166 HEAT OE HIGHER LAYERS OF AIR, 



The progress thus made is not the same in dif- 

 ferent seasons. Being least, and in many glaciers 

 becoming almost nothing, in winter, it increases 

 towards the spring, and shows itself most strongly 

 when the masses of snow fallen during the winter 

 begin to melt, and the glacier-rivulets are rein- 

 forced by the water now coming down from all 

 the mountain-sides, and pouring in at the border 

 of the glacier. In summer they decrease again. 



According to the observations of Agassiz, the 

 greatest advance takes place in general at the 

 upper parts of the glaciers. The yearly advance 

 at the lower end amounts, in some of the Alpine 

 glaciers, to more than three hundred feet, in others 

 to less. It seems that the most massive glaciers 

 move forward most rapidly. Agassiz has 

 reckoned, from the advance of the glacier of the 

 Aar, that its ice must take one hundred and 

 thirty-three years to travel from the limits of the 

 firn down to the source of the Aar. 



Very lately some naturalists have tried to at- 

 tribute the motion of the glacier-ice to other 

 causes than those above treated of. Thus by some, 

 especial stress is laid on the somewhat readier 

 yielding to external pressure, which distinguishes 

 glacier-ice from common ice; by others, on an 

 expansion due to the freezing of the water absorbed 

 by the glacier-ice. 



That these theories are untenable, and even 



