18 Rev. 0. Fisher on the Surface Elevations and other 



level of no strain exceeds the surface-temperature by b-^ - 



degrees only, which, with the high estimate of 7000° for 

 solidification, gives an excess of 228° at present ; so that no 

 rock can ever have been pressed up from a depth where its 

 temperature has been much higher than that of boiling 

 water. Neither can it be replied that it may have been 

 otherwise in former ages, because the above expression for the 

 excess varies as the square root of the time and must have 

 been less hitherto than it is now. 



The very slight changes, either through cooling or com- 

 pression, which the rocks above the level of no strain have 

 undergone since solidification, render it improbable thai 

 their effects would be observable. The fact that the rocks now 

 being compressed were once extended would not have any 

 effect upon the calculated mean height of the surface-eleva- 

 tions ; because their former compressive extension was at the 

 cost of their thickness. If, however, the effects were sufficient, 

 they might be traced petrologically. But it must be re- 

 membered that the thickness of the sedimentary strata must 

 be in most places greater than the depth of the level of no 

 strain. 



The property of the level of no strain enables us to calculate 

 the contraction of the radius of a solid globe, cooling according 

 to the supposed law; because the mean contraction from the 

 centre to that level is the same as that of the circumference 

 there. Thei'efore the mean contraction of z in the interval 



ez °-£ dt=ez °2rVl dt > 



.".in the whole time * it will be 



P/ a3/> A, 



or 



3 b 



2 I 

 m 



e I - V«(2r^-|m^). 



The term in — is negligible. 



Hence the contraction from the centre to the level of 



strain is . 3& 



e rr b s/ifct = g - - or ; 

 2 2 a 



which, with the assumed values, gives 

 6-3272 miles. 



