240 GRAVITY AND LEVITY 



what it contained in the water, is exactly equaF 

 to the difference which exists between the den- 

 sity of the-water, with relation to the rarity of 

 the air ; the quantity of which may be ascer- 

 tained by means of a pair of scales, suspended 

 by a beam, with \<teights which are placed in 

 one of the scales, as the standard of that differ- 

 ence. It is owing to the same cause, that 

 a cork is heavy in air, and light in water ; 

 and that iron is heavy in water, but light in 

 quicksilver ; that a bullet of lead, and a bullet 

 of feathers, of equal bulk, fired, at the same time, 

 from the mouth of the same cannon, take very 

 different directions. The feathers float in the 

 air, and are often balanced in it, but the lead, 

 on the contrary, after the impelling force is lost, 

 which it had received from the expansion 

 of the air, which the gunpowder contained, 

 gradually sinks to the surface of the earth ;* 

 and, as the pressure downwards is the natural 

 measure, or test, which exists between the differ- 

 ent degrees of density, and of rarity, which differ- 

 ent bodies within the same bulk possess ; so the 



* That a body loses as much of its own weight, as the 

 weight of the body which it displaces, is a fact which was 

 known as far back as the time of Archimedes : although the 

 story is a common one, it nevertheless, is very decisive. 

 Hiero, king of Syracuse, suspecting that a crown which he had 

 ordered a goldsmith to make fur him of pure gold, contained 



