466 Description of a Manometer* 



is to be adjusted successively in two positions which form x 

 right angle with each other. This plummet is attached to : 

 a moveable brass scale H, to which we give only m .04 to 

 m .05 or' extent. This scale embraces, by means of two 

 rings b b, not shut, the barometrical" tube: it may thus be 

 placed at any height on the barometer, and preserve the 

 position- which is given to it. It is used to determine the 

 quantity which the height of the column of mercury has 

 varied in the course of an experiment: if this quantity 

 exceeds the limits of this scale, which is not very probable,, 

 it may be shifted so as to measure at several times the wholtr 

 variation observed. The absolute height of the mercury is 

 taken at the commencement of the experiment on a barometer, 

 and we fix one of the extremities of the scale H at the sum- 

 mit of the mercury at this moment. The small branch of 

 the syphon is furnished with a scale, in order also to ob- 

 serve the difference of the height of the mercury from the 

 commencement to the end of the experiment. When the 

 experiments require it, we give to the tube a length which 

 exceeds much that of the common barometers, and it may 

 he augmented sufficiently to indicate a pressure double that 

 of the atmosphere. 



The plate E has at C a stop-cock intended to give .issue 

 to the air of the apparatus when we wish to examine it ; 

 and this stop-cock is adjusted in such a manner that we 

 may repeat these experiments as often as we jud^e it neces- 

 sary in the course of an experiment, without fearing to 

 change the nature, or even the state of compression, of the 

 air of the manometer. For this effect, the stop-cock has 

 ■above Its collet at L (figures 2, 3,4, and 5) two nut-screws, 

 one internal and one external. On the latter is mounted a 

 copper salver M, which we fill with distilled water: the 

 glass tube N, graduated and furnished with a copper socket 

 at O, is adjusted upon the internal screw, after having been, 

 also filled with distilled water : the extremity of its screw 

 is furnished with a round of leather, which we compress. 

 On opening the stop-cock the water of the tube is dis- 

 placed by the air, which escapes from the manometer, and 

 when we perceive that a sufficient quantity has entered into 

 the tube, we shut the stop-cock. Upon unscrewing the 

 tube, the volume of the air which has entered generally 

 changes, and occ upies a smaller or larger space, in proportion 

 as it underwent in the manometer a pressure weaker or 

 stronger than that of the atmosphere. But we remove the 

 tube by plunging the finger into the water of the bason, and 

 closing with its extremity the orifice of the lube, and we 



do 



