1862.] on the Motion of Glaciers, 415 



by experiment that the retarding force of friction was independent of 

 the body's velocity, which consequently went on constantly increasing ; 

 but this result involved the condition that the surfaces of the body and 

 of the plane which were in contact, should not be affected by their 

 action against each other. Now this condition was not fulfilled in the 

 case of the glacier. The temperature of its whole mass beyond very 

 small depths was found by observation and theory to remain constantly 

 at the freezing point, and in that state the cohesive power of ice was 

 found to be considerably more feeble than at lower temperatures. 

 In fact, it must necessarily, at the lower surfiice of the glacier, be con- 

 stantly in a state of disintegration from its thawing. Hence, as each 

 successive thin layer at the bottom of the mass is converted into water, 

 the mass of the glacier is let down, as it were, and advances through 

 a corresponding small space along its inclined bed. At the same time 

 this advance must depend on the rate at which the ice melts, and as 

 that rate is very slow and uniform, the motion of the glacier must be 

 slow and uniform likewise. 



This important conclusion was not left to mere abstract mechanical 

 reasoning. Mr. Hopkins had verified it by a /ery simple experiment. 

 He placed a lump of ice on a rough paving slab, which could easily 

 be placed at different inclinations to the horizon. When the tempera- 

 ture of the surrounding atmosphere and the slab were below that of 

 freezing, the lump of ice remained stationary on the slab at all inclina- 

 tions less than about 20° ; at greater inclinations it moved down the 

 slab, as in ordinary cases, with an accelerated motion. But when the 

 temperature of the slab and of the air was above the freezing tempera- 

 ture, the ice descended with a very slow and uniform motion, even at 

 inclinations as small as half a degree.* The greater the inclination 

 of the plane and the greater the weight of the sliding mass, the greater 

 was the uniform velocity with which it moved. Mr. Hopkins regarded 

 this experiment as supplying a complete answer to the objections above 

 stated against the sliding motion of glaciers. 



That glaciers do slide over their beds was indeed established 

 on indubitable evidence. It was easily seen, on examining almost 

 any glacial valley, that its glacier had been formerly continued 

 beyond the position of its present extremity, far below which, along 

 the sides and bottoms of the valley, were observed the rounded 

 and striated surfaces frequently so conspicuous at the in mediate 

 termination of the glacier, where they were manifestly due to the 

 sliding of its general mass. All valleys in which ancient glaciers 

 were believed to have existed likewise bore the same testimony. 

 But the most direct evidence of this fact is to be found in the 

 observations of Professor Forbes, and those of Dr. Tyndall, from 

 which it appeared that, of the whole observed motion of the upper 

 surfaces of the glaciers at the points where the observations were made, 

 about one-half was due to the sliding of the general mass, and the other 



♦ The experiment was exhibited during the discourse. 

 Vol. III. (No. 35.) 2 f 



