Contraction during Cooling of a Solid Earth. 145 



where the right-hand side depends on the collisions between 

 the two kinds of gas in the layer, s being the semi-sum of the 

 diameters. From these we obtain 



In the special case, when the masses and diameters are 



equal in the two gases, the diffusion-coefficient (the multiplier 



d C2 Q 

 of -j-y above) has the value 



It is therefore inversely as the density, and directly as the 

 square root of the absolute temperature. And in the case of 

 two infinite vessels, connected by a tube of length I and sec- 

 tion S, and containing two gases whose particles have equal 



masses and diameters, the rate of flow of either is .-—j 1*785 



Is/ h 



in mass per unit of time. 



Other cases are treated ; and among these it is shown that 

 with equal masses, and constant semi- sum of diameters, differ- 

 ence of diameters favours diffusion. 



XVIII. On the Amount of the Elevations attributable to Com- 

 pression through the Contraction during Cooling of a Solid 

 Earth. By Rev. 0. Fishee, M.A., F.G.S.* 



IT is now thirteen years since I first published in the 

 4 Transactions of the Cambridge Philosophical Society'f, 

 and, at a later date, in my ' Physics of the Earth's Crust ' j, 

 an attempt to estimate the mean height of the elevations 

 which compression, resulting from the contraction due to 

 cooling, might give rise to upon the earth considered as a solid 

 globe. 



A remark from my friend Mr. Davison, who is working on 

 this subject, has suggested to me that the investigation I have 

 given is not quite satisfactory ; and I now offer the following 

 as an improvement. 



If we are to attribute the corrugations which we meet with 

 in the earth's crust to compression arising from the secular 

 cooling of a solid globe, we must assume, as I have tacitly 



* Communicated by the Author. 



t Vol. xii. pt. 2. Read Dec. 1, 1873. 



