Description of Sang^s Hyjpsometer. 



191 



tioned to you when having the pleasure of visiting you at Bays- 

 water. It is sent by post at the same time as this letter. 



The instrument was tried on some houses and trees here, and 

 it gave their height (especially the houses) with great accuracy. 

 It is rather difficult to manage at first, but after a few trials it 

 becomes quite easy. The method is as follows : — 



By means of a small hook (if a knot of white cloth be attached 

 to it, so much the better), fix the end of a tape line to the bole 

 of the tree, at exacdy the height of the observer's eye from the 

 ground. Retire from the tree, letting the tape line unwind until, 

 by using the instrument, the top of the tree and the end of the 

 tape line are seen quite close together. Add the height of the 

 observer's eye to the length of the tape line, and the sum is the 

 height of the tree. Now, the difficulty is, to catch the image of 

 the top of the tree in the instrument, and it is this which requires 

 a few trials, although any person who has been accustomed to 

 use a sextant will do it at the very first. Hold the instrument 

 by one of the milled ends, taking care that the fingers do not 

 project over any of the holes, and that the 

 brim of the hat is out of the way. Apply the 

 eye to the round hole marked a '\x\.Jig. 3]., 

 and look through in the direction of the small 

 square hole &, the instrument being held so 

 that the line joining « 6 is about level, while 

 the large square hole c is turned toward 

 the sky. You will then see some object 

 directly through the small hole, and at the 

 same time the image of some other object, 

 the light from which enters the large aper- 

 ture, and, after being reflected by the two 

 mirrors inside, passes into the eye. What- 

 ever two objects are thus seen in contact, 

 subtend at the eye an angle of 45°, as in 

 jig. 32. ; so that, if 

 one of them be the 

 end of the tape 



line on a level, or nearly so, with the 

 observer's eye, while the other is the top 

 of a tree, supposed to be growing 

 straight up, the distance from the eye 

 to the bole of the tree will be exactly 

 equal to the distance from the end of 

 the tape line to the top of the tree. 

 , You will thus observe that the accuracy of the measurement 

 depends on the tree being erect from the ground. On sloping 

 ground the measurer would require to go out from the tree in 



o 4 



32 



