c. The directions published by Mr. Airy, although they may be of great practical utility in any iron-built vessel that may navigate the English Channel, or, in fact, around the British Isles, where the dip does not vary * Barlow on Magnetic Attraction, p. 182. AND THE MARINES S COMPASS. 57 much ; yet the plan he has proposed and the directions he has given are not applicable for distant regions. His plan is to find the local magnetism of the vessel upon its compass, by swinging the ship in the usual way, and then to correct the local attraction of the ship and her contents, by means of permanent mag- nets, placed at a convenient distance from the compass. His method therefore is, to correct the inductive mag- netism of the malleable or cast iron fabric of the vessel, by means of permanently magnetic steel bars : that is, to correct or cancel in one hemisphere, by a constant quantity, a magnetic agency that may vanish, or from being positive will become negative, in the other hemisphere.* A TABLE of the Compass Deviations of H.M S. Erebus, as ascertained before her departure from Chatham, (dip 69 12? N.), and after her arrival at Hobart Town, where the dip of the needle is 70 40' S. Ship's Head, by Compass. ERRORS OF COMPASS. Ship's Head, by Compass. ERRORS or COMPASS. England. Dip 69 12' N Australia. Dip 70 40' S England. Dip 69 12' N Australia. Dip 70 40' S N. 0. 6 W. 1.10 W. S. 0.28 W. 0.49 E. N. by W. 1.12 , 0.24 S,byE. 0.19 E. 0. 1 W. N.N.W. 2. 1 , 0.40 E. S.S.E. 0.48 0.38 N.W. by N. 2.10 , 1.54 S.E. by S. 1.23 1.12 , N.W. 3. 3 , 2.10 S.E. 1.53 1.35 , N.W. bv W. 3.28 , 2.58 S.E, by E. 2.21 ,, 2.35 , W.N.W. 3.51 , 3.18 E.S.E. 250 317 , W. by N. 4. 9 , 3.39 E. by S. 3.17 3.12 , W. 4.19 4.15 , E. 3.42 3.38 , W. by S. W.S.W. 4.40 4. 3 4.13 , 4.27 E.byN. E.N.E. 4.53 3.46 3.54 , 3.30 S.W. by W. a s.w. 3.24 2.45 4.39 4. 6 N.E. byE. N.E. 3.18 2.59 3.21 3.12 S.W. by S. 2. 8 3.36 N.E. bv N. 2.16 2.50 S.S.W. 1.34 2.30 N.N.E. 1.39 2.26 S. by W. 0.52 1.39 N. byE. 0.49 2.19 * The Astronomer-Koyal took offence at the concluding sentence of this paragraph, and wrote rather an angry letter on the subject. His letter and my rejoinder, with matters relative thereto, will be found in the Appendix (B). 58 THE MAGNETISM OF SHIPS, The above figures have been extracted from the Philosophical Transactions for 1843. It is evident the deviations have changed their signs from east to west, and vice versa ( 12). We observe, too, that the points of no deviation do not exactly coincide in the two hemispheres. The points of maximum deviation have also shifted a little. These points at Chatham were W. by S. and E. by N. ; in Van Dieman's Land at E. by N. and S. W. by W. This may in some measure be accounted for, by the ship arriving direct from the north to the southern hemisphere ; and time is required for iron to part with the magnetism it has taken up ( 17) ; and this opinion is fully borne out by the fact that, on the Erebus returning to Hobart Town, from the vicinity of the south magnetic pole, her compass errors were somewhat greater than is shewn in the above table, and her points of maximum compass errors were at east and west. The Erebus was a wooden ship, with a few guns, cast iron shot, cast iron ballast, wrought iron tanks, chain cables, anchors, &c. ; and we may safely assume, from the foregoing tabu- lated results, that the magnetism of the ship's metals changed its character by passing from one magnetic hemisphere to the other ( 19), and that the induced poles of the ship's iron changed places in it. The Erebus was commanded by Captain Sir James C. Ross, who was assisted by several competent ani qualified officers. The ship was fitted out for the express purpose of making magnetic observations in the South Atlantic and Antarctic Oceans, for the advancement of science, and, we trust, for the benefit of mariners. The ship and officers were supplied with AND THE MARINER S COMPASS. 59 useful instruments, of extreme delicacy and accuracy, and the results obtained and recorded will be received without that mistrust or doubt which attaches to state- ments made by persons of equivocal qualification or experience. As doubts, however, may still be enter- tained by sceptics, relative to an actual change in a ship's induced magnetism, by passing from one^ mag- netic hemisphere to another, we shall give the results obtained in another vessel. The Terror, discovery ship, accompanied the Erebus to the Antarctic regions ; * and the following table exhibits the changes Tier magnetism underwent: A TABLE of the Compass Deviations of H.M.S. Terror, in England and in Van Dieman's Land. Ship's Head, by Compass. ERRORS OF COMPASS. Ship's Head, by Compass. ERRORS OF COMPASS. England. Dip 69 12' N Australia. Dip 70 4(X S England. Dipe^li^N Australia. Dip 70 40' S N. 0.11 W. 0.42 W. S. 0. 8E. 0.11 E. N. by W. N.N.W. 1.35 .. 2.31 0.34 E. 1.21 S. by E. S.S.E. 0.51 1.42 0.52 W. 1.56 N.W. by N. N.W. 3. 9 3.58 2.21 3.26 S.E.by S. S.E. 2.30 3. 9 2.38 3.29 N.W. by W. 4.39 3.57 S.E.byE. 3.40 4. W.N.W. 5. 8 4. 2 E.S.E. 4.34 4.43 W. by N. 5.35 4. 7 E. by S. 4.57 4.28 W. 5.55 4.37 E. 5.22 4.24 W. by S. w.s.w. 5.17 4.39 4.45 4.53 E.byN. E.N.E. 5.50 5.22 4.11 4. 7 3.W. by W. 3.50 5.23 N.E.byE. 4.27 3.27 s.w. 3. 8 4.24 N.E. 337 3. 2 S.W. by S. 2.24 3.32 N.E. by N. 2.37 2.37 s.s.w. 1.38 2. 4 N.N.E. 1.40 2.11 S. by W. 0.55 1.37 N.byE. 0.33 1.26 The reader will now see, that the Terror's compass deviations exhibit the same character as the deviations * The Erebus and Terror, the two ships mentioned above, are the same vessels with which Sir John Franklyn sailed on another expedition to the Arctic regions, and which are still missing, notwithstanding search being made for them. 60 THE MAGNETISM OF SHIPS, of the Erebus ; the stowage and general equipment of both ships being nearly alike. 30. In iron-built vessels, the compass must always be liable to great derangement and irregularity, and less dependence should be put upon their dead reckon- ing. In ships built generally of timber, the local attraction upon the compass presents nearly a uniform character, although the amount of deviation may vary in different ships, and with different cargoes. The rudder is fixed at the stern, the steering wheel is near the rudder,* and upon the upper deck; consequently, the compass must be placed near the helmsman, that is, upon the upper deck, and near the after end of the vessel. The principal quantity of iron in a ship will, therefore, be before and below the compass, and the nearest inductive magnetic poles in the iron will act more powerfully on the compass than the more distant and opposite poles. The result is that, in our hemis- phere, and in the majority of ships, the north point is drawn forward, and in the south magnetic latitude, it is the south point of the compass that is drawn for- ward by the ship's local attraction ; and the greatest effect takes place when the ship's head is nearly east or west. The amount of attraction or repulsion of iron upon a ship's compass will depend upon the quantity, mass, or magnitude of the metal, and its distance from the compass-needle. The disturbing magnetic action of the iron increases, as its distance from the compass diminishes, in the inverse duplicate ratio of the dis- tance ; that is to say, if we place an iron bar at four feet from the compass-needle, its magnetic action will AND THE MARINER'S COMPASS. 61 only amount to one-fourth part of what it would be at two feet, and one-sixteenth of what its force would be at one foot distant ; and so on, the force increasing in the inverse duplicate proportion of the distance. We see, then, that a very small quantity of iron, as an iron bolt in the corner of a hatchway, or skylight, if near the binnacle, may act upon the compass as powerfully as a gun would act when secured in a port at the side of the ship, or fifty tons of iron stowed in her hold. 31. The derangement of a compass, by the mag- netic action of masses of iron, may be ascertained at sea by its oscillation, and by its indicating different bearings of a distant object, when the ship's head is in different directions at the same anchorage. Another sign of the existence of local attraction in a ship at sea is noticed when beating to windward, say with a northerly wind, when the ship appears, by the compass, to lie within four or five points of the wind. Whereas, when beating to the southward, she may appear to be no closer to the wind than six or seven points. These anomalous appearances in the direction of the ship's head arise from the north point of the compass-card being drawn forward, on both tacks, by the local mag- netism of the vessel. Whenever these symptoms appear, a compass should be placed on the forecastle of the ship, and the magnetic direction of the ship's head on both compasses noted. The one in the bin- nacle will have its north point drawn forward, and the compass forward will have its north point drawn aft. Hence the correct magnetic bearing, or direction of the ship's head, will be intermediate. When doubts exist in a merchant ship about the correctness of the 62 THE MAGNETISM OF SHIPS, course, the above plan, of carrying a compass forward and comparing it with the one abaft, affords an excel- lent check against any local attraction that may arise from receiving a new cargo, or from making changes in the stowage in a vessel. The magnetism which the iron within a ship receives from the earth, in all latitudes, will act upon the steer- ing compass in the following manner : 1st. In north magnetic dip, the higher or upper parts of the iron being north poles, the north point of the compass-card (which is a south magnetic pole,) will be drawn forward in the vessel, and the south point will be repelled towards the stern, and hence the compass will indicate a course further to the northward than the ship steers ; consequently, the ship will be to the southward of her reckoning.* 2nd. In south magnetic dip, the highest or upper parts of the iron will possess south magnetic poles, and the south point of the compass needle (which is a north magnetic pole,) will be drawn towards the ship's head, and the north point repelled towards the stern ; and hence the compass will indicate a course further to the southward than the ship steers, and she will be found to the northward of her reckoning. 3rd. In north magnetic dip, and by reason of the changeable polarity of the iron in a vessel (Jig. 7, 25), as for example in a man-of-war, the north point of the compass-card is drawn towards the lee side, and the south point is attracted towards the weather side, * In many merchant vessels of the present day, there is much iron about the wheel and steering apparatus abaft the compass. In such vessels, the north point of the compass-needle may be drawn aft in north magnetic dip. AND THE MARINER'S COMPASS. 63 whenever the ship is inclined by the force of the wind on her sails ; or, in fact, by any other means, as by shifting her cargo ( 26). 4th. In south magnetic dip, and when a ship is inclined from an upright position, the south point of the compass is drawn to leeward, and the north point is drawn to windward, by the induced magnetic poles of iron being transferred from end to end of a gun or bolt in a ship's side, &c. But when a ship rolls from side to side, in regular succession, the compass-card obeys the magnetic impulses of the changeable polarity in surrounding objects, and goes on in regular oscillations. 32. These are generally the conditions of the local magnetism of all sailing vessels, and of almost all wooden-built steam vessels, whose compasses have not been corrected by artificial means, or removed beyond the sphere of the ship's local magnetism. If the local attraction of a ship has been correctly ascertained by swinging her round, and the amount of local magnetism noted upon each of the two-and-thirty points of the compass, then these corrections may be safely applied to the courses, so long as the ship continues to be navigated in the same amount of magnetic dip ; but if the ship's local attraction has not been correctly found, we may deduce the following practical results from the principles we have been propounding : 1st. Almost all ships will be found nearer to the magnetic equator than the dead rekoning will place them. Thus, in England, or in north magnetic latitude, ships get to the southward of their reckoning; but at the Cape of Good Hope, or coast of Brazil, they generally get to the northward of their reckoning. 64 THE MAGNETISM OF SHIPS, 2nd. With regard to the change which takes place in the polarity of the iron in a vessel as she changes her position, and which involves important conside- rations to the navigator, we may deduce the following general rules. 3rd. In north magnetic latitude, and when a ship is on a wind and steering a northerly course, she is liable to be to leeward of her reckoning ; but when steering to the southward, she is liable to be to windward of her computed position. 4th, In south magnetic latitude, and when a ship is on a wind, and consequently inclined, she is liable to be to leeward of her reckoning when standing to the southward ; but liable to be to windward when standing to the northward. So that in any magnetic latitude, whether north or south, a man-of-war, when on the wind, and steering towards a magnetic pole, is liable to be to leeward of her reckoning ; but when standing towards the mag- netic equator, and inclined by the force of the wind on her sails, will get to windward of her place by dead reckoning, when all other things are equal (figs. 7&8). 33. We must go a little further into our subject, and remind our readers, that the general mass of the metals, being in every ship below and before its steering compass, the magnetic needle is acted on most power- fully by the nearest magnetic pole of the iron before it ; and that the greatest effect is produced when the compass-needle is nearly parallel to the ship's beam, or at right angles to the keel. If the ship be perfectly upright, and both sides perfectly alike with regard to the iron entering into her fabric, then, when her head, AND THE MARINER'S COMPASS. 65 or rather her keel, is in the direction of the magnetic meridian, the local attraction on the compass is = ; but when the ship's head is either magnetically east or west, the local attraction is a maximum; the vessel being supposed perfectly upright. But if the ship be inclined from an upright position, the induced polarity of the iron in the vessel is transferable from side to side ; and the local effect upon the compasses, under these conditions, is greatest when the ship's head is on the magnetic meridian, and least when either east or west. Now, when the ship's course is either N.E., S.W., N.W., or S.E., it is evident the local magnetism of the general mass of the vessel and her contents, as ascertained when perfectly upright, may either go to cancel or combine with the magnetism arising from a change in the ship's inclination from starboard to port, although the direction of the ship's keel may not change. Our investigations are assuming rather a complicated appearance, but we shall endeavour to' make ourselves understood. Ex. gra. Let a ship be steering a north-east compass course from the Longships to Milford, then it is pos- sible that the north part of her compass may be drawn forward half a point by the local magnetism of her contents, when the ship is quite upright ; consequently, although the compass might indicate a north-east course, she would actually be steering a N. E. \ E. course. Let it now be granted, that the wind may change to north-west, and that, by the ship's inclination, the north point of the compass -card may be drawn half a point towards the lee side (fig. 7) ; the ship, if continuing to be guided in her course by the com- F 66 THE MAGNETISM OF SHIPS, pass, would now be steering at N.E.b.E. instead of N.E. If the wind be south-east, and the ship inclined to port, and the north point of her compass be drawn half a point to leeward, this quantity, arising from the change in the magnetic polarity of the guns, knees, or bolts in the vessel, would cancel the other kind of local attraction ; and, under our conditions, the compass course would be correct on the starboard tack, but one point in error on the larboard tack ; and in this way the quantity that the north point of the compass-card might be drawn forward, by the general magnetism, might be either cancelled or doubled by the quantity that the north point might be drawn or driven to lee- ward, by the ship's inclination. A commander of a vessel, unacquainted with these magnetic anomalies, might, in making one passage, pronounce his compass free from error; and on another occasion might find his vessel one mile to leeward of the reckoning, for every five of her distance run. What would he do ? Why, place his errors to the account of a strange and unaccountable current. On the 30th of July, 1842, H.M.S. Vanguard, Captain Sir David Dunn, being some 80 or 100 miles south-west from the Lizard, and being sure of their position, shaped a course for the Lizard, with a moral conviction of correctly making the lights ahead. The weather was perfectly clear, with a fresh breeze from the north-west ; but the ship got a long way to leeward of her intended course. On her arrival at Plymouth, the circumstance was mentioned to the writer, who gave Sir David Dunn and the officers a practical de- monstration of the way whereby the north point of the AND THE MARINEH'S COMPASS. 67 Vanguard's compasses was drawn forward as well as to leeward, on a north-east course with a north-west wind. Those who have clear conceptions of magnetism, and who know something of the composition and reso- lution of forces, will have no difficulty in compre- hending how these results must influence a ship's reckoning ; but those who have not paid attention to the subject, would, in order to follow our reasoning, require practical illustration by means of a model. It is one thing to enunciate a principle, and another to make the reader comprehend it. 34. I might easily quote recorded instances of errors arising to the reckoning of ships under my immediate charge, in order to bear out the conclusions to which we have arrived ; but I deem it more fitting to refer to cases of disaster at sea, with which the public may be better informed cases where ships have run ashore with fair winds, steering compass courses in- tended to lead them clear of all danger, but which really led them to destruction. If it can be shewn, as I have shewn, that, when the local magnetism of ships is generally of such a character as to cause the vessel to deviate from her intended course in one particular direction, the mariner is forewarned of his danger : he knows on what side of the course the ship is likely to diverge, and takes his precautions accordingly, even if he should be in a ship where no pains have been taken about the steering apparatus, and where nothing what- ever has been done to determine the deviation of the needle on the different courses the ship is liable to steer. The British and North American mail steamer, F 2 68 THE MAGNETISM OF SHIPS, Columbia, sailed from Boston, on the 1st of July, 1843, for England; and, shaping a course for Seal Island, on the 2nd ran directly upon it, and became a total wreck. The sea was quite smooth at the time, although the weather was hazy. All the crew and passengers were saved by the boats. A committee of the House of Commons, appointed to enquire into the cause of shipwrecks, summoned witnesses to give evi- dence relative to the loss of the Columbia, and that evidence is worthy of notice, although it does not appear that the committee derived much information from it. A fine well-appointed steamer, leaving a harbour on one day, and, with fine weather and smooth water, run- ning the next day directly on an island, for which a correct course had been shaped, offered a problem of some interest to solve ; for the magnetism of the ship and the compass were involved in it. If the case had been investigated by a committee of experienced seamen and practical navigators, they would have disentangled extraneous evidence, and arrived at the facts of the case, which are very in- structive. The captain stated, in his evidence before the com- mittee on shipwrecks, that, during the two years he had been employed in the Company's steamers, the ships were always found to be about 20 miles to the south- ward of their reckoning, in crossing the Bay of Fundy, between Seal Island and Boston Lighthouse, when shaping a compass course of E. \ N. or W. \ S. ; that this constant southerly set of the ships was ascribed to an "outset," or southerly current ; that the Columbia AND THE MARINER'S COMPASS. 69 shaped the ordinary course of E. J N. by compass, on the 1st of July, and on the 2nd ran upon Seal Island, when they expected to be, as usual, many miles to the southward of it. The conclusion they then drew was, that the outset, or southerly current, had ceased to run. Similar evidence was given by other witnesses, and the committee appeared satisfied. One member asked if the Columbia's compasses were correct, and was informed that they were. The witness was asked how he knew the compasses were correct ; and the reply was, that the Columbia had recently had her compasses made correct by a scientific person at Liverpool, who swung the ship round, and was a whole day about it, and who placed magnets near the compass. This in- formation appeared to satisfy the committee, and may probably please those who undertake to adjust the compasses of ships, and gain a living by the process. In reading over the evidence, it appears clear enough, that the mail steamers employed by the Company were always found to the southward of their reckoning, in crossing the Bay of Fundy ; and this southerly set of the ships was considered as the result of a current set- ting out of the Bay. The fact is, that these steamers would be, as far as compass-deviations are concerned, very similar to other steamers. Their engines, &c. would attract the north point of the compass-needle forward, and thereby cause the vessels to diverge in a more southerly direction than their compass indicated the courses steered. If these vessels' compasses had never been adjusted by magnets or other means, their deviations in a southerly direction would have con- tinued up to the present time ; or, if the Columbia's 70 THE MAGNETISM OF SHIPS, compass had not been touched by the scientific person at Liverpool, in all probability she would have cleared Seal Island, and gone on safely in crossing the Bay of Fundy, and recording in her log-book the hypothetical " southerly set." If the gentleman who adjusted the Columbia's compass, and made it correct by magnets, had even said to the master, " I found your compass had considerable errors, and no doubt you generally found the ship to the southward of her reckoning. I have now made the compass sensibly correct, so that for the future your ship will not be so frequently found to the southward of her intended course." An obser- vation of this kind would have opened the officer's eyes, and the loss of the steamer might have been avoided. The true bearing of Seal Island from Boston Lighthouse is N. 74 E., distance 225 miles ; and the variation of the compass is about 10 degrees westerly. Now, the course shaped by the Columbia, as stated by the captain, was E. JN., to which if we add 10 of westerly variation, we obtain E. 15 37' N. ; or N. 74 23' E., consequently the course the ship shaped was direct for Seal Island. That the compass was correctly adjusted, there is little doubt, and the steerage of the vessel, in light winds and smooth water, must have been good, otherwise the vessel would have cleared the fatal island. A deviation of less than 5 on an E. \ N. course, for a distance of 225 miles, would set the vessel 20 miles from her intended direction. It is therefore probable, that the Columbia's compass had originally an error of 4 or 5 degrees on an E. \ N. or W. \ S. course. It is well known and the public have paid pretty AND THE MARINER'S COMPASS. 71 handsomely for their knowledge of the fact that our ships, when running up Channel with a fair wind, run ashore more frequently on the coast of France than on the English coast ; that is, they get to the south- ward of their reckoning. The coasting steam-vessels that regularly ply between Cork, London, Plymouth, and Dublin, know from experience without knowing why that they must steer from the Start to St. Catha- rine's E. J S. by compass, and from St. Catharine's to Start, W.b.N. J N. This they regularly do, traversing the same line in perfect safety, by steering courses not given in their sailing directions, nor diametrically opposite to each other. It will be in the recollection of our readers, that the West India mail packet, Solway, left the harbour of Corunna on a fine evening, and about two hours after rounding the light-house, ran upon the rocks and went down, along with the greater part of her passengers and ship's company. The steamer shaped a compass course to clear the Island of Cis- argas, but her compass was affected by the local magnetism and machinery of the vessel, and in the same way that the Irish packets' compasses are affected. The north point was probably drawn a point forward on a W.N.W. course ; and consequently, the vessel, instead of going W.N.W., was making a W.b.N. compass course ; that is, she got to the southward of her intended route, ran upon sunken rocks, and in a few minutes went to the bottom. 35. In the month of November, 1842, several vessels were either lost, or in great jeopardy, near Boulogne. A ship called the Reliance, of 1550 tons, from India, having about 120 persons on board, and 72 THE MAGNETISM OF SHIPS, laden with a valuable cargo, ran on shore on the coast of Merlimont, when it was believed the vessel was on the English coast. This fine ship was destroyed, and about 115 persons drowned. The Reliance, like many others, was a long way to the southward of her reckon- ing, by steering a compass course that should have led her along the English shore. A good deal has been written about the loss of this fine ship ; but no sufficient cause has been assigned. We are, however, enabled to shew how this fine Indiaman, laden with teas and other Indian productions, should be misled by her compass, and run upon the coast of France, even with a south- west wind. The following extract is from a newspaper, dated 1st April, 1843: "During the last ten days, Mr. Kent and his associates, who purchased the wreck of the Reliance, near Boulogne, have been busily employed in their endeavours to bring the wreck to land; they have found a chronometer, several silver and plated dishes, and a large iron tank, 46 feet long by 8 feet deep, and 6 feet wide." Having made enquiries about this huge tank, I was informed by a gentleman who knew the ship, and had seen the tank, that it was placed abaft the mainmast, and before the binnacle, and probably 18 feet below the compass. Now, here was a tank of malleable iron that would exert a magnetic action upon the compass as powerful as that of a solid mass of iron of the same linear dimensions ; that is, equal to 2208 cubic feet of iron, which would weigh 468 tons. When the Reliance was running up Channel, she was probably steering a compass course of E.b.S., and the wind being on the starboard quarter, the weather binnacle would be used. AND THE MARINER'S COMPASS. 73 Now, the ship would have an inclination to port, and all that part of the huge iron tank that lay on the larboard side of the ship would attract the south point of the compass, and the weather end would draw the north point forward ; because, if a line had been drawn trans ver sly through the body of the tank, instead of being in the plane of its magnetic equator, its starboard or southernmost end would have been elevated 25 or 30 above it (see Jig. 5) ; and therefore the ship was steering a compass course, under such circumstances as to be under the maximum magnetic influence of this immense tank. The tea and other things stowed above the tank would not cut off, or in any way impede its action on the steering compass, whose north point must have been drawn forward very considerably; for, at the time the ship struck, they supposed the vessel off Dungeness, eight or nine leagues further to the northward. If this huge iron tank had never been placed in the after hold of the Reliance, it is probable she would have arrived safely. Its presence must necessarily have produced a deviation in her compasses, and in the direction we have indicated ; for a cargo of empty iron tanks, when stowed in the hold of one of Her Majesty's Naval transports, has drawn the north point of the compass 18 degrees forward on a W.N.W. course. In the year 1810, H.M. Ships Nymphe, Pallas, and L'Aimable ran ashore on the south side of the Frith of Forth, when steering a course intended to lead them up to Inch Keith. How many of our men-of-war have been lost off the coast of Holland, at a time when the pilots believed the ships further to the north- 74 THE MAGNETISM OF SHIPS, ward! There is no current setting upon the Dutch coast ; on the contrary, there is actually an off-set by reason of the discharge from the rivers of Germany. That ships navigating high northern latitudes do ge- nerally get to the southward of their calculations is beyond any doubt ; and that these errors in the reckon- ing arise from local magnetism is equally certain. 36. Three of the West India mail steam - packets have been wrecked, and three others of the same Com- pany have run on shore, but were floated again.* Now, if these six vessels, being under the full power of their engines, and steering a course by order of their com- manders, which courses ran the ships ashore instead of clearing adjacent dangers, it is evident, that the courses ordered to be steered were either wrong courses, or else the vessels' compasses were under the influence of local attraction; and as the whole of these vessels were to the southward of their computed positions in north magnetic latitudes, it is fair to infer, that their compasses were deranged by the local magnetism of the steamers ; that is, the north points of their com- passes were drawn forward, and the south points repelled aft. It is not in the latitude alone that these errors arise, the longitude is equally affected ; for let a ship be steering a south-west course by her compass, and a distance of, say 100 miles ; then, if he compass be so acted on, by the ship's magnetic attraction, that its north point be drawn forward half a point, the difference of latitude made will be greater, and the * Medina on Turk's Island, Isis at Porto Rico, Solway near Corunna, Medina at Saba, Teviot at Owers, Tweed on Turk's Island; also a vast number of other steamers of the Royul Navy and Mercantile Marine. This was written in 1843. AND THE MARINER'S COMPASS. 75 departure less, than if the compass indicated a correct course. In 1849, a merchant vessel took in a cargo of ma- chinery for the Mediterranean. The master, although unacquainted with magnetism or compass deviations, possessed a good share of prudence and common sense. On sailing from Ipswich, and shaping courses from one buoy to another, he found his vessel did not go in the direction he wished her to go ; in fact, he always missed the buoys ; and he made this note in his log, " I begin to think our cargo affects the compasses." He got, however, to the North Foreland, and in steering a southerly course parallel to another vessel, he hailed her, and found that both ships' compasses were alike ; but in going through the Gull-stream, he made an- other entry in his log, "The sailing directions give the course through the 'Gull' to the Downs, S.W.JW., and our course by the compass is W.b.S." This is exactly the kind of observations that seamen should make. This vessel steered by the land along the shore as far to the westward as Beachy Head, when the wind came aft, and a course was shaped W.N.W. for the Isle of Wight. Studding sails were set for the night ; but towards morning, land was reported right ahead, on the French coast. Sail was shortened, the ship hauled to the wind on the starboard tack, for the English coast, which was made in due time. The prudent master then groped along shore to Plymouth, in order to learn what should be done. He employed an agent, who applied to the writer, who ascertained, that on easterly and westerly courses the compass was three points in error, but correct at north 76 THE MAGNETISM OF SHIPS, and south. The agent wrote to the owner of the vessel, who replied, " I cannot but think that Captain 's tale is a very strange one, as to his compasses being out of order through the attraction of the cargo. I have lately laden several vessels with the same sort of cargo, but I have never heard of such a tale as this of Captain 's before." Be that as it may, we trust the prudent captain did not in any way suffer for the steps he took to prevent risk to ship and cargo. 37. The quality and amount of the inductive magnetism, which the iron in a ship receives from the earth, are always of the same kind and intensity as that of the magnetic hemisphere where the vessel may happen to be. When the needle dips towards the south magnetic pole, the higher parts of the iron on board will have a south magnetic polarity, and will draw the south point of a compass-card forward in a vessel when steering an easterly or westerly course ; in a word, the magnetic phenomena, which we have described at some length, will, in south magnetic latitude, be found of an opposite kind to that in corresponding dips in north magnetic latitudes ; here ships will generally get to the northward of their dead reckoning. The southern seas, however, are not covered with vessels as are the waters north of the equator. The relative proportions of land and water in the two hemispheres of the earth are very unequal ; for it may be seen by a map of the world, that the equator marks to the southward about nine-tenths of South America, one -third of Africa, Madagascar, Australia, New Zealand, and the Polynesian Islands. If we THE MARINEK'S COMPASS. 77 analyze the naval statistics, and add together the whole shipping of these southern countries, it is presumed that England alone will be found to possess a greater number of vessels in its coasting trade. We frequently hear of vessels, when in south mag- netic latitude, getting to the northward of their dead reckoning, on the coast of Australia, near the Cape of Good Hope, or in South America. Several valuable ships have got on shore in Africa through errors in the reckoning ; that is, by being further to the northward than was expected.* H.M.S. Thetis sailed from Rio de Janeiro, on the 4th December, 1830, having on board 800,000 dollars, for England. On the evening of the 5th December, she ran upon Cape Frio, and was wrecked. The weather, after the Thetis left Rio, was rather tempes- tuous ; it blew strong from the south-east ; a course was shaped N.E.b.E. by compass, and the ship ran against the cliffs of Cape Frio, her studding-sail boom- irons striking fire from the rocky cliffs, at a time when her dead reckoning placed the vessel thirty miles from the land. Whenever enquiries are made about the loss of ships that have run ashore, we invariably find the currents of the ocean are assigned as the cause. It is even probable that a current may have set the Thetis to the * The author speaks from personal experience, having passed round the Cape of Good Hope sixteen times. Many merchant vessels have got ashore on the south coast of Cape Colony, and several of her Majesty's ships have grounded or been wrecked here, by steering wrong courses; namely, the Birkenhead, Rhadamanthus, &c. One large steamer was wrecked on Cape Reaffe, and another narrowly escaped the same fate on the Bird Islands. 78 THE MAGNETISM OF SHIPS, westward ; but we are certain that the local magnetism of the ship would have the effect upon her steering compasses of indicating a course more easterly and less northerly than the ship was steering. We have already demonstrated, that vessels steering a north-easterly course in the English Channel invariably get to the south-eastward of their reckoning. Now, the Thetis being in south magnetic latitude, would diverge to the north-westward of her reckoning, and it is not necessary to have recourse to an imaginary current to account for her loss. 38. Old captains of ships,* who have made many voyages from England to India, and who have had to cross the trade winds, will, on reference to their logs, find that, on one side of the equator, errors in the lon- gitude, by dead reckoning, have accumulated, and on the other side these errors have disappeared; the ship's longitude by dead reckoning, in the end, agree- ing with that by chronometer. The ordinary track that ships pursue, in their outward voyages from England to India, is to the south-west; and so long as they continue to steer towards the magnetic equator, they are, for reasons already given, liable to be to windward of their reckoning, that is, to the eastward : but when they have passed the magnetic equator, and advance towards the south magnetic pole, where the needle dips to the southward, and being on the larboard tack, they are liable to be to leeward or to the westward of the reckoning, by reason of the ship's local magnetism having changed its character on crossing the magnetic equator. * We refer to wooden-built ships, under canvas. AND THE MABINER'S COMPASS. 79 H.M.S. Malabar, 74 guns, Captain Sir George Sar- torius, left Plymouth in June, 1842, for Rio Janeiro. On the outward passage, her longitude by dead reckon- ing was carried on in her log, and the errors went on increasing in north magnetic latitude, but diminished as she approached Rio. This ship returned to Plymouth in May, 1843, her longitude by dead reckoning from Rio Janeiro to the Lizard being carried on in the log, and the error amounted to 9 36' ; i.e. the ship was nine degrees and thirty-six miles to the westward of her reckoning. The Malabar was a small 74-gun ship, built of teak wood, and armed with heavy guns ; she was moreover not a stiff ship, but inclined with a fresh breeze some ten or fifteen degrees. She had to cross the south-east as well as north-east trade winds, and consequently had to sail near 4,000 miles on the star- board tack. On her way from Rio to the magnetic equator, the Malabar, like the Thetis, would get to the northward and westward of her reckoning ; and when she entered into the north magnetic hemisphere, her inclination to port would cause all her lee guns to attract the north point of the compass, and the wea- ther guns to attract the south point. In this way, and during the transit of the north-east trade wind, and probably for a distance of 2,500 miles, the ship's com- pass might indicate a course nearly one point more weatherly than the vessel made through the water. Mr. Barlow, the master of the Malabar, and others, informed the writer, that whilst the ship was standing to the northward on the starboard tack, they found a difference of 5 between, observations, made for the variation, upon the weather side of the poop deck, and 80 THE MAGNETISM OF SHIPS, similar observations made upon the lee side of the same deck at the same time ; hence it resulted, that observations made for the sun's azimuth and variation of the compass were not to be depended on. We are sometimes told, that a binnacle compass will not be influenced by a ship's local attraction, if the direction of her keel coincide with the direction of the magnetic meridian. But this opinion is incorrect, and founded on the supposition, that the local magnetism of a ship may be referred to a central point near the middle of the vessel, and that it remains constant ; * an opinion at variance with the doctrine we have detailed, and the demonstrations we have given. The errors that creep into a ship's reckoning no doubt frequently arise from currents. Navigators may likewise get out, in their reckoning, by making allowances for currents that do not exist ; but the great source of errors in navigation consists in bad steerage, and ignorance or inattention to the magnetic attraction and repulsion of the iron in a vessel, which derange the compass courses. Practical Rules for ascertaining the Deviations of the Compass, which are caused by the Iron in Ships, were published by order of the Lords Commissioners of the Admiralty, in 1842 and 1852, where, in 8, I find the following sentence relating to the points upon which there are no deviation : " Experience, however, has shewn, that when the points of no deviation in any vessel have been once determined in a compass, placed as above directed, and always used in the same place, they may be regarded as constant, provided that no ex- traordinary or unusual change be made in the amount * Barlow on Magnetic Attraction, edition 1824, p. 307. AND THE MARINER'S COMPASS. 81 or distribution of the iron in the ship." The words here quoted are also to be found in a work by Captain Johnson,* a work containing much valuable informa- tion obtained from observations made in H.M. ships, where good observers and excellent instruments were to be found. At pp. 104-5 of the second edition of Captain Johnson's book, we find tabulated deviations, as found in three iron steam-vessels, and five wooden steam-ships of the Royal Navy, which had their devia- tions ascertained in England, and afterwards at distant parts of the world, where they were swung for the purpose of again ascertaining their compass errors on each point. I have examined these tables with some care, and find that, in almost every case, the points of no deviation, and points of greatest deviation, have not remained constant, even though the ships have not passed from one magnetic hemisphere to another. From the tables above referred to, I have extracted and tabulated the compass errors of H.M.S. Centaur, swung first in England, and then near the magnetic equator; H.M.S. Geyser, swung first in England, and afterwards at the Cape of Good Hope, where the dip is about 53 S. ; and lastly, the Acheron, swung first in England, and secondly in New Zealand. This table will satisfactorily demonstrate, that not only do the points of maximum and minimum compass deviations change places on the compass-card, but that the de- viations from plus become minus quantities, a short * Practical Illustrations of the necessity for ascertaining the Deviations of the Compass. By Captain Edward Johnson, R.N., F.K.S., Superin- tendent of the Compass Department of the Royal Navy, p. 91. London : Potter, Poultry, 1852. G 82 THE MAGNETISM OF SHIPS, time after the ships pass from one magnetic latitude to another. TABLE shewing the changes of Compass Deviations which took place in the undermentioned steam-ships, after leaving England and being reswung at the following places ; viz., the Magnetic Equator, Cape of Good Hope, and New Zealand. The dash ( ) denotes the course on which the devia- . tion = ; the asterisk (*) that on which the deviation is greatest. Direction of Shir's Head, Standard Compass. CENTAUR, s. v. Com. Fanshawe. GEYSER, s. v. Com. Brown. ACHERON, s. v. Capt. Stokes. England, 1849. Dip 69 IV N. Fernando Po, 18SO. nonr MftOMtk Equator. England, 1847. i>ii> <;>!- .v C. Good Hope, law. Dip 58 S. % SB New /ealand um DipTlKC.M S. N 2.20 E 3.40 5.40 6.50 8. *8.10 7.20 7.35 6.40 5.40 4.40 3.40 2. 1. 0.30 W 1.30 2.45 3.50 5.20 6. 1.55 E 3.30 3. 3.30 4.10 4.40 *4/X) 3.30 2.10 1.30 o'.io w 0.30 130 1.20 1-30 1.10 2.30 (?) 1.30 1.35 2. 6 W 0. 2.20 E 4.30 6.50 7.20 7.50 !l.5 I *9.10 9.10 9.10 7.40 630 5.30 4.40 3. 1.40 0. LOW 2.20 1.50 W 2.10 2.30 2.15 2. 1.50 1.40 2.3.0 3.30 *3.30 330 3.15 i. : 0.30 0.25 0.20 0.55 E 2.10 2.20 3. 6 E 4.40 6.35 K15 8.35 8.40 *8.50 8.30 8.10 7. 5 5.45 4.25 3. 1.40 1. T5 W 2.10 3.40 4. 5 4.57 0.12 W 1.19 1.20 0..04 1. 1 0.32 E 2.57 1.57 1.54 2.52 321 *3.55 3,01 3.52 3.52 3.38 1.2,5 1.19 1.10 1.30 N. by E NNE N.E. by N. . N.E... N.E. by E E.N.E E by N E E. by 8 E S E 8.E. byE 8E S.E. byS 8 S E S.byE 8 8. by W s.s.w 8.W. by 8 sw 6.30 7.30 7.40 *8.10 8. 7.40 6.40 5.40 4.30 3.10 1.50 0.40 E 1.45 2.20 2.20 *3.10 2.30 2.20 2.20 2.20 2.10 1.20 0. 1.20 E 3.30 5. 6.30 8. 9.20 10.20 10. 11.0 9.40 8.20 ,, 6. 4.40 *2.30 2.20 2.10 1.40 1.10 0.30 0.10 W 0.35 1. 0.45 0.30 1.10 5.50 6. 7.20 *7.20 7. 5 6.35 6.10 5.30 4.25 3. 3 1.40 020 E 2.10 1.32 0.43 W 0.49 0.13 0. 4 0.58 1. 8 0. 8 0.42 1-32 0.12 S W by W W 8 W W.byS W W.byN W N W NW byW N W N W. by N NNW . ... N. by W The above table shews, that when the ships' heads are at north or south by standard compass, there are considerable errors. In the Centaur, the north point AND THE MARINER'S COMPASS. 83 of her compass is attracted towards the starboard side. In the Geyser, her compass is attracted to port. In the Acheron, whilst in England, the north point of her compass was attracted to starboard, but when she had reached New Zealand, her compass, at north or south, was attracted towards the port side of the ship ; and almost all the signs of compass deviations had been changed. In these ships, it is evident there was an unequal distribution of induced magnetism on each side of the ships ; and hence the apparent complexity and irregularity in the amount of compass deviations on the same courses in different latitudes. The resultant points, or points of least and greatest deviation, it is evident, have varied remarkably. For example, the greatest error of the Geyser's compass in England was, on a N.W. bW. course, and = 11 westerly; at the Cape, and on a W.N.W. course, it was = 10' westerly. In the Acheron, the greatest errors were near E.N.E. and W.S.W. in England, but at New Zealand her least errors were N.E. b E. and W.S.W. ; whence we may infer, that these resultant points, instead of re- maining constant, do actually change places, in some ships, when passing from one magnetic hemisphere to another of an opposite kind, but nearly of an equal amount of dip or inclination. Captain Johnson, in his second edition, at pages 106 and 108, makes favourable mention of two pamphlets, detailing methods proposed for reconstructing a ship's compass deviation tables at sea. These methods are grounded on the assumption, that the compass courses of greatest and least errors (resultant points), remain G 2 84 THE MAGNETISM OF SHIPS, constant everywhere ; but as the tables I have referred to clearly shew, that, in iron as well as in wooden ves- sels, the points of greatest and of smallest compass errors may change 50 or 80 degrees upon the compass- card, the intelligent mariner will do well to resort to frequent amplitudes and azimuths for determining the errors of his compass ; for, in all cases, the difference he finds between the calculated and observed compass bearing of the sun will be the quantity he should apply to correct his course at the time of observation. The variation, properly so called, and the deviation caused by the ship's local magnetism, will always be involved together in the observations he makes, whe- ther by amplitude or azimuth. 39. Being in command of a king's store-ship (the vessel referred to in 1), filled with new iron water tanks, and bound down Channel for Plymouth, on the 30th December, 1818, I was of opinion, that these wrought iron tanks would exert an additional influence upon the compass. A W.N.W. compass course was shaped from St. Catharine's point for the Start. There was a fresh breeze at east, with clear weather ; but at daylight, instead of making the Start, it bore N.N.E, 21 miles. In this case, the ship was at least 8 leagues further to the southward than she ought to have been, by steering W.N.W. I was not then aware of the fact, discovered by Professor Barlow, that an iron tank would exert a magnetic influence equal to that of a solid of iron of the same linear dimensions. On the 26th of March, 1803, H.M.S. Apollo sailed from the Cove of Cork (now Queen's Town), with a convoy of seventy sail of merchant ships ; on the 2nd AND THE MARINER'S COMPASS. 85 of April, at three in the morning, the frigate and forty sail of her convoy went on shore, on the coast of Portugal, at a time when they imagined themselves three degrees to the westward of their real position. The loss of these ships may be ascribed to the local attraction of the frigate upon her compass ; for about thirty sail of the convoy had, during the night, wore to the N.W., and thereby escaped destruction. The Apollo and her convoy had been steering for several days in a south-west direction. The north point of the frigate's compass being attracted forward, would thereby indicate a more westerly course than the con- voy was making, and hence the melancholy loss of so many ships together. Being assistant master- attendant of Plymouth dock- yard, in the year 1833, I was sent to Pembroke to launch and equip H.M.S. Royal William, 120 guns, for removal to Plymouth. I had orders not to put to sea without two steam -frigates, that had directions from the Admiralty to join me at Milford. They ul- timately arrived, and towed the ship down Milford Haven and out to sea. Sail was then made on the three-decker, the two steamers, and the Pantaloon. Symptoms were seen of the wind promising to draw round from N.W. to W. and to blow fresh; and I shaped a course for Scilly, instead of for the Land's End. The large ship, being light, clean, and well rigged, out-sailed the other vessels ; but Capt. Oliver, of the Dee, getting within hail, enquired how long I intended to steer that course. I replied, " Till I make Scilly." The wind increased its force, and the Royal William her speed to 11 or 12 knots an hour, and the 86 THE MAGNETISM OF SHIPS, two steamers and brig were left far astern. The line- of-battle ship's compasses were perfectly correct. She made Scilly light, bore up for the Longships, passing between them and the Seven Stones, rounded the Land's End, and ran up Channel with a strong wind backing to the S.W. ; which enabled her to sail direct into Hamoaze without stopping. The two steamers and brig had continued their course in the track of the large ship, under full steam and a crowd of canvas, and in a dark and rather stormy night made the breakers of Scilly under their bows. The brig tacked; the steamers backed and shortened sail, and stood to the N.W. till daylight; then search was made for the line-of-battle ship, which was nowhere to be found. In the course of the day, they reached Plymouth, and not finding the ship in the Sound, were alarmed for her safety. It now turned out, that the compasses of all these vessels were under the influence of local attraction. They had never been swung, nor had their compass errors been examined in any way. The compass courses they steered, in following the Royal William, ought to have carried the steamers and brig far to the westward of Scilly. Their compasses indicated wrong courses : the compass in the large ship was correct. I was ordered to return to Pembroke for another line-of-battle ship, taking a passage in the steam-ship Salamander, commanded by Capt. Austin, now super- intendent of packets at Southampton. The adventure of the Royal William induced us to make many observations on the local attraction of the steamer, on our passage to Pembroke. AND THE MARINER'S COMPASS. 87 We took the Camel, dockyard lighter, in tow, and, rounding the Longships, shaped a course for St. Ann's Point, with a firm conviction on my part of making St. Go van's Head. The wind shifted to the S.W. with thick weather ; but being prepared in every way for making land, at a distance of half or three-quarters of a mile, we went boldly on, keeping a sharp look out ahead. As we neared the Welsh coast, we were amused as well as instructed by the careful master of the lighter astern, standing upon the bowsprit and waving his hat to keep more to port. At last, and without leave, he cast off the tow rope, and hawled up two or three points. We went on as before, and, as was expected, made St. Govan's Head instead of St. Ann's Point. The master of the lighter knew by his compass (which was right) that the steamer was steering a wrong course. Our object was to prove how dangerous it was to place reliance on a compass in a steam-ship, when her local attraction was unknown : and my object in recording so many instances, even at the risk of being tedious, is to impress upon the minds of naviga- tors the imperative necessity of being ever watchful and careful of their steering apparatus. The general use of chronometers, the correctness of our logarithmic tables for practical astronomical purposes ; the accuracy of our astronomical and mathe- matical instruments used for naval purposes, have left us little to desire for all the purposes of practical navigation, so long as the state of the weather will allow us to make astronomical observations. But the possession of good instruments, charts, and books has 88 THE MAGNETISM OF SHIPS, made navigation so easy and accurate in clear weather, that the necessary care and attention, under ordinary circumstances, to the helm, log, lead, and look-out, have been sadly neglected by the majority of seamen. The result has been, that worse dead reckonings are now kept than ever were kept before. The general adoption of iron into the construction and equipment of ships, deranges the compass courses, and a record is made on the log-board, of courses that the ship never steered ; that is, the log -board only shews a compass course, instead of the magnetic course, uninfluenced by the local magnetism of the ship or her cargo. When an observation for the latitude is obtained at noon, this latitude enables the navigator to correct his longitude by dead reckoning, and also to work out his sights for the chronometer ; but when a ship has been a day or two without obtaining an observation to determine her latitude, there may be a considerable error in the latitude by account. Now, if sights be obtained for determining the longitude by chronometer, and the mean time for these sights be obtained by applying to the calculation a latitude which is not the latitude of the place where the observations were made, it is evident that the longitude obtained by such a calculation must be wrong. H.M. Ship Challenger was wrecked on the coast of South America, in consequence of placing too much confidence in calculations of the above description. She had not obtained an observation for her latitude for two days ; her latitude by account was erroneous to the amount of thirty-four miles, and this latitude being used for working sights obtained for her chronometer, AND THE MAKINER'S COMPASS. 89 the computation gave a longitude one degree to the westward of the ship's place.* 40. The makers as well as the managers of mariners' compasses should thoroughly understand the ele- mentary principles of magnetism. The compass, like chronometers and other useful machines, should be submitted to some test of its efficiency after it is made, and before it is offered for sale, or brought into use at sea. The compass, although less costly than even a common watch, is infinitely more useful than the best chronometer. A compass is not subjected to any trial or test of its accuracy of manufacture, its magnetic intensity, or the amount of its friction on the pivot upon which the card traverses, and the amount of its directive power, compared with the weight of the needle and its card. Generally, we may remark, that neither the vendor nor the purchaser of a compass knows much about these matters ; the former being satisfied if he realise a good profit, and the latter being pleased if he purchase a handsome article. The essential qualities of an efficient compass are, great directive power combined with little weight or friction on the pivot; the compass-bowl being freely slung in jimbals attached to the box, so as to preserve a horizontal position under all cases of the ship's rolling or pitching. The steel of the needle should be of pure metal, of uniform hardness throughout, and magnetised to saturation, and the magnetic intensity of a compass-needle should be preserved by all possible * Vide sentence of Court Martial appointed to enquire into the causes of the loss of the Challenger. Sumner's method for verifying a ship's position affords a check upon the reckoning, in cases similar to the Challenger, when the dead reckoning cannot be depended on. 90 THE MAGNETISM OF SHIPS, care. We have said, that a compass-card should be submitted to some test. Now, a very fair and efficient test of the magnetic energy of a compass-needle is to try if two similar cards will, by their mutual magnetism, support each other's weight ; that is to say, if the north point of the needle of one card be applied to the south point of a similar one, and their mutual attrac- tion be such as to support the weight of the card, the magnetic intensity of such needle may be regarded as sufficiently strong if they mutually support each other's weight, along with the cards they respectively carry. Now, the magnetic intensity or power of every compass- needle should remain permanently a constant quantity, and this can only be accomplished by each ship being supplied with a pair of artificial magnets, of sufficient power to renovate the magnetism of the compass- needles, whenever the test might indicate that they required re-touching. The magnetism of a single needle is, probably, best preserved by allowing it to traverse freely on its pivot, or else to be stowed in a direction parallel to that of the magnetic dip ; for if it be placed, say, with its south pole (or north point) towards the south pole of the world, or vice versa, its magnetic intensity will decrease ; or, if a compass be placed near to a large mass of iron, in such a manner that the magnetic polarity of the iron may act so as to control the magnetism of the compass, to a certain extent, by acting in a contrary way to that of terrestrial magnetism, then will the magnetism of the compass-needle be deteriorated. If, for example, the south point of a compass-needle were placed near to the upper end of a vertical iron pillar, the needle AND THE MARINER'S COMPASS. 91 would be deprived of a portion of its magnetism ; but if the north point of the card be placed near, or in contact with the upper end of the pillar (in north dip), the magnetic energy of the compass-needle would be augmented. We have mentioned, that the magnetic poles of the same name or kind, in any two similar and equal artificial magnets, repel each other. There is a con- stant effort exerted between them to obtain the mastery, and so they mutually destroy their magnetism ; and if two such magnets be so situated for a considerable time, their magnetism will be nearly destroyed. There was formerly a common steering - compass, made by John Syeds, in June 1810, in the binnacle of the Plymouth Breakwater light- vessel ; it was the only compass in the vessel, and had been twenty-eight years in her. No care had been taken of this card, which had remained in its box for so many years, and yet it retained a considerable amount of magnetism. If this card, instead of having one needle only, had been fitted with two or more equal and similar parallel needles, we venture to assert, from the principles we have explained, that the magnetic force of these needles would, in a comparatively short period of time, have been reduced to the natural standard of the earth's magnetism. The spare compass-cards, carried to sea in ships, should be stowed in boxes, and their opposite poles connected by pieces of soft iron, in the judicious manner recommended by Professor Barlow, and practised in the Royal Navy. 41. We have already shewn, by experiment, how the changeable polarity of the inductive magnetism, in 92 THE MAGNETISM OF SHIPS, the metals within a ship, either draws the compass- needle quietly aside from its correct magnetic bearing, in smooth water, or else causes the compass-card to maintain a constant oscillation of a point or two, on each side of the course, when a ship rolls heavily from side to side in stormy weather ( 25). These trouble- some oscillations give rise to the most serious obstacles to good steerage. There are but few helmsmen to be found who can steer a ship in stormy weather, when the compass-card is swinging about with every roll or lurch of the ship. And when, " With labouring throes, she rolls on either side, And dips her gunnels in the yawning tide; Her joints unhinged in palsied langour's play, As ice-flakes part beneath the noontide ray : The gale howls doleful through the blocks and shrouds, And big rain pours a deluge from the clouds ; From wintry magazines that sweep the sky Descending globes of hail impetuous fly, High on the masts, with pale and livid rays, Amid the gloom portentous meteors blaze." * It is under such circumstances as are described by Falconer, that we are taught to appreciate the worth of a good helmsman, and the value of an efficient compass. One card is exchanged for another, and weak needles are loaded with heavy weights, in order to lessen the oscillations ; but neither brass bars, nor brass rings, wax, paper, nor talc, can cure the evil: for as we increase the weight and friction of the card, we only make it the more sluggish and unfit for the helmsman's use, who, instead of being guided by the compass * Falconer's Shipwreck. AND THE MARINER'S COMPASS. 93 in the binnacle, must ever and anon keep looking ahead at the clouds, the waves, or the stars ; for he finds, that a sluggish compass does not indicate a change in the direction of the ship's course, until some time after that change has taken place.* 42. The brass-box compass having been found to be more steady than wooden-boxed compasses, the latter have been almost entirely laid aside, without sufficient reason; for it is admitted, that a compass- needle mounted in a wooden box is more sensitive than one in a brass box, when all other things are equal. Let us enquire how this happens. f The comparative magnetic inductive susceptibility of the metals is very considerable ; that of copper to mahogany as 29 : 0,37, or as 78 : 1 nearly. Now, all magnets have a power of communicating a certain portion of magnetism to substances brought within their spheres of action. Thus a magnetic needle enclosed in a mahogany box would communicate a magnetism to the box ; the north pole of the needle imparting a south polarity to the wood near it, and the south pole giving out a north polarity to that part of the wooden box opposite to the south pole of the magnetic needle. There would, therefore, result a certain amount of attraction between the ends of the magnetic needle, and those parts of the wooden bowl nearest to the needle ; and as the magnetism is taken up in less time than it is parted with, the induced magnetism of the box would tend to retard any oscillation of the needle. The magnetism * See Extract from Official Report on Compasses, in Appendix G. f The Chinese avoid the use of metal in the mounting of their compasses. 94 THE MAGNETISM OF SHIPS, of mahogany is very small indeed, and can only be detected by such delicate and elegant instruments as were used by Sir W. Snow Harris ; but the magnetic inductive susceptibility of copper or brass being about 80 times greater than that of wood, its effects become sensible and apparent. We see, then, that a copper or brass-mounted compass is more steady in a gale of wind, because its box is inductively magnetised from its enclosed magnetic needle ; and, therefore, although it be really more steady in its vibrations, it is also more sluggish in its motion than it would be if mounted in wood instead of metal. Compasses mounted in metal may also be more steady in stormy weather, by the screening influence of the metals surrounding the card ; take, for example, a gun on each side of the binnacle, where the polarity changes at every roll of the ship ; then the changeable magnetism given out by the guns may be in some measure screened by the surrounding box and rings of the compass. It is therefore prudent and proper that ships should be supplied with brass or copper-boxed compasses for use in bad weather; but they should have at least one compass mounted in wood, for use in light winds and smooth water, especially when the effect of local magnetism has not been corrected, or magnetic oscillations cut off by artificial means. Her Majesty's government determined on obtaining a set of superior steering compasses for the Royal Navy, to supersede the imperfect instruments formerly supplied by contract, and still in use in some ships. The Compass Department was placed under the super- intendence of an intelligent and efficient naval officer, AND THE MARINER'S COMPASS. 95 whose duty it was to determine the local magnetism of ships on the Home Station; to supervise magnetic experiments, furnish ships with tables of their compass deviations, and recommend every known means for the improvement and preservation of the compass. Every ship should have a standard compass, of a superior description, fixed in some convenient part of the ship, and raised above the ordinary level of the binnacle, in order that bearings, amplitudes, or azimuths, may be the more conveniently taken by it. The compass course of the binnacle should be referable to the standard compass, and corrected accordingly, the local attraction of the ship on each point being previously found on the standard compass, as the ship swings round the horizon. These arrangements might prove highly advantageous and economical to the naval department, by lessening the number of casualties ; for the money value of even a single steam vessel, or large frigate, would defray the expense of keeping up a set of compasses for the Royal Navy for ever. The steering apparatus, instead of being, as heretofore, consigned to the care of the boatswain, and stowed away in his store-room, with iron hooks and thimbles, chain cable gear, &c., and adjacent to the carpenter's and gunner's store rooms, crammed with all kinds of metals, will in future be placed in the master's charge, who, being entrusted with the navigation of the ship, is of course the proper officer to have the care of the mariner's compass the most important of all machines. When compasses were served in by contract, the contractor's aim was the making of money. He made 96 THE MAGNETISM OF SHIPS, his needles, not of pure hard steel, but of soft iron pointed with steel. Such needles were easiest made and most readily magnetised, and they required more frequent repair and cleaning. Being stowed away without care or attention, these needles soon lost their magnetic energy, and were returned from ships to the dock-yards rusty and unserviceable; they were sent back to the maker for repair, for which their was a price, and also a price for re-touching weak needles. The consequence of all this was, that the expense of the compass department, imperfect as it was, was greater than it will be in future.* 43. The generality of sea-faring men are not so well informed about magnetism as they should be. How can they, since philosophers differ in opinion about their respective theories ! We have touched but lightly on these theories, as our object has been rather to teach the navigator a few of the fundamental prin- ciples of terrestrial and inductive magnetism, upon which the practical utility of the mariner's compass depends. These principles should form a part of the navigator's education : they are essential to the prac- tice of his art. When Mr. Norman, the compass maker, discovered, in the year 1580 ( 9), the tendency a magnetic needle had to dip and depart from a horizontal position, when balanced by him, previous to the needle being mag- netised, he found it necessary to add a weight to the south arm of the needle, to restore the card to its level. He ma'de the first dipping needle ; but it * Tills practice is still pursued in the merchant service. The ship- chandler deals in compasses : the mathematical instrument maker seldom. 97 never occurred to him, that, by adding a weight to the south semi -circle of his steering compass - cards, he was adding to the errors of the mariner's compass. Strange to say, the practice has been continued up to the present time. Philosophers, too, have lent their sanction to ancient practice ; so that the compass has been actually made mechanically incorrect, by er- roneous views being taken of the philosophy of its mechanism. We are taught to believe, that all heavy bodies tend or gravitate in a direction perpendicularly downwards towards the earth's centre, and that all ponderous bodies, when suspended by a string, or supported on a pivot, will have their centres of gravity directly under the point of suspension; that is to say, the point of suspension and centre of gravity will be in the same vertical plane or direction. This opinion is fallacious. It is only true of bodies that are not magnetic. A magnetic needle, when freely suspended on a point or pivot, has its centre of gravity drawn from beneath its point of support in all magnetic latitudes ; and hence a compass-needle, with the card and cap attached to it, may be regarded in a constant state of mechanical instability, unless it be at the mag- netic equator, where the needle has no tendency to dip. When a plain unmagnetised steel needle is fitted to its cap and card, and placed upon its pivot in the compass-bowl, its centre of gravity will necessarily be directly under its point of support ; because gravity is the only force acting upon it. If all its opposite parts be similar and equal, the card will be perfectly hori- zontal. Let the needle be magnetised, and a new H 98 THE MAGNETISM OF SHIPS, force is introduced ; the north end of the needle dips or points downwards, and the south point of the needle is elevated; and, consequently, the centre of gravity of the card has changed its relative position with the point of suspension. The magnetism has brought about this change, without adding any weight to the needle: the north point of the needle is at- tracted downwards by the earth's magnetism, and the south point is repelled upwards by terrestrial mag- netism. The usual means adopted to remedy this de- flection of the compass-needle is, to fix sliding weights of brass to the needle, to restore the card to a level, and thus make the card apparently in equilibrium. In order to make my meaning more clear, let the following figures represent sections of a common com- pass-needle, with its card and cap resting upon the pivot. First, when properly balanced, then when mag- netised, and finally when re-adjusted by a weight. Fig. 1. Fig. 1 is the steel needle p N S devoid of magnetism : by the laws of mechanics, its centre of gravity and point of suspension in the agate P are in the same vertical. Fig. 2 is the same needle after being magnetised, and dipping towards the north. Its centre of gravity G is now drawn from under the point of suspension. It is now to the southward of a perpendicular let fall from its point of suspension at P. AND THE MABINER'S COMPASS. 99 Fig. 3 is the same needle restored to its horizontal level, by means of the weight W, fastened to the needle near its south point, which weight apparently restores the needle to its equilibrium ; but in reality determines the mechanical centre of gravity of the mass to be nearer to the south than the north point of the needle. It is a self-evident fact, that the south part of the card is made heavier than the opposite part; and therefore their common centre of gravity is not in the same vertical with the point of suspension P, although it may appear to be so. The south arm of the needle is actually made heavier than the north arm, by the weight W.* And this is the condition in which all ships' steering-compasses are made, up to the present time. The mariner's compass, then, is an imperfect instru- ment, in a state of constant instability, by reason of the conflicting forces of gravity and magnetism ; yet no inconvenience arises in its use, in Jine weather and smooth water. But when the sea runs high, and the ship is rolling or plunging about among the waves, the compass, which is always fixed at a considerable height above the ship's axis of rotation, is carried backward, forward, and in every direction through the air, at the rate of 20 or 30 feet per second. Matter, by its inertia, resists any change endeavoured to be made in its state, whether of rest or of motion. This is admitted on all sides, although doubts have been entertained about * The weight W will always be a measure of the magnetism of the needle N S, when multiplied by the leverage from the pivot P. H2 100 THE MAGNETISM OF SHIPS, the amount or quantity of force required to put a body of a given weight in motion, or, when moving with a given velocity, to stop it. Some have maintained, that the moment of inertia, or rather the momentum of a body in motion, is as the simple velocity ; whilst others have contended, that the force of a body in motion must be proportional to the square of the velocity with which it moves. 44. It appears to me, that every effect produced must necessarily be proportional to the cause of that effect ; and that, when a body is in motion, we must take into consideration the quantity of matter in the body, and the velocity with which it moves. Now, the force arising from the quantity of matter in the body must necessarily be proportional to the quantity of matter; and the force arising from the velocity of the motion (as a cause) is necessarily pro- portional to the velocity of the motion. The whole force, then, arising from the quantity of matter, and the velocity of its motion, must be proportional to these two causes taken together; and, therefore, in bodies of equal weight, having equal quantities of matter, and moving with equal velocity, their momenta will be equal. If the force of a ponderous body in motion were as the square of its velocity, then would a cannon ball of 24 pounds, when moving with a velocity of 100 feet in a second, be arrested in its flight by coming in collision with a ball of only 6 pounds, and moving in an opposite direction with a velocity of 200 feet in a second. We know experimentally, that it would require a bullet of 12 pounds in weight, and moving with a velocity of AND THE MARINER'S COMPASS. 101 200 feet in a second to stop it. To imagine that the motive force of a body of a given weight, when moving with a given velocity, is as the square of that velocity, is to imagine that the force which arises from the velocity is equal to the square of itself.* If this reasoning cannot be controverted, it is useless to suppose that any arrangement of sliding weights upon the needle of a steering-compass, whether they be placed nearer to or further from the point of suspension, can be made to balance the card on a level, and at the same time make the inertia of the opposite semi-circles of the card perfectly equal under all cir- cumstances. If our reasoning be applied to a compass-card with an unmagnetised needle, mounted on its pivot in the usual way, in a compass-bowl, its centre of gravity would be in the same vertical as its point of sus- pension ; all its parts would be in equilibrium by the force of gravity ; and although the card might vibrate by reason of its pendulous centre of gravity when put in motion, yet there would be no tendency in the card to oscillate or swing in a horizontal plane ; because the inertia of all its opposite parts would be equal, and the centre of its inertia would be in the same vertical line as the point of support: in a word, there would be no reason or cause why the card should not remain on its pivot, without turning round or oscillating upon it. Let now the needle receive the magnetic touch in north magnetic latitude. The needle arranges itself in the direction of the magnetic meridian ; the needle dips, * See Appendix I. 102 THE MAGNETISM OF SHIPS, and drives the common centre of gravity of the card to the southward of its point of suspension. There is now a greater quantity of matter to the southward than there is to the northward of the pivot's point; and if we put the compass in motion alternately in an east and west direction, the south semi-circle of the card lags behind, by reason of its superior weight ; and, if we bring it suddenly to rest, the south part of the card goes on, by reason of its momentum being greater than that of the opposite semi-circle, the directive force of the magnetised needle striving to get the mastery of the disturbing mechanical force arising from gravity, and the unsupported position of the common centre of gravity of the card and its appur- tenances. 45. In high magnetic latitudes, where the dip is great, the mariner's compass, in its present construction, must be regarded as a very imperfect instrument, by reason of the disturbing forces that exist between gravity and magnetism. Gravity acts in a line perpen- dicularly downwards ; but magnetism acts in a line parallel to the direction of the magnetic dip, and varies in different latitudes from a horizontal direction, at the magnetic equator, to the vertical one at the magnetic poles. When a compass-needle is but slightly magnetic and the card is comparatively heavy, such a compass will be very sluggish in smooth water, but tolerably steady in stormy weather ; but if the card be a light one, and its needle a powerful magnet, then will the compass be sensitive and serviceable in smooth water and fine weather, but unsteady and of little use in a storm. The reasons are obvious enough : a weak mag- AXD THE MARINER'S COMPASS. 103 netism in a heavy card but slightly deflects the centre of gravity from its point of support ; whereas a pow- erful magnetism in a light card has the effect of deflecting the centre of gravity very considerably, and consequently, the momenta of inertia in the opposite parts of the card must be very unequal, when the ship rolls heavily in high magnetic latitudes, and is steering a course near the direction of the magnetic north or south points of the compass. All kinds of plans have been tried to diminish this oscillation of sea compasses. Extra gimbals have been applied ; cards have been placed in rectified spirits of wine ; heavy weights have been added ; compass-cards have been mounted on their pivots like toad-stools on their stalks, and supported by metal braces like a lady's parasol, or a gentleman's parapluie ; chain cables have actually been shackled on to the lower side of compass needles, in order that the ends of the chains might drag about in the compass-bowl, and as it were moor the needle in the direction of the ship's course. Of course, all these patent contrivances were of no practical utility. How could they be ? Can the physician cure the malady without knowing something of the nature of the patient's disease ? Captain Johnson, superintendent of the compass department, states, at pages 77 80 of the 2nd edition of his book, the result of a trial of different kinds of com- passes in bad weather, on board the Garland steamer, between England and Ostend, in 1850, and made by direction of the Lords Commissioners of the Admiralty. The following compasses were then tried in the Garland: 1. A standard compass on its elevated MM l ME MAGNETISM ol SUM'S, j)ill;tr. '2. Mr. Dent's large compass in the binnacle. .'{. A lnjiud compass by Mr. Preston. 4. A compass on treble gimbals, with point of .suspension elevated one quarter of mi inch above the plane of the card; its pivot passing through a brass collar si'inilar to Mr. Walker's plan (hut eer ainly not my plan). . r >. A compass with an edge bar needle, ruby point, u spe- culum metal cap, and four ivory pins on the lace of the card, their points being j. of an inch from the glass cover when the card was on level, or parallel to the cover on Captain Johnson's plan. C. A compass l>y Messrs, d'rey and Keau, of Liverpool, having ii card suspended in double central gimbals, with two needles on axes so as to dip. The results obtained, as stated by Captain Johnson, who was accompanied by Mr. Dent, in the Garland, are as follows : That Nos. 2 and 3 were the steadiest under all circumstances ; but that when the vessel was running before the sea, or with the sea on the quart t -r, Mr. Dent's compass became unsteady: that compass No. 6, on Captain Johnson's plan, and No. 4, in some measure resembling one of mine, were sulliciently steady to steer by, till the sea became turbulent and struck the ship, when the standard compass vibrated twelve points : Nos. 4 &t 5, from one to one and a half points : No. 6 (the Liverpool dipping compass) swung half round: and ordinary compasses spun wholly round. To ivmed\ these oscillations of the Admiralty com- passes, Captain Johnson states, that he has applied four glass beads, suspended from the glass compass- bowl cover, to within | of an inch of the card.* * II.".. wi tin. I it officially reported, in 18AO, Unit in a heavy HCII th* AND THE MARINER'S COMPASS. 105 I am at a loss to understand why the Admiralty, or Compass Committee, did not get a bond fide compass on my plan, to be tried with the before-mentioned compasses ; or why the many compasses fitted on my plan in Dcvonport Yard, and sent to sea in H.M. ships, lia\c not been returned to Devonport, for examination or re-adjustment by the artificer whom I was directed to instruct by the Lords Commissioners of the Ad- miralty, about eight years ago. 10. We have now shown, that the mariner's com- pass, in its ordinary construction, is an imperfect instrument, and we have explained the nature of its imperfection; we have also shown in what way the induced magnetism of a ship's iron acts upon the compass. It remains to be shown how the mariner's compass itself may be rectified, and how the magne- tism of a ship's iron may be cut off from acting upon the compass, by means of the same kind of materials tliat disturb it in different magnetic latitudes, and not hy permanent magnets. I stated in a former part of our subject, that an efficient compass should possess a great magnetic energy, with small weight and friction on the point of suspension ; and the reader will have noticed that these conditions appear to be incompatible : they may, however, be united in the same compass. A small ('onimill.-,' ^tmi.hml compuHH vibrated 12 points,aml that glass hoods were to he 8UH|H'H(1<>(1 from the cover of the steering compass, eo as to touch and retard an unsteady compass-card. Ten years ago, the writer made an official statement, that the standard compasses at 25 each possessed the yiitid at a g y at jz; 1 I 2 lengths. Y< 3 8 P 2 lengths. 3 K 3 1 2 lengths. in. gr. in. gr. o in. gr- 1 6i 6-25 17 12 fy 625 18 1 6 625 14 2 626 15 13 62-5 17 2 6^ 680 14 3 620 18 14 " 630 18 3 780 14 4 620 18 15 630 15 4 6j 725 13 5 625 16 16 " y 630 17 5 6, 755 10 6 620 17 17 625 17 6 6. 750 14 7 625 18 18 1 " 630 15 7 6 J 780 17 8 630 17 19 625 19 8 (; 785 16 9 630 19 20 jL 625 18 9 6 685 13 10 620 18 21 635 18 10 6 J 735 16 11 625 18 22 n 62-5 16 11 ej 735 16 Number 1 of Mr. Cox's needles was & pattern needle, sent into the Dockyard as a guide for the workmen, by which pattern, two dozen were to be made of the same length and weight. Mr. Cox sent in only one dozen, instead of two dozen, for trial and transmission to the Admiralty. The reader, on looking over the table, will see in the columns of deflection, the magnetic and directive power of each needle in proportion to its weight and length, &c. The whole of Mr. Cox's needles were made shorter and heavier than his pattern 118 THE MAGNETISM OF SHIPS, needle ; and yet their magnetism, by deflection, appears inferior to lighter needles, tested at a greater distance from the deflected compass. A needle of the same weight and power as No. 20, when fixed to its card and cap, &c., weighed 1080 grains, and was sent on board the Recruit as a binnacle compass, on my plan, for trial, with about 15 other kinds of compasses. I weighed the Recruit's standard compass-cards, and compared their deflecting power with mine, and the following is the result ; viz. 1st. Mr. Walker's card . . 1 needle, 1080 grains, deflection 18 at 2 lengths. 2nd. Admy. card in .Recruits 1570 17 3rd. Ditto ditto ditto, 12 2460 23 These compound needles for the Recruit's standard compass were made of Strasburg steel, manufactured in a particular way,* for being fitted as main- springs for chronometers. It will be evident, by reference to the above results, that the directive power of the com- pound needles, as compared with that of the single needle, No. 1, are really small in proportion to the weights they have to carry; and yet, in this vessel, these heavy cards were among the most unsteady in the brig in bad weather ; whilst my powerful but light card, with its compensation for dip, and its double suspension, was the steadiest as a steering instrument, and was sought for by the quarter-masters and helsmen, when other experimental compasses failed them at night. The question, then, does not involve itself into one of additional friction, but is dependent on the principle. I was the first to explain and demonstrate * See Appendix, H. AND THE MARINER'S COMPASS. 119 to the Lords Commissioners of the Admiralty, a prin- ciple which compass makers and the Admiralty Com- mittee are unwilling to admit, and ashamed to deny or publicly discuss.* 52. Experience has proved, that our earth gives out, or imparts magnetism to every thing upon its surface, just in the same way that magnets commu- nicate their magnetism to bodies capable of receiving it, when placed within their sphere of action. Iron of every kind has, of all known substances, the highest capacity for receiving magnetism, and hardened steel has the greatest capacity for retaining it. We have shewn (21) that a cast iron shot or sphere becomes pervaded by magnetism received from the earth inductively. That the shot has a north and a south pole, with a magnetic equator, where the at- traction = 0. That the ball resembles a little world, and acts magnetically upon a small delicate compass- needle, by attraction or repulsion, just as we please ; for in changing the position of the shot, we alter the position of its magnetic poles, as well as of its mag- netic equator. If the directions given at 21, jig. 4, be followed, and a line be drawn, or a thread tied round the ball, parallel to A B, or at right angles to the dip C D, then this line A B will represent the magnetic equator, and will divide the ball into two magnetic hemispheres ; so that every part of the ball above the line A B will attract the north end of a small compass-needle, and under the line the north point will be everywhere repelled. I know of no experiment tjiat so clearly * For reports on my compasses, in several ships, see Appendix G. 120 THE MAGNETISM OF SHIPS, demonstrates the nature of induced terrestrial mag- netism as this one. The reader will bear in mind, that every article of iron (cast or wrought), whatever its form or magnitude may be, will be magnetic by induction from the earth, and have two poles of op- posite kinds, just like our shot ; and whose axis will be parallel to the dip at the place of observation or experiment. It is on this account that an iron ship exhibits such powerful effects upon her compasses. A floating iron ship may be regarded as an inductive floating magnet, liable to have its poles changed by every alteration of course, or of every change in the inclination of the vessel, or transfer to another magnetic latitude. The reader, in looking back to 21, and figs. 1, 2, and 3, will observe that the bolts or bars, placed in different positions, act differently on each other, or on a small magnetic needle ; but these bars are made magnetic by the earth, in the same way as the iron sphere is mag- netised by it. The positions of the induced poles of any regular mass of iron will always be referable to the magnetic dip at the place of observation ; for example, on the earth's magnetic equator, the polar axis of the iron in a ship, instead of approaching the vertical, as in high latitudes, would actually be in a horizontal direction, and parallel to the magnetic meridian. There are upwards of 1000 tons of iron employed in the construction and equipment of a first-rate man-of- war, and many merchant vessels carry cargoes of that metal, stowed in all kinds of ways. Need we be sur- prised, then, that the steering compasses are deranged by the magnetic action of ships and their contents ? AND THE MARINER'S COMPASS. 121 53. The deviations in any ship will be the result- ant of all the magnetic forces of the guns, bolts, bars, anchors, tanks, &c., acting on the compass in its place on each course ; and these agencies not only change in direction and amount, as the ship changes her geographical position, but they alter as the ship turns round in any place, and change with every roll or inclination of the vessel. A large spike nail near the binnacle may even act on the compass as powerfully as the best bower anchor, in its place at the cat-head. This subject is one of much complexity. Magnetism of this kind is a mysterious agency, known only by the effect it produces ; perplexing and often bewildering the mariner in the use of his compass, in dark or foggy weather, when he is unable to correct his reckon- ing or adjust his compass by astronomical observations. When a ship is upon the magnetic equator, her compass-needle has no tendency to dip ; and when near the magnetic pole, the needle, however free to move, has no tendency to horizontal direction : it would tend to point perpendicularly downwards, and be useless for purposes of navigation. We seldom hear complaints of compasses near the magnetic equator ; for here the magnetic line (dip) coincides with the magnetic meri- dian, and all the iron in a ship will have its poles in a direction parallel to the compass-needle, or nearly so. But when ships advance into high latitudes, the dip increases ; the directive energy of the compass becomes less and less ; whilst the induced magnetism of the ship's iron increases, and its disturbance is greater, and the deviation of the compass or its local attraction is augmented. 122 THE MAGNETISM OF SHIPS, Powerful compasses then require additional weights, to preserve horizontality in the cards ; and this apparent balancing actually increases the difference between the weights of the north and south semi-circles of the compass-cards ; the balance weights making the differ- ence between them : the ship as she rolls or rocks from side to side, carries her compasses through the air, alternately in opposite directions, and the north and south sides of the needle and card being actually made of unequal weights, a mechanical oscillation of the compass-card, in a horizontal plane, arises, which sea- men might with propriety compare to the swinging of a topsail, or topgallant yard, when the braces are let go, and one yard-arm is made heavier than the other by a man upon it. Another great inconvenience arises to produce errors and oscillation in a compass, from the induced magnetic poles of the iron in a ship passing from side to side, by the inclination or rolling of the vessel. This disturbance is greatest when the course is north or south, and least when steering east or west. This agency has already been noticed, and demonstrated in 25, by suitable diagrams. The mariner, then, has two disturbing forces to contend with in the use of his steering or azimuth compass : 1st, The mechanical error arising from the centre of gravity of the needle being drawn by the earth's magnetism on one side of the pivot of support ( 43, fig. 2). 2nd, The change which is constantly taking place in the position of the induced poles of every article of iron in the ship, as she changes her latitude, course, or seat in the water. With regard to the mechanical error, it has been AND THE MARINEK'S COMPASS. 123 shewn ( 46, and Appendix) that this has been got rid of. But some may argue, that the very small weights that are really required for a compass of the ordinary construction, to counterbalance the ten- dency of the needle to dip, are altogether unim- portant and insignificant. It may be said in reply, if these sliding weights are unimportant, why apply them? The fact is, they are important; they must be of sufficient magnitude to preserve the card in a horizontal position ; and they are actually found to cause considerable oscillation in high latitudes : whereas near the equator no inconvenience is experienced in any compass. A powerful compass-needle, magnetised to saturation, will require six or eight grains near its ends, to counteract the tendency to dip ; and if any sensitive needle be moved alternately in an east and west direction, by hand or otherwise, the needle will oscillate from its meridian, and we can predict the direction taken by either of its poles : for in any latitude whatever, the heaviest end of a compass-needle will be opposite the dip ; and this end of the needle, by the well known laws of mechanics, will first resist motion : but having once acquired motion, the heaviest end will be disposed to go on at the end of the motion. 54. About ten years ago, I made a sectional model of a frigate, with iron guns run out in their respective ports, on each side of a small compass, placed on the quarter deck. The model itself was entirely free from iron, and was intended to shew the way in which a compass was acted on by the magnetism of the guns alone, either as producing a permanent error in smooth water, or an oscillation in a sea-way. This experiment 124 THE MAGNETISM OF SHIPS, was made at the Trinity House and at the Admiralty : it was convincing and conclusive. At the request of Admiral Sir George Cockburn, I shewed how this oscil- lation was prevented, by placing a piece of soft iron in such a position as to have its poles changed with the rolling motion, and to operate in such a manner as to counteract the effect of the guns. In rolling the model, with its guns in an east and west direction, the compass remained perfectly steady. An Admiralty order was immediately sent to the Port-admiral at Plymouth, that I should prepare a pendulous appara- tus * for counteracting the magnetic disturbance of the guns, &c., of the St. Vincent, then lying as flag-ship at Portsmouth, but without tanks and many of her guns. The harbour of Portsmouth is not deep enough for such ships to lie afloat in with everything on board ; and I suggested to their lordships, that an experiment made under the conditions referred to would not be of much scientific utility ; that errors and other conditions should be ascertained, before a remedy could be pro- vided; and I named the Caledonia, of 120 guns, as a proper vessel to .experiment upon, she having her full equipment on board. An order was accordingly issued to try the amount of the Caledonia's magnetism when inclined each way ; but she was ordered to sea, and this experiment was tried in the St. Vincent, on her arrival at Plymouth, in September, 1844. This interesting experiment, although originating with me, was not made in the way I should have made it. It was made under instructions issued to the superintendent of the compass department, who had * See Appendix I. AND THE MARINER'S COMPASS. 125 orders to communicate with me on the subject. I of course attended to the experiments, recommended some to be made, and noted every thing : and although all was not done that ought to have been done for the advancement of science, we obtained sufficient data to shew, that the principles I had previously propounded were borne out by the experiments made in the St. Vincent; namely, that the changeable polarity in the ship's iron would be apparent as the ship was either turned round, or inclined on each side from an upright position. We will now describe the nature and extent of these experiments, which, being made in a ship of three decks, with every thing on board, are not devoid of interest. The St. Vincent mounted 1 20 guns ; had a complement of 1000 men on board, with three months' provisions, and 410 tons of water stowed in her hold ; she had 307 malleable iron tanks, and 263 tons of cast iron ballast. The ship was under sailing orders, and all her stores were complete. The steering compasses of all ships are necessarily placed near the helmsman, at the stern of the vessels. The compass then is above and abaft the great masses of iron in a ship ; hence it results, that the north point of the compass-needle is, in our hemisphere, drawn forward by the magnetic attraction of the ship's iron ; whilst the south end of the needle is driven aft by repulsion ( 21). Our object was, first to discover how much, or through how many degrees the St. Vin- cents compasses deviated, as the ship was turned round in a perfectly upright position, with every thing in its proper place, as if the ship had been at sea. This 126 THE MAGNETISM OF SHIPS, operation had nothing new in it, being similar to the usual process pursued for determining the compass deviations generally. The correct magnetic bearing of a distant object is first ascertained. It should be sufficiently distant, so as not to have its bearings sen- sibly altered by a ship turning round in a circle, of which her own length is the radius. In the case of the St. Vincent, we selected a high hill or tor on Dartmoor, as our object of standard bearing, distant from us about twelve miles. 55. The ship's head being placed upon the mag- netic meridian, or in a north and south direction by the standard compass, she was gradually turned round, carefully stopped, and firmly held in the direction of every one of the two-and-thirty points of the compass, until observation and comparison was made of the correct, and the compass bearings of the aforesaid tor on Dartmoor. The result of this experiment proved, that the greatest amount of local attraction of the St. Vincent, upon her steering compass in the binnacle, did not exceed half a point, when perfectly upright. This kind of deviation, in wooden ships, is almost always greatest when the ship's head is nearly east or west ; and it diminishes or entirely disappears at north or south. A deviation of half a point, although appa- rently small, is yet of too great amount to be neglected in navigation. A large ship like the St. Vincent might easily run 120 miles from sunset to sunrise; and a deviation from her course, of half a point, would give an error of twelve miles in her dead reckoning. It may excite surprise that a three-decker, containing at least 1000 tons of iron, should have her compasses AND THE MARINER'S COMPASS. 127 so slightly affected; when we meet with three times the amount of deviation in small vessels, with very little iron about them. The fact is, that in large vessels, their huge dimensions enable the mariner to place his steering compass at a greater distance from the iron than he can possibly do in a small craft. In the experiments under consideration, the St. Vincent's standard compass was 18jft. from the nearest carronade, and 53 feet from the nearest tank in her hold. Magnetic attraction varies with the distance. The force it exerts is found to be inversely as the square of the distance from a gun, &c. ; so that should a gun, at 18 feet from a compass, cause a deviation of only one degree, it would, at half that distance, or 9 feet, produce a deflection of 4 degrees ; and at 4J feet from the binnacle (not an unusual distance in small vessels), 16 degrees. We need not, therefore, be sur- prised that a large ship enables its navigator to remove his compass so far from the iron usually stowed in her, that the amount of compass deviations is less than that in smaller vessels. The next object we had in view was, to find if an inclination of the vessel to either side would exhibit any difference in the magnetic compass bearing of the distant object on Dartmoor, as compared with the bearings taken on each point, when the ship was up- right ; that is, if the magnetism of the ship would be transferred from side to side by the inclination. The guns and other things were secured in their places, as at sea ; one hundred tons of water, in wooden casks, - were procured from the shore, and placed upon the larboard side on the main-deck. This water, with the 128 THE MAGNETISM OF SHIPS, men's hammocks, inclined the ship 8J degrees to port. The ship's head was placed at north, arid we found that the muzzles of all the guns on the starboard side attracted the north point of the compass, and the breeches of all the guns on the larboard side attracted the same point: thus shewing clearly, that the north polar magnetism of the guns had moved along their length to the highest part of each ( 25). On going down into the hold among the iron tanks, which were stowed in two tiers and in close contact, we found that all the tanks on the starboard side of the ship attracted the north point of the compass, and those on the lowest or opposite side of the ship repelled the north, and attracted the south end of a compass-needle. When a delicate compass was placed amidships, on the square of the main hatch-way on the orlop deck, it exhibited an easterly direction of 1J point; the north point being drawn to starboard and the south point to port, by the magnetic agency of the tanks, and in an opposite way to that produced by the guns. The ship was then swung to each point successively round the compass, and observations were made upon compasses placed as follows ; one on front of poop, one in each binnacle, and one in the bread room. The compass in the bread room had previously been placed in the orlop, but it gave early indication of being powerfully influenced by the spindle of the capstan or tanks ; and it was removed (not at my request) to the bread room, at the greatest possible distance from the disturbing agency. 56. The result of this experiment of swinging the ship, when inclined 8J degrees to port, did not exhibit any great amount of change in the bearings by AXD THE MARINER'S COMPASS. 129 the poop compass ; but it was evident, that the amount of compass deflection, arising from the ship's incli- nation, was due to the difference between the varying polarity of the ship's wrought iron tanks, and her cast iron guns, &c. For all the guns attracted the north point of the. compass towards the lee side, and the two tiers of tanks, being in contact with the iron ballast and each other, acting as one mass, attracted the same point towards the weather side ; the magnetism of the tanks having the mastery over the guns ( 24). At section 33, we theoretically discussed the effect that a change of wind might have upon a ship's compass deviations, although her course remained constant any where between the cardinal points of the compass ; and that by the force of the wind on her sails, the ship, by a change of inclination, might even cancel her up- right compass error on one tack, and double it on the other ; the actual direction of the ship's head remain- ing unchanged. Now, the experiments made in the St. Vincent proved the correctness of previous con- clusions.* For example, when the ship's head was north-east by the standard compass, on its pillar on the poop, the ship's head was N. 41 E. by the binnacle compass (when on an even keel, or upright), and at N. 46 E. by the same compass when the ship was inclined 8J degrees to port. When the ship's head was south-west, or in an opposite direction, the binnacle compass in the upright position gave S. 49 30' W., and in the inclined position only S. 47 W. ; so that in the first instance the incline deviation was greater than the upright deflection ; and in the second case it was less. See Appendix D, K 130 THE MAGNETISM OF SHIPS, The quantities we have quoted appear small, and practically would be of small consideration, so far as a day's dead reckoning is concerned ; but the results obtained are nevertheless important, as developing a principle which neither Act of Parliament nor order in council can invalidate. I regret that the superintendent of the compass department did not deem it necessary to have the St. Vincent inclined the other way, and swung round with an inclination to starboard. He said his orders were to swing the ship round; and having done it in one direction, with one inclination, he deemed his orders fulfilled. Scientific conside- rations required that this huge three-decker should have been turned round with an inclination both ways. It was not done ; but the tabulated results obtained, enable me to give a cursory view of the induced polar magnetism of the ship having undergone a change by her inclination; and if no other experi- ments had been made, these tabulated results would have been inserted here. There are, however, more convincing and satisfactory experimental results ob- tained, which will appear in the sequel. I will now sum up such evidence as may be worth recording, relative to the magnetic trials made on board the St. Vincent, in September, 1844: 1st. That, by the ship's inclination, the induced polarity of the tanks and iron ballast passed from side to side ; the highest side attracting the north end of a compass-needle, and the lee or lowest side repelling the north, and attracting the south point. 2nd. That the 307 iron tanks (capable of holding 410 tons of water), resting upon 263 tons of cast iron AND THE MARINER'S COMPASS. 131 ballast, did exert as much magnetic influence upon the ship's compasses as 3263 tons of iron in the hold ; for an iron tank acts magnetically as a solid.* 3rd. That, although the nearest gun was only 19 feet from the standard compass, and the nearest tank in the hold 53 feet from it, the magnetism of the tanks and ballast, deflecting the compass in one direction, was greater than the magnetism of the 120 guns, bolts, knees, and other things acting upon the ship's compass in an opposite direction. 4th. That the actual compass deviation in the Si. Vincent, as demonstrated by the experiments in her inclined position, was due to the difference between the magnetic action of the guns or bolts, and the tanks and ballast. Since the distant tanks, resting on the cast iron ballast, exerted a greater influence on the ship's com- pass, in her inclined position, than the action of all the guns, &c., by drawing the north point to windward, and thereby obtaining the mastery over the guns' magnetism, as demonstrated in 25 ; I think it will be admitted, that an augmentation of guns, or a diminution in the tanks, would bring these induced magnetic antagonistic forces, acting on a ship's com- pass, nearer to an equality ; and thereby render a compass more steady when a ship is rolling. We may also fairly assume, that if a line-of-battle ship landed her guns, the incline deviations of her compass would be augmented, and her compass would be rendered more unsteady in stormy weather. These * Barlow on Magnetic Attraction. K2 132 THE MAGNETISM OF SHIPS, deductions appear to me to be perfectly legitimate ; but let us look for corroborative evidence. 57. H.M. ship Thunderer was ordered to land her lower-deck guns, in 1843, and convey a regiment to the Mauritius. She was. supplied with a powerful standard compass, fixed on a pillar on the front of her poop ; and this compass had cards of different kinds, and some additional contrivances to fetter the oscilla- tions, if necessary. Many of her cabin and upper- deck guns were struck down into the orlop, so that the Thunderer's remaining guns would act but slightly, as she rolled from side to side. This ship had also two tiers of wrought iron tanks, stowed above her iron ballast ; and being a ship of two decks, (instead of three,) her standard compass was fixed a deck lower than that in the St. Vincent; consequently the two- decker's poop compass was nearer to her tanks than the poop compass of the three-decker ; and therefore, the magnetism, or local attraction of the Thunderer's tank, as a disturbing agency on her compass, would be very considerable, and greater than that found by experi- ment in the St. Vincent. In steering a northerly or southerly course, there would be two disturbing forces to produce oscillation: 1st, Th& mechanical errors of the compass already mentioned ( 43), whereby the heaviest or loaded end of the needle would, at every .roll of the ship, swing towards the lee side : 2nd, The opposite end of the needle, by the ship's induced mag- netism, would be attracted by the magnetism of the highest or weather side of the ship, or weather side of her tanks in the hold. In our northern hemisphere, we have seen, that the north point of the compass 133 needle would be attracted (as in the St. Vincent) towards the highest or weather side, and the south point of its needle, with its load, would be dragged by its magnetism, and mechanically driven by its load, towards the lee side ; and all this by a constant suc- cession of impulses, as the ship rolled from side to side ; these two forces combining to produce an unusual oscillation. When the ship approached the magnetic equator, these disturbing forces would diminish or vanish in a short time ; but after passing the line of no dip, and advancing into south magnetic latitude, the polarity of the ship's iron would undergo a change ( 31), the dip being south, and the disturbing forces we have described would be reversed, but equally powerful in opposite but corresponding dips.* When the Thunderer returned from the Isle of France to Plymouth, in 1843, I was kindly furnished with a copy of the officer's report and observation upon the working of the standard compass, which was constructed under the auspices of the compass com- mittee. By that report, which I insert in the Appendix (C), it appears, that in latitude 44 N. whilst the ship was rolling 20 on each side of an upright position, and steering a southerly course, the standard compass oscillated 68. That in latitude 33 south, whilst the ship was scudding in a S.E.b.S. direction, in a heavy sea, and going about 12 knots an hour, the standard compass oscillated as much as 90, and became useless as a steering instrument ; whilst the old common and weak single-needled dockyard compass, from its abso- lute deficiency of magnetism, was tolerably steady. * See Tables of Erebus and Terror, pp. 57 & 59. 134 THE MAGNETISM OF SHIPS, The report further states, that the committee compass was more steady in the southern than in the northern hemisphere, "probably from the circumstance of the course being generally to the eastward or westward." Having written to the Admiral on the subject of these compass oscillations, I was ordered to present myself, and explain the cause thereof at the Admiralty. I accordingly went to London ( 49) ; but previously- received opinions in scientific quarters were looked upon with favour, whilst my explanations were regarded as doubtful or unimportant. It was then that the addi- tional friction principle began to be entertained by the compass committee. In 1847, the Asia, 84 guns, a sister ship to the Thunderer, was fitted to bear the flag of Rear- Admiral Hornby. She had steering compasses, as well as a standard compass, on the committee's plan. En- countering bad weather in the Chops of the Channel, her compasses were found to be useless, and the Admiral had to write on the subject.* We can now explain how these powerful compasses become useless in stormy weather, when in reality a good instrument is most wanted. In 1844, the Caledonia, 120 guns, commanded by Captain Milne, now a Lord of the Admiralty, was on a cruise at sea. Captain Milne watched the action of the Caledonia's compass oscillations, and he informed me, that the north point of her compass-card went towards the weather side, at the end of each roll: whence we may infer, that the Caledonia's compass oscillated from causes similar to those acting in the St. * She put back to Plymouth. AND THE MARINER'S COMPASS. 135 Vincent, the Thunderer, and the Asia ; and that these vibrations can only be cured by the mechanical con- trivances I devised, and by magnetic compensations made by antagonistic magnetic forces, introduced by means of soft iron correction. 58. In the year 1846, an iron sloop of war, named Recruit, was commissioned in the Thames by Com- mander (now Captain) Adolphus Slade . Her compasses were adjusted at Greenhithe, by Captain Johnson, who, as usual, supplied a table of deviations for each of the two-and- thirty points of her standard compass. When the vessel got to sea, the officers found that these deviations were inapplicable to correct the courses under sail. On arriving at Spithead, Commander Slade applied to have his compass re-adjusted, and the Admi- ralty directed that the assistant to the superintendent of the compass department should proceed to Plymouth in the Recruit, there to swing and re-adjust for com- pass deviations. On the passage down Channel, with foul winds and hazy weather, difficulties were experienced in finding the vessel's position; and these difficulties being men- tioned to me by Commanders Slade and Strange, I wrote, through the Admiral-superintendent, to the Admiralty, adverting to the rather unsatisfactory expe- riments made in the St. Vincent ; and requesting that, since difficulties had arisen in navigating the Recruit, and as she had to be swung, I might be allowed to determine the nature and amount of her compass deviations when inclined, as well as when upright, and to have these operations performed in my own ivay, without unnecessary interference. 136 THE MAGNETISM OF SHIPS, I obtained the sought-for authority,* and the vessel was moved into Barnpool, after being re-stowed, when she was swung by me in three positions. It was predicted in high quarters, that these ex- periments would be disposed of like those of the St. Vincent, and that the subject deserved no serious consideration. The result of the experiments, however, commanded attention. Commander Strange was sent down to repeat the experiments, and their Lordships ordered subsequent and similar experiments to be made in several iron ships, by Captain Johnson, who has recorded some of them in his valuable work, published in 1847. The attraction and repulsion of an iron ship's sides are seldom alike ; that is to say, the magnetical and mechanical axes of the ship, when her head is north or south, do not coincide, but cross the direction of the ship's keel. In the Recruit, this difference was about 5 ; in the JBirkenhead, it was 7 ; in the Styx and some other vessels, it amounts to 6 or 7, and sometimes to 8. The following table of the Recruit's deviations under different conditions, and the table at page 82, clearly prove that the compass points of no deviation have in every case changed considerably. These changes in the 'resultant points' arise from the variable inequality of the magnetism of the sides of the ships. The points of no errors in the Birkenhead, when in England, were near N.N.W. and S.S.E. ; but at the Cape, her compass would have errors on these points. Before the ship struck on the rocks off Point Danger, she had been steering a S.S.E. compass course, as was stated by the surviving helmsman ; and there is ground for our belief, that the officers who perished had placed * See Appendix K. AND THE MARINER S COMPASS. 137 too much confidence in the 8th practical rule for ascer- taining compass deviations ; which states, that the points of no deviation, when once ascertained, may be regarded as constant. The following Table exhibits the Recruit's Compass Deviations, when upright, and when inclined 8 both ways. Correct Magnetic Courses. Deviations of Compass. Deviations with Iron apparatus applied to partially correct Com- pass, when ship upright. When vessel upright. When vessel 8 to Port. When vessel 8 to Starbd. N. 4.15 East. 2. 6 East 9. East. 1. West. N.byE. 11.15 5.15 13. 0. N.N.E. 13. 8.30 19.30 . 2.30 East. N.E. by N. 15.30 10.15 20.18 3.45 N.E. 17. , 12. , 20.30 3. N.E by E. 18. 14.30 , 21.45 . 2.15 E.N.E. 18.30 16.15 22.30 , 1.45 E. by N. 18.45 , 13.45 22.15 2.45 E. 16.53 15. . 16.30 , 2. E. by S. 13.25 16.15 14. , 2.15 E.8.E. 13. . 13.10 , 9.15 1. SE.byE. 6.45 12.15 . 2.45 1.30 S.E 5. , 9.30 , 3. OWest. 2. S.E. by S. 2.45 . 7.15 10.4-5 2. 8.S.E. 5. OWest. 7.15 12. . 2. S.byE. 6.45 2.45 14. 2. 8. 6. , 1. OWest. 14. " 2. S. by W. 8.30 , 0.45 16.45 . 0. s.s.w. 9.30 , 2. . 18. 0.30 S.W. by S. 9. , 6.15 19.45 1.30 S.W. 9.15 . 7. . 17.30 . 2.30 S.W.bvW. 11.15 . 7.15 . 16.15 . 4. W.S.W. 11. , 9. , 14.0 1. W. byS. 10. , 10.15 12.15 . 0. W 8. , 10.30 , 11. , 1.30 West. W. by N. 6.45 , 10.15 7.15 , 3.30 W.N.W. S. . 8. 7.30 4.30 N.W.byW. N.W. 5.15 . 3. , 6.15 7. 4.15 . 1.20 , 4. ,' 3.45 N.W.bvN. 1.15 , 3. 0.15 East. 3.15 N.N.W. 0.30 East. 2.30 , 4.30 , 3.30 N.byW. 3. 1.15 6.15 3.30 Total amount of Deviations 296. 3 258.10 401.53 69.4.5 The Recruit had wooden beams, and iron topsides, and was armed with twelve long 32-pounder guns. On repeatedly swinging her round, we observed, 1st, that when her head was placed in a northerly direction, 138 THE MAGNETISM OF SHIPS, all the compasses abaft were more sensitive than when her head was in a southerly direction, probably on account of the brig's magnetism and terrestrial mag- netism acting in conjunction on northerly, but in opposition on southerly courses. 2nd, that on changing the ship's direction from point to point, a few minutes of time were required for the new compass deviation to be fully developed. The binnacle compass was placed before the wheel, and close to the after side of the -cabin skylight, affording but small means for fixing any iron materials for correcting the compass deviations ; and the means I did use for that pur- pose were without any regular authority. We were at a distance from the Dockyard, and time pressed upon us ; the services of the vessel being wanted at Madeira. Commander Slade, on receiving such a list of devia- tions for his compasses, enquired officially by what compass he was to keep his reckoning, or by what means he was to correct his courses ; * and was ordered to adhere to those deviations found and furnished by Captain Strange, which did not sensibly differ from the deviations found by Captain Johnson at Greenhithe. * When the Recruit left Plymouth, after being swung, on taking leave of Captain Slade, I jocosely said to him, " If you do not actually lose your iron-sided craft, do let us hear how you get on." I received a letter from him, dated from St. Michael's, 17th September, 1847, which began thus : "MY DEAR WALKER, I will ease your conscience, by telling you that we have found land. You could hardly have expected us to do so, after having be-devilled our compasses. Joking apart, our deviations have altered on many points. We are never easy when running, or making land, and have to trust to sights; but in the absence of sun, under such circumstances, I have made up my mind to trust to the binnacle compass. As eels become reconciled to skinning, so we get accustomed to faithless needles " The Recruit had sixteen different kinds of experimental compasses on trial, by order of the Board. AND THE MARINER'S COMPASS. 139 The following official report was made by me to the Commander-in- Chief at Plymouth : " Bovisand, 4th February, 1847. " Sin, In obedience to tbeir lordship's order of the 23rd November last, 'to cause the Recruit to be heeled as much as may be convenient, in order to try the effect that the in- clination may have upon the deviation of the compass, to report the results of the experiments, and to submit any observations I may wish to make upon the subject;' I have the honour to inform you, that the experiments have been made, and the results are exhibited in the annexed table. The ship's head was placed upon correct magnetic bearings, and not upon those points that a compass in the binnacle would indicate. For this purpose, the standard compass was used as a theodolite, to measure the angle that the ship's keel made with a distant object. The observers were the master and master's assistant, selected by the Commander, who, with myself, checked the ob- servations ; and in this way many sources of error were avoided. " It will be seen by the table, that the upright and incline deviations are very different. "When the ship heels to starboard, on a N.E. course, the error of the compass is greater than when upright; and when she heels to port, the error is less. In some directions, the difference may amount to a point, by a change of inclination from side to side. "Having ascertained these facts, I prepared an iron ap- paratus, viz., a sphere, cylinder, and pendulum, with a view to correct or dimmish the errors of the compass, which, in this iron ship, are very remarkable and complicated. I prepared this apparatus in the Dockyard, and fixed it in the brig ; but was surprised to find that the iron hull of the vessel had absorbed or weakened the magnetic energy of my apparatus ; so that the influence of the iron ship had greatly reduced the energy of my iron apparatus, thereby shewing that iron acts magnetically upon iron. It was therefore necessary to augment my apparatus, since the form and fitting of the Eecruit does not admit of a choice of distance. 140 THE MAGNETISM OF SHIPS, " On fixing this apparatus and swinging the ship, it was found an easy matter to make the compass correct, with soft iron, upon the cardinal points ; but the errors were considerable upon intermediate points. My aim was ultimately to make the eiTors on an average as small as possible, and thus place the Recruit in the condition of a wooden vessel, where the errors being small, no serious damage can arise by neglecting them, or even by applying them the wrong way, as I fear is sometimes done. " The results obtained have shewn that I was not wrong in the principle I had propounded, although I am not satisfied with the corrections. We had completed our operations, and were unable to account for appearances in the results obtained in swinging the ship inclined to port, when we discovered that the brig's skylight had been built, and her binnacle placed three or four inches nearer to one side than the other; and this circumstance is sufficient to account for the north point of the needle being drawn to one side of the vessel when her head is upon a north or south line. Assuming the skylight and binnacle to have been in the midship line, I fixed my apparatus accordingly ; but the original error in the skylight and binnacle has to some extent vitiated the experiments. The errors, how- ever, are greatly reduced in the upright positions ; and I am desirous of having a trial at sea, in order to settle an important principle in connection with the correction of a ship's compass. I found experimentally, about twenty-eight years ago, that the ship's iron at the Cape of Good Hope, produced a deviation in an opposite direction to the deviation in England ; so that if a vessel left England with an official table of deviations, and applied these corrections to her compass during the passage, her compass might, by such application, be made correct at starting, but might be three points in error at the Cape. I have insisted that such would be the case, either by using tables of corrections as found here, or by the application of permanent magnets to correct the compass. But I am of opinion, the soft iron corrections will cease to act upon the compass, as the other iron in the ship loses its power of disturbing it. It is, AND THE MARINER'S COMPASS. 141 therefore, highly desirable that this point should be settled by an experiment in a ship like the Recruit (a vessel of most complicated magnetism), with the apparatus just as it is; and when she returns, the binnacle and apparatus could be fixed where it should be upon a midship, and fore and aft line. " I am, Sir, " Your most obedient humble Servant, "Admiral Sir John West, K.C.B., "WILLIAM WALKER. Commander-in-Chief, Plymouth." 59. We have endeavoured to shew how very distrustful antient mariners were of all those who meddled in any way with the compass ; and how con- servative they were in adhering to old opinions, and rejecting new ones ; even when supported by experi- mental proofs. Highly gifted individuals of the present time, in their endeavours to generalise facts with unsound data, have now and then contributed to throw doubts upon the principles they themselves have, in other parts, propounded. Hand-books have been pub- lished for the guidance of navigators, so far as regards the compass and its deviations : artificial magnets were at one time to correct ships' compasses all the world over : an iron disc was to perform the same desirable end : the troublesome horizontal vibrations of the com- pass were actually to be got rid of altogether, by the adoption of compass-cards having 4, 8, or 12 needles of the greatest possible magnetic power: deviation tables have been supplied to ships for correcting their courses ; and other kinds of tables have been printed for reforming deviation tables, should changes take place in their amount on the points of greatest de- viation ; these latter tables being grounded on the 142 THE MAGNETISM OF SHIPS, assumption that the points of greatest and least devia- tion remained constant. Unfortunately all these plans have partially failed. Very excellent "Practical Rules" have from time to time been published by authority of the Admiralty, for the information of navigators, relative to the local attraction of ships and their magnetic action upon the compass ; but these rules have required, and will again require revision, so soon as opinions become settled on these subjects. In the 10th "Practical Rule for ascertaining the deviations of the compass," we find the following information given (edition 1842): "Experience has shewn, that the amount of deviation on the several points varies in distant regions ; and that in the southern hemisphere it may even become westerly on the points on which it was easterly, and in the northern hemisphere easterly where it was westerly." In 1847, we find the words in italics of the above sentence altered as follows : " It frequently becomes westerly." And in the last edition of Captain Johnson's work, of 1852, there is another new wording in the 10th practical rule, running thus : " It generally becomes west- erly." These alterations evidently shew, that opinions or conclusions have not yet been arrived at ; yet these changes of phraseology teach us to be tolerant of the opinions of others. He would be a bold man who would now stand forth and attempt to publicly vindi- cate the practice of correcting, by permanent magnets, the ever-changing inductive magnetism in an iron ship ; for experience has taught the mariner, that his compass, corrected by magnets at home, becomes very erroneous AND THE MARINER'S COMPASS. 143 abroad ; that although his compass error at London or Liverpool may be made = 0, yet at Sydney or New Zealand its error may be = 3 or 4 points ; and that instead of the errors being corrected by magnets here, they may even be doubled elsewhere. Some fifteen years ago, one of the Niger Expedition iron ships, named the Soudan, had her compasses very correctly adjusted by magnets, &c., in England, and on arriving at Sierra Leone she was re-swung, and as much as 12 and 14 degrees of error were found on some points of her compass, where no errors whatever were found in England at her departure.* A gentleman who professes to correct compasses at Liverpool, wrote me in 1847: "A master of an iron vessel, sailing from this port to Bombay, told me that the compass near the wheel, corrected by magnets, was out one or two points on getting near the line ; but on coming or going northward, it gradually came back again." The iron Royal Mail Packets, that now pass with such rapidity between the two hemispheres, experience the inconvenience of permanent magnet correction. Some commanders take the magnets away from their compasses when they get well to the south- ward, and approach the line, and replace them as they return northward, and by this means avoid the doubling of compass errors. 60. If a ship were supplied with a table of com- pass deviations in England, and if the navigator were to continue the application of such a table to correct his courses in the same amount of dip in the southern hemisphere ; then might he double his compass errors, * See Appendix B. 144 THE MAGNETISM OF SHIPS, instead of correcting them. If a ship had one com- pass corrected by magnets, and another of the same kind, in the same ship, and under the same amount and description of disturbance, had its errors correctly recorded in a deviation table; and if the navigator were to assume that his corrected compass remained always perfectly right, by applying his recorded de- viations constantly to correct the courses steered by the other compass, he might find his two dead reckon- ings agree. The iron steamer Adelaide, that left Plymouth early in January, 1853, had her steering compass corrected by magnets, and another in a standard position on the top of her saloon, for which a deviation table had been supplied. These compasses will greatly differ during the voyage, and the uncor- rected compass will be the most to be relied upon. Permanent magnets for compass corrections are, however, very convenient for our steamers employed in the home trade ; and so long as the ships are kept upriyht, or nearly so, no great danger need be appre- hended ; unless the magnets lose a portion of their correcting power, or some alteration be made in the ships or their machinery. In iron vessels, the compass deviations abaft may on some courses amount to five or six points. These great errors are generally to be found in iron screw- vessels, that have their steering wheels and binnacles near the stern, and just before an upright iron stern- post, with its iron rudder working on it. These vertical masses of iron are powerful inductive magnets by position, and conduct upwards the magnetism of the lower parts of the vessel ( 24), thereby acting power- AND THE MARINER'S COMPASS. 145 fully on the compass ; attracting the north point towards the vessel's stern, and not forward, as is the case in the majority of wooden ships. Now, magnets are convenient for counteracting these great deviations, in such iron screw-ships as may be employed in the home trade ; but large iron ships of any kind, in- tended for distant employment, should not have their steering-wheel and binnacle too far aft, for the reasons just given; unless means were devised for moving counteracting magnets, so as to adjust the binnacle compass at any time, as a mere matter of convenience for the helmsman. The course in all cases should be shaped and recorded in the log from a standard com- pass, placed in a permanent position, where there is no error, or where errors have been corrected by iron alone, or where small errors, arising from change of geographical position, or from the ship's inclination when under sail, are indicated by a deviation compass. When two compasses are placed in one binnacle, or when two binnacles in the same ship are too near to each other, they will be within their sphere of mutual attraction, and become erroneous on all courses, except at north, south, east, or west. On all other courses, the compasses will be in error, arising entirely from their mutual influence on each other ; and yet the two compasses, if similar, will be found to agree. Any body may try the experiment, by placing two compasses on a table, and turning the table slowly round. The compasses will agree when they bear north, south, east, or west of each other, but on all other points they would give a bearing of a distant object, differing from the bearing of the same object when the two instru- L 146 THE MAGNETISM OF SHIPS, ments bear north, south, east, or west of each other. The conclusions to be drawn are, that only one compass should be used at a time, and that one should be placed, if possible, exactly amidships. If, however, the ship be large, and steered by a wheel of large diameter, then the position of the helmsman will not be amidships, and two binnacles may be necessary ; as is the case in large ships of war, where the binnacle compasses are kept four feet apart, in order to avoid compass errors from too close proximity. We have endeavoured to shew how the position of an irregular mass of iron may either attract or repel the mariner's compass, and divert it from that correct magnetic bearing it would indicate if acted on by the magnetism of the earth alone. This kind of compass disturbance is called, compass deviation, to distinguish it from the variation. The local deviations of a ship, arising as they do from the magnetism of the ship and her contents, are in some measure under control, and may be cancelled by counteracting iron, or by means of permanent magnets, so long as a ship continues in seas where the magnetic dip and magnetic intensity of the earth do not sensibly vary. The variation, however, is altogether beyond the control of man. It varies irre- gularly from time to time in different parts of the world. There is even an annual, a monthly, and a diurnal variation of the compass, but of too small amount to influence a ship's reckoning. The variation appears to arise from some periodical and physical changes going on in the earth itself, which still remain among the unexplained mysteries of nature. 61. We have endeavoured to explain, at consider- AND THE MAKINER'S COMPASS. 147 able length, how a ship's compass acquires errors from the local attraction or repulsion of iron in its vicinity ; and how this iron acquires magnetism and polarity by its position in the ship, and its position with regard to the magnetic dip ; that these relative positions change, as the ship turns round, inclines from side to side, or passes from one hemisphere to another, or from one magnetic latitude to another. The earth's magnetic intensity is also found to be inconstant. Hence it results, that although the stowage of a ship may remain undisturbed, and the relative positions of her compass, and every article of iron in her, remain constant during a voyage, her deviations and local attractions will constantly vary; even from plus to minus quantities, and vice versa. We need not insist upon this, since the tables we have given demonstrate the fact. The local magnetic disturbances of a ship's compass are decidedly variable quantities, and any attempt to correct a compass permanently, subject to so many changeable disturbing forces, cannot be effected by a counter-application of constant forces, as has been attempted with permanent magnets, and found to fail. Neither can compasses be rectified by the application of any hypothetical or unsound principles of terres- trial magnetism. Local attractions, as found existing in almost all vessels, on certain points of their courses, in smooth water and an upright position, as well as those annoying compass oscillations which are so detrimental to good steerage and correct reckonings, can only be correctly cancelled or cut off by counteracting the disturbing forces, and by a proper application of materials of the same quality or kind as those that L2 148 THE MAGNETISM OF SHIPS, derange the free and correct pointing of a ship's com- pass. The disturbing and correcting materials should be of the same kind, in order that similar inductive magnetic changes may arise in the metals that disturb, as well as in those intended to correct a ship's compass,' as she may pass from one magnetic latitude to another. If space were ample, we might instruct our readers by recording instances of ships being wrecked and lives lost through sheer pedantic ignorance, neglect, or indifference to professional detail. Our aim has been to avoid, as much as possible, unpleasant recitals and unpalatable quotations ; giving only such proofs as may be necessary for supporting the views we entertain of the magnetism of ships, and its operation on the mariner's compass. If steering compasses in ships have occasionally been made erroneous through ignorance or neglect, they may sometimes have been corrected by accident or design, in the equipment, stowage, or general arrange- ment of the iron on board. Brief reference has been made (1) to observations recorded by the Author on the English coast and at the Cape of Good Hope, in a vessel containing much iron of different kinds, and to a letter he wrote to Admiral Sir Byam Martin, relative to the effect that iron lower masts would have on the steering compasses of a frigate. In that letter we asserted, that the keel and lower half of the Phcetori s iron mainmast " would attract the south point of the compass ; the head and upper half being at too great a distance to counterbalance the influence of the heel and parts below." When the Phaton was swung at Spithead, in February, 1825, and her compass devia- AND THE MARINER'S COMPASS. 149 tions tabulated and discussed by Mr. Bennett, of the school of Naval Architecture, it was found that her compass errors were very small indeed; that the greatest deviation did not exceed J of a point (1 40'). Mr. Bennett, in discussing the effects produced by the iron mast, says, " that the guns, $"