142 Review of the Controversy Regarding the Motion of Glaciers. 



its solidity," the infinitesimally small amount of heat which can be 

 transmitted into it from a film of overlying water at 32°.* 



It may also be worth while to obr-erve in passing, though not a*? 

 affecting the theory in question, that the Ijottom of a glacier must have 

 a tem-perature lower than 32°, on account of the weight of the overlying 

 mass. The experiment of Prof. W. Thomson of Glasgow, in 1849, 

 showed that the melting point of ice is lowered about 0.013° for every 

 additional atmosphere brought to bear upon it. Assuming, then, the 

 weight of glacier ice at 56 lbs. per cubic foot, the pressure will increase 

 at the rate of nearly one atmosphere for every 40 feet of depth. 

 Consequently, at a depth of 400 feet, when a pressure of 10 atmospheres, 

 or 150 lbs., on the square inch would \)c reached, the melting point of 

 ice would be not 32° but 31.87°, and if the bottom of the glacier lay at 

 this depth the lower surface must melt whenever that point is attained. 

 This is the origin of the stream Avhich issues in winter from beneath 

 the ice. The heat of the earth melts the ice in contact with it, and 

 keeps it in a constantly thawing condition, though its own surface is 

 below 32°. Were it not that a great part of tlie water thus produced 

 solidifies again, as soon as released from the pressure, Ihese winter 

 streams would be larger than they are. 



Though not logically concerned in Mr. Croll's theory, it may be as 

 Avell also to consider the effect of radiant heat upon the ice of a glacier. 

 That radiant heat passes into ice more rapidly than the heat of con- 

 duction is of course true. But even here we fear Mr. Croll's theory 

 has little to expect. The intensity of solar heat among the Alpine 

 glaciers is matter of constant remark from travelers in Switzerland. 

 "I scarcely ever," says Forbes, "remember to have found the sun 

 more piercing than at the Jardin." On this Mr. Croll himself 

 remarks: " there is no proof from this that the glacier experiences any 

 change of temperature. The sun shines down with piercing rays 

 and the traveler is scorched, the glacier melts on the surface, but it 

 still remains cold as ice. The scorching rays are withdrawn and the 

 traveler is subjected to radiation on every side, from (query to) 

 surfaces at the freezmg point." f 



It may, at first sight, seem thai so great an amount of solar heat, 

 falling directly upon a glacier, must have some effect upon it ; but a 

 little consideration of the properties of ice toward radiant heat is 

 sufficient to dispel the illusion. Notwithstanding the transparency 



-It is not antecedently more improbable that ice should be capable of existing at temper- 

 atures slightly above 32°, than that water should be capable of remaining liquid in certain 

 circumstances at temperatures below the freezing point, which is a thing of constant occurrence. 



t L. E. and D. Fhil. Mag., Sept., 1870. 



