It is often necessary in designing a machine to know the weight of its parts, and any good engineer's handbook will give the weights per cubic inch of all the metals. Not all kinds of iron weigh exactly the same, since different processes of manufacture use different amounts of the materials of which it is made. The same thing is true of all metals, so only the approximate weight is given in the following table which shows some of the metals used in construction and their approximate weights per cubic inch.