502 



NA TURE 



[Shl'l EMBER 24, 1903 



ation times, whether it has moved, or where it now is, the 

 answer must be " I do not know." It is true that Ross in 

 183 1, by a single observation, considered he had fixed its 

 position, and I believe hoisted the British flag over the spot, 

 taking possession thereof ; but he may or may not have set 

 up his dip circle over a position affected by serious magnetic 

 disturbance, and therefore we must still be doubtful of his 

 complete success from a magnetic point of view. Although 

 eminent mathematicians have calculated its position, and 

 Neumayer in 1885 gave a place to it on his charts of that 

 year, we have still to wait for observation to settle the 

 question, for one epoch at least. 



Happily, I am able to repeat the good news that the Nor- 

 wegian, Captain Roald Amundsen, sailed in June last with 

 the express object of making a magnetic survey of Ross's 

 position and of the surrounding regions, in order to fix the 

 position of the north magnetic pole. Furnished with suitable 

 instruments of the latest pattern, he proposes to continue 

 his investigations until 1905, when we may look for his re- 

 turn and the fulfilment of our hopes. 



So far as we can now see, the south magnetic pole cannot 

 be approached very nearly by the traveller, and we can only 

 lay siege to it by observing at stations some distance off but 

 encircling it. We have our own expedition on one side of 

 it, and now with the return of the Gauss to South Africa in 

 June last, we have learnt that that vessel wintered in lat. 

 66° 2' S., long. 89° 48' E., a position on the opposite side of 

 the supposed site of the magnetic pole to that of the Dis- 

 covery. We may now pause to record our warm congratu- 

 lations to Dr. von Drygalski and his companions on their 

 safe return, accompanied by the welcome report that their 

 expedition has proved successful. 



In addition to the British and German expeditions, there 

 are the Swedish expedition and the Scottish expedition. 

 Therefore, with so many nationalities working in widely 

 different localities surrounding it, we have every reason to 

 expect that the position of the south magnetic pole will be 

 determined. 



The Secular Change. 



When in the year 1600 Gilbert announced to the world 

 that the earth is a great magnet, he believed it to be a stable 

 magnet ; and it was left to Gellibrand, some thirty-four 

 years later, by his discovery of the annual change of the 

 magnetic declination near London, to show that this could 

 hardly be the case. Ever since then the remarkable and 

 unceasing changes in the magnetism of the earth have been 

 the subject of constant observation by magneticians and of 

 investigation by some of the ablest philosophers in Europe 

 and America. Year after year new data are amassed as to 

 the changes going on in the distribution of the magnetism 

 of the earth, but as yet we have been favoured by hypotheses 

 only as to the causes of the wondrous changes which the 

 magnetic needle records. 



These hypotheses were at one time chiefly based upon a 

 consideration of the secular change in the declination, but it 

 is now certain that we must take into account the whole of 

 the phenomena connected with the movements of the needle, 

 if we are to arrive at any satisfactory result. Besides, it will 

 not suffice to take our data solely from existing fixed 

 observatories, however relatively well placed and equipped, 

 and valuable as they certainly are, for it now appears that 

 the secular change is partly dependent upon locality, and 

 that even at places not many miles apart differences in re- 

 sults unaccounted for by distance have been obtained. 



The tendency of observation is increasingly to show that 

 the secular change of the magnetic elements is not a world- 

 wide progress of the magnetic needle moving regularly in 

 certain directions, as if solely caused by the regular rotation 

 during a long series of years of the magnetic poles round the 

 geographical poles, for if you examine Map No. i, showing 

 the results of observations during the years 1840-80 as re- 

 gards secular change, you will observe that there are local 

 causes at work in certain regions, whilst in others there is 

 rest, which must largely modify the effect of any polar rota- 

 tion. 



Allow me to explain further. The plain lines on Map No. i 

 indicate approximate regions of no secular change in the 

 declination, and the small arrows the general direction (not 

 the amount) in which the north-seeking end of the hori- 

 2ontal needle was moving during those forty years. The 



NO. 1769, VOL. 68] 



foci of greatest change in the declination, with the approxi- 

 mate amount of annual change in the northern hemisphere, 

 are shown in the German Ocean and N.W. Alaska, in the 

 southern hemisphere off the coast of Brazil, and in the South 

 Pacific between New Zealand and Cape Horn. The two foci 

 of greatest annual change in the dip are shown, one in the 

 Gulf of Guinea where the north-seeking end of the needle 

 was being repelled strongly upwards, the other on the west 

 side of Tierra del Fuego, where the north-seeking end of the 

 needle was being attracted strongly downwards. 



It is remarkable that the lines of no change in the de- 

 clination pass through the foci of greatest change in the dip. 

 If the needle be repelled upwards, as at the Gulf of Guinea 

 focus, it will be found to be moving to the eastward on the 

 east side of the whole line of no change in the declination 

 from the Cape of Good Hope to Labrador ; to the westward 

 on the west side. If the needle be attracted downwards, as 

 at the Tierra del Fuego focus, it will be found moving to the 

 westward on the east side of the whole line of no declination 

 from that focus to near Vancouver Island ; to the eastward 

 on the west side. 



A similar result may be seen in the line passing through a 

 minor focus of the dip near Hong Kong. 



Judging from analogy there should be another focus of 

 change in the dip in lat. 70° N., long. 115° E., or about the 

 position assigned to the Siberian focus of greatest force. 



On Map No. 2 are shown lines of equal value of the de- 

 clination — the red lines for the year 1880, the black lines for 

 the year 1895. From these, when shown on a large scale, we 

 may deduce the mean annual change which has taken place 

 in the declination during the fifteen years elapsed. 



In this map we are reminded of the different results we 

 obtain in different localities, for if a line be drawn from Wel- 

 lington in New Zealand past Cape York in Australia to 

 Hong Kong, little or no change will be found in the neigh- 

 bouring region since 1840. Again, the line of no change in 

 the declination shown on Map No. i to be following much 

 the same direction as the great mountain ranges on the west 

 side of the American continent has hardly moved for many 

 years according to the observations available. 



On the other hand, let us now turn to an example of the 

 remarkable changes which may take place in the declination 

 unexpectedly and locally. The island of Zanzibar and the 

 east coast of Africa were constantly being visited by our sur- 

 veying-ships and ships of war up to the year 1880, observ- 

 ations of the declination being made every year at Zanzibar 

 during the epoch 1870-80. The results showed that from 

 Capetown nearly to Cape Guardafui the annual change of 

 that element hardly exceeded i'. 



During the succeeding years of 1890-gi observations were 

 made by the Germans at Dar-es-Salaam and some other 

 places on the neighbouring coasts, with the result that the 

 declination was found to be changing at first 3' annually, 

 and since that period it had reached 10' to 12' at Dar-es- 

 Salaam. Subsequent observations at the latter place in 

 1896-98 confirmed the fact of the great change, and in addi- 

 tion our surveying-ship on the station, specially ordered to 

 " swing " at different places in deep water off the coast, 

 generally confirmed the results. It is remarkable that whilst 

 such great changes should have taken place between Cape- 

 town and Cape Guardafui, Aden and the region about the 

 straits of Bab-el-Mandeb seem to be comparatively unaffected. 



Local Magnetic Disturbance. 



In Map No. 2 normal lines of equal value of the declin- 

 ation are recorded, and so far as the greater part of the 

 globe covered by water is concerned, we may accept them as 

 undisturbed values, for we have yet to learn that there are 

 any local magnetic disturbances of the needle in depths 

 bevond 100 fathoms. 



When, however, we come to the land, there is an increas- 

 ing difficulty in finding districts of only a few miles in ex- 

 tent where the observed values of the magnetic elements at 

 different stations therein do not differ more widely than they 

 should if we considered only their relative position on the 

 earth as a magnet. Take Rucker and Thorpe's maps of the 

 British Isles and those of the United States, for example, 

 where the lines of equal value are drawn in accordance with 

 the observations, with the result that they form extraordinary 

 loops and curves differing largely from the normal curves of 

 calculation. 



