In doing this, a piece of wood must be used to hammer on in driving them in and out, since driving them with the bare hammer, a piece of metal, or a mandril, stretches the skin and enlarges the diameter across the bore; then when the brasses are bored and the stretched skin is consequently removed, the brass resumes its original shape, and hence becomes loose in the strap or box. In fitting brasses to their places, leave them a little too tight, since all brasses contract a little across the bore in consequence of the process of boring. This rule applies also to journal-boxes of cast-iron or any other metal.