80 A HISTORY OF 



of the same weight ; and this will instantly show the difference ; fo, 

 the true ponderous metal will sink, and the false bulky one will be 

 sustained in proportion 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. Seventy-one pounds and a half of quicksilver is found to bo 

 equal in bulk to a hundred pounclsweight of gold. In the same pro 

 portion sixty of lead, fifty-four of silver, forty-seven of copper, forty 

 fiv< of brass, forty-two of iron, and thirty-nine of tin, are each equa\ 

 to a hundred pounds of the same most ponderous of all metals. 



This method of precisely determining the purity of gold, by weigh- 

 ing in water, was first discovered by Archimedes, to whom mankinc 

 have been indebted for many useful discoveries. Hiero, king of Sici 

 ly, having sent a certain quantity of gold to be made into a crowi., 

 the workman, it seems, kept a part for his own use, and supplied the 

 deficiency witli a baser metal. His fraud was suspected by the king, 

 but could not be detected ; till applying to Archimedes, he weighed 

 the crown in water; and, by this method, informed 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 destroy 

 that level. From hence, therefore, it will easily be inferred, that the 

 deeper any body sinks, the gre.'.ter will be the resistance of the de 

 pressed fluid beneath. It will be asked, therefore, as the resistance 

 increases in proportion as the body descends, how comes the body, 

 after it is got a certain way, to sink at all ? The answer is obvious. 

 From the fluid above pressing it down with almost as great a force as 

 the fluid beneath presses it up. Take away, by any art, the pressure of 

 the fluid from above, and let only the resistance of the fluid from be- 

 low be suffered to act, and after the body is gone down very deep, the 

 resistance will be insuperable. To give an instance : a small hole 

 opens in the bottom of a ship at sea, forty feet, we will suppose be- 

 low the surface of the water ; through this the water bursts up with 

 great violence ; I attempt to stop it with my hand, but it pushes the 

 hand violently away. Here the hand is, in fact, a body attempting to 

 .sink upon water, at a depth of forty feet, with the pressure from above 

 taken away. The water, therefore, will overcame my strength ; and 

 will continue to burst in till it has got to its level : if I should then 

 dive into the hold, and clap my hand upon the opening, as before, I 

 should perceive no force acting against my hand at all : for the water 

 above presses the hand as much down against the hole, as the water 

 without presses it upward. For this reason, also, when we dive to 

 the bottom of the water, we sustain a very great pressure from above, 

 it is true, but it is counteracted by the pressure from below ; and the 

 whole acting uniformly on the surface of the body, wraps us close 

 round without injury. 



As I have deviated thus far, I will just mention one or two proper- 

 ties more, which water, and all such like fluids, is found to possess. 

 And, first, their ascending in vessels which are emptied of air, as in 

 *ui common pumps for instance. The air, however, being the agent 



