4:6 NICHOLLS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE

the skylight for lifting the cylinder covers and pistons. The upper
block has two sheaves rigidly attached to each other but difiering
slightly in diameter. An endless chain passes over the sheaves the

Fig. 3, — Swinging the Derrick.

rims being made with "snugs" to take the links of the chain to prevent

2R
it from slipping. The power gained is given by ~ — , where R is the

— r