830 Mr. Hopkins o« the Mechanism of Glacial Motion. IV. 



The experiment was also made somewhat tliiferently, by 

 placing the central rods so that they reached only half the 

 lenjith of the iroiiffh. When these rods were made to move 

 as before, with the mass immediately superincumbent upon 

 them, that portion along the middle of the trough under which 

 the rods did not pass was extended, and the curved fissures 

 across the whole surface were there produced in a manner ex- 

 actly accordant with theory. 



Another system of lines also resulted from the motion com- 

 municated to the mass. A great number of extremely small 

 ridges forming discontinuous lines, as represented in the pre- 

 ceding diagram, appeared on the surface, their directions being 

 at right angles to the lines of fissure. They were manifestly 

 due to the compression in directions at right angles to them, 

 these directions being in fact the directions of maximum press- 

 ure, as determined by theory. That such was the case was 

 made further evident by continuing the motion of the central 

 portion, and watching the gradual development and increased 

 number of these lines. They were best developed when the 

 surface of the mass had become slightly crisped before the mo- 

 tion was given to it. 



To obtain more distinct evidence respecting the existence 

 of the internal pressure in question, I cut the mass so as to 

 form an artificial fissure of some width, along one of these 

 lines of elevation, and therefore perpendicular to the direc- 

 tions of the original fissures first described. When the mo- 

 tion of the central part of the mass was continued, the artificial 

 fissure was soon entirely closed up, while the width of the 

 natural fissures continually increased. The evidence was per- 

 fectly conclusive. 



According to theory, the directions of the natural fissures 

 and that of the artificial one mentioned in the last paragraph, 

 are those in which there is no tendency in the particles on 

 one side of a vertical plane to slide past the contiguous par- 

 ticles on the opposite side. This was perfectly verified by 

 experiment; for after the movement which opened the na- 

 tural fissures and closed the artificial one intersecting them 

 at right angles, the direction of the artificial fissure, as well as 

 that of each natural one, remained unbroken at the point of 

 intersection. There had not been the smallest sliding motion 

 like that here contemplated, in the direction of either fissure, 

 although there was nothing to resist the tendency to sucli 

 motion, the cohesion being entirely destroyed along the lines 

 of fissure. This conclusion, it will be recollected, is in di- 

 rect opposition to the theory of Professor Forbes, according 

 to which the direction of the artificial fissure above described 



