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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 maximum 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 where 

 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- 

 ties and differential motions resulted from these internal pressures 

 and tensions, they must be produced in those directions in which 

 there is the maximum tendency to produce them, i. e. in directions 

 inclined at 45° to those of the crevasses. All these directions might 

 be supposed to be (as they would generally be in a glacier) nearly 

 horizontal. These conclusions, Mr. Hopkins repeated, were as cer- 

 tain as that of the parallelogram of forces, and no theory which con- 

 tradicted them could possibly be true. 



Professor Forbes was the first to recognize the law which esta- 

 blishes a certain relation between the veined structure and the cre- 

 vasses. He asserts that, as a matter of observation, the crevasses 

 intersect the structure at right angles; consequently the blue veins 

 must be perpendicular to the directions of maximum pressure, and 

 could not coincide (such being the law) with the directions in which 

 differential motion must necessarily take place, if it should take place 

 at all. The law above enunciated exactly accorded with the conclu- 

 sions above stated, and also with Dr. Tyndall's views, who asserts, 



