GEOLOGY. 221 



shearing force and the weight of a given volume of a glacier, so 

 that it may just descend by its weight only. Now it is possible 

 to investigate mathematically what that relation is. I have made 

 that investigation. (See Pliil. Mag., May, 1809.) 



" The resistances opposed to the displacement of a glacier are, 

 (1) Those which oppose themselves to the shearing of one sur- 

 face of ice over another, which is continually taking plaee 

 throughout the whole mass, by reason of differential motion ; (2) 

 The friction of the superimposed laminae of ice upon one another, 

 which is greater in the lower than the upper ; (3) Abrasion of 

 ice on the bottom and sides of the channel of the glacier. If it 

 descends by the weight only, then the work of its weight in its 

 descent through any distance must at least equal "the sum 

 of the works of all these resistances. It is of course impossible 

 to represent this relation mathematically in respect to an actual 

 glacier having a variable direction and an irregular channel and 

 slope ; but in respect to an imaginary one having a constant 

 direction and a uniform channel and slope, it is possible. I have 

 made that calculation, and it results from it that the unit of shear 

 in ice (that is, the force necessary to make one square inch of 

 ice shear over another square inch) must not be more than 1 

 pound, that a glacier may descend by its weight only. If the 

 unit of shear in ice be more than that, the glacier cannot descend 

 by its mere weight on a slope like that of theMer de Glace. But it 

 is a great deal more than that. It requires from 60 pounds to 120 

 pounds to shear one square inch of ice over another square inch. 

 The ice of the Mer de Glace cannot therefore descend by its weight 

 only ; it does not shear easily enough. It must be ice of about the 

 consistency of soft putty to descend by its weight only, for that 

 substance shears with a pressure of from 1 pound to 3 pounds 

 per square inch. 



"Ice, therefore, if it be fluid, is not fluid enough, and if it be 

 solid, it is too solid to descend by its weight only. There must be 

 some other force to help it down besides its weight, certainly 

 45 times greater, and, possibly, 90 times. This result is directly 

 opposed to the viscous theory and to that known as the theory of 

 regulation, both of which attribute the descent of glaciers to their 

 weight as the only cause. It reveals the existence of some other 

 force." 



This force Canon Moseley finds in the heat from the sun, and 

 the effect is produced by the dilatation of the ice within the mass, 

 where it cannot be converted into the liquid state. He found that 

 a sheet of lead, placed on an inclined plane and exposed to the 

 rays of the sun, gradually worked its way down, on account of the 

 contraction and dilatation of the mass with changes of tempera- 

 ture. 



" How much heat entering the surface of a glacier is necessary 

 to this result has been made the subject of calculation. Supposing 

 the depth of the ice to be the same as that at Tacul, its motion at 

 different depths, that which Tyndall found it to be there, and its 

 surface motion, that which he measured lower on the Mer de 

 Glace, at Les Fonts, and supposing the resistance to shearing of 



