A Catechism of Familiar Things; / Their History, and the Events Which Led to Their Discovery. / With a Short Explanation of Some of the Principal Natural Phenomena. For the Use of Schools and Families. Enlarged and Revised Edition.

by Anonymous

It means the weight of a body as compared with the weight of an equal bulk of some other body taken as a standard—commonly water.

Why do we say that certain metals—as, for example, platina or gold—are heavier than others, say, lead or iron?

Because the former have a greater specific gravity.

But is not a pound of gold as heavy as a pound of lead?

Yes; but a lump of gold will be heavier than a lump of lead of equal bulk.

Can we explain by this what we call floating?

A body will float in water if its gravity is less than that of water; for example, wood floats for this reason in water, and a balloon in the air.

Why does a portion of the floating body sink below the surface of the water?

Because the body in order to float must displace a portion of water equal in weight to the whole floating body.

But why do iron steamers float—iron being heavier than water?