486

MODERN STEAM PRACTICE.

bored with a number of holes, so that the plate can be shifted toaccommodate the head. Others use T bolts, which are let into theboss, the nut being cross cut at the top for a split pin to passthrough, which prevents it from turning round; a plan likewiseadopted when the bolts are tapped into the boss.

Propeller blades are made of cast iron or Steel, and recently

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bronze has been used for this purpose. The advantage inusing bronze for propellers lies in the greater durability and thesmoothness of the skin. Manganese bronze has now been in use