342 Professor Forbes's Fourteenth Letter on Glaciers. 



that varations of temperature below 0° centigrade, or 32° Fahren- 

 heit, produce almost inappreciable changes in the rate of motion of 

 the ice. Hence, from this circumstance alone, we should deduce 

 that in the higher parts of the glacier (where, for example, it freezes 

 almost every night in summer) the variations of velocity should be 

 least, and indeed comparatively small at different seasons. This 

 is well illustrated by comparing the summer motions of the stations 

 D, A, and C, mentioned in the first part of this section,* with their 

 annual motion, which exhibit a much slighter excess in favour of 

 the summer period than in the lower stations which we are now dis- 

 cussing. The same thing was observed by M. Agassiz's surveyors 

 on the glacier of the Aar. Their position of observation far up on 

 the glacier of the Aar, in a spot having a mean temperature near 

 the freezing point if not lower, had a summer daily motion of 7'99 

 inches, and a mean daily motion during the whole year of 6*41 

 inches.f Now at station C, or the Pierre Platte, on the Mer de 

 Glace, the mean motion for July 1842 was 10 inches, and for the 

 whole year, 1842-43, it was 8*56 inches. It is quite evident that 

 the motion of any point in the midst of a glacier is controlled by that 

 of those which precede and follow it, and that it does not necessarily 

 result, either that all must at once suffer a similar increase or di- 

 minution of speed, or that the times of maxima and minima, or 

 even the general form of the annual curve, shall be the same. 



* See the Philosophical Transactions, 1846, p. 179. This part of the section 

 is not reprinted here. 



t Comptes Rendus, Dec. 9, 1844. 



