118 [May 22, 



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 

 also two other directions through P characterized by a peculiar pro- 

 perty. If we take two adjoining particles, P and P', in the line of 

 maximum tension, that tension will exert a greater effort than there 

 will be in any other direction to separate those particles ; or if the 

 internal force be the maximum pressure, those points will be more 

 compressed together than in any other direction. In the two direc- 

 tions (now to be denned) the forces on P and P', acting perpendicu- 

 larly to the line joining those particles, will exert a greater tendency 

 than is exerted in any other direction, to separate them by making 

 one slide tangentially past the other, and then to twist and contort 

 any internal elementary portion of the mass. These two directions 

 are perpendicular to each other, and bisect the angles between the 

 directions of maximum tension and maximum pressure. This problem 



