162 HISTORY OF 



we then bore a hole in its top, into which we 

 plant a narrow tin pipe, of about thirty feet high : 

 by pouring a quart of water into this, at the top, 

 as it continues to rise higher in the pipe, it will 

 press more forcibly on the bottom and sides of 

 the hogshead below, and at last burst it. 



Still returning to our simple instrument of de- 

 monstration. If we suppose the obstacle at the 

 bottom of the pipe to be moveable, so as that the 

 force of the water can push it up into the other 

 arm ; such a body as quicksilver, for instance ; 

 here it 'is evident, that the weight of water 

 weighing down upon this quicksilver in one arm, 

 will at last press it up in the other arm, and will 

 continue to press it upward, until the fluid in 

 both arms be upon a par. So that here we ac- 

 tually see quicksilver, the heaviest substance in 

 the world, except platina and gold, floating upon 

 water, which is but a very light substance. 



When we see water thus capable of sustaining 

 quicksilver, we need not be surprised that it is 

 capable of floating much lighter substances, ships, 

 animals, or timber. When any thing floats upon 

 water, we always see that a part of it sinks in the 

 same. A cork, a ship, a buoy, each buries itself 

 a bed on the surface of the water ; this bed may 

 be considered as so much water displaced ; the 

 water will, therefore, lose so much of its own 

 weight as is equal to the weight of that bed of 

 water which it displaces. If the body be heavier 

 than a similar bulk of water, it will sink; if light- 

 er, it will swim. Universally, therefore, a body 

 plunged in water, loses as much of its weight as 



