146 Mr. Hopkins on the Mechanism of Glacial Motion. 



by the upward current, from the horizontal impetus of the 

 wind and consequently calm, need not be so spacious as to 

 require more than a very few minutes for it to be passed over 

 by a storm travelling at the rate of thirty miles per hour. 

 Supposing the confluent blasts to proceed only from two op- 

 posite quadrants, the extent of the storm, in front and in rear 

 of its path, would of course be doubled, so as to require forty 

 miles distance to effect a reduction of the velocity from one 

 hundred miles per hour to five miles. 



I am, Gentlemen, 



Respectfully yours, 

 Philadelphia, November 23, 1844. Robert Hare. 



XV. On the Mechanism of Glacial Motion. 

 By W. Hopkins, Esq., M.A., F.R.S., 8^c. 



To the Editors of the Philosophical Magaziyie and Journal. 

 Gentlemen, 

 TN my preceding communication I gave an explanation of 

 ^ the manner in which the general motion of glaciers may 

 be accounted for according to the sliding theory. I now pro- 

 ceed to consider more particularly the mechanism of this 

 motion. 



1. The most important facts in the motion of glaciers, 

 hitherto determined, appear to be the following : — 



( 1 .) The motion of each point on the surface of a glacier is 

 approximately parallel to the longitudinal axis of the glacier, 

 and the sides of the glacial valley, whenever the axis and sides 

 are approximately parallel, as is very generally the case in 

 those portions of glaciers which have been subjected to de- 

 tailed observations, and which are descending along valleys of 

 well-defined and determinate forms. If the valleys converge 

 or diverge in descending, the motion of different points of the 

 glacier will partake of the same character. 



(2.) If we take any number of points in a transverse line 

 perpendicular to the axis of the glacier, the velocity of those 

 points will be greatest which are nearest to the centre of the 

 glacier. 



(3.) This increase of the velocity, in passing from the sides 

 to the centre, takes place much more rapidly near the sides 

 than in other parts of the glacier. 



(4-.) The motion (at least during summer) is greater near 

 the lower than it is near the upper extremity of a glacier. 



(5.) The motion is greater in warm than in cold weather. 



(6.) Nothing has been positively ascertained by observation 

 respecting the relative velocities of the upper and lower surfaces 



