14 SECOND LETTER ON GLACIERS. [1842. 



of movement should be ascertained inductively from the observed 

 motion, carefully and numerically ascertained at different points. 

 It is because authors have considered the problem as too simple 

 a one to require a systematic analysis, that we find little or 

 nothing done in this respect ; and it may be affirmed, without 

 any disrespect to the ingenious persons who have assigned pro- 

 bable causes for the movement of these masses of ice, that their 

 solutions have been, like the astronomical theories of the earlier 

 cosmogonists, based upon somewhat vague analogies with better 

 understood phenomena, as when the analogy of magnetic attrac- 

 tions seemed to offer a parallel to the mechanism of the heavens 

 in the theory of Gilbert, and that of fluid currents gave rise to 

 the Cartesian vortices. The Newtonian theory was based upon 

 its coincidence with the empirical laws of planetary motion. 

 We have as yet no empirical laws of glacier motion, conse- 

 quently no proper mechanical theory can as yet be adequately 

 tested. I endeavoured to point out in my lectures how a 

 mechanical theory might be deduced from observation, and how 

 these observations might be practically made. I believe that I 

 have also obtained, for the first time, the numbers on whose 

 importance I insisted. I am not aware that any one had 

 hitherto proposed to determine the diurnal velocity of a given 

 point of a glacier with reference to three co-ordinates. The 

 analogy with the empirical laws of astronomy is both striking 

 and just ; an exact acquaintance with the path described by any 

 molecule of a glacier, will almost as certainly lead to a know- 

 ledge of the cause of its motion, as the theory of gravitation 

 sprung from the three laws of Kepler. We have to deal, 

 indeed, with an effect more complex and varied ; but the results 

 contained in my last letter already show how much of numeri- 

 cal precision may be attained. I have already determined the 

 diurnal motion of 10 points of the Mer de Glace with a probable 

 error, not exceeding, I think, a quarter of an inch in any case ; 

 and when these observations shall have been pursued, as I 

 expect to do, until the end of September, there will be a toler- 

 able basis for sound speculation. 



