1846.] MUTUAL INFLUENCE OF THE PARTS OF A GLACIER. 139 



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 [on the Mer 

 de Glace] D, A, and C, mentioned in the first part of this section 

 [page 123], 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 discussing. The same thing was observed 

 by M. Agassiz's surveyors on the glacier of the Aar, who at 

 first saw, in this not very great inequality, an objection to my 

 theory. On a more searching investigation, however, the objec- 

 tion disappears, as in their later writings they have acknowledged. 

 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.* 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 diminution of speed, or that the times of maxima 

 and minima, or even the general form of the annual curve, shall 

 be the same.f This leads to an important practical result which 

 we shall follow out in the next section. 



* Comptes Kendus, Dec. 9, 1844. 



f [These considerations will go far to explain any seeming irregularities in the 

 results, not only of these tabular motions, but of those more recent observations 

 which will be cited in a later part of this volume [Sixteenth Letter on Glaciers]. 

 Both the glaciers on which continuous measures have been taken in summer and 

 winter present an enormous variety in elevation and declivity at different parts of 

 their course. The points of observation to which the tables of page 128 refer, are 

 some thousand feet below the level of the greatest portion of either glacier to which 

 they belong. The warmth of spring, and the consequent gorging of the glacier with 

 water derived from thawing snow, occurs, in the localities observed, weeks, or even 

 months, before the same causes are in full action on the general course of the glacier. 

 Hence a local acceleration of the points of observation, followed later by an accelera- 

 tion due to the increased supply from the middle and higher regions. The result 

 may be either a double maximum of velocity (which occurs in some instances), or a 



