1846.] CONSTANCY OF THE RELATIVE VELOCITIES. 149 



insensible gradations.* (5.) When we compare the motion of a 

 given point of a glacier any day of one year and the same day 

 of another, the probability is that the velocity will be exactly 

 the same, if the season be equally hot or cold ; hence, surely, a 

 most unexpected result, which I first announced in 1842, that a 

 few days observation of a glacier will enable any one to compare 

 its mean rate of motion over its various parts and with different 

 glaciers.^ Thus, the motion of a point marked D 2 on the Mer 

 de Glace, was in 1842, from August 1 to August 9, 16J inches 

 daily ; from August 9 to September 16, 18 inches ; now next 

 year, 1843, one observation at the same point in August gave 

 16 inches ; and in 1844, one observation in September gave 17J 

 inches. But still further (6.) The very law of flexure of the ice 

 is the same from year to year : a series of stations across the ice 

 at the Montanvert gave, in 1842, the following (simultaneous) 

 relative velocities : \ 



1-000 1-332 1-356 1-367 



The same points being recovered in 1844, the relative motions 

 were (by a single observation of the space moved over in five 

 days) 



1-000 1-339 1-362 1-374, 



ratios almost the same, but slightly increasing, which corresponds 

 with the fact mentioned above (3), that when the absolute velo- 

 cities are greater, the relative velocities are so too, which was 

 here the case, for the velocity denoted by 1*000 was a little 

 greater in the second case than in the first. 



Tensions and Thrusts. The occurrence of open crevasses 

 plainly indicates the existence of strains in the ice of glaciers 

 producing disruption, at least partially. Hence some writers 

 have precipitately inferred that the whole glacier must be in a 

 state of tension ; an uncertain inference surely in a problem of 



* See 5 of this paper. 



f [Regard must however be paid to the circumstance that the different parts even 

 of the same glacier are not exactly simultaneously affected by change of season. 

 See Note to page 139.J 



\ Travels, 1st edit. p. 146. 



[The continuation of these observations to 1846 is given in the Thirteenth 

 Letter below.] 



