296 ANNUAL KEPOET SMITHSONIAN INSTITUTION, 19 30 



rain, air, wind, and water. Glaciers advance after heavy winter 

 precipitations followed by low summer temperatures and cloudiness. 

 They retreat following slight winter snowfalls, high sunniier tem- 

 peratures, and clear sky. 



It has been said that the cause of the principal variations in the 

 present size and condition of glaciers in Alaska is due to variations 

 in altitude, in latitude, in precipitation, and in direction of slope 

 (Tarr and Martin, 1914) . Fluctuations in Alpine glaciers have been 

 traced back to about 1600 A. D.; in Norwegian glaciers, to about 

 1700 A. D. Yearly measurements were begun on the Rhone Glacier 

 in 1874 and on other glaciers in various parts of the world since 

 1894. Weather changes are not immediately recorded at the termi- 

 nal end of an ice mass, since glaciers vary as to size of the neve field, 

 depth of the catchment basin, width of the fern field, length of the 

 ice tongue, etc. For example, small glaciers in the Alps began to 

 advance in 1909 and 1910; medium-sized glaciers in 1912 and 1913, 

 while long valley glaciers, such as the Aletsch, did not advance until 

 1920, when it moved forward 120 feet, and a greater amount in 1921. 

 The variability of the weather thus causes glaciers to advance or 

 retreat, but not all at the same time; for, being of different sizes, 

 their respective masses, as well as other factors, tend to govern their 

 movements. 



SOME PROPERTIES OF ICE 



Under the influence of heat, ice behaves as most solids do, con- 

 tracting when cooled, expanding when heated. As regards the evap- 

 oration of ice, it was shown by Barnes and Vipond in 1909 that it 

 goes directly into vapor without passing through a preliminary 

 liquid phase. The melting point of ice is lowered by increase of 

 pressure. The rate at which this occurs is 0.0075° C. for every 

 atmosphere of pressure. This fact Avas theoretically predicted by 

 James Thomson in 1849 and demonstrated by Sir W. Thomson (Lord 

 Kelvin) in 1850. 



Although there is no rise of temperature accompanying the melt- 

 ing of ice, a definite quantity of heat is absorbed — namely, about 80 

 calories per gram, or 78.818 thermal units. The same amount of 

 heat is evolved when water becomes ice — that is, the amount of heat 

 required to convert ice into water, or vice versa, would raise the same 

 amount of water through 80° C. This is the latent heat of fusion 

 of ice or the latent heat of water. Because of this fact, ice is the 

 most difficult of all solids to melt, as regards the amount of heat 

 energy required to be put into it in order to effect fusion, and water 

 the most difficult liquid to freeze of all substances, owing to the 

 relatively large amount of the latent heat needed. It is fourteen 

 times as great as for lead and twenty-eight times as great as for 

 mercury. 



