106 VISCOUS THEORY OF GLACIER MOTION. [1845. 



The theodolite was planted at a fixed point on the ice Q, just 

 within the crevassed portion, which intervened between it and 

 the western shore of the glacier. This point of fundamental 

 and constant reference was fixed by an exactly vertical hole 

 pierced with an iron jumper, or blasting iron, one inch in 

 diameter, and was frequently deepened in order to preserve the 

 centre as exactly as possible in the same vertical line in the ice. 

 The theodolite was centred over it at every observation by 

 means of a plummet, which nearly filled the cylindrical hole 

 and permitted an adjustment, which one day with another 

 might be accurate to about one-tenth of an inch. No stick was 

 placed in the hole, but when not in use it was covered by a 

 large flat stone, which effectually prevents congelation in ordi- 

 nary weather.* The adjustment of the theodolite on the ice is 

 always a matter for patience, but I succeeded in rendering it 

 perfectly stable when once erected, by inserting the three feet 

 in cavities in the ice, and filling them carefully with ice chips. 



The theodolite, placed at Q, was pointed with its vertical 

 wire on the well-defined angle of an erratic block Ql on the 

 opposite eastern bank of the glacier, above Les Echellets (see 

 Plate III. fig. 2). By causing the telescope to traverse in a 

 vertical circle, a transverse line joining the points Q, Ql, was 

 determined, and several stations were fixed in the compact ice 

 eastwards of Q, at distances from it of 30, 60, and 90 English 

 feet, and subsequently at 120 and 180 feet. These were num- 

 bered in succession (1), (2), (3), (4), (5), and the permanence of 

 their positions in the ice was secured as before by carefully 

 driving vertical holes two feet deep, which were occasionally 

 deepened, and covered with flat stones when not in use. As these 

 points were in succession nearer to the centre of the glacier, 

 they were expected to move with gradually increasing velocity 

 in advance of the imaginary line Q, Ql, drawn across the ice. 



* On one occasion this precaution having been neglected (in the case of a 

 different mark on the ice), the hole was found completely frozen up after exposure 

 to a day or two of severe weather in the month of August. It was, however, 

 recovered hy observing the beautiful stellar form of the ice- crystallization. 



