Mr. Hopkins on the Mechanism of Glacial Motion. IV. 329 



building. Tiie. quantity was such as to form a uniform layer 

 of about two inches above the rods and to fill up the inter- 

 spaces between them. The whole was then left to acquire the 

 requisite degree of solidity. The motion was communicated 

 by applying a force to the central rods alone, which, with the 

 superincumbent mortar adhering to them, were thus made to 

 move, while the lateral rods and mortar remained at rest. In 

 this manner a straight transversal line drawn on the surface 

 of the mortar became a loop, the form of which could be regu- 

 lated at pleasure, and the mass was brought into a state of 

 constraint exactly similar to that superinduced in a glacier by 

 the excess of its central over its lateral motion. The results 

 were different according as the mortar was comparatively soft 

 or compact. 



I. When the mortar was left for twenty-four hours or up- 

 wards, it acquired a considerable degree of solidity, so that, 

 when such a motion had been communicated to its central 

 portion as to give a very slight curvature to the transverse 

 line above-mentioned, a system of parallel fissures began to 

 be formed in each lateral portion, in directions making angles 

 of 45° with the axis of the trough, as represented in fig. 1. 



Fig. 1. 



This is exactly accordant with my theoretical deduction for the 

 case in which there is neither longitudinal nortransversal exten- 

 sion or compression (Second Letter, art. 20,p. 161), as was the 

 case in the experiment ; for, from the manner in which the mo- 

 tion was communicated, there could be no longitudinal exten- 

 sion or compression, and transversal compression could only 

 arise from the relative motion of the centre, and could there- 

 fore only be of the second order of small quantities, the mo- 

 tion itself being considered a small quantity of the first order. 

 The incipient fissures were not curved but straight, as they 

 ought to be, according to theory, under the above conditions. 



