384 Mr. 0. Fisher on the Thermal Conditions and 



supposed, the same both for the ice and rock. Hence equa- 

 tion (2) becomes, 



for the rock, F = K ^, 



for the ice, F = &/3, 

 whence 



p k 60 



Now the ratio of the conductivities will be the same whatever 

 system of units we employ. 



Referring to Professor Everett's i Illustrations of the C.G.S. 

 System of Units,' we find the mean of K for three kinds of 

 rock in situ, as determined by Sir Wm. Thomson, to be *00581, 

 and the mean of k for ice "00218 ; whence 



p 218 60 



= •04441. 

 Hence, for the ice, 



2=5 =-04441, 



e ' 



in which expression a is the temperature of the surface in con- 

 tact with the rock, and b is the temperature of the surface ex- 

 posed to the air, e being the thickness of the ice. 



If, therefore, we wish to find the thickness of ice which will 

 just be sufficient not to induce melting at the bottom, we must 

 put 32 for a ; and if, with Dr. Croll, we suppose the mean 

 temperature of the surface exposed to the air to be 0° F., we 

 must put for b. Whence 



-=•4441; 



e 



.-. £=743 feet. 



If the thickness of the ice be less than this, no melting will 

 take place ; but if greater, there will be melting at the junction 

 of the ice and rock. 



Here no account has been taken of the lowering of the 

 melting-point by pressure. But that is easily allowed for. 

 For, comparing the height of a column of ice whose pressure 

 is equivalent to a column of mercury of 30 inches (or one 

 atmosphere), it is 37 feet. 



Since, then, the melting-temperature is lowered by 0'0137° F. 



