Mr. H. Spencer on the Form of the Earth. 195 



plete obedience to them. The mud in our streets stands in 

 ridges behind the passing cart-wheel — when scraped together 

 it appears Mquid and assumes a horizontal surface. On the 

 spade of the excavator, clay retains its square figure and its 

 sharp angles ; but when made into a bulky embankment, it 

 will, if the slope be insufficient, spread itself out on one or 

 both sides of the base, occasionally continuing to slip until it 

 assumes an inclination of six to one. 



A comparison of the physical powers of large and small 

 animals exhibits a series of facts of an analogous character. 

 A flea jumps several hundred times its own length, and is un- 

 injured by collision with any obstacle. The great mammals, 

 on the other hand, seem to possess no agility whatever ; and 

 a concussion borne by the insect with impunity would smash 

 an elephant to a jelly, Between these extremes may be ob- 

 served a gradation in the ratios of power to bulk ; so that 

 commencing with the smaller creatures, every increment of 

 size is, cceter is paribus, accompanied by an under-proportionate 

 increase of strength, until we arrive at that limit (to which the 

 elephant has evidently approximated) where the creature is no 

 longer capable of supporting its own framework. 



These and innumerable like facts point to the inference 

 that fluidity and solidity are to a great extent qualities of 

 degree ; that the cohesive tenacity of any piece of matter bears, 

 as the mass of that matter is increased, a constantly decreasing 

 ratio to the natural forces tending to the fracture of that mat- 

 ter; and that hence any substance, however solid to our per- 

 ceptions, only requires to have its bulk increased to a certain 

 point, to give way, and become in a sense Jluid before the 

 gravitative and other forces. 



However repugnant to that "common sense" for which 

 some have so great a respect, this proposition is capable of a 

 very simple demonstration. 



The strength of a bar of iron, timber, or other material 



1 • , , • . BD^ „ , • 



subjected to the transverse stram varies as —j— ; rJ bemg 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 as the fraction y will remain con- 

 stant the strength will vary as D-, or (since D bears always 

 the same proportion to B and L) as B'^ or L^. Hence in 

 similar masses of matter the resistances to the transverse strain 

 are as the squares of the linear dimensions. Tlie same law 

 still more manifestly applies to the longitudinal strain, when 



