34- CORRECT CHAMBER BAROMETER. 



Use of the pis- any position assigned to it by the operator. Tlie use of this 

 apparatus is almost too obvious to stand in need of an ex- 

 planation ; its office is to place the surface of the mercury 

 in the leg D E F on a level with O o, when an observation 

 is to be taken with the instrument. This is done by placinj*- 

 the finger upon G, and pressino- the piston downwards, 

 when the barometer is rising, which operation must be con- 

 tinued until the quicksilver coincides with ,0 o. On the 

 contrary, when the mercury falls in the leg B C, the piston 

 must be drawn up by means of the handle G L, so as to 

 produce a coincidence in the opposite branch of the tube, 

 A necessary similar to that just now mentioned. One circumstance must 

 thrconstruc- ^ attended to in the construction of the instrument ; for it 

 tion of the in is necessary that the middle of the piston K, which is mark- 

 b rumen . ^j ^j^l^ ^ dotted line in the figure, should coincide with 

 O o, as oft as the barometer stands at 39'5 inches. The 

 method of providing for this coincidence will be easily 

 pointed out by an example : suppose, when the tube is filled, 

 that the middle of the piston and the surface of the mer- 

 cury in the leg D E F coincide with O o, while the height 

 of the barometer is 29*00 or { an inch less than 29*5 ; in 

 this case, raise the piston until the mercury in B C stands 

 at 28*5, or as much below 29*0 as 29*0 is below 29*5 : this 

 being done, pour mercury into the leg D E F, so as to 

 make the opposite column rise again to 29*0, and the re- 

 quired coincidence will be provided for. On the other hand, 

 suppose the instrument to be adjusted as above described, 

 and the height of the barometer to be 30-0, or | an inch 

 above 29'5: in this case push the piston downwards, until 

 the column in B C stands at 30*5, or as much above 30*0 as 

 30*0 is above 29*5 : this being done, take a quantity of mer- 

 cury out of the leg D E F just sufficient to make the oppo- 

 site column fall again to 30*0, and you will have secured the 

 coincidence required. 

 Remarks re- I have observed in a preceding part of this letter, that the 



specting the piston of my barometer is If inch in length, and ~ an inch 

 the piston. i» diameter ; but the reader is not to imagine, that these 

 dimensions are fixed by necessity, for they may be varied at 

 pleasure; and the following rule will determine the one, 

 wlien tlie other is given. 1st, When the diameter of tie 



piston 



