146 SOME APPLICATIONS OF 
that a lengthening of the day even by several hours will be incapable 
of altering the form of the solid earth; another believes that the solid 
earth will probably change its form just as easily as the sea” (p. 421). 
If Mr. Blytt should ever have further occasion to consult physicists 
on this or any allied point, he would find an exact definition of such 
terms as solid acertain amount of protection from é priori speculations. 
Mr. Blytt’s own principal view seems due in part to an erroneous inter- 
pretation of Tresca’s experiments on the flow of metals under pressure. 
They do not in reality justify his statement ‘“ By reason of the enor- 
mous pressure which prevails in the interior of the earth, it must be 
supposed that masses from a certain depth are more or less in a plastic 
state” (p. 417). It was in fact pointed out some years ago by the Rey. 
Osmond Fisher * that the existence of an orifice from which the metal 
can flow constitutes a complete difference between the conditions of 
Tresca’s experiments and the state of a body subjected to nearly uni- 
form pressure all round. 
Mr. Blytt apparently does not stand alone in believing Sir W. Thom- 
son to hold that the solid earth is incapable of altering its form as the 
rotation alters, and that if possesses the same eccentricity as when it 
solidified. Prof. Darwin in Nature, 1886, vol. XXxXtIv, pp. 420-423, seems 
also to put this interpretation upon a passage he quotes from § 830 of 
Thomson and Tait’s “ Natural Philosophy.” Supposing this interpre- 
tation correct, Prof. Darwin’s opinion that Sir W. Thomson does not 
allow “a sufficient margin for uncertainties” expresses only a part of 
the objections I should entertain. I find it difficult however to believe 
that Sir W. Thompson, who elsewhere gives data for the eccentricity 
produced by rotation in solid spheres of steel, can actually suppose no 
change at all in the eccentricity to follow an alteration in the angular 
velocity. Still it must be confessed that though the passage contains 
the statement, “ It must necessarily remain uncertain whether the earth 
would from time to time adjust itself completely to a figure of equi- 
librium adapted to the rotation,” its most natural interpretation is that 
given by Prof. Darwin. I need hardly say that the conclusion that 
the earth, however solid, would retain a constant eccentricity while the 
rate of rotation varied, seems to me directly opposed to the conclusions 
to which the elastic solid theory leads. 
Prof. Darwin hinself, in his paper in Nature refers to Tresca’s ex- 
periments and thinks it probable there would be from time to time a flow 
of material as the anguiar velocity altered. One of the “ uncertain- 
ties” he refers to is the possibility that, in accordance with Dr. Croll’st 
views, a greater rapidity of denudation in equatorial than in polar 
regions may have reduced the eccentricity markedly below the value it 
possessed when the earth solidified. He does not seem however to 
* Physics of the Earth’s Crust, 1st edition, 1881, foot-note p. 120. 
t Climate and Time (1885), p. 336. 
