GRAVITY AND LEVITY OF GASES. 



forced and unnatural situation, and is restored 

 to space free and unconfined, its power is 

 displayed by its activity : it neither acts by 

 virtue of its density, or rarity, of its gravity or 

 levity; it does not, like incompressible bodies, 

 confine itself within the same limited bounds, 

 nor press upwards or downwards only, from its 

 levity or gravity ; but expands to an indefinite 

 extent, from a centre to the whole circum- 

 ference, equally in every direction. The pres- 

 sure which is produced on the surrounding 

 medium, by the expansible power of gases, is as 

 different from the pressure which is produced 

 by liquids, as it is from the pressure produced 

 by solids. 



While the pressure downwards of a liquid, 

 placed in a medium rarer than itself, is limited 

 and confined to the perpendicular height only ; 

 the pressure produced by the weight of solid 

 bodies, on the contrary, congregates, as it were, 

 from the whole circumference, to one point, at 

 the bottom. The surface of a plank, an inch 

 in diameter, will suffer as much pressure from 

 a column of water, of an inch in diameter, and 

 of the height of twenty feet, as it will do from 

 a column of water of the same height, whose 

 diameter is as v/ide as the ocean itself. The 

 pressure, on the contrary, of solids, compre- 

 hends the aggregate quantity of matter con- 



