CH. III. ARCHIMEDES— SPECIFIC GRAVITY. 2.? 



will no longer weigh the same, because the water round 

 them buoys them up just as much as it would buoy up the 

 quantity of water which they displace. 



Now, the gold ball takes the place of as much water as 

 would weigh one ounce if you could take it out and weigh it 

 in the air. So the gold ball is buoyed up 07ie ounce by the 

 water round it, and accordingly you see it only pulls the 

 marker down 18 ounces instead of 19. But the silver ball, 

 although it weighs the same, is larger, and takes the place of 

 nearly two ounces of water, therefore it is buoyed up nearly 

 two ounces, and only pulls the marker down to 1 7. Now, as 

 the crown weighs the same as the two balls, its shape is of 

 no consequence ; if it was made all of gold it would take as 

 much room, and be buoyed up as much as the gold ball. If 

 it was all silver it would be buoyed up as much as the silver 

 ball, and therefore, as it pulls the marker down half-way 

 between 17 and 18 ounces, it must be half gold and half 

 silver. 



In this way Archimedes showed how we can learn the 

 weight of any substance compared to an equal bulk of water, 

 and this is called the * specific gravity ' of the substance. 



He also invented a screw for pumping up water, which is 

 still called the ' screw of Archimedes.' 



Archimedes was unfortunately killed in the city of 

 Syracuse when it was besieged by the Romans during the 

 second Punic war. The General Maecenas had given special 

 orders that his life should be spared ; but he was so deeply 

 engaged in solving a problem that he heard nothing of the 

 din of war around him, and a common soldier not being 

 able to get any answer from him, killed him without knowing 

 who he was. 



