If you fill it with water, however, you increase its weight without adding anything to the amount of water it displaces,--or rather, as you let water into all the hollow space, you lessen by that much the amount of water it must displace in sinking without taking away anything from its weight, and so it sinks; or, if you break the teacup you lessen the amount of water it must displace without lessening its weight, and so it sinks in that case, too.

Do you understand that ?" "Yes, I think I do," said Tom; "but I don't exactly see how it applies to the drift-pile." "I'll explain that presently.

I want to make it plain first that the ability of a thing to float depends not on its weight, but on its weight as compared with that of a like bulk of water.

This comparative or relative weight is called _specific gravity_, and in measuring the specific gravity of substances water is taken as the standard usually, though sometimes gold is used for that purpose.

Now to come to the drift-pile.