PLASTIC NATURE OF GLACIER ICE. 253 



6. A fact not less important than any of the preceding, and 

 equally well-established, not only by observations of my own, 

 but by those of succeeding experimenters, is this : that in- 

 creased temperature of the air favours the motion of the ice ; 

 and generally whatever tends to increase the proportion of. the 

 watery to the solid constituents of a glacier, as mild rains, and 

 especially the thawing of the superficial snow in spring. The 

 velocity does not, however, descend to nothing even in the 

 depth of winter. Indeed, in the lower and most accessible 

 portions of the Mer de Glace (or Glacier des Bois) and the 

 Glacier des Bossons, the ratio of the winter to the summer 

 motion is almost exactly 1:2. On endeavouring to establish a 

 relation between the velocity of the glacier and the temperature 

 of the ambient air, we find that these two quantities diminish 

 together in an almost regular manner down to the freezing- 

 point ; below which the velocity seems to remain constant.* 



The circumstances of motion detailed in the six preceding 

 propositions appear to be reconcileable with the assumption of 

 what may be called the Viscous or Plastic Theory of glacier 

 motion, and with that alone. 



Plastic Nature of Glacier Ice. Notwithstanding the ap- 

 parent paradox of calling a vast mass of coherent ice a semi- 

 fluid body, there is something about a glacier which almost 

 inevitably conveys to the mind the idea of a stream. This may 

 be traced in the descriptions of unscientific tourists, of poets, 

 and of some of those who have addressed themselves more 

 seriously to the question of the real nature of these bodies. To 

 the latter class of observers belong Captain Basil Hall and 

 Monseigneur Rendu, Bishop of Annecy, who had much more 

 than hinted at the possibility of a true mechanical connection 

 between the descent of a glacier and that of a mountain torrent, 

 or of a stream of lava. But until the actual conditions of 

 motion were reduced to rule, it was impossible to know how 

 far the analogy was real or apparent. 



* Philosophical Transactions, 1846, p. 191 ; and Edinburgh Philosophical 

 Journal, 1847 [page 138 of this volume]. 



