44 MAN AND THE GLACIAL PERIOD. 



by the interest of this visit, Forbes returned to Switzer- 

 land in 1842 and began a series of independent investiga- 

 tions upon the Mer de Glace. After a week's observa- 

 tions with accurate instruments, Forbes wrote to Professor 

 Jameson, editor of the Edinburgh New Philosophical 

 Journal, that he had already made it certain that " the 

 central part of the glacier moves faster than the edges in 

 a very considerable proportion, quite contrary to the 

 opinion generally maintained." This letter was dated 

 July 4, 1842, but was not published until the October fol- 

 lowing, Agassiz's results, so far as then determined, were, 

 however, published in Comptes Eendus of the 29th of 

 August, 1842, two months before the publication of 

 Forbes's letter. But Agassiz's letter was dated twenty- 

 seven days later than that of Forbes. It becomes certain, 

 therefore, that both Agassiz and Forbes, independently 

 and about the same time, discovered the fact that the 

 central portion of a glacier moves more rapidly than the 

 sides. 



In 1857 Professor Tyndall began his systematic and 

 fruitful observations upon the Mer de Glace and other 

 Alpine glaciers. Professor Forbes had already demon- 

 strated that, with an accurate instrument of observation, 

 the motion of a line of stakes might be observed after 

 the lapse of a single clay, or even of a few hours. As 

 a result of Tyndall's observations, it was found that 

 the most rapid daily motion in the Mer de Glace in 1857 

 was about thirty-seven inches. This amount of motion 

 was near the lower end of the glacier On ascending the 

 glacier, the rate was found in general to be diminished ; 

 but the diminution was not uniform throughout the 

 whole distance, being affected both by the size and by the 

 contour of the valley. The motion in the tributary gla- 

 ciers was also much less than that of the main glacier. 



This diminution of movement in the tributary glaciers 

 was somewhat proportionate to their increase in width. 



