1845.] DETAILS OF THE EXPERIMENT ON THE HER DE GLACE. 107 



But as the theodolite stationed on the glacier at Q must 

 partake of its motion whilst the mark Ql on the bank remained 

 at rest, the visual line QQ1 would appear to revolve towards 

 the origin of the glacier, and hence the relative advance of the 

 points (1), (2), etc., would seem too rapid. To estimate the 

 correction for this error the velocity of the glacier at Q must be 

 determined, and also the distance QQ1. For the former purpose 

 the following method was adopted. When an observation at 

 station Q had been completed, by pointing the telescope on Ql 

 and observing the apparent advance of the points (1), (2), etc., 

 the telescope was reversed in the Y's, or turned 180 towards 

 the western moraine, upon which it indicated from day to day 

 a new position, owing to the angular revolution of the line 

 joining the fixed point Ql and the moveable point Q. The 

 point Q2 in the topographical sketch (Plate III. fig. 1) indi- 

 cates the point where the visual line touched the moraine at the 

 commencement of the observations on the 9th of August 1844. 

 By the application of a scale or a similiar method, the apparent 

 advance of Q referred to the moraine Q2 was regularly measured. 

 It is thus obvious that these apparent motions were too great 

 (by the property of diverging lines) in the ratio of the distance 

 Ql Q2, to Ql Q ; and hence it became necessary to ascer- 

 tain the position of Q2 as well as Ql. For this purpose a 

 base-line of 300 feet was measured on the ice parallel to the 

 length of the glacier,* [and the following results obtained] : 



Q Ql = 2569 feet. Q Q2 = 550 feet. 



The apparent motion of Q measured on the moraine is greater 

 than the true motion in the ratio 2 5 2 6 1+ 9 5 5 or | nearly. The 

 actual motion of Q is readily deduced, as well as the apparent 

 rotation of the visual line QQ1. Thus during 16*75 days, the 

 duration of the experiment, the apparent advance of Q referred 

 to the moraine Q2 was twenty-three feet six inches, which at 



the distance Ql Q2 (3119 feet) subtends an angle of 25' 54", 



or almost exactly one and a half minutes daily. 



* [The subordinate details are here abridged from the Philosophical Transactions.) 



