Mr. Hopkins on Glacial Theories. 313 



supposed to take place by pressure and differential motion, indepen- 

 dently, as far as Mr. Hopkins understood, either of infiltration or 

 melting of any portions of the ice. 



Both Professor Forbes and Dr. Tyndall had endeavoured to eluci- 

 date the ijhsenomena of glacial motion by means of a semifluid sub- 

 stance descending down a trough inclined to the horizon. For the 

 purpose of ascertaining the direction of greatest extension and com- 

 pression of the substance when thus put in motion, the latter gen- 

 tleman described circles on its surface while still at rest, and ob- 

 served the compressions and extensions of the radii when the mass 

 was in motion. He thus found that the lines of greatest extension 

 were inclined at angles of 45° to the axis of the trough ; each such 

 line pointing centrally and downwards, or laterally and upwards, 

 while the lines of greatest compression were perpendicular to them. 

 In the case of a glacier descending a canal-shaped valley, the former 

 of these directions would manifestly be that of greatest tension ; 

 and this is precisely the result which Mr. Hopkins had obtained both 

 by exact mechanical reasoning and by experiment fifteen years ago ; 

 and it was thus that he was able to explain (and he was the first to 

 do so) the formation of crevasses making angles of 45° with the axis 

 of the glacier, and directed centrally and upwards, i. e. at right 

 angles to the lines of greatest tension. There was also another 

 result to which Mr. Hopkins was led by an exact consideration of 

 the problem, but which he had also elucidated by experiments, as 

 described in the letters above alluded to in the Philosophical Maga- 

 zine. There were not only directions of maximum and minimum 

 pressures and tensions at each point of the mass, there were also 

 two other directions inclined at 4 5° to the former, not recognized by 

 either of the above-mentioned experimenters, in which there is a 

 maximum tendency to produce the differential motion, to which Prof. 

 Forbes ascribed an actual rupturing of the ice and consequent forma- 

 tion of the veined structure. In the directions of maximum and 

 minimum pressures or tensions, this tendency to produce a differ- 

 ential motion altogether vanished. The truth of these results was 

 just as certain as that of the parallelogram of forces. In more com- 

 plicated cases than that above supposed of a glacier descending 

 down a canal-shaped valley, the absolute directions of these differ- 

 ent lines would vary with the conditions of the glacial mass, and the 

 external pressures to which it was subjected ; but the important fact 

 was, that there must in all cases exist at each point of the mass a 

 direction of maximum tension or of minimum pressure, and a direction, 

 perpendicular to it, of minimum tension or maximtan pressure, and two 

 other directions inclined to each of the former at 45°, along which 

 there is a maximum tendency to produce the kind of differential 

 motion above described. It was moreover manifest that wliere 

 crevasses were formed there must be tension ; and equally manifest 

 that the directions of such crevasses must at least approximate to 

 perpendicularity with the directions of maximum tension, and therefore 

 to coincidence with those of maximum pressure. Also, if discontinui- 



