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LXXXT. Description of a Manometer, hy means of which 

 we may ascertain the Changes which take place in the 

 Elasticity and in the Composition of a determinate Volume 

 of Air. By M. C. L. Berthollkt*. 



jl he appellation of maho/meter has been given to various 

 instruments whieii have been contrived for ascertaining the 

 differences of density of the strata of the atmosphere ; for 

 we cannot determine by the barometer the variations which 

 depend upon heat and upon the hygrometrical state. 



Otto Guerick describes a manometer, which was after- 

 wards given by Bovle as his own invention ; but neither of 

 these writers distinguishes its use from that of the barome- 

 ter. Varignon, Fouehi and Gerstner have since given 

 various manometers. 



These instruments have been generally used for ascertain- 

 ing the changes of density in the air, by means of the dif- 

 ference between an empty globe and one full of air, but 

 sealed hermetically, and put in equilibrium with a metallic 

 weight; for, when the density of the external air changes, 

 the globe undergoes a change in its weight, which answers 

 to that which takes place in a volume of air equal to that 

 which it occupies, while the metallic weight, which is of a 

 small volume on!v, remains sensibly the same. 



Bouguer employed a different method for comparing the 

 densities of the atmospheric airf : he used a pendulum 

 which he made to oscillate at various heights, in order to 

 judge, from the losses of motion experienced by the pen- 

 dulum in a given time, of the resistance of the air, and 

 consequently of its density. His experiments seemed to 

 him to confirm the opinion which he had been led to form, 

 viz. that, from the height at which the barometer supports 

 itself at 16 inches, to that at which it supports itself at 21 

 inches, there is a constant relation between the densities of 

 the air and the weights which compress it ; but that this 

 relation varies from that height down to the level of the sea, 

 which he attributes to a difference in the elasticity of the 

 molecules of the air. This error mav proceed from the 

 difficulty of obtaining results free from uncertainty by means 

 of the pendulum used by Bouguer, as has been proved by 

 M. de Saussurej, and from his neglecting to reckon the 

 effect of the heat, and of the hygrometrical stale of the air. 



We might with propriety resort to these methods of 



* From Mcmmrci de In SurK'' dTArnttil, torn. i.p. 2F2. 



f Man. dt CAcud. des Sciences, 1753. J Joutn, de Physique, 1790. 



• ascertain ins 



