62 THE FORMS OF WATEK IN 



motion. He showed, moreover, that it is not necessary 

 to wait a year, or even a week to determine the motion 

 of a glacier; with a correctly-adjusted theodolite he 

 was able to determine the motion of various points of 

 the Mer de Glace from day to day. He affirmed, and 

 with truth, that the motion of the glacier might be 

 determined from hour to hour. We shall prove thi? 

 farther on (162). Professor Forbes also triangulated 

 the Mer de Glace, and laid down an excellent map of 

 it. His first observations and his survey are recorded 

 in a celebrated book published in 1843, and entitled 

 6 Travels in the Alps.' 



154. These observations were also followed up in 

 subsequent years, the results being recorded in a series 

 of detached letters and essays of great interest. These 

 were subsequently collected in a volume entitled ' Occa- 

 sional Papers on the Theory of Glaciers, 5 published in 

 1859. The labours of Agassiz and Forbes are the two 

 chief sources of our knowledge of glacier phenomena. 



§ 21. The Theodolite and its TJse^ Our own 

 Measurements. 



155. My object thus far is attained. 1 have given 

 you proofs of glacier motion, and a historic account of 

 its measurement. And now we must try to add a 

 little to the knowledge of glaciers by our own labours 

 on the ice. Resolution must not be wanting at the 

 commencement of our work, nor steadfast patience 



