liflE EARTH. 1^5 



nine of tin, are each equal to a hundred pound of the same 

 most ponderous of all metals. 



This method of precisely determining the purity of gold, by 

 weighing in water, was first discovered by Archimedes, to whom 

 mankind have been indebted for many useful discoveries. Hiero, 

 king of Sicily, having sent a certain 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 till 

 he applied to Archimedes, who 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 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 : 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 

 below be suffered to act, and after the body is gone dowTi 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, below the surface of the water ; through this the 

 water bursts up with great violence ; 1 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 overcome 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 pres- 

 sure from above, it is true, but it is counteracted by the press .la 



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