176 TWELFTH LETTER ON GLACIERS. [1846. 



the motion of the points (1), (2), (3), are observed. Let 6 be 

 the angle under which X and x are seen from one of these points. 

 Let^> be the linear transversal movement of the point as seen 

 from X (deduced from the apparent angular motion and the 

 known distance of X). Let q be the similar quantity with 

 respect to x, which will have the same or contrary sign with p, 

 as the apparent motion from the two stations is in the same or 

 in contrary directions. Then the total motion of the point ob- 

 served (which is assumed to be small relatively to the dimen- 

 sions of the triangle which has X, a?, for two of its corners) 

 will be 



r = Y/^ 2 + (q cotan Q p cosec of 



and the angle (p) of the direction of motion with the visual line 

 through X, is found by this equation 



q 



sm 9 = - 

 r 



I shall take the liberty of addressing you again, as to the 

 farther observations which I have been able to make, in another 

 letter. I remain, etc. 



Largs, Ayrshire, 16th Sept. 1846. 



TWELFTH LETTER ON GLACIERS. Addressed 

 to PROFESSOR JAMESON.* 



On the Extraordinary Increase of the Glacier of La Brenva from 1842 to 1846 

 Its Motion in Winter, as observed by M. Guicharda Observations on the 

 Veined Structure in 1846, and Experimentum Crucis respecting its Origin 

 with measurements of the motion of the Ice Analogy from the motion of 

 the Khone. 



MY DEAR SIR In continuation of the results of my recent 

 journey, of which I communicated a part in my Eleventh 

 Letter, I shall first give some account of the phenomena 

 observed on a fresh visit to the glacier of La Brenva, on the 



* Head to the Royal Society of Edinburgh, 7th December 1846. Edinburgh 

 Philosophical Journal for January 1847. 



