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li 



Tlie manometer B with baro- 

 meter tube A is (see fig. 2) fastened 

 in a groove of a solid board 7^ by 

 means of Cailletet cement. This 

 board is ]>art of an adjusting table 

 ,S', ANJiicli can j-otate round an 

 horizontal axis by means of the 

 screw R. This axis passes through 

 the jioints of support of two sets 

 of screws Z, oidy one of which 

 is to be seen in the figure 2. The 

 mirror of the manometer has 

 been [)lacetl in such a way, that 

 the axis mentioned is j)arallel 

 to its fi-ont and coincides with 

 its michlie. The air tra[) I'', in 

 wiiich the barometer tube ends, 

 is by means of an India rubber 

 tube }>rovi(led witli two clamps 

 (r and H and connected with a 

 mercury resei'voir K. 



The principle, on \\ Inch this 

 manometer rests, is as follows: 



If the barometer tube is filled 

 \\\{\\ Hg as far as in the bulbs BB 

 and if the pressure is the same 

 in the two legs of the manometer, 

 we can cause the two glass points 

 to just touch the mercury mirrors, 

 which are now in one horizontal 

 plane, by using simultaneously the 

 clamp H') and the screw R. This |)oint can be very accurately 

 determined, as we can observe the reflected images at the same time 

 with the points and so it is as if we saw twice two points approach 

 each other. If the light of an incandescent lamp <i, concentrated by 

 a lens, was thn.wn on the manometer through a mirror iiu a great 

 accuracy of adjustment could l)e reached after some practice, when 



1) The clamp G serves for the rough adjustment. 



~) As it proved necessary to prevent heating ot the manometer as much as 

 possible, a thick plate of asbestos it was adjusted between the incandescent lamp 

 and the manometer; in this plate a glass vessel w was fastened with a solution 

 of alum for the absorption of the rays of heat. 



Fis. 1. 



