Barometer. 355 



air be exhausted from the longer branch, till the pressure is re- 

 duced successively one half, two thirds, &c., the portion of air 

 confined by the drop of mercury will expand, driving the drop 

 before it, and will occupy successively, double, triple, &c., of its 

 original bulk. We infer, therefore, universally, that the space 

 occupied by any given portion of air is reciprocally proportional to 

 the pressure. 



In order that this law may hold true, however, in the strict- 

 est sense, it is to be remarked that the air must be perfectly dry ; 

 for the small quantity of aqueous vapour, which is ordinarily 

 found mixed with the atmosphere, is not condensed by pressure 

 according to the same law, as will be shown hereafter. 



The instrument represented by figure 230, is called a ma- 

 nometer. It is used for the purpose of measuring tlie elastic force 

 of other gases besides the atmosphere ; and they are all found 

 to be condensed and expanded according to the above law. 

 This important property was discovered by Mariotte, and is 

 frequently referred to under the name of the law of Mariotte. 



469. Recurring to the first experiment above described, the 

 pressure exerted upon the portion EC of confined air, when the 

 recurved part of the tube is just filled with mercury, is that of 

 the atmosphere, or g A h. But this pressure is resisted and 

 counterbalanced by the elasticity of the confined air, which by 

 supposition is of the same density with that immediately sur- 

 rounding the apparatus. We may take g A /t, therefore, as the 

 measure of the elastic force of the air in question. This force 

 remains the same so long as the air continues of the same densi- 

 ty and the same temperature. If a manometer be removed from 

 one place to another, care being taken not to change the state 

 of the confined air, the product g A h which represents the elas- 

 tic force does not undergo any change. But if the gravity g va- 

 ries as we remove from one place to another, the height h of the 

 mercury will also vary in the inverse proportion to g, the den- 

 sity A of the fluid being supposed to remain the same. It will 

 hence be perceived that the variations in the heights of the mer- 

 cury in the manometer are capable of rendering sensible the 

 variations of gravity, and may even be employed in determining 

 the augmentations or diminutions of this force arising from 

 changes of distance with respect to the centre of the earth. 



