THE EARTH T9 



Still recurring to our pipe, let us suppose one of its arms ten times 

 is thick as the other; this will produce no effect whatsoever upon the 

 obstacle below, which we supposed hindered its rise in the other arm ; 

 because, how thick soever the pipe may be, its contents would only rise 

 to its own level ; and it will, therefore, press the obstacle with a force 

 equal thereto. We may, therefore, universally conclude, that the 

 bottom of any vessel is pressed by its water, not as it is broad or 

 narrow, but in proportion as it is high. Thus the water contained 

 in a vessel not thicker than my finger, presses its bottom as forcibly 

 as the water contained in a hogshead of an equal height ; and. if we 

 made holes in the bottoms of both, the water would burst out as 

 forceful from the one as the other. Hence we may, with great ease, 

 burst a hogshead with a single quart of water ; and it has been often 

 done. We have only,* for this, to place a hogshead on one end, fill 

 ed with water; 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 demonstration. If wo 

 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. Now, 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 actually 

 see quicksilver, the heaviest substance in the world, except gold and 

 platina, 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 sub- 

 stances, ships, animals, or timber . When any thing floats upon wa- 

 ter, we always see that a part of it sinks in the same. A cork, a ship, 

 a buoy, each buries itself in a bed on the surface of the water ; this bed 

 may be considered as so much water displaced ; the water will, there- 

 fore, lose SQ 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 simi- 

 lar bulk of water, it will sink ; if lighter, it will swim. Universally, 

 therefore, a body plunged in water, loses as much of its weight as is 

 equal to the weight of a body of water of its own bulk. Some light 

 bodies, therefore, such as cork, lose much of their weight, and there- 

 fore swim ; other more ponderous bodies sink, because they are 

 heavier than their bulk of water. 



Upon this simple theorem entirely depends the art of weighing 

 metals hydrostatically. I have" a guinea, for instance, and desire 

 to know whether it be pure gold ; I have weighed it in the usual 

 way with another guinea, and find it exactly of the same weight, 

 but still I have some ''suspicion, from its greater hulk, that it is not 

 pure. In order to determine this, I have nothing more to do than to 

 weigh it in water with that same guinea that I know to be good, aud 



* Nollet's Lectures. 



