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THE HALLEY LECTURE, 1913 
 
 
 The Earth's Magnetism 
 
 By 
 
 L. A. J3AUER, M.A., Ph.D., D.Sc., 
 
 Director, Department of Terrestrial Magnetism, 
 Carnegie Institution of Washington 
 
 Delivered in the Schools of the University of Oxford, 
 May 22nd, 1913 
 
 Reprinted from BEDROCK, October, 1913 
 
A3. 
 
 BEDROCK, Vol. II., No. 3, October, 1913, ^. 273291. 
 
 THE EARTH'S MAGNETISM* 
 
 By L. A. BAUER, M.A., Ph.D., D.Sc., 
 
 Director, Department of Terrestrial Magnetism, Carnegie 
 
 Institution of Washington 
 
 IT is indeed a great privilege and pleasure to give a lecture at 
 Oxford, where Edmund Halley, whose name the Founder has so 
 wisely coupled with this Lectureship, laboured devotedly in the 
 interest of science ; and to be permitted, in some small measure, 
 to pay the debt of Terrestrial Magnetism, and my own personal debt 
 as well, to this illustrious investigator. 
 
 Halley's varied scientific activity and his wide sympathies were 
 well set forth by the Halley Lecturer f of two years ago, who had as 
 his subject an astronomical one "The Stars in their Courses." 
 The theme of last year's lecture, { " Large Earthquakes," by that 
 zealous pioneer, Professor Milne, again exemplified both the scope of 
 this Lectureship and the fact that Halley's interest and achievements 
 in geophysical science, though not generally so well known as his 
 astronomical discoveries, were no less great. The subject of the 
 lecture to-night "The Earth's Magnetism " is one in which 
 Halley's name stands out pre-eminent among the early students of 
 the science. As it is a large subject and one in which there might 
 be much discursive rambling, we shall do well to limit ourselves 
 
 * The fourth " Halley Lecture," delivered in the Schools of the University 
 of Oxford, on May 22nd, 1913 ; illustrated by lantern slides. 
 
 t Professor H. H. Turner, D.Sc., D.C.L., F.R.S., Savilian Professor of 
 Astronomy, University of Oxford (see BEDROCK, Vol. I., No. 1, April, 1912, 
 pp. 88107). 
 
 t Published in BEDROCK, Vol. L, No. 2, July, 1912, pp. 137156. 
 
 273 
 
 M809706 
 
BEDROCK 
 
 somewhat to choose our starting point and then proceed in certain 
 definite directions. 
 
 The adopted flag of the Chinese Republic consists of five stripes, 
 partly because, as I am told, in China all good things are five five 
 seasons, five principal grains, five genii, five relationships that make 
 up life, and five points of the compass : north, south, east, west, 
 and centre. For, to the Chinese, the starting-out point is as important 
 as the point to which, or direction in which, a journey is made. 
 So it also must be with us to-night. 
 
 According to the regulations governing this lecture, it is to be 
 known as the " Halley Lecture on Astronomy and Terrestrial 
 Magnetism." " Astronomy shall include Astrophysics, and Terres- 
 trial Magnetism shall include the physics of the external and internal 
 parts of the terrestrial globe." This lecture might, therefore, with 
 propriety cover the whole range of investigation in terrestrial and 
 cosmical magnetism. However, we must limit ourselves to those 
 particular lines of research in our subject in which Halley himself 
 was chiefly interested. It so happens that these are the very lines 
 also in which I have been given the opportunity to continue and 
 expand the work begun by him. 
 
 After Halley had made two attempts to establish a working 
 theory respecting the distribution of terrestrial magnetism and the 
 cause of its striking change with the lapse of years the so-called 
 secular variation he must have reached the conclusion that the 
 elusive problem of the Earth's magnetism would be more profitably 
 advanced by additional facts than by further speculation. That, 
 paraphrasing Seneca, to avoid making a false calculation of matters, 
 it were better to advise with Nature rather than with opinion. 
 Accordingly we find him setting out in October, 1698, in command 
 of a sailing ship, the Paramour Pink, and cruising in her under orders 
 from the British Government, back and forth, north and south, in 
 the Atlantic Ocean for two years, observing almost daily, sometimes 
 several times in a day, the angle which the compass needle makes 
 with the true north and south line the angle known to the man of 
 science as the magnetic declination, to the mariner and surveyor 
 as the " variation of the compass." 
 
 This is memorable as being the first scientific expedition sent out 
 by any country with the specific object of improving existing 
 
 274 
 
THE EARTH'S MAGNETISM 
 
 knowledge regarding certain facts of the Earth's magnetism. Not 
 until somewhat over two centuries later did it occur again, that a 
 sailing ship traversed the oceans with the chief purpose of making 
 magnetic observations.* In July, 1905, there sailed from the port 
 of San Francisco, California, a chartered sailing yacht, the Galilee, 
 sent under the auspices of the Carnegie Institution of Washington, 
 on the sole mission to determine the magnetic elements at sea, for 
 the benefit of both the mariner and the man of science, as was also 
 the purpose of Halley's voyages. Four years later, in 1909, a 
 specially built non-magnetic vessel, likewise under the auspices of 
 the Carnegie Institution of Washington, left New York for St. Johns, 
 Newfoundland, and thence proceeded to Falmouth, along practically 
 the same track followed by Halley's ship. Since then this vessel, 
 the Carnegie, has circumnavigated the globe and has repeatedly 
 intersected the course of the Paramour Pink in the Atlantic Ocean. 
 In view of the historic interest thus attaching to Halley's magnetic 
 expedition, it will be well worth our while to use this as our starting 
 point or centre, the fifth point in the Chinese compass. The 
 instructions given Halley, as far as they pertained to his obser- 
 vational work, were as follows : 
 
 " Whereas his Majesty has been pleased to lend his ' Pink the 
 Paramour ' for your proceeding with her on an expedition to 
 improve the knowledge of the Longitude and variations of the 
 Compasse, which shipp is now completely Man'd, Stored, and 
 Victualled, at his Majesty's charge for the said Expedition : you 
 are therefore hereby required and directed to proceed with her 
 according to the following instructions : 
 
 " You are to make the best of your way to the southward of the 
 Equator, and there to observe on the East Coast of South America, 
 and the West Coast of Africa, the variations of the Compasse with 
 all the accuracy you can, as also the true situation both of 
 Longitude and Latitude of the Ports where you arrive. 
 
 " You are likewise to make the like observations at as many of 
 the islands in the seas between the aforesaid Coasts as you can 
 (without too much deviation) bring into your Course ; and, if the 
 season of the year permit, you are to stand soe f arr into the South 
 till you discover the Coast of the Terra Incognita, supposed to lie 
 
 * Valuable magnetic data have been secured by various expeditions since 
 Halley's time, but either the magnetic work was merely incidental, or formed 
 part of a general scientific programme, or was combined with some geo- 
 graphical object, such as Arctic or Antarctic exploration the memorable 
 Erebus and Terror expeditions, for example. 
 
 275 
 
BEDROCK 
 
 between Mongolan's Straits and the Cape of Good Hope, which 
 Coast you carefully lay down in its true position. In your return 
 home you are to visit the English West India Plantations or as 
 many of them as conveniently you may, and in them make such 
 observations as may contribute to lay them down truely in their 
 Geographicall Situation. And in all the Course of your voyage 
 you must be carefull to omit no opportunity of noting the variation 
 of the Compasse, of which you are to keep a Register in your 
 Journal." 
 
 Curiously enough, Halley, though a prominent member of the 
 Royal Society, never contributed a paper to it, nor did he publish 
 anything elsewhere, on these voyages of his, his observations, or 
 resulting conclusions. Not until 1775 were Halley 's journal and 
 observations published, and then by Alexander Dairy mple in his 
 Collection of Voyages chiefly in the Southern Atlantick Ocean, from the 
 manuscript in the possession of the Board of Longitude at London. 
 Halley appears to have contented himself with laying down the 
 results of his work on a chart entitled " A new and correct Sea 
 Chart of the Whole World, showing the Variations of the Compass 
 as they are found in the year 1700." This chart is often briefly 
 referred to under the title " Tabula Nautica." The first edition, 
 published probably in 1701, covered only the ocean the Atlantic 
 traversed by Halley himself ; for the later edition, as the chart 
 was now to cover the greater part of the globe, he had to collect and 
 utilise observations made by others. No printed reference to the 
 early edition, either by Halley or by anyone else, prior to my 
 discovery of a copy in the British Museum in 1895, has thus far 
 come to light. Yet this particular chart, termed by me the 
 " Atlantic Chart," to distinguish it from the later one the " World 
 Chart " is especially interesting, as it contains the routes followed 
 by the Paramour Pink. Airy, when he reproduced Halley 's " World 
 Chart " in the Greenwich observations of 1869, was seemingly not 
 aware of the " Atlantic Chart." * 
 
 The only description of Halley's chart by himself, thus far found, 
 is that either attached to certain editions of the chart or contained 
 
 * Those interested in the history of the Halley charts may be referred to 
 the various articles by L. A. Bauer in Nature, May 23rd, 1895, p. 79, and in 
 Terrestrial Magnetism, January, 1896, and September, 1913; the last named 
 reference also contains a compilation of the magnetic results obtained on 
 Halley's expedition. 
 
 276 
 
THE EARTH'S MAGNETISM 
 
 on an accompanying leaflet. This, however, is very brief and was 
 chiefly intended to instruct mariners in the use of the chart. Halley 
 points out that in certain regions where the " Curves " run suitably, 
 they may be used " to estimate the Longitude at Sea thereby." 
 To his lines of equal " magnetic variation " he gave no distinctive 
 name, simply referring to them as the " Curve Lines." Thus he 
 says : " What is here properly New, is the Curve Lines drawn over 
 the several Seas, to show the degrees of the Variation of the Mag- 
 netical Needle, or Sea -Compass." He does, however, use the term, 
 " Line of No Variation." For some time these lines were referred 
 to by others as the " Halleyan Lines." Hansteen, a century later, 
 introduced the term, " isogonic lines," which is now generally 
 adopted. According to Hellmann there is reason for believing that 
 some attempts had been made before those of Halley to give on a 
 globe or a map a graphical representation of the direction in 
 which a compass needle points. It is conceded, however, that 
 Halley 's was the first successful attempt ; his " variation chart " 
 was the first magnetic chart based on sufficient observational data 
 to give it immediately both practical and scientific value.* 
 
 After the publication of his chart the most important contribu- 
 tion to the observation-material of terrestrial magnetism at the 
 time Halley made no further attempt to establish a theory or to 
 improve on his early magnetic speculations. He appears finally 
 to have adopted the view so clearly formulated by Professor 
 Turner f : 
 
 " that the perception of the need for observations, the faith 
 that something will come of them, and the skill and energy to act 
 on that faith that these qualities all of which are possessed by 
 any observer worthy the name, have at least as much to do with 
 the advance of Science as the formulation of a theory, even of a 
 correct theory." 
 
 We find Halley embracing every occasion 
 
 " to recommend to all Masters of Ships, and all others, Lovers 
 
 * Mountaine and Dodson, the authors of the second and revised edition 
 (1744) of the Halley Chart, and of the third (1756), published in connection 
 with the latter a small tract : " An Account of the Methods used to describe 
 Lines on Dr. Halley'' s Chart of the terraqueous Globe, showing the variation of 
 the magnetic needle about the year 1756 in all the known seas .... London, 
 1758, 4." This tract was again published in 1784. 
 
 t Pres. Address, Sec. A., Brit. Assoc. Adv. Sci., 1911. 
 
 277 * 
 
BEDROCK 
 
 of Natural Truths, that they use their utmost Diligence to make, 
 or procure to be made, Observations of these Variations in all 
 parts of the World, and that they please to communicate them to 
 the Royal Society, in order to leave as compleat a History as may 
 be to those that are hereafter to compare all together, and to 
 compleat and perfect this abstruse Theory." 
 
 Consulting the minutes of the Royal Society, it is found that 
 Halley communicated, from time to time, the results of magnetic 
 observations received from various expeditions, as also the values 
 of the magnetic declination observed by himself, at London, viz. : 
 
 "1701, May 7. Mr. Halley tried the experiment of the 
 Variation of the Needle this day, with the two needles he had 
 with him in his late Voyage : and by the one the Variation was 
 7 40', by the other 8 00' W. 
 
 " 1702, July 8. Mr. Halley observed the Variation of the 
 Needle, which was found to be 8J Westward, or very near it. 
 
 " 1716, May 24. Dr. Halley reported that he had drawn a 
 Meridian Line on the stone erected in the Society's yard before 
 the repository and that the Variation was found at present to be 
 full twelve degrees." 
 
 These observations of the magnetic declination of 1701, 1702, and 
 1716 are perhaps printed here for the first time and are nob found in 
 any of the compilations of magnetic declinations at London 
 published thus far. Only Halley's earlier observations, namely, 
 those of 1672 (2 30' W.), 1683 (4 30' W.), and of 1692 (6 00' W.), 
 having been given by Halley himself in his printed papers of 1683 
 and 1692, have become known to compilers. 
 
 CHANGE or THE MAGNETIC DECLINATION IN THE ATLANTIC 
 OCEAN SINCE HALLEY'S CHART. 
 
 In view of the fact that the two vessels the Paramour Pink and 
 the Carnegie both being primarily dependent for their motive 
 power upon the prevailing winds in the Atlantic Ocean, have 
 followed nearly identical courses, it will be a matter of no little 
 interest to compare the values of the magnetic declination given 
 on Halley's chart for 1700 with those obtained by the Carnegie in 
 her cruises of 1909 10. We find first that over the entire Atlantic, 
 from 50 N. to 40 S., the north end of the compass needle in 1910 
 was to the west of the compass direction of 1700, by amounts 
 varying with locality. Thus for various important ports the 
 
 278 
 
THE EARTH'S MAGNETISM 
 
 approximate change was as follows : New York, 2'9 W. ; St. 
 Johns, Newfoundland, 14- 6 W. ; Falmoutb, England, 10-4 W. ; 
 Funchal, Madeira, 15- 6 W. ; Bermuda, 10'5 W. ; Porto Rico, 
 7-6 W. ; Para, Brazil, 14-6 W. ; Rio de Janeiro, 20-8 W. ; 
 Buenos Aires, 13'0 W. ; Cape Town, 16-2 W. 
 
 If we follow a line passing through the points of maximum change 
 in the Atlantic Ocean, we find for the following points : 
 
 VALUES OF THE MAGNETIC DECLINATION IN 1700 AND 1910. 
 
 Latitude. 
 
 Longitude. 
 
 Ptird m ou r Pi n k , 
 1700. 
 
 Carnegie, 
 1910. 
 
 Secular Change 
 (1910-1700). 
 
 50-4 N. 
 
 30-4 W. 
 
 ll-3 W. 
 
 29'5 W. 
 
 18-2 W. 
 
 35-9 N. 
 
 47-0 W. 
 
 4-0 W. 
 
 22-l W. 
 
 18-1 W. 
 
 21-0 N. 
 
 30-9 W. 
 
 0-6 W. 
 
 19-2 W. 
 
 18-6 W. 
 
 5-9 X. 
 
 35-8 W. 
 
 2-5 E. 
 
 16-5 W. 
 
 19-0 W. 
 
 40-6 S. 
 
 25-2 W. 
 
 10-7 E. 
 
 17'5W. 
 
 28-2 W. 
 
 We see, accordingly, that the compass direction, in the course of 
 time, suffers large changes ; for the region and time -interval 
 considered the changes vary from about 3 off New York to 28 in 
 the Atlantic Ocean about midway between Buenos Aires and Cape 
 Town. Even these amounts may not represent the total or 
 maximum change during the period in question. 
 
 Equally to be noted with these large changes with time is the 
 important fact that the amount of change is as dependent upon 
 locality as is the prevailing compass direction itself, which for over 
 four centuries has been known to be anything but " true to the 
 pole." 
 
 We have thus had impressed upon us this important fact : Two 
 sailing vessels cruising in the Atlantic Ocean from port to port- 
 the one in 1700 and the other in 1910 were forced by the prevailing 
 winds to follow very closely identical courses. If, however, these 
 two vessels had been directed to follow certain definite magnetic 
 courses, and if we may suppose that they had such motive power 
 as to render them independent of the winds, then their respective 
 paths would have diverged considerably. For example, if the 
 Carnegie had set out from St. Johns, Newfoundland, to follow the 
 same magnetic courses as those of the Paramour Pink, instead of 
 
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BEDROCK 
 
 coming to anchor in Falmouth Harbour, she would have made a 
 land -fall somewhere on the north-west coast of Scotland. In brief, 
 while the sailing directions as governed by the winds over the 
 Atlantic Ocean are the same now as they were during Halley's time, 
 the magnetic directions or bearings of the compass that a vessel 
 must follow to reach a given port have greatly altered. To quote 
 from the suggestive essay on Terrestrial Magnetism by John F. W. 
 Herschel : * 
 
 " The configuration of our globe the distribution of tempera- 
 ture in its interior the tides and currents of the ocean the general 
 course of winds and the affections of climate whatever slow 
 changes may be induced in them by those revolutions which 
 geology traces yet remain for thousands of years appreciably 
 constant. The monsoon, which favours or opposes the progress 
 of the steamer along the Red Sea, is the same which wafted to and 
 fro the ships of Solomon. Eternal snows occupy the same regions, 
 and whiten the same mountains and springs well forth at the 
 same elevated temperature, from the same sources, now as in the 
 earliest recorded history. But the magnetic state of our globe 
 is one of swift and ceaseless change. A few years suffice to alter 
 materially, and the lapse of half a century or a century to 
 obliterate and completely remodel the form and situation of those 
 lines on its surface, which geometers have supposed to be drawn, 
 in order to give a general and graphical view of the direction and 
 intensity of the magnetic forces at any given epoch." 
 
 REGARDING LONGITUDE DETERMINATIONS AT SEA. 
 
 One important result of Halley's voyage and of the publication 
 of his chart was the awakening of renewed interest in the improve- 
 ment of methods for determining the longitude at sea. Recalling 
 Halley's instructions, we note that one of the objects of his 
 expedition was " to improve the knowledge of the Longitude." 
 
 When the discovery was made that the magnetic declination 
 varied from place to place, the idea immediately occurred to 
 Columbus, as also to Cabot, that the longitude might be determined 
 at sea by means of this fact. Antonio Pigafetta, who accompanied 
 Magellan on his first voyage around the world in 1522, definitely 
 proposed, in his book on navigation, this method of longitude 
 determination. The line of no magnetic declination, which at that 
 
 * Essays from the Edinburgh and Quarterly Reviews, with Addresses and 
 Other Pieces, by Sir John F. W. Herschel, London, 1857, pp. 69 70. 
 
 280 
 
THE EARTH'S MAGNETISM 
 
 time passed through the Azores, was regarded as the natural meridian 
 from which to count longitude. When later it was found, as was 
 first remarked by J. de Acosta in his Historia Natural . . . Sevilla, 
 1590, that there were four such lines, it was again thought that 
 these quadrantal divisions could be utilised for reckoning longitudes. 
 In 1674 Charles II. appointed a commission to examine into the 
 pretensions of a scheme devised by Henry Bond for ascertaining the 
 longitude by the " variation of the compass." 
 
 Halley's chart, however, definitely showed that it would be, in 
 general, futile to attempt to determine the longitude by means of 
 an element so variable and so irregular in its distribution as is the 
 magnetic declination. Nevertheless, the hope that some magnetic 
 phenomenon might yet serve to aid in the solution of this problem 
 did not die immediately. 
 
 In 1721 we find William Whiston, Newton's successor at Cam- 
 bridge, installing dip circles on a number of vessels, with instructions 
 to observe diligently the magnetic dip in order to determine whether 
 by means of this element the longitude could be better found at 
 sea than by the magnetic declination ; he likewise hoped thus to 
 determine the latitude at sea. 
 
 It is also interesting here to note that when Dr. Johnson was 
 at Oxford, he gave in 1756 to the Bodleian Library a thin quarto 
 of twenty-one pages, entitled An Account of an Attempt to ascertain 
 the Longitude at Sea by an exact Theory of the Variation of the 
 Magnetical Needle, etc., by Zachariah Williams, published at London 
 in 1755 ; Johnson entered it with his own hand in the Library 
 Catalogue. Boswell relates that Johnson himself wrote the English 
 version for Williams, and, in order to make it more extensively 
 known, also had an Italian translation prepared by his friend, 
 Signor Baretti. 
 
 For fully three centuries the idea that the longitude could be 
 determined at sea with the aid of some magnetic element, though 
 proved to be fallacious, served a most useful purpose by furnishing 
 the necessary incentive to observe the magnetic elements. This is 
 a striking illustration of the soundness of the position taken by 
 Maxwell when he said : "I never try to dissuade a man from trying 
 an experiment ; if he does not find what he wants, he may find out 
 something else." It was indeed true of these magnetic longitude 
 
 28! 
 
BEDROCK 
 
 seekers, that they failed in their purpose, but they contributed data 
 of inestimable value to the advancement of our knowledge of the 
 Earth's magnetism. 
 
 Before leaving this subject it might be said that Halley himself 
 proposed an astronomical method for solving the longitude problem 
 and, with Newton, he was responsible for the Act of 1714 offering a 
 reward to any person who should devise a satisfactory method for 
 the determination of the longitude at sea. He also improved some 
 of the instruments used in navigation. 
 
 Another result of Halley's various voyages deserves mention here, 
 though not immediately concerned with the subject of our lecture, 
 namely, his theory of the cause of the trade winds.* On certain 
 editions of his " Variation Chart," there was given, in addition to 
 the lines of equal magnetic variation, a " View of the Generall and 
 Coasting Trade Winds and Monsoons or shifting Trade Winds." 
 
 COMPLEXITY OF THE EARTH'S MAGNETISM. 
 
 Reference has already been made to Halley's attempts, before 
 his magnetic expedition, to establish a theory respecting the 
 phenomena of the compass needle. Thus in 1683 he published in 
 the Philosophical Transactions of the Royal Society " A Theory of 
 the Variation of the Magnetical Compass," and in 1692, in the same 
 Transactions, " An Account of the Cause of the Change of the 
 Variation of the Magnetic Needle." 
 
 In these papers Halley rejected the hypothesis which had been 
 accepted up to that time and on the basis of which elaborate tables 
 of the magnetic declination had been constructed by previous 
 investigators, namely, that the directions assumed by a compass 
 needle in various parts of the Earth could be accounted for by a 
 simple magnetisation parallel to a diameter so that the magnetic 
 poles would be diametrically opposite to each other. While the 
 conclusion reached by him that " the whole Globe of the Earth is 
 one great Magnet having four Magnetical Poles, or Points of Attrac- 
 tion, near each Pole of the Equator Two," has, in a certain sense, 
 been found to be incorrect, nevertheless, this view appears to have 
 been the first definite recognition of the heterogeneity or complexity 
 of the Earth's magnetic condition. 
 
 * See Miscellanea Curiosa, Vol. I., pp. 6180, and PL 2. 
 
 282 
 
THE EARTH'S MAGNETISM 
 
 The increased knowledge gained from magnetic surveys since 
 Halley's time has taught that the more carefully a country has been 
 explored, i.e., the nearer together the points at which the magnetic 
 elements have been determined, the greater is the number of irregu- 
 larities usually shown by the so-called isomagnetic lines ; indeed, 
 regions have been found where no system of lines can adequately 
 and correctly represent the prevailing magnetic conditions. We 
 have learned that the regularities in the distribution of the Earth's 
 magnetism, far from being normal features, as was once thought, 
 are, instead, the abnormal ones, and that the irregularities are the 
 normal and to-be -expected phenomena. 
 
 The magnetic forces, as measured at any given point on the 
 Earth's surface, appear, according to various analyses, to be the 
 resultant effects of : (1) a general or terrestrial magnetic field due 
 to the general magnetic condition of the whole Earth ; (2) a general 
 terrestrial disturbing cause which distorts at the place of observation 
 the general magnetic condition of the Earth ; (3) a disturbing 
 effect continental in extent ; (4) a regional disturbance effect due 
 to low-lying magnetised substances ; and (5) a local disturbance due 
 to the magnetised masses in the immediate vicinity. 
 
 No formula has as yet been established which will represent the 
 observational facts within the error of observation, in fact not even 
 with sufficient accuracy for the practical purposes of the surveyor 
 and of the mariner. 
 
 THE EARTH'S MAGNETIC POLES. 
 
 We have noticed that Halley, as the result of his study of the 
 observations of the magnetic declination, as far as they had become 
 known up to 1683, reached the conclusion that the Earth had " four 
 Magnetical Poles, or Points of Attraction." Some confusion has 
 arisen as to the precise meaning which Halley attached to his 
 " Poles." Owing to his alternative term " Points of Attraction " 
 certain eminent writers have sought to identify Halley's supposed 
 four Magnetic Poles with the four foci of maximum total magnetic 
 force, whose existence appeared to be indicated when, near the middle 
 of the nineteenth century, it became possible to construct a chart of 
 the lines of equal magnetic force. By this incorrect inference these 
 authors have unwittingly credited Halley with a discovery which, 
 
 283 
 
BEDROCK 
 
 in the absence at the time of any observation whatsoever respecting 
 the strength of the Earth's magnetic force, he could not possibly 
 have made. The real merit and purport of Halley's deduction has 
 thereby been obscured. The observation-material at Halley's 
 disposal, before he himself enriched the material during his voyages, 
 consisted of some miscellaneous observations of the compass direc- 
 tion and a few values of the magnetic dip. As has been said, there 
 were no observations of the magnetic force, for the art of measuring 
 this element had not yet become known. 
 
 Scrutinising carefully his scanty observation-material, Halley 
 noticed that the direction of the compass needle did not change from 
 place to place in the simple way it would if, for example, the Earth 
 had two Magnetic Poles diametrically opposite each other. In the 
 latter case, the needle would set itself tangent to the great circle 
 passing through the Magnetic Poles and the place of observation. 
 If, then, the compass direction were known at two places, sufficiently 
 far apart, the points of intersection of the two great circles drawn 
 respectively tangent to these compass directions would be the two 
 diametrically opposite Magnetic Poles. It is such points of inter- 
 section " points of convergence," as Hansteen later called them 
 which Halley had in mind as " Magnetic Poles." He was the first 
 to perceive clearly the fact abundantly verified since that the 
 various points of convergence as found from successive pairs of 
 compass directions, in the manner just described, do not fall together 
 as they should on the basis of a simple or regular magnetisation of 
 the Earth. However, it appeared to Halley, and the same con- 
 clusion was reached over 100 years later by the illustrious Norwegian 
 magnetician, Hansteen, that the several points of convergence 
 grouped themselves, in a general way, about two main centres, 
 
 " near each Pole of the Equator Two ; and that, in those parts 
 of the World which lie near adjacent to any one of those Magnetical 
 Poles, the Needle is govern'd thereby, the nearest Pole being 
 always predominant over the more remote." 
 
 It will not be well to lay greater stress upon this deduction, nor upon 
 those in his 1692 paper, where he seeks to account for the existence 
 of his four " Magnetic Poles " and for the secular variation, than to 
 say that Halley drew the best possible conclusions the material at 
 his disposal permitted. In fact, his conclusions were not materially 
 
 284 
 
THE EARTH'S MAGNETISM 
 
 improved upon until a century and a half later, when a much more 
 complete knowledge of the distribution of the Earth's magnetism 
 had been gained, and when the various mathematical attempts 
 which had been made to compute the magnetic elements, on the 
 basis of more or less intricate hypotheses as to the Earth's 
 magnetisation, had been found to be inadequate. Some later 
 investigators, indeed, might have spared themselves considerable 
 pains had they previously familiarised themselves more thoroughly 
 with Halley's work. 
 
 When we to-day speak of the Earth's Magnetic Poles, it is 
 generally recognised that those points on the Earth's surface are 
 meant where the dipping needle stands precisely vertical and where 
 the magnetic dip is, accordingly, 90. This definition permits, with 
 the aid of the dipping needle, of a precise determination of the 
 Magnetic Poles, though, of course, it must not be understood that 
 these Poles are mathematical points ; the area over which the dip 
 may be found to be 90, within the instrumental means of deter- 
 mination, may, in fact, be several miles square. A more or less 
 extensive magnetic survey of the region round about would be 
 required to eliminate the possibility of disturbing influences owing 
 to local deposits of iron-ore. At these " Poles," since the magnetic 
 force exerted by the Earth is all up and down, with no side component, 
 a compass needle would have no directive force acting upon it. 
 Some distance before reaching the Magnetic Pole it would become 
 sluggish, and directly over the Pole itself it would be of no more use 
 than a brass needle to indicate any definite direction. 
 
 Excluding for the present the purely " local magnetic poles " 
 caused by extraordinary local deposits of attracting masses, all 
 observations to date show that there are but two such points (or 
 areas) where the dipping needle stands vertical, one in the Northern 
 Hemisphere, located by Captain James Clark Ross, in June, 1831, 
 in latitude 70'l N. and longitude 96'8 W.* and the other in 
 the Southern Hemisphere, lying, according to the observations 
 of the recent Antarctic expeditions, about in latitude 72*7 S. and 
 
 * During Captain Amundsen's completion of the Northwest Passage, 
 1903 07, he also made observations with a view to locating the North 
 Magnetic Pole, but the resulting position has not yet been published. 
 
 285 
 
BEDROCK 
 
 longitude 156 E. The Magnetic Poles, therefore, are, on the average, 
 about 1,200 miles from the geographical poles. Owing to the 
 asymmetrical distribution of the Earth's magnetism, the Magnetic 
 Poles are not diametrically opposite each other, even if the positions 
 given applied to the same year ; in fact, the perpendicular distance 
 from the Earth's centre to the chord connecting the Magnetic Poles 
 is about 750 miles. 
 
 Let us suppose now that one explorer starts out from Oxford, 
 where the compass points at present about 16 west, and follows 
 always the direction shown by the north end of the compass needle, 
 whereas another starts north from Washington, where the compass 
 bears about 5 west, and follows likewise the direction of the compass 
 needle. The paths thus traced out by them are the so-called 
 " magnetic meridians," which, owing to the irregular way in which 
 the Earth is magnetised, would not be straight lines or arcs of great 
 circles, but more or less devious lines. Could these magnetic 
 meridians be followed into the Arctic regions, they would be found 
 to intersect at the North Magnetic Pole. 
 
 Owing to the irregular distribution of the Earth's magnetism, 
 the points of greatest intensity of the total magnetic force depart 
 widely in their locations from the Magnetic Poles. Thus there are 
 in the Northern Hemisphere two distinct maxima of total magnetic 
 force, one in the north-east of Siberia and the other in Canada to the 
 south-west, approximately, of Hudson's Bay. A magnetic survey 
 of the latter region is being made this summer by an expedition sent 
 out by the Department of Terrestrial Magnetism. 
 
 Do THE MAGNETIC POLES MOVE ? 
 
 Possibly the most frequent question asked of those engaged in 
 magnetic work is : "Do the Magnetic Poles move with the lapse of 
 years, and if so, why ? " Unfortunately, as has already been shown, 
 there are no direct observations as yet on which to base a definite 
 statement. But it would be singular, indeed, if these points 
 remained fixed and were not affected by fluctuations such as are now 
 known from three centuries of observations to exist in every one of 
 the Earth's magnetic phenomena. It is quite possible, in fact, that 
 the Magnetic Poles pass through certain motions even in the course 
 of a day or suffer displacements during magnetic storms. 
 
 286 
 
THE EARTH'S MAGNETISM 
 
 The diagram below shows the changes in the direction of the 
 compass (magnetic declination), as well as in the direction of the 
 dip needle (magnetic inclination), as far as known, for London, 
 Baltimore, and Boston. Imagine yourself, if you will, standing at 
 the centre of a great magnetised needle so suspended as to be free 
 to assume the direction actually taken by the lines of magnetic force 
 at the place of observation, and let us suppose you are looking 
 towards the north-pointing end of the needle. Could you gaze long 
 enough, you would see a curve described in space by the observed 
 end of the needle. This curve would lie on a sphere whose radius 
 
 Curves showing the Secular Change in the Magnetic Declination and in the 
 Dip at London, Boston, and Baltimore. [Drawn for supposed length of 
 freely suspended magnetic needle of about 50 centimetres, or nearly 20 inches.] 
 
 is the half-length of the suspended needle and for graphical 
 representation we may take a central projection of it on a plane 
 tangent to the sphere at about the middle point of the curve. The 
 curves here given were constructed by me with the aid of the 
 accumulated observations up to about 1895 ; the course followed by 
 the needle since 1895 will be discussed later. 
 
 A number of interesting and instructive facts follow from these 
 curves ; time will permit us to give our attention only to the chief 
 ones. It is seen that at London, for example, the compass reached 
 its maximum easterly direction of about 11 in the year 1580, hence 
 during the middle of Queen Elizabeth's reign ; thereafter the 
 easterly direction began to diminish until about 1658, the year of 
 
 287 
 
BEDROCK 
 
 Cromwell's death, when the needle bore due north and then swung 
 over to the west, continuing to do so until it reached a maximum 
 westerly direction of somewhat over 24 in about 1812. Hence in 
 the interval of about 232 years (1850 1812) the compass direction 
 changed at London from 11 E. to 24 W., or 35. At the present 
 time it points about 15J W., or nearly 9 less than in 1812, and a 
 most interesting question doubtless immediately occurs to all of 
 us : Will the freely suspended magnetic needle ever return precisely 
 to a direction taken at some previous time or is there any definite 
 cycle of changes which will repeat itself from time to time ? 
 
 Here again no wholly definite answer can be given, primarily 
 because of the fact, as will be seen from the diagram, that, if there 
 be such a cycle, it embraces many more years than are covered thus 
 far by the interval of observation. For some European stations, 
 e.g., Paris and Rome, the observation-interval is somewhat longer 
 than at London, but still not long enough for definite prediction 
 as to the future course of the magnetic needle. 
 
 The diagram shows also that in the United States the changes 
 in the compass direction, as far back as they are known, have not 
 been as great as those during the same time at London. Thus, at 
 Baltimore, for example, the compass appears to have reached a 
 maximum westerly amount of about 6*1 near 1670, and a 
 minimum of f in 1802, after which, instead of passing through 
 a zero value as at London in 1658, and swinging to the eastward, it 
 turned back and began to increase its westerly direction until at 
 the present time the amount is about 6J. Thus, at this station the 
 compass direction passed from a maximum to a minimum in about 
 132 years and the total change was but 5|, or only one-sixth to 
 one -seventh of that at London. 
 
 In brief, the facts revealed by the known compass changes in 
 my country cannot be brought in harmony with those witnessed in 
 your country, unless we assume that the length of the cycle of 
 complete change is many times longer than merely twice the period 
 between a maximum and a minimum bearing of the compass. 
 There are evidences, furthermore, into which we cannot go here, to 
 indicate that the cycle of change at one station is not of the type 
 which would result were we to close the apparently nearly completed 
 curve at London by uniting the two ends in some simple manner. 
 
 288 
 
THE EARTH'S MAGNETISM 
 
 On the contrary, the evidences point to cycles within cycles and to 
 the probability that the secular variation curve, instead of being a 
 single closed curve, may consist of smaller loops within a larger one, 
 etc. ; it is even questionable whether there ever will be exact 
 closure of the curve. 
 
 There is at present another matter of no little interest with regard 
 to England which should be pointed out here. It will be seen from 
 the London curve that the dip of the needle below the horizon 
 reached its maximum amount of 74-4 in about 1688. At this time 
 the compass changed its direction the maximum amount of 13' per 
 year. The curve would seem to indicate that the time of a minimum 
 dip is now approaching ; this phase has already occurred at Pawlowsk 
 and seems to be now taking place at Potsdam and is travelling west- 
 ward. Whether it will reach London and when cannot be answered 
 definitely. However, it is a matter of no little interest, in this 
 connection, to observe that the annual amount of change in the 
 compass direction has in recent years received a remarkable accelera- 
 tion in this part of the Earth. Thus, as is shown by the magnetic 
 observatory records, it has almost steadily risen from 4' per year in 
 1902 to about 9' per year in 1912 ! Whether this portends an early 
 approach of the phase of minimum dip at London is one of the many 
 interesting questions continually arising respecting the perplexing 
 phenomena of the Earth's magnetism. The course of the needle 
 since 1890 has been about as shown by the arrow ; thus in 1910 the 
 magnetic declination was approximately 15' 9 W. and the dip 
 was 66-9. 
 
 One thing more. Note that for each of the three curves as far 
 as drawn, the motion of the freely suspended magnetic needle has 
 been clockwise, i.e., the same as the motion of the hands of a watch. 
 This fact, as shown by the curves in other parts of the world, con- 
 structed with the aid of the available observations, appears to hold 
 generally in both the Northern and Southern Hemispheres, except 
 for certain retrograde motions which thus far have not been of the 
 same extent as the direct one, although, of course, it is not affirmed 
 that they may not become so later. Such retrograde motions are 
 at present being experienced in certain parts of the United States. 
 Thus, for example, the compass pointed in 1910, 6*25 W. at Balti- 
 more and 13*35 W. at Boston, and in the same year the magnetic 
 
 289 
 
BEDROCK 
 
 dip was 70*9 at Baltimore and 73'l at Boston. If we plot these 
 values on the diagram, we shall find that the curves for Boston and 
 Baltimore, instead of progressing in the direction of the arrows, 
 passed through a secondary crest about 1895 and then bent over to- 
 the left ; how long this will continue cannot be foretold. [Several 
 slides in illustration of the various facts of the secular change were 
 shown.] 
 
 The question as to the cause of the remarkable changes from 
 time to time in the Earth's magnetic condition, as indicated by 
 these curves, has been a fruitful source of speculation since 1634, 
 when Gellibrand definitely proved the fact that the compass direc- 
 tion varies from year to year. Some of the best minds have been 
 engaged with the discovery of the cause, but the riddle is still 
 unsolved. Hence, as regards the actual motions of the Earth's 
 Magnetic Poles and the precise cause or causes, we may still say 
 with Halley that these are " Secrets as yet utterly unknown to Man- 
 kind, and are reserv'd for the Industry of future Ages." 
 
 A mathematical analysis of the accumulated material shows 
 that, in order to find an adequate explanation of the secular variation 
 of the Earth's magnetism, we must reckon with systems of magnetic 
 or electric forces having their seats both below and above the 
 Earth's crust. There would also appear to be some evidence that, 
 in addition to a motion of the Magnetic Poles or magnetic axes of 
 the Earth, we may also have to take into account a possible diminu- 
 tion in the Earth's magnetic moment or intensity of magnetisation, 
 
 THE ORIGIN OF THE EARTH'S MAGNETISM. 
 
 Before concluding this lecture, we ought, perhaps, in the few 
 minutes remaining, to say something regarding the status of the 
 ever -recurring question as to the origin of the Earth's magnetism. 
 Assuming that the magnetism of our planet is uniformly distributed 
 throughout its mass, it is found that the average intensity of 
 magnetisation is only about T oioo of ve3 T m 'g nl y magnetised hard 
 steel. Professor Fleming, in his very suggestive popular lecture on 
 the " Earth, a Great Magnet," given at the meeting in 1896 of the 
 British Association for the Advancement of Science, made this 
 statement : 
 
 " Taken as a whole, the Earth is a feeble magnet. If our 
 globe were wholly made of steel and magnetized as highly as an 
 
 290 
 
THE EARTH'S MAGNETISM 
 
 ordinary steel -bar magnet, the magnetic forces at its surface 
 would be at least 100 times as great as they are now. That might 
 be an advantage or a very great disadvantage." 
 
 If, however, we could penetrate the Earth's crust, we would find 
 at a distance of only about 12 miles a temperature so great that, 
 according to present laboratory facts, all magnetisation would 
 necessarily cease. Hence, if the Earth's magnetic field arises from 
 an actual magnetisation of the substances composing the Earth, 
 these substances must be confined within a comparatively thin 
 shell. But the question immediately arises : Is this argument 
 correct ? May it not be that just as the point of liquefaction is 
 raised by increased pressure, so is also the critical temperature of 
 magnetisation. It may thus occur that the effect due to increase 
 of pressure with depth of penetration more than balances that duo 
 to increased temperature. There are at present no wholly decisive 
 experiments which may be drawn upon to answer this query. 
 
 The hypothesis that the Earth may be an electromagnet also meets 
 with difficulties when we attempt to account for the origin, direction, 
 and maintenance of the required currents. In spite of the accumu- 
 lated facts of over three centuries, we are still unable to say definitely 
 to what the Earth's magnetic field is really due. Perhaps we may 
 not be able to solve the riddle until the physicist answers for us the 
 questions : What is a magnet ? What is magnetism, in general ? 
 
 In the Devil's Dictionary by Ambrose Bierce, published in 1911, 
 the following definitions are given : " Magnet, n. Something acted 
 upon by magnetism. Magnetism, n. Something acting upon a 
 magnet." In explanation the author cynically remarks : " The 
 two definitions immediately foregoing are condensed from the works 
 of 1,000 eminent scientists, who have illuminated the subject with a 
 great white light, to the inexpressible advancement of human 
 knowledge." * 
 
 * These definitions and accompanying remarks may have had their origin 
 in the following interesting anecdote told in the American Review of Reviews 
 for August, 1909, of the late Professor Simon Newcomb, by Mr. A. E. Bostwick, 
 associate editor of the Standard Dictionary. Of the definitions in physical 
 science for this dictionary, Newcomb had general oversight, and on one 
 occasion he took exception to the definitions framed for the words " magnet " 
 and " magnetism," as based, in the absence of authoritative knowledge of 
 the causes, simply upon the properties manifested by the things. After 
 
 291 
 
BEDROCK 
 
 A line of thought first suggested by Schuster and Lord Kelvin, 
 that every large rotating mass, due to an as yet undiscovered cause, 
 may be a magnet, should be considered in conclusion, though we 
 may do so but briefly. If this be true, then magnetism is not 
 confined to our planet alone, but all celestial bodies are surrounded 
 by magnetic fields. Thus far no laboratory experiment, possibly 
 owing to lack of required sensitiveness in the measuring instruments, 
 has detected any magnetic field arising solely from rotation. 
 Schuster and Swann have recently discussed the character and 
 magnitude of the effects from the possible causes which may operate 
 if the Earth's magnetic field be related in some manner to its 
 rotation. 
 
 In 1900 03 Sutherland propounded a theory for the origin of 
 the Earth's magnetism, which, briefly stated, is this : We know that 
 electricity is an essential constituent of matter and that in every 
 atom, if it be electrically neutral, there are equal amounts of negative 
 and positive electricity. So with the whole Earth. Since it is 
 almost electrically neutral, suppose that the total negative charge, 
 while practically equal to the total positive one, occupies a slightly 
 different volume from that of the positive charge, or, in brief, that 
 the volume densities of the two body charges differ slightly. Then, 
 because of the rotation of the electric charges with the Earth, a 
 magnetic field arises. I have recently repeated Sutherland's 
 calculations and, as I had previously found that the Earth's intensity 
 of magnetisation increased systematically towards the equator, I 
 have included a term to represent a possible effect similar in its 
 distribution to that arising were the Earth's centrifugal force the 
 operating cause. The computations show that to satisfy the known 
 phenomena of the Earth's magnetism, the volume density of the 
 negative charge must be smaller than that of the positive, or, in 
 other words, the Earth's total negative charge must be distributed 
 through the larger sphere, and, if that be the whole Earth itself, 
 then for the chief term involved in the magnetic potential, the 
 surface of the sphere containing the positive charge need be, on the 
 
 writing and erasing alternately for an hour or more, he finally confessed, 
 however, with a hearty laugh, that he himself could offer nothing better 
 than the following pair of definitions : Magnet, a body capable of exerting 
 magnetic force, and Magnetic Force, the force exerted by a magnet. 
 
 292 
 
THE EARTH'S MAGNETISM 
 
 average, only 0*4xlO~ 8 cms., i.e. four-tenths of the radius of an 
 ordinary molecule, below that of the Earth's surface to give a 
 magnetic field of the required strength. Taking the average atomic 
 weight of the Earth's substance in round numbers as 50, the mean 
 volume density of either charge would be about 3'3xl0 12 electro- 
 static units. 
 
 At present there is little hope that a magnetic field, caused just as 
 supposed, can be detected in the laboratory. For a sphere of 15 cms. 
 radius, rotating 100 times a second, the magnetic intensity at the 
 poles would be but one hundred-millionth part (10~ 8 ) of that of the 
 Earth. We thus see that the quantities involved in the solution of 
 one of the great problems confronting the student of the Earth's 
 physics the origin of the Earth's magnetic field may be of such a 
 minute order as to be beyond the ken at present of the laboratory 
 experimentalist. Perhaps the effects become appreciable in the 
 case of the Earth because of the fortunate fact that it is a body 
 of sufficient size and angular velocity. 
 
 On the other hand, the geophysicist is at a great disadvantage in 
 that he is unable to bring his Earth-magnet into the laboratory and 
 to experiment upon it to reverse the direction of rotation, for 
 example, and see what would happen ! Fortunately for him, 
 however, Nature comes to his relief somewhat and performs experi- 
 ments for him on his great magnet on a world- wide scale, by pro- 
 ducing in an incredibly short time, manifold, and at times startling 
 variations and fluctuations in the apparently fixed magnetisation 
 of the Earth. Thus, on September 25th, 1909, there occurred the 
 most remarkable magnetic storm on record, during which, within a 
 few minutes, the Earth's magnetic moment, or intensity of mag- 
 netisation, was altered by about one-twentieth to one-thirtieth part. 
 The Earth's magnetic condition was below par for fully three months 
 thereafter. As this severe storm was accompanied by a brilliant 
 display of polar lights, this is the most appropriate place to recall 
 that Halley made the first suggestion of a connection between the 
 aurora borealis and the Earth's magnetism. 
 
 It is firmly believed that a long step forward will have been taken 
 toward the discovery of the origin of the Earth's magnetism when 
 once we have found out what causes it to vary in the surprising 
 manner shown by the secular or long-period changes, by the magnetic 
 
 293 
 
BEDROCK 
 
 storms, and by the numerous other fluctuations, such as the diurnal 
 variation, for example. The keynote of modern investigation in 
 terrestrial magnetism, as in the biological sciences, must surely be 
 the study of the variations and mutations ! 
 
 Is it not probable that the very features of the Earth's magnetism 
 regarded at one time as defects the " constant inconstancies," as 
 an early writer quaintly put it will instead become sources of help 
 and inspiration from totally different points of view or in some 
 entirely different line of thought ? Who knows of what import the 
 riddles of the Earth's magnetism, characterised by eminent physicists 
 as being, next to gravity, the most puzzling of natural forces, may be, 
 not simply to the magnetician alone, but to all interested in the 
 steady progress of the physical sciences ? Thus Schuster suggests 
 that " atmospheric electricity and terrestrial magnetism, treated 
 too long as isolated phenomena, may give us hints on hitherto 
 unknown properties of matter." " The field of investigation into 
 which we are introduced," says Maxwell, " by the study of terrestrial 
 magnetism, is as profound as it is extensive." And, says Sabine, 
 one of England's greatest and most enthusiastic magneticians : 
 " Viewed in itself and its various relations, the magnetism of the 
 Earth cannot be counted less than one of the most important 
 branches of the physical history of the planet we inhabit." 
 
 BRADBURY, AGNEW, & CO. LD., PRINTERS, LONDON AND TONBRIDGE. 
 
UNIVERSITY OF CALIFORNIA LIBRARY 
 BERKELEY 
 
 Return to desk from which borrowed. 
 This book is DUE on the last date stamped below. 
 
 ASTRONOMY LIBRARY 
 
 9 1966 
 \ 
 
 JUIX1 195 
 
 LD 21-100m-7,'52(A2528sl6)476