124 Mr. H. H. Howorth on 



a frost, its motion is checked ; if its fluidity is increased by 

 a thaw, the motion is instantly accelerated. Its motion is 

 greater in summer than in winter, because the fluidity is 

 more complete at the former than at the latter time. The 

 motion does not cease in winter, because the winter's cold 

 penetrates the ice as it does the ground, only to a limited 

 extent. It is greater in hot weather than in cold, because 

 the sun's heat affords water to saturate the crevices : but 

 the proportion of velocity does not follow the proportion of 

 heat, because any cause, such as the melting of a coating of 

 snow by a sudden thaw, as in the end of September, 1842, 

 produces the same eflect as great heat would do. Also, 

 whatever cause accelerates the movement of the centre of the 

 ice, increases the difference of central and lateral motion. 



Meanwhile, Agassiz continued his researches, and in 

 Desor's elaborate report of his experiments on the Aar 

 Glacier, we find a reiteration of most of his views, and an 

 elaborate defence of the infiltration theory, with many 

 experiments cited to prove it. On one point he had to 

 give way, however, and to confess that the experiment made 

 by driving a series of six stakes in a line across the glacier 

 as a test of its motion, shewed that the advance of the 

 glacier caused them to arrange themselves in a curve whose 

 convexity was inclined downwards. This, as M. Desor 

 reports, far from confirming the opinion which M. Agassiz 

 had previously hazarded, that the margins advance more 

 rapidly than the middle, shewed that the centre advances 

 more rapidly than the sides, almost to even double the extent, 

 being in the ratio of 245 feet to 125 in one case and of 269 

 to 160 in the other {Edin. Phil. Jour., XXXYI. \^^, Biblio- 

 theque Universelle de Geneva, iS4^f Nos. 88 and 89). 



He, however, still maintained, apparently, that the upper 

 part of a glacier moves more slowly than the lower, and that 

 in winter a glacier is virtually stationary (id.). 



In the autumn of 1843, Forbes was again in Switzerland, 



