CLOUDS AND RIVERS, ICE AND GLACIERS. 61 



oy M. Escher de la Linth to determine the motion of 

 a series of wooden stakes driven into the Aletsch 

 glacier, but the melting was so rapid that the stakes 

 goon fell. To remedy this, M. Agassiz in 1841 under- 

 took the great labour of carrying boring tools to his 

 ' hotel,' and piercing the Unteraar glacier at six different 

 places to a depth of ten feet, in a straight line across 

 the glacier. Into the holes six piles were so firmly 

 driven that they remained in the glacier for a year, and 

 in 1842 the displacements of all six were determined. 

 They were found to be 160 feet, 225 feet, 269 feet, 245 

 feet, 210 feet, and 125 feet, respectively. 



151. A great step is here gained. You notice that 

 the middle numbers are the largest. They correspond 

 to the central portion of the glacier. Hence, these 

 measurements conclusively establish, not only the fact 

 of glacier motion, but that the centre of the glacier, like 

 that of a river, moves more rapidly than the sides. 



152. With the aid of trained engineers M. Agassiz 

 followed up these measurements in subsequent yeat*s. 

 His researches are recorded in a work entitled ' Systeme 

 glaciaire,' which is accompanied by a very noble Atlas 

 of the Glacier of the Unteraar, published in 1847. 



153. These determinations were made by means of a 

 theodolite, of which 1 will give you some notion im- 

 mediately. The same instrument was employed the 

 same year by the late Principal Forbes upon the Mer de 

 (xlace. He established independently the greater centra) 



