When a body floats in a liquid, its weight is equal to the weight
of liquid which it displaces, and, when it is immersed in a liquid, its
weight is reduced by the weight of the liquid it displaces. Archimedes
discovered the idea that substances differ in density. Applied to Hiero's
crown, the argument runs thus:
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A pound of gold will displace a certain quantity of water;
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A pound of silver will displace a different quantity, because its density
is different and therefore the space it occupies in the water is different.
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