Physics and Mathematics to Geology. 343 



form due to its rate of rotation, as certainly and promptly as 

 if it were liquid/' The other writer, Mr. A. Blyti *, amongst 

 other references to the paper says, " I believe that Spencer is 

 the first who expressed the opinion that even a solid earth can 

 change its form.''' 



Mr. Spencer, after some statements as to the relative 

 strength and agility of large and small animals, such as 

 elephants and fleas, formulates the general result that the 

 strength — called also "resistance to fracture 9i — of a solid 

 structure varies as the square of its linear dimensions, while 

 the u agencies antagonistic to cohesive attraction/'' i. e, gravi- 

 tational and " centrifugal " forces, &c, vary as the cube. 

 Excepting a statement that this is obviously true of simple 

 longitudinal and torsional stress, the following is the sole 

 proof of his very general law supplied by Mr. Spencer : — 

 " The strength of a bar of iron, timber, or other material sub- 

 jected to the transverse strain varies as BD 2 /L ; B 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 the same ; then . . . the strength will vary as D 2 ..." 

 (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 fluidity 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 



i7mk^ th of the earth ' s bulk " (P- 19ti )- 



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 gravitational and centrifugal forces, he might, if 

 Maxwell's view be correct, have added to the denominator 

 of his estimate as many O's as the printer could spare ; but if it 

 is the rupture of an elastic solid or its transformation into a 



* Phil. Mag. May 1889, p. 415. Translated from Kijt Magazin for 

 Naturvidenskaberne, 13d. xxxi. 1889. 



