432 



NA TURE 



lSeptember 7, 1893 



elevation and depression, to which he invites the attention of 

 mathematicians 10 determine whether it would not produce a 

 change of 15" or 20° in the position of the pole. 



The invitation was duly accepted by Sir Wm. Thompson — • 

 now Lord Kelvin — and by Prof. G. H. Darwin. The former, 

 by a process which he Joes not explain, convinced himself that 

 a vira causa existed in the distortion of the earth, as shown by 

 j»eological and other evidence, ^uf(icient to produce large devia- 

 tions in the position of the axis. To quote his own eloquent 

 words, " Consider the great facts of the Himalayas and .\nde'', 

 and Africa, and the depths of the Atlantic, and America, and 

 the depths of the Pacific and Australia ; and consider further 

 the ellipticity of the equatorial section of the sea-level, esti- 

 mated by Capt. Clarke at about one-tenth of the mean ellipticity 

 of meridianal sections of the sea-level. We need no brush from 

 the camel's tail to account for a change in the earth's axis ; we 

 need no violent convulsions, producing a sudden distortion on 

 a great scale, with change of axis of maximum moment of 

 inertia, followed by gigantic deluges ; and we may not merely 

 admit, but assert as highly probable, that the axis of maximum 

 inertia and the axis of rotation, always very near one another, 

 may have been in ancient times very far from the present geo- 

 graphical position, and may have gradually shifted through 10, 

 20, 30, or 40 1 r more degrees, without at any time any per- 

 ceptible sudden disturbance of either land or water." (British 

 Association Reports, 1876, Sections, p. II). 



Prof. G. H. Darwin has made this the subject of an elaborate 

 mathematical investigation (Phil, Trans. 1877, p. 271). As 

 the basis he takes the earth as we find it, assuming that the 

 elevations of the continents and depressions of the ocean repre- 

 sent the kind and amount of distortion to which the earth has 

 been subjected in the course of its past history. The mean 

 elevation of the continents being about i !00 feet, and the mean 

 depth of the oceans about 15,000 feet, it follows that in order 

 tp convert an ocean bed into a continent, or vice versa, an 

 elevation or subsidence of 16,000 feet must have taken place. 

 This would not, however, correctly represent the distortion of 

 the eaith, for the waters of the ocean tlowing into the depres- 

 sions would considerably modify the result. Taking into 

 account the density of water as compared with the surface rocks, 

 it appears that an extreme elevation of 16,000 feet from the 

 bottom of the ocean to the surface of the supposed continent 

 would be equivalent to an effective elevation of about 10,000 

 feet on a sealess globe. In case of a perfectly rigid globe, the 

 only deformation which could take place would be that due 

 to a redistribution of the surface materials. For a given 

 elevation with a corresponding depression the maximum effect 

 upon the position of the earth's axis would be produced when the 

 elevations occurred in latitude 45° in two diametrically opposite 

 quarters of the earth with corresponding depressions in the 

 remaining quarters. In such a globe Prof. Darwin's analysis 

 showed that the pole could never have wandered more than 3° 

 from its original position as a consequence of the continents 

 and oceans changing places. If, however, the earth is suffi- 

 ciently plastic to admit of realjustment to new forms of 

 equilibrium by earthquakes or otherwise, possible changes of 

 10° or 15° may have occurred. 



This would, however, require such a complete changing about 

 of the continents and ocean«, with maximum elevations and 

 depressions in precisely the mo5t favourable places, as has cer- 

 tainly never occurred wilhin geologic time. In fact, the 

 evidence indicates that the continental areas have always 

 occupied about the same position as now. 



It would appear, therefore, that the geologist must give up 

 this hypothesis of great changes in latitudes as a factor in the 

 earth's development, unless, indeed, some other cause can be 

 found of sufficient potency to produce the desired result. Such 

 an agency is, perhaps, alluded to by Prof. Arthur Schuster in 

 his address before Section A of the British Association a year 

 ago (Nature, 1892, Aug. 4, p. 327). He propounds this 

 question: "Is there sufficient matter in interplanetary space 

 to make it a conductor of electricity ? " He adds that he be- 

 lieves the evidence to be in favour of this view ; but the con- 

 ductivity can only be small, for otherwise the earth would 

 gradually set itself to revolve about its magnetic poles. If such 

 an action were admitted, we must suppose the poles of revolu- 

 tion and magnetic poles would long since have been brought 

 into practical coincidence, unless this consummation were frus- 

 trated by changes in the position of the latter. 



However all this may be, the question before the practical 



astronomer is this — Have we any reliable evidence showing that 

 progressive changes in the position of the pole are now taking 

 place? If this question were submitted to a jury composed of 

 twelve good men and true from the astronomical profession, 

 the chances would be largely in favour of a verdict in agreement 

 with Laplace's decision seventy years ago. 



At the International Geodetic Conference held in Rome ten 

 years ago, Mr. Fergola brought forward a plan looking to a 

 systematic study of this and other questions connected with 

 changes of terrestrial latitudes. This plan, which was favourably 

 received, consisted in a scheme for simultaneous series of ob- 

 servations at pairs of observatories on nearly the same parallel 

 of latitude, but differing widely in longitude. The instruments 

 were to be prime vertical transits, ar.d the same stars to be 

 employed at each of the two stations. Several pairs of observa- 

 tories were designated by Fergola as being favourably situated for 

 the purpose. Among others, Washington and Lisbon, the 

 difference of latitude being 11' 7", that of longitude 4h. 31111. 

 It is understood that efforts in this direction were made at 

 Washington, but the necessary cooperation at the other end of 

 the line was not secured, and the plan came to naught. It has 

 not come to my knowledge that the scheme was at that time 

 seriously considered at any of the other points selected. 



Fergola gave a tabular statement which at that time seemed 

 to show small but progressive diminutions of latitudes ia 

 Europe and North America. This table, with some additions 

 — the latter enclosed in brackets — is as follows : — 



38 53 



48 50 



IS 27 



41 53 



40 51 



54 42 



51 



54-26 

 5409 



46-6;! 

 45 '4^ 



50-71 

 5056 



38-43 

 38-17 

 37 '9^ 



NO. 1245, VOL. 48] 



i 



In all these cases there is an apparent diminution during the 

 present century. A similar tendency is shown by the observa- 

 tions of Peters, Gylden, and Nyren at Pulkowa, also by my own 

 observations at Bethlehem since 1875. Instances are not want- 

 ing, however, where this diminution fails to manifest itself. 

 Possibly most of the discrepancies shown here may be referred 

 to periodic changes, the existence of which is no longer in Joubt. 

 It is by no means impossible or improbable that small local 

 changes of latitude may occur due to slipping of the superficial 

 strata of the earth's crust. That such lateral movements have 

 taken place in times past in connection with mountain upheaves 

 is, without doubt, true. That they are still going on in certain 

 localities is probable ; whether they are of sufficient magnilude 

 to admit of measurement can only be determined by ob.erra- 

 tion. 



When we remember how few points there are on the surface 

 of the earth, whose latitude was determined even no longer ago 

 than fifty year?, within one or two seconds of the truth, probably 

 we should suspend judgment for the present with reference to 

 the whole subject of progressive change*. 



We come now to a phase of our suliject with reference to 

 which we can speak with some confidence, viz. periodic 

 changes. 



That in the case of a perfectly rigid earth, theory pouiis to 

 the existence of such a periodic change, completing its cycle in 

 about ten months, has been long understood. In connectJioii 

 with the general problem of the motion of a free body under 

 the action of any system of forces, the consideration of whicti 



