192 Description of Sang's Hypsometer. 



such a direction that the tape line was perpendicular to the stem, 

 but this could be judged sufficiently well by the eye to give the 

 height, if even a very high tree, nearly correctly. The heights 

 of those houses I tried were given within an inch, which was no 

 doubt owing to their being perfectly upright on a level court- 



The principle of the instrument is quite simple, being exactly 

 the same as that of the sextant or quadrant, only that the mirrors 

 are fixed at a certain angle instead of being movable. Thus, 

 in Jig. 33., a is the eye, b a mirror partly 

 silvered, and c a larger mirror wholly 

 silvered. A ray of light r, falling on 

 the mirror c, is reflected from it in the 

 direction c 6, and again reflected from 

 the mirror h in the direction b a to the 

 eye; at the same time another ray of 

 light comes from an object o direct to 

 the eye at «, without being reflected. 

 From the nature of reflected light, the angle r a o is equal to 

 twice the inclination of the mirrors, and is constant, however 

 much the whole instrument may be moved in the plane of the 

 objects, as you will easily perceive by catching the reflection of 

 the candle in the instrument, and moving it in the plane of the 

 milled ends. 



I am sure this very portable instrument will be useful for 

 measuring single trees, or buildings, which are as far asunder as 

 they are high, but I am afraid it will not work well in a close 

 wood, on account of the operator not having room to retire as 

 far from the trees as their height. If this is 3^ 

 found to be the case, the remedy is to construct /^| 



another instrument in which the mirrors are 

 placed so as to give an angle of 63° 26' 05". 

 In this case the height of the trees will be 

 equal to twice the length of the tape, added to 

 the height of the observer's eye. (Seey^. 34.) Of 

 course a small deviation from squareness in the 

 trees and tape line will make a greater error 

 than with the instrument sent, but still it will 



give a result near enough for all practical pur-_Ji ^ — 



poses. 



I have only to add, that the mirrors are made of common 

 window glass selected as the most even from among a great many 

 pieces, but still they are not quite flat. I had some glass from 

 London perfectly true and flat, but so dim and badly polished as 

 to be unfit for use. 



Kirkcaldy, Jan, 31.1840. 



