114 Mr. H. H. Howorth on 



These passages which I feel bound to quote, in conse- 

 quence of the fierce polemics which have arisen about them, 

 prove that the learned Canon, who afterwards became Bishop 

 of Annecey, fairly grasped the main feature of glacier motion, 

 which had been hitherto neglected, namely, its differential 

 motion. With the modesty of a real student, he does not 

 claim to have proved his case experimentally, but appeals 

 to future observers, who should do so to settle the question. 

 It was not long before the necessary experiments were made. 

 In the summer of 1841, Principal Forbes was invited by 

 M. Agassiz, who was then studying the Aar glacier, to pay 

 bim a visit there. In the course of this visit he realised the 

 necessity of applying precise measurements at different 

 points to the movement of glaciers, so as to definitely settle 

 what the nature of the movement was. The next year he 

 paid a second visit to the Alps, and having made his way to 

 the Mer de Glace at Chamounix, and having pierced a hole 

 in the ice, and planted his theodolite in it, he proceeded to 

 determine its position with respect to three fixed co-ordinates. 

 These having been obtained, three marks were made on 

 rocks, thus giving the absolute position of the point experi- 

 mented upon. On returning the next day he found that 

 the red mark first made, showed that the glacier had advanced 

 i6'5 inches during the previous 26 hours. Thus the diurnal 

 motion of a glacier was obtained for the first time froni 

 direct observation {Travels through the Alps of Savoy ^ 129), 

 During the next four days Forbes satisfied himself by 

 similar methods, (i) that glacier motion is approximately 

 regular and continuous ; (2) that it is nearly as great during 

 the night as during the day ; (3) that an increase of motion 

 observed on the 20th, 29th, and 30th was due to the heat of 

 the weather ; and (4) and most important, that the centre of 

 glaciers moves quicker than the sides, quite contrary to what 

 had been supposed previously. These conclusions he com- 

 municated to Professor Jamieson in his " First Letter on 



