78 Royal Society. 



of the atmosphere is unaccompanied by a corresponding change in 

 the mean motion cf the glacier. 



The glacier moves with different velocities at different depths, the 

 surface-motion being faster (probably two or three times) than that of 

 the deepest part. The motions at different depths cannot but be re- 

 lated to one another : so that as the influence of variations of tempe- 

 rature is felt on the surface, it cannot but be felt throughout the glacier. 



If every change of solar heat is associated with a corresponding 

 change of glacier-motion, it seems to follow that the two are either 

 dependent upon some common cause, or that the one set of changes 

 is caused by the other ; and the former of these conclusions being 

 inadmissible, we are forced on the latter. It is not necessary to show 

 how it is that changes of external temperature penetrate glaciers. Of 

 the power of the sun upon them there are, however, evidences in the 

 ablation of surface constantly going on and in the preservation of the 

 ice which is covered by the stones of a moraine, which sometimes 

 forms an icy ridge from 50 to 80 feet high, and some hundred feet in 

 width. 



"The sun's rays," says Tyndall*, "striking upon the unpro- 

 tected surface of the glacier, enter the ice to a considerable depth ; 

 and the consequence is that the ice near the surface of the glacier is 

 always disintegrated, being cut up into minute fissures and cavities 

 filled with water and air, which, for reasons already assigned, cause 

 the glacier wdien it is clean to appear white and opaque. The ice 

 under the moraines, on the contrary, is usually dark and transparent. 

 I have sometimes seen it as black as pitch, the blackness being a 

 proof of its great transparency, which prevents the reflexion of light 

 from its interior. The ice under the moraines cannot be assailed in 

 its depths by the solar heat, because this heat becomes obscure be- 

 fore it reaches the ice, and as such it lacks the power of penetrating 

 the substance. It is also communicated in great part by way of con- 

 tact instead of by radiation. A thin film at the surface of the mo- 

 raine ice engages all the heat that acts upon it, its deeper portions 

 remaining transparent and intact." 



It matters not to the argument how little below freezing the tem- 

 perature of a glacier may be. So long as the ice exists in a solid state 

 and is capable of being penetrated by the solar heat, it cannot but 

 dilate and contract. Its central portions, lying folded in ice 100 feet 

 thick above and below, may well, however, be conceived to retain 

 some of the cold of the region from which they have descended. The 

 observations of Agassiz on the temperature of the Aar Glacier are not to 

 be relied upon, because the access of damp external air to the borings 

 in which they were made, and of water percolating the disintegrated 

 ice of the surface, was not effectually stopped. The included 

 thermometers could not but under these circumstances show zero, 

 although the temperature of the surrounding ice was below it. For 

 the water freezing on the walls of the boring, the latent heat thereby 

 given out, would raise the temperature of the air about the bulb to the 

 freezing-point, and this water being continually renewed, the quick- 

 silver would always be kept at that point. 



* Glaciers of the Alps, p. 294. 



