Mr. J. Croll on the Cause of the Motion of Glaciers. 155 



shearing-force must be very much less than that of ice ; yet im- 

 mense masses of boulder-clay will lie immoveable for ages on the 

 slope of a hill so steep that one can hardly venture to climb it, 

 while a glacier will come crawling down a valley which by the 

 eye we could hardly detect to be actually off the level. Again, a 

 glacier moves faster during the day than during the night, and 

 about twice as fast during summer as during winter. Pro- 

 fessor Forbes, for example, found that the Glacier des Bos 

 near its lower extremity moved sometimes in December only 11*5 

 inches daily, while during the month of July its rate of motion 

 sometimes reached 52*1 inches per day. Why such a difference 

 in the rate of motion between day and night, summer and winter ? 

 The glacier is not heavier during the day than it is during the 

 night, or during the summer than it is during the winter; 

 neither is the shearing-force of the great mass of the ice of a 

 glacier sensibly less during the day than during the night, or 

 during the summer than during the winter ; for the temperature 

 of the great mass of the ice does not sensibly vary with the 

 seasons. Then, if this is the case, gravitation ought to be as able 

 to move a glacier during the night as during the day, or during 

 the winter as during the summer. At any rate, if there should 

 be any difference it ought to be but trifling. It is true that, 

 owing to the melting of the ice, the crevices of the glacier are 

 more gorged with water during summer than during winter ; and 

 this, as Professor Forbes maintains*, may tend to make the 

 glacier move faster during the former season than during the 

 latter. But the advocates of the regelation theory cannot con- 

 clude, with Professor Forbes, that the water favours the motion 

 of the glacier by making the ice more soft and plastic. The 

 melting of the ice, according to the regelation theory, cannot 

 very materially aid the motion of the glacier. 



The fact that the rate of motion of a glacier depends upon the 

 amount of heat that the ice is receiving shows that heat in some 

 way or other stands related as a cause to the motion of the 

 glacier. 



But the point under consideration is, If the ice of a glacier 

 shears in the solid state, as is generally supposed, has Canon 

 Moseley proved that a glacier cannot descend by its weight only ? 

 I have carefully read the interesting memoirs by Mr. Mathews 

 and Mr. Ball in reply to Canon Moseley ; and although I agree 

 with the most of their remarks regarding the unsatisfactory nature 

 of Mr. Moseley's own theory of glacier-motion, yet I am unable 

 to perceive that any thing which they have advanced materially 

 affects his general conclusion as regards the commonly received 

 theory. If the ice of a glacier shears, nothing which I have yet 

 * Occasional Papers, pp. 166, 223, 

 M2 



