MOTION OF A GLACIER ITS LAWS. 251 



motion varied at different points of the length of the same 

 glacier, being on the whole greatest where the inclination of its 

 surface is greatest. As the season advanced, he observed 

 notable changes in the rate of motion of the same part of the 

 ice, and connected it by a very striking direct relation with the 

 temperature of the air. These facts were established during the 

 summer of 1842, and promptly published. [See the first four 

 Letters on Glaciers, reprinted in the present volume.] By means 

 of occasional observations during the following winter and spring 

 by his guide, Auguste Balmat of Chamouni, and by a more full 

 comparison of the entire motion of a glacier for twelve months 

 with its motion during the hot season of the year, another gene- 

 rally received error was rectified : the motion of the glacier 

 continues even in winter, and it has a very perceptible ratio to 

 the summer motion. Last of all, it was found that the surface 

 of a glacier moves faster than the ice nearer the bottom or bed.* 



These and some minor laws of motion are undoubted ex- 

 pressions of the way in which glaciers move ; and having been 

 successively confirmed by succeeding observers, seem to admit 

 of but one expression in the form of an approximate theory, 

 " A glacier is an imperfect fluid or a viscous body, which is 

 urged down slopes of a certain inclination by the mutual 

 pressure of its parts."f The analogy subsisting between the 

 motion of a glacier and that of a river (which is a viscous 

 fluid ; were it not so, its motion would be widely different) 

 will be best perceived by stating more precisely its laws of 

 motion in order. 



1. Each portion of a glacier moves, not indeed with a constant 

 velocity, but in a continuous manner, and not by sudden sub- 

 sidences with intervals of repose. This, of course, is character- 

 istic also of a river. 



2. The ice in the middle part of the glacier moves much 

 faster than that near the sides or banks ; also the surface moves 

 faster than the bottom. Both these facts obtain in the motion 



* Eleventh Letter. Edinburgh Philosophical Journal, 1846. [Reprinted in 

 this volume.] 



f Travels in the Alps, p. 365. 



