4: THE ICE AGE IN NORTE AMERICA. 



creased, the relative influence of friction in retarding the 

 motion is rapidly diminished. The friction on the sides of 

 a glacier two miles wide is no greater than that upon one a 

 quarter of a mile in width, though the cross-section is eight 

 times as large. A cross-section of the Mer de Glace at Les 

 Moulins is estimated to be one hundred and ninety thousand 

 square yards ; whereas a cross-section of the Muir Glacier, in 

 Alaska, a mile above its mouth, is upward of one million 

 square yards. 



Though observation shows that ice actually moves as if it 

 were a fluid, the scientific imagination is tasked to the utmost 

 to conceive how such motion can be consistent with other 

 manifest qualities of the material ; for in many conditions ice 

 seems as brittle as glass and as inelastic as granite. The 

 mystery is probably solved, so far as such questions are ever 

 solved, by attention to the facts already referred to concern- 

 ing the behavior of ice at its melting-point. When ice passes 

 into water, an immense amount of heat is absorbed in the 

 process, which yet does not produce any effect upon the 

 thermometer. If a hole be bored in the surface of a melting 

 glacier, and a thermometer inserted, it will stand at 32° Fahr. 

 If the same thermometer be inserted in the subglacial stream 

 issuing from the ice front, it will stand at the same point. 

 Yet the absolute difference between the heat contained in 

 the particles of ice, and that contained in the particles of 

 water, is 144° Fahr. — so much heat being occupied in keep- 

 ing the substance in a liquid form. Ice is also transparent 

 to the rays of heat as it is to the rays of light. Scoresby 

 amused himself, in the arctic latitudes, by making lenses of 

 ice with which to concentrate the sun's rays and set com_ 

 bustible substances on fire. 



The fusing-point of ice is also modified by pressure. 

 Under pressure the freezing-point of water may be lowered 

 two or three degrees ; but upon the removal of the pressure, 

 the water will instantly become solid. This has been demon- 

 strated in various ways. M. Boussingault, for example, filled 

 a hollow steel cylinder with water, having a bullet loose with- 



