230 Royal Society : — 



convey to the minds of others the most remote idea of regelation, as 

 a property of ice at a particular temperature. No better proof can 

 be given of this than the general conviction which appeared to flash 

 across the mind of every glacialist when he first heard of Professor 

 Tyndall's experiment, that the recognition of the property of instan- 

 taneous regelation was a well-marked and important discovery, which 

 had at once completely removed a great stumbling-block in glacial 

 theory. In fact, the viscous theory assigns no physical cause for 

 the reunion in question. All we could do, before the publication of 

 those experiments, was to infer from the observed facts that ice did 

 possess some property which facilitated the reunion of separate pieces 

 in contact ; but this was like the attempt to define viscosity by an 

 appeal to the phenomena which that property was intended to ex- 

 plain. Regelation has, in fact, no connexion with viscosity, but stands 

 in direct antagonism to it. 



An imperfect plasticity in ice has sometimes been spoken of. The 

 fact is, all solid bodies may be said to have an imperfect plasticity, if 

 w r e chose to admit this vagueness in scientific language, since all are 

 capable of greater or less extension or compression. As to the apparent 

 plasticity inferred from the motion of glacial masses, and arising from 

 the crevicing of the ice as already explained, it has no relation what- 

 ever to real plasticity. Such crevices are the necessary consequences 

 of the external forces acting on the glacier, and are as essential to the 

 theory of regelation as they are unconnected with any property of 

 plasticity. 



The author then briefly describes the experiment, by which it is 

 shown that ice will slide down an inclined plane at an inclination to 

 the horizon less than that of any known glacier, provided its lower 

 surface be in that state of disintegration in which it will necessarily 

 be when its temperature = zero (C.). The motion is then slow and 

 uniform. That glaciers do slide over their beds, has been established 

 as clearly as it can be by the comparatively few observations which 

 have been made on the subject ; and every existing glacial valley, 

 and every valley which is believed to have been such at former 

 geological periods, testify to the truth of that conclusion. The 

 author also explains that both theory and observation agree in the 

 result that the temperature of the lower surface of a glacier of any 

 considerable depth in the latitude of the Alps must necessarily be 

 = zero (C). He regards this sliding motion as far too important a 

 part of the whole motion of a glacier to be neglected in any com- 

 plete theory of that motion. 



The author then proceeds to investigate certain properties of the 

 internal tensions and pressures at any point (P) in the interior of a 

 mass held in a state of constraint by external forces. He shows 

 that at every point (P) there are three determinate directions, at 

 right angles to each other, in which the direct tension is such that 

 in one of them it is a maximum, in another a minimum, and in the 

 third neither a complete maximum nor a complete minimum ; it is 

 convenient to call this the mean axis. The tensions or pressures in 

 these directions are called principal tensions or pressures ; there are 



