28

NICHOLLS'S SEAMANSHIP AND NAUTICAL'KNOWLEDGE

cavities filled with solidified grease which is only liberated when the
sheave is working. See grooves in Fig. 75.

The simple machines are the pulley block, the wheel and axle, the
lever, the wedge and the screw. All other mechanical appliances are
practically a combination of one or more of those simple machines
modified in form and application to meet particular requirements.

Fig. 80.—Metal Block.

Fig. 81—Cargo Block.

In a simple fixed Motionless pulley (Fig. 82), if W represents a weight
of 1 Ib. due to the downward force of gravity, and P represents a spring
balance held in the hand, the balance will register 1 Ib., thus demon-
strating that a power or force of 1 Ib. has to be exerted to equalise the
weight of 1 Ib. If the weight of 1 Ib. be overcome by exerting more power
at P eo that W moves slowly upwards, the balance will fully register
1 Ib, whilst W moves up the same distance as P moves down. * The
downward force at 0 mfl be 2 Ibs. No power is gained by this system
and a single pulley is only adopted in practice for convenience generally
as a leading block.
% Arrange the single pulley so that iib is movable as in Fig. 8& Secure