THE EARTH. 



55 



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 an hogshead with 

 a single quart of water; and it has been often 

 done. W e have only," for this, to place an 

 hogshead on one end, filled with water : we 

 then bore a hole in its top, into which we 



Elant a narrow tin pipe, of about thirty feet 

 igh : 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 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 quick- 

 silver, 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 upwards, until the fluid in both arms be up- 

 on a par. So that here we actually see quick- 

 silver, the heaviest substance in the world, 

 except gold and platina, floating upon a wa- 

 ter, which is but a very light subsbince. 



When we see water thus capable of sus- 

 taining 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 water, 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 sur- 

 face 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 

 lighter, it '.vill 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 therefore swim; other more pon- 

 derous bodies sink, because they are heavier 

 than their bulk of water. 



Upon this simple theorem entirely depends 



a IN 7 ollet:'s Lectures. 

 NO. 6. 



the art of weighing metals hydrostatically. I 

 have a guinea, for instance, and desire to 

 know whether it be pure gold ; I have weigh- 

 ed 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 

 bulk, that it is not pure^ In order to deter- 

 mine this, I have nothing more to do than to 

 weigh it in water with that same guinea that 

 I know to be good, and of the same weight : 

 and this will instantly show the difference ; 

 for the true ponderous metal will sink, and 

 the false bulky one will be sustained in pro- 

 portion to the greatness of its surface. Those 

 whose business it is to examine the purity of 

 metals, have a balance made for this purpose, 

 by which they can precisely determine which 

 is most ponderous, or, as it is expressed, 

 which has the greatest specific gravity. Se- 

 venty-one pound and a half of quicksilver, 

 is found to be equal in bulk to a hundred 

 pound weight of gold. In the same propor- 

 tion sixty of lead, fifty-four of silver, forty- 

 seven of copper, forty-five of brass, forty-two 

 of iron, and thirty-nine of tin, are each equal 

 to an hundred pound of the same mosi pon- 

 derous of all metals. 



This method of precisely determining the 

 parity of gold, by weighing in water, was first 

 discovered by Archimedes, to whom mankind 

 have been indebted for many useful discove- 

 ries. Hiero, king of Sicily, having sent a cer- 

 tain quantity of gold to be made into a crown, 

 the workman, it seems, kept a part for his 

 own use, and supplied the deficiency with a 

 baser metal. His fraud was suspected by 

 the king, but could not be detected; lill he 

 applied to Archimedes, who weighed the 

 crown in water; and, by this method, inform- 

 ed the king of the quantity of gold which was 

 taken away. \. 



It has been said, that all fluids endeavour 

 to preserve their level ; and, likewise, that a 

 body pressing on the surface, tended to de- 

 stroy that level. From hence, therefore, it 

 will ensily be inferred, that the deeper any 

 body sinks, the greater will be the resistance 

 of the depressed fluid beneath. It will be 

 asked, therefore, as the resistance increases 

 in proportion' as the body descends, how 

 comes the body, after it has got a certain 

 way, to sink at all ? The answer is obvious : 

 Q 



