June i8, 1896J 



NATURE 



161 



each of the stages of growth a batch of larvrc was removed and 

 surrounded by dark twigs, and at the end of the stage restored 

 to the green leaves. By comparing the colours of the mature 

 larva; in the different batches, it is possible to determine the 

 period of larval susceptibility. 



The following forms of variation in butterflies of the genus 

 Helkoiiiiis, of Tropical America were exhibited by Mr. W. F. 

 H. Blandford. (l) Variation in Hiliioniiis erato., L. There 

 are three main types with the basal patch of the hind-wings, 

 respectively, red, blue, or green. The green form is dominant 

 in Panama ; it occurs throughout Central America, but not in 

 South America, except sparingly in C"lombia and Venezuela. 

 At Sao Paulo, on the Upper Amazons, the blue form alone 

 occurs, or the basal patches may be obsolete. (2) Variation in 

 HeHionius thelxiope, Hlibn., and Hiliioiiiiis vesta. Cram. Both 

 forms occur together, and are very variable in Cayenne and the 

 Lower Amazon Valley. Further west definite parallel geo- 

 graphical races occur of both. Heliccitius thehiope is connected 

 by intermediate forms in Cayenne with Hcluomus iiiclpoiiieiie, 

 L. , a widely distributed species, which occurs in the Amazon 

 Valley at Santarem and Obydos only, and is not found in the 

 humid forest. In Bolivia .M?/«co«2>« TCiA; merges mKo Heliconius 

 phytlis. 



Mr. W. Saville Kent showed interesting photographs and 

 specimens illustrating the natural history and ethnology of 

 Australia. The Hon. Walter Rothschild exhibited a group of 

 recently described and other rare Birds of Paradise and Bower- 

 birds. 



During the evening four lantern demonstrations were given in 

 the meeting room of the Society. Prof .K. C. Haddon showed 

 a series of slides illustrating the evolution of the cart, and 

 another which illustrated the evolution of the Irish jaunting-car. 

 The Altels avalanche, which occurred in September 1895, was 

 described with photographs by Dr. Tempest Anderson, and 

 Prof. Herkomer gave a demonstration of his new gravure 

 process. Prof. Dewar dealt with liquid air. and showed the 

 following experiments illustrative of low temperature effects : — 

 Filtering liquid air ; vacuum vessels boiling at 350° F. 

 below the freezing point ; colour and absorption spectra ; 

 spheroidal state ; solid alcohol ; frozen soap-bubble ; distilling 

 mercury and phosphorus ; liquefaction and solidification of 

 gases ; fusible metal spring ; brittle indiarubber and its 

 expansion by cold ; the diamond burning in liquid 

 oxygen ; magnetic oxygen ; photographic action and phos- 

 phorescence ; ignition by means of a lens of liquid air ; cooling 

 a vessel 380° F. below the freezing point, until the air of the 

 room condenses on the surface to the liquid state. 



The Lords of the Committee of Council on Education have 

 arranged foi the public exhibition, in the Western Galleries of 

 the Science Museum at South Kensington, of a number of the 

 objects shown at the soiree. The exhibition will remain open 

 to the public for about a fortnight. 



ON THE ROTATION OF THE EARTH} 



T^HE recent discovery of periodical variations of terrestrial 

 ■'• latitudes demands a revision of the actual theory of the 

 rotation of our planet. This theor)-, based upon the hypothesis 

 of the absolute rigidity of the earth, admits of variations of this 

 kind, but very difi'erent in their laws from those of the observations. 

 The period of revolution of the terrestrial poles given by the theory 

 is one of about ten months. That which the observations give us 

 lasts nearly fourteen months. Still further, the attentive analysis 

 of the observations of the latitudes, executed of late by Mr. 

 Chandler, shows us that the movement of the terrestrial poles is 

 compounded of two others, of which the periods are, the one of 

 430 days, and the other of twelve months. 



Following the order of the ideas established in the science by 

 the celebrated co.smogenic hypothesis <if Laplace, we ought to 

 attribute this disagreement of the theory and the observations 

 to the interior fluidity of the earth. But the illustrious physicist. 

 Lord Kelvin, does not admit that the fluid nucleus of the earth 

 may be of considerable enough dimensions. The greatest part 

 of the astronomers of our day adhere to this opinion. They 

 refer the said discordance to the terrestrial globe being elastic. 



t .\bridged translation of a paper by Th. Sloudski, Professor at the 

 University of Moscow {Bulletin dt la SocUt^ Imperiate des Naturalistes de 

 Moscou. .^Iln^e 1895, No. 2). 



NO. 1390. VOL. 54] 



In considering the hypothesis of a thin rigid crust of the earth 

 as contrary to all given physics, the celebrated English physicist 

 affirms in his memoir " On the Rigidity of the Earth," Phil. 

 Trans., 1863, and in the first edition of the "Treatise on Natural 

 Philosophy" (§§ 847 and 84S), that this hypothesis is also 

 incompatiljle with the observations of the precession and of 

 the nutation. On subsequently withdrawing certain of these 

 astronomical objections, he has replaced them by some 

 others. 



To be able to appeal to objections of this kind, the theory of 

 the rotation of the earth considered fluid in its interior ought to 

 have been previously established. Lord Kelvin has not done it. 

 He has limited himself to enunciating in general terms the prin- 

 cipal propositions of this theory. To be able to judge of the 

 .said objections of the celebrated English physicist, the theory in 

 question must be previously established. 



The problem of th^ rotation of the earth — supposed fluid in its 

 interior — was approached by W. Hopkins in 1839 (P/iil. Trans., 

 1S39-40-42); but the state in which hydrodynamics then was 

 found, did not permit the English savant to treat the matter in a 

 satisfactory manner. The more recent attempts to solve this 

 difiicult problem have not been more successful. 



We shall endeavour in the present article to give a more 

 perfect solution of this important problem. To render this 

 task more easy, we shall assume that the nucleus of the earth is 

 homogeneous, and of the form of a planetary ellipsoid. 



The success of our task is assured by the beautiful researches 

 of our clever geometrician, Prof. N. Joukovsky, relative to the 

 movement of a solid body with cavities filled with an incom- 

 pressible homogeneous fluid. We have only to apply these 

 researches to our special problem. We hope to lessen the 

 difficulties of this application by the supposition that the rotatory 

 motion of the entire terrestrial mass differs very little from the 

 uniform rotation. The proposition of the celebrated Laplace, 

 relative to the effect of friction of the fluid parts of the earth 

 upon its rotatory motion, affords us a solid foundation for the 

 said suppo.sition {"Qiuvres Completes de Laplace," tome v. 

 p. 283). 



We shall commence our article with an abridged exposition of 

 the theory of the rotation of a solid body, which has a cavity 

 filled with an incompressible homogeneous fluid. In the develop- 

 ment of the principal formula; of this theory we shall employ 

 the most simple method, that of the illustrious Poisson. We 

 shall equally profit by them in our transformations of the hydro- 

 dynamical equations. 



(The final paragraphs, after thirty large octavo pages of intricate 

 mathematics, are as follows.) 



We have taken our problem with some considerable restric- 

 tions relative to the form, to the position, to the structure, and 

 to the movement of the terrestrial nucleus. This renders 

 almost useless the detailed comparison of our results with the 

 given astronomical ones. We will only say some w^ords relative 

 to one of these results, of which the generality is indubitable. 



The hypothesis of a fluid nucleus of the earth being admitted, 

 and the exterior forces neglected, the movement of the terres- 

 trial poles ought to be composed of two periodic movements. 

 The period of the former of these movements is perhaps of twelve 

 or fourteen months, that of the second ought to be pretty nearly 

 a day. 



The astronomical observations do not show us this second 

 movement of the poles. Is not this a reason for taking excep- 

 tion to the hypothesis of the fluidity of the earth in its interior? 

 By no means. It is in the first place possible that the 

 smallness of the amplitude of the movement in question may 

 make it unrecognisable. The smallness of the factors /to, v^' 

 renders this supposition probable. Secondly, it may also be 

 admitted that the want of the appropriate observations causes 

 us to ignore for the present this movement, although its ampli- 

 tude may be appreciable. One may also suppose that the 

 period of the movement in c|uestion, from the usual order of 

 astronomical observations, ajjpears to us to be a period of twelve 

 or of fourteen months. For instance, should the said period be 

 equal to twenty-four sidereal hours exactly, and the observations 

 of the latitude of any astronomical observatory be made every 

 midnight during a good many years, the result of them will be 

 the period of twelve months. 



This last supposition appears to us worthy of attention, 

 because according to our opinion the explanation of the period 

 of twelve months by meteorological causes, as is adopted at 

 present by some astronomers, wants probability. 



