314 Cambridge Philosophical Society : — 



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, 

 from his own observations, that the laminas of blue and white ice 

 (and especially in those places in which the structure originates) 

 conform to the law of perpendicularity to the direction of greatest 

 pressure. So far from there being any tendency to produce ruptures 

 and fissures lying in the planes of the lamina; in these positions, 

 they were the only positions entirely free from such tendency. And 

 hence it became so difficult to conceive how the laminated structure 

 could possibly originate in actual discontinuities such as those to 

 which Professor Forbes had ascribed them, whether we suppose the 

 blue laminffi to be produced by subsequent infiltration or any other 

 process. 



According to Dr. Tyndall's views, the law stated in the preceding 

 paragraph was an essential consequence of physical causes, to which 

 the production of the laminae was referred. He had shown, experi- 

 mentally, tliat if a piece of ordinary ice be subjected to direct pres- 

 sure, it will melt along fine lenticular laminae perpendicular to the 

 direction of the pressure. A similar process is supposed to take 

 place in the neve, or in any part of the glacier where the structural 

 laminae originate. Professor W. Thomson had offered an explana- 

 tion of this phsenomenon on thermal principles. It had been shown 

 by himself and his brother that the melting temperature of ice is 

 lowered by compression. Now if, in Dr. Tyndall's experiment, the 

 ice were a perfectly homogeneous substance, every portion would 

 be equally compressed; and if the uncompressed mass were only 

 just above the melting temperature, the whole would melt under the 

 compressing force. But no substance is perfectly homogeneous ; 

 and consequently the internal pressures in the experiment would 

 not be perfectly equable ; and those portions of the ice which were 

 subjected to the greatest pressure would melt the soonest, and pro- 

 duce the aqueous laminae above mentioned in the experiment, or the 

 laminae of blue ice in the glacier. In the latter case the laminar 

 portions must first be supposed to melt, the air-bubbles to escape. 



