226 GRAVITY AND LEVITY 



if it contains more matter than it displaces, it 

 falls ; if less, it rises ; it is not to the body 

 alone, or to the medium alone, to which these 

 effects are to be ascribed, but to the mutual 

 relation which exists between the one and the 

 other. The natural measure, or test, which 

 exists of the different quantities of matter in 

 different bodies, is ascertained by the different 

 degrees of pressure downwards which they 

 produce ; so the pressure upwards of dif- 

 ferent bodies, is the test of the comparative 

 degrees of rarity in the particles of matter, con- 

 tained within the same bulk, with relation to 

 the density of the particles of matter by which 

 they are surrounded. Bodies such as these, 

 instead of remaining suspended in equilibrio, 

 instead of pressing downwards, are found uni- 

 formly to press upwards, and to rise, with a 

 force which is equal to the difference which 

 exists between the rarity of the materials of 

 which they are composed, and the density 

 of the matter in which they are immersed: 

 it is from this cause, that bodies are lighter 

 in a dense, than in a rare medium; and 

 heavier in a rare, than in a dense one ; 

 why they are lighter in quicksilver, than in 

 water ; in water, than in air ; lighter near the 

 surface of the earth, than in the most elevated 

 regions of the air ; and, finally, lighter in a 

 Receiver full of atmospheric afr, than in an 



