CONDITIONS OF CONTINENTAL ICE. 215 



downwards more rapidly than the heat is travelling 

 upwards. We must therefore conclude that under- 

 ground heat is confined to a very thin layer of the cap 

 at the bottom, and that its effects, either in melting the 

 ice or in raising its temperature, are so trifling that 

 they may be practically disregarded in our present 

 inquiry. 



It must further be observed that when it is stated 

 that underground heat will maintain at the melting- 

 point the ice in contact with the ground, it is not 

 meant that it will maintain it at the temperature of 

 32°, for, as Prof. James Thomson discovered, the 

 temperature at which the ice melts is lowered by 

 pressure at the rate of about 0'0137° F, for every 

 atmosphere of pressure. In the present case the 

 pressure depends upon the thickness of the ice ; so 

 that, if the sheet be 1400 feet deep, the melting-point 

 will be 31 0, 5 ; if half-a-mile deep, it will be 31° ; if 1 

 mile deep, 30° ; and so on* 



Heat derived through the Upper Surface. — It follows, 

 from what has already been shown, that the greater 



* The melting-point does not, however, vary uniformly with the 

 pressure; for Mousson (Ann Chim. et Phys., 3rd series, lvi., p. 257, 

 1859) found that it required a pressure of 13,000 atmospheres to 

 lower the melting point to zero F., whereas, if the melting-point had 

 decreased in proportion to the increase of pressure, a pressure of 

 2337 atmospheres would have been sufficient. Boussingault 

 succeeded in lowering the melting-point 11° below zero F., but the 

 amount of pressure employed was not determined (Ann. Chim. et 

 Phys., xxvi., p. 544, 1872). 



The fact that the melting-point of ice would be lowered by pressure, 

 or rather that pressure would prevent freezing, was suggested nearly 

 a century ago by Dr. Charles Hutton, Professor of Mathematics in 

 the Military Academy of Woolwich. From certain experiments on 

 the expansive force of ice, made in Canada by Major Williams, in 

 the year 1784-85, Dr. Hutton makes the following remarks : — 



"From these ingenious experiments we may draw several con- 

 clusions : — First. We hence observe the amazing force of the 



