: PHYSICS AND MATHEMATICS TO GEOLOGY. 145 
being the breadth, D the depth, and L the length. Suppose the size 
of this bar to be changed, whilst the ratios of its dimensions continue 
thesame; then - - - thestrengthwillvaryasD? - - - ”(p.195). 
The following is the conclusion drawn by Mr. Spencer: ‘ Viewed by 
the light of this principle, the fact that the earth is an oblate spheroid 
does not seem to afford any support to the hypothesis of original fluid- 
ity as commonly understood. We must consider that in respect of 
its obedience to the geo-dynamic laws, the earth is fluid now and must 
always remain so; for the most tenacious substance with which we are 
acquainted, when subjected to the same forces that are acting upon the 
earth’s crust, would exceed the limit of self-support determined by the 
above law, before it attained ;z50+ 50-000 Of the earth’s bulk” (p.196). 
Perhaps if one knew what Mr. Spencer means by ‘the limit of self 
support,” and what is the exact distinction he draws between “ fluidity 
as commonly understood ” and * fluidity in respect of obedience to geo- 
dynamic laws,” one might be in a position to form some estimate of his 
degree of physical insight; but so far as I can see all he satisfactorily 
shows is an extraordinary agility in jumping to conclusions. If his 
meaning is that deformation must accompany the action of gravita- 
tional and centrifugal forces, he might, if Maxwell’s view be correct, 
have added to the denominator of his estimate as many 0’s as the 
printer could spare; but if it is the rupture of an elastic solid or its 
transformation into a plastic state to which he refers, as seems almost 
certain from the context, he must have formed an extremely low esti- 
mate of what strains a solid can stand. 
In the same passage Mr. Blytt refers to Mr. Peirce,* Sir J. W. Daw- 
son,t and Prof. J. EK. Todd { as holding that a solid earth will alter its 
shape if the rate of rotation vary. The views of Mr. Peirce I have not 
seen, but the other two writers mentione * regard the solid earth itself 
as changing shape only by means of a succession of what we may term 
catastrophies. Their views seem identical with those which Mr. Blytt’s 
translator ascribes to him in the following words: ‘‘The sea adjusts 
itself in accordance with the smallest change in the length of the day 
- -  - But the solid earth offers resistance to change of form, and 
begins to give way only when the tension reaches a certain amount ” (p. 
418). Mr. Blytt makes several distinet references to the subject, and 
his remarks are not perhaps always strictly consistent. This, however, 
is hardly to be wondered at, since he gives as the result of his investiga- 
tions: “As has been stated, there prevails - - - a disagreement as 
to how far the earth will change its form, in case the centrifugal force 
varies. Thomson is most inclined to believe that it will not. Darwin 
is of opinion that it will. And among other physicists whom I have 
consulted a similar divergence prevails upon this point. One thinks 
* Proc. Amer. Acad. Arts and Science, 1873, vol. vi, p. 106. 
+ Story of the Earth and Man, ninth edition, pp. 291, 292. 
t American Naturalist, 1883, vol. Xvi1, pp. 15-26, specially pp. 18, 19. 
H. Mis. 334, pt. 1 10 
