64 THE FOEMS OF WATER IN 



with its surroundings in a note-book, so as to be able 

 immediately to recognise it on our return to this place. 

 Imagine a straight line drawn from the centre of the 

 telescope to this point, and that this line is permitted 

 to drop straight down upon the glacier, every point of 

 it falling as a stone would fall; along such a line we 

 have now to fix a series of stakes. 



158. A trained assistant is already upon the glacier. 

 Ee erects his staff and stands behind it ; the telescope 

 is lowered without swerving to the right or to the 

 left ; in mathematical language it remains in the same 

 vertical plane. The crossed fibres of the telescope 

 probably strike the ice a little away from the staff of 

 the assistant ; by a wave of the arm he moves right or 

 left ; he may move too much, so we wave him back 

 asrain. After a trial or two he knows whether he is 

 near the proper point, and if so makes his motions 

 small. He soon exactly strikes the point covered by 

 the intersection of the fibres. A signal is made which 

 tells him that he is right ; he pierces the ice with an 

 auger and drives in a stake. He then goes forward, and 

 in precisely the same manner takes up another point. 

 After one or two stakes have been driven in, the 

 assistant is able to take up the other points very 

 rapidly. Any requisite number of stakes may thus be 

 fixed in a straight line across the glacier. 



159. Next morning we measure the motion of all the 

 stakes. The theodolite is mounted in its former posi« 



