GLACIERS. 



673 



are no such bands, as the tributaries making it do not descend in 

 cascades. (Tyndall.) This difference of velocity between the middle 

 and sides of a glacier has been proved also by direct experiment. 



The rate of movement depends, of course, upon the slope. Accord- 

 ing to different observations, it varies from five to over fifty inches 

 a day ; and in some places a glacier may be so embayed as to lie 

 almost without motion. A rate of eight to ten inches a day is most 

 common i it is equivalent to 243^ to 304 feet a year, or one mile in 

 about twenty-two to seventeen years. 



Forbes deduced, from his measurements made at two stations on each of the 

 Bois and Bossons Glaciers, the following results. The first station on the Bois 

 Glacier was near its upper part, where the rapidity is unusually great, and the 

 other near its lower extremity. 



Boss. II. 



Motion from Nov. '44 to Nov. '45.. 



Mean daily motion 



Mean daily motion in summer, 

 April to October 



Mean daily motion in winter, Oc- 

 tober to April 



Bois I. 



Bois II. 



Boss. I. 



847.5 ft. 

 27.8 in. 



220.8 ft. 

 7.3 in. 



657.8 ft. 

 21.6 in. 



37.7 in. 



9.9 in. 



28.0 in. 



19.1 in. 



4.7 in. 



15.8 in. 



489.1 ft. 

 16.1 in. 



22.2 in. 

 10.7 in. 



This table shows, further, that the rate of motion is about twice as great in 

 summer as in winter. 



The maximum in July at the upper station on the Bois Glacier was 52.1 

 inches ; in December, 11.5 inches. The rapidity at the same place is not always 

 the same in different years. Thus, at one station on the Mer de Glace, Forbes 

 obtained for the daily motion in 1842-43, 1843-44, 1844-46, the amounts 8.56, 

 9.47, 10.65 inches. A knapsack lost in the Talefre Glacier (t, in fig. 948) after 

 ten years was found 4300 feet distant; the slope here of this high glacier was 

 14° 55' (Forbes). 



The rate (1) at the upper surface, (2) half-way to the bottom, and (3) at the 

 bottom, was found by Tyndall to be in one case 6 inches, 4.59 inches, and 2.56 

 inches, in a day; and the rapidity at the middle above, to be one-half faster 

 than along the sides. 



The power of motion in a glacier depends on — 



(1.) The capability it has, to a limited extent, of sliding along its 

 bed, but only portions at a time. 



■ (2.) A degree of plasticity in ice, in consequence of which the 

 glacier can adapt itself to any uneven surface ; for ice at a tempera- 

 ture near 32° F. may be moulded by pressure into almost any shape. 

 A heavy oblong mass supported at one end may be bent even into 

 a short arch by its own weight. Kane mentions in his "Arctic Ex- 

 plorations" the case of one table of ice, eight feet thick and twenty 

 or more wide, supported only at the ends, which, between the 

 middle of the months of March and May, became so deeply bent 



44 



