VISCOSITY THEORY OF FORBES. 



61 



and sides of their channels. They have their shallows and their deeps, 



their narrows and their lakes, their cascades, their rapids, and their 



tranquil portions. Fig. 52 shows a glacier 



running through a narrow gorge into a 



wide lake of ice, and again through another 



gorge. There is this difference, however, 



between a glacier and a water-current, viz., 

 that, while the latter 

 conforms to even the 

 minutest and sharpest 

 outlines, the former con- 

 forms only to the larger 

 or more gentle. In this, 

 a glacier acts like a stiff, 

 viscous fluid. 



7. The Line of Swiftest Motion is more sinuous 

 than the Channel. — We have already seen that this is 

 true of rivers (page 24). The line of swiftest current 

 is reflected from side to side, increasing the curves by 

 erosion. The same has been recently proved by Tyn- 

 dall to be the case with glaciers. Fig. 53 represents 

 a portion of a sinuous glacier, like the Mer de Glace : 

 the dotted line represents the line of swiftest motion. 



Fig. 52. 



Theories of Glacier Motion. 



There are few subjects connected with the physics of the earth 

 which have excited more interest than that of glacier-motion. The 

 subject is one of exceeding beauty, and not without geological im- 

 portance. Passing over several very ingenious theories which have 

 now been abandoned, the first theory which was conceived in the true 

 inductive spirit, and which explains the differential motion, is that of 

 Prof. James Forbes. 



Viscosity Tlieory of Fortes. 



Statement of the Theory. — According to Forbes, ice, though appar- 

 ently so hard and solid, is really, to a slight extent, a viscous body. In 

 small masses this property is not noticeable, but in large masses and 

 under long-continued pressure it slowly yields, and will flow like a stiffly 

 viscous fluid. In large masses like a glacier, this steady, powerful press- 

 ure is furnished by the immense weight of the superincumbent ice. 



Argument. — It is evident that this theory completely accounts for 

 all the phenomena of glacier- motion, even in their minutest details. 

 A glacier, beyond all doubt, moves like a viscous body, but it is still a 

 question whether it does so by virtue of a property of viscosity. The 



