Description of a Manometer* 459 



ascertaining the density of the strata of the atmosphere, 

 if any doubts remained as to the nature of the air, the pro- 

 portions of its constituent parts, and the law which its di- 

 latation follows by the elevation of temperature.: but at 

 present, as we have precise information on these subjects, 

 and as the uncertainties which may remain on the indica- 

 tions of the hygrometer are much smaller than those which 

 ought to result from the methods mentioned ; it is more 

 expeditious and more certain to adhere to the barometer, 

 combining its indication with that of the thermometer and 

 hygrometer. 



The case is not the same with the manometer destined 

 to determine the changes which take place in the elasticity 

 of a quantity of air contained in a vase. Saussure directed 

 towards this object the apparatus to which he gave the 

 name of manometer, and by means of which he made some 

 most important observations*: it is simply a barometer, 

 the bulb of which is contained in a bell-glass which is her- 

 metically closed, and into which we may introduce the sub- 

 stances which mav afTect the elasticity of the air, by an 

 aperture in the neck of the bdl-glass, but by establishing, 

 at that instant, the communication between the internal 

 and external air. 



While the communication with the external air is inter- 

 rupted, the barometer is insensible to the variations of the 

 atmosphere, and it undergoes no change in its elevation ex- 

 cept by the increase or diminution of the elasticity. 



This is the manometer, the applications of which I wished 

 to extend, and which I endeavoured to adapt to the obser- 

 vation of the phenomena which take place during vegeta- 

 tion, and generally those which animal and vegetable sub- 

 stances present, during life or after death, relative to the at- 

 mosphere with which they are surrounded. 



In the first place we perceive that the barometer which 

 performs the functions of the manometer, indicates. the 

 quantities of gas which are disengaged or absorbed in a 

 given time ; and as it is easy to ascertain a change even of 

 one thousandth part in the height of the barometer, we 

 may determine a change of one thousandth pari in the 

 quantity of the contained air, by the absorption or extrica- 

 tion of a gas. 



But in order to make this estimate, there must be a ther- 

 mometer suspended internally to indicate the same tem- 

 perature with that of the first observation : or if the tem- 



* Essais sur CHygrometrie, p. 109. 



peraturt 



