March 2, 18 76 J 



NATURE 



355 



equatorial protuberance, any modification on its stirface would 

 have to be on an enormous scale to produce any sensible effect 

 upon its axis of revolution. But, after all, is the earth, strictly 

 speaking, a spheroid ? — and are not some of the arguments and 

 dicta based upon its spheroidal character founded on a fallacy? 

 For it does appear to me a fallacy to treat as one homogeneous 

 spheroid, a body partly consisting of a mass of solid or quasi- 

 solid matter of irregular form, and partly of a liquid mass in 

 constant motion, irregularly distributed over a portion of its 

 surface. No doubt the contour of the liquid portion is, accord- 

 ing to established geometrical laws, almost that of a regular 

 spheroid ; but its distribution, except in the case of inland seas, 

 can have but little to do ^^-ith the regulation of the movement of 

 the solid body on which it rests. It is true that Laplace has 

 maintained that "whatever may be the law of the depth of the 

 ocean, and whatever the figure of the spheroid which it covers, 

 the phenomena of precession and nutation will be the same as if 

 the ocean formed a solid mass with this spheroid ; " but do the 

 position of the axis of revolution and its permanence in one spot 

 come under the same category as precession and nutation ? It 

 certainly appears to me that the position of the axis of revolution 

 must mainly depend upon the form of the mineral portion of the 

 globe, and be but in the slightest degree affected by the distribu- 

 tion of the ocean, the specific gravity of which is moreover only 

 about one-fifth of that of the more solid portion. 



With regard to the permanence of the axis of rotation, if it must 

 of necessity coincide with the axis of figiire, and if the figure of 

 the mineral portion of the earth, in consequence of upheavals and 

 depressions, of the wearing away of continents and the transport- 

 ation of their constituents by mechanical or even chemical mean", 

 is being constantly changed, so as to acquire a nsw axis, then the 

 axis of rotation must also as constantly be undergoing a change of 

 position. 



Let us now glance at some of the irregularities of form of the 

 more solid part of the globe as at present existing. The difference 

 between the polar and equatorial diameters of our globe has been 

 calculated at about 26 miles, or about 13 miles in the radius ; but 

 at the equator itself, little more than one-fifth of the circumference 

 of the globe is dry land, and nearly four-fifths are sea ; and this 

 sea is by no means shallow, as the soundings taken by the 

 " Tuscarora," the "Challenger," and other exploring vessels will 

 prove. Leaving those taken near land out of the calculation, I 

 find that 48 soundings in the Pacific, between 15° and 30° north 

 latitude, give an average depth of 2,634 fathoms, or 5,268 yards, 

 that is to say, within a few yards of three miles. The South 

 Pacific does not appear to have been so well explored ; but 

 across the Atlantic, in the equatorial regions between 10° N. and 

 10° S., I find that an average of 32 soundings gives a mean 

 depth of 2,309 fathoms, or 4,618 yards, while, in one spot in 

 lat. 15° S., Sir James Ross did not find the bottom with 

 a line of 4,600 fathoms, or nearly 5|^ miles. In the Indian 

 Ocean, within the same limits, 20 soundings give an average of 

 2,468 fathoms, or 4,936 yards, or more than 2^ miles. Taking 

 these soundings as fair representations of the depth of the sea in 

 the neighbourhood of the equator, it appears that we may at once 

 reduce the equatorial diameter of the more solid part of the 

 globe by from 5 J to 6 miles over nearly four- fifths of its circum- 

 ference ; that is to say, we may reduce the usually accepted 

 equatorial protuberance from about 13 miles to a little over 10. 

 It is not within my province to inquire whether the fact of so large 

 a portion of the equatorial protuberance being of so much less 

 SI- ecific gravity thin if it were composed of mineral matter, will 

 in any way affect the established calculations with regard to the 

 precession of the equinoxes and the nutation of the poles, or, what 

 is of more importance to us, the inferences with regard to the 

 crust of the earth which have been thence deduced. 



But while so large a portion of the surface of the land is, in 

 the equatorial regions, so much below the normal level, there are, 

 especially in the northern hemisphere, large tracts of land which, 

 like the great plateau of Tibet, are some thousands of feet above 

 it. The average elevation of the whole of Asia has, indeed, been 

 estimated at 377 yards, or nearly a quarter of a mile above the 

 sea-level. The depth of the ocean in non-equatorial regions must 

 no doubt be taken into account ; but practically, the spheroidicity 

 of the globe, on which the stability of the pole has been held to 

 depend, may be regarded as, even at the present time, consider- 

 ably less than is usually supposed. When, however, we come to 

 think of the enormous elevations and depressions which some 

 parts of the gh.be have undergone during geological time, it is by 

 no means difficult to imagine conditions under which the general 

 average, so to speak, of the surface, would approach much 



more nearly to the form of a sphere, and the globe would become 

 much more sensitive of any disturbances of its equilibrium ; 

 but wliether the globe is a sphere or a spheroid, it is hard to see 

 why disturbances of its equilibrium should not affect the position 

 of its axis of rotation. 



Taking our globe with the distribution of land and water as at 

 present existing, I should like to inquire of mathematicians what 

 would be the theoretical result of such a slight modification, geo- 

 logically speaking, as the following : — Assume an elevation to 

 the extent, on an average, of 4,000 feet over the northern part of 

 Africa, the centre of the elevation being, say, in 20° north 

 latitude. Assume that this elevation forms only a portion of a 

 belt around the whole globe, inclined to the equator at an angle 

 of 20°, and having its most northerly point m the longitude 

 of Greenwich, and cutting the equator at 90° of east and west 

 longitude. Asnime that along this belt the sea-bottom and what 

 little land besides Africa it would traverse were raised 4,000 feet 

 above its present level over a tract 20° in width. Assume further 

 that the elevation of this belt was accompanied by corresponding 

 depressions on either side of it, so as to leave the total volume of 

 the mineral portion of the earth unaffected. Would not such a 

 modification of form bring the axis of figure about 15° or 20° 

 south of the present, and on the meridian of Greenwich, that is 

 to say, midway between Greenland and Spitzbergen ? and would 

 not, eventually, the axis of rotation correspond in position with 

 the axis of the figure ? 



If the answer to these questions is in the affirmative, then I 

 think it must be conceded that even minor elevations within the 

 tropics would produce effects corresponding to their magnitude ; 

 and also that it is unsafe to assume that the geographical 

 position of the poles has been persistent throughout all geological 

 time. 



It is not the first time that I have insisted upon this point ; for, 

 some ten years ago, I pointed out another possible means of 

 accounting lor a change in the geographical position of the axis 

 of the earth. My hypothesis was, however, founded on the 

 assumption of the globe consisting of a comparatively thin crust, 

 with an internal fluid nucleus over which the crust would slide 

 when, from any geological cause, its equilibrium was disturbed. 

 To this it has been objected ^ — ist. That there would be a ten- 

 dency in the transfer of sediment from one part of the globe to 

 another, and in the various elevations and depressions of land 

 simultaneously, to balance each other ; and 2nd, that the friction 

 over the nucleus would be too great, and that, owing to the earth 

 being a spheroid and not a perfect sphere, any motion pf the crust 

 would be attended by great resistance, and the bending and rend- 

 ing of its mass. 



To these objections it may be replied that the effects of the 

 transfer of sediment from one place to another, and of elevations 

 and depressions of land going on at the same time, are just as 

 likely to be doubled by the depressions taking place in the same 

 hemisphere as the elevations, but on opposite sides of the pole, 

 as they are to neuti alise each other ; and, 2ndly, that with a 

 comparatively thin crust, the readjustment to a fresh position on 

 a nucleus so slightly spheroidal as that supposed to exist in 

 the earth, is not accompanied by any great change of form, or 

 certainly not more than what the contorted rocks all over the 

 world have undergone. 



I am not, however, on the present occasion, going to attempt 

 to prove that the assumption involved in my hypothesis is 

 reasonable. How we are to account for all the vast oscilla- 

 tions of the earth's surface, which we find to have been going 

 on ever since the earliest geological perioi up to the present 

 day, on any assumption more reasonable, I will leave for others 

 to determine. I have already called attention to the bearing 

 which recent researches in solar physics have upon this subject, 

 and I am content to leave the matter as it stands, in the hope 

 that before many years have passed, we may learn more either 

 in its proof or disproof. 



The moral which I wish to draw from all that I have just said is 

 this : — That so long as there is a possibility, not to say a proba- 

 bility, of the geographical position of the poles having changed, 

 it is premature to invoke intense glacial periods to account for all 

 the glacial phenomena which may be observed. Much as we must 

 esteem the labours of M. Adhemar and Mr. Croll, and others 

 who have gone so deeply into the question of glaciation — enor- 

 mous as have been the effects of ice in this and other countries — 

 there are many who cannot but feel that the ice-caps mvoked 

 almost transcend our powers of belief, and who will bg grateful 

 to any astronomer or mathematician who will bring the pole 

 » Lyell's " Prindplef," nth edition, vol. ii. p. 209. 



