52 Mr. Murray vn a Barometer for measuring Jltitudes. 

 Explanation of the Figure. 



A is a square phial which contains the supply of mercury, 

 the surface of which is represented by the clotted line. 



B is a tube graduated say from 31 to 20 

 inches, commencing from the inferior orifice, the 

 numerical value decreasing in the ascent. The 

 lower end of this tube, which is seen immersed 

 in tlie quicksilver, and which slides up and 

 down through an air-tight collar of leathers at 

 a, is ground to fit a small socket at 6 : c and d 

 are two ground stoppers, accurately adjusted. — 

 Glass stop-cocks will be more difficult and less 

 simple. 



C is a glass tube to cover the instrument and 

 to screw on at a. 



Before commencing die ascent, the stopper d 

 is removed, and the altitude of the mercury in 

 a good barometer ascertained. This is noted — 

 the stopper is replaced, and it may be advisable J p^d 



to tie a slip of bladder (or a ground glass cap) 

 over it, for additional security : the stopper c 

 is at some time removed, and subsequently ad- 

 justed. These being done, the graduated tube 

 B is pressed down until its lower orifice reposes 

 in the socket f and the glass envelope is screwed over the 

 collar of leathers at a, which is externally provided with a small 

 screw for this purpose. 



The summit of the acclivity being gained, the cover is un- 

 screwed, and the tube elevated until the level of the surround- 

 ing mercm'y is on a line with that indicating the altitude of the 

 barometer set out with (e. g. 30 inches) : the stopper c being 

 now removed, the mercury will ascend and determine tlie ele- 

 vation ; the difference between, say 30 inches, and that where 

 the mercury now reposes, will give the altitude above the point 

 of departure. 



The thermometer, it is needless to add, as in the common 

 construction, must accompany the present instrument, and an 

 affixed spirit level determine the plane. 



If the instrument is pcrfeetlif air-tight, the original supply of 

 air obtained on the level of the sea will subserve the purpose 

 any length of time. But it may be requisite to verify the in- 

 dications of the instrument by taking advantage of every op- 

 portunity presented by an accurate and good barometer. 



XVII. No- 



