e other side of the
discs. A short piece of one piston rod is shown in fig. 54.

[Illustration: Fig. 55. THRUST BEARING. Messers. Bourne & Co.]

[Illustration: Fig. 56. COUPLING CRANKS. Messers. Bourne & Co.]

652. _Q._--How is the thrust of the screw shaft received?

_A._--The thrust of the screw shaft is received upon 7 collars, each 1 inch
thick, and with 1 inch of projection above the shaft. The plummer block for
receiving the thrust of the shaft is shown in fig. 55, and the coupling to
enable the screw propeller to be disconnected from the engine, so that it
may revolve freely when the vessel is under sail, is shown in fig. 56. When
it is required to disengage the propeller from the engine, the pins passing
through the opposite eyes shown fig. 56, are withdrawn by means of screws
provided for that purpose, and the propeller and the engine are thenceforth
independent of one another.

[Illustration: Fig. 57. LINK MOTION. Messrs. Bourne & Co.]

653. _Q._--Will you describe the arrangement of the valve gearing?

_A._--The end of the screw shaft, after emerging from the bearing beside
the disc, is reduced to a diameter of 4 inches, and is prolonged for 4-1/2
inches to give attachment to the cam or curved plate which gives motion to
the expansion valve. This plate is 3-1/2 inches thick, and a stud 3-1/2
inches diameter is fixed in the plate at a distance of 5 inches from the
centre of the shaft. To this stud an arm is attached which extends to a
distance of 2 inches from the centre of the shaft in the opposite
direction, and the end of this arm carries a pin of 2-1/2 inches diameter.
From the pin most remote from the centre of the shaft, a rod 2-1/2 inches
broad and 1 inch thick extends to the upper end of the link of the link
motion; and from the pin least remote from the centre of the shaft, a
similar rod extends to the lower end of the link of the link motion. This
link, which is represented in fig. 57, is 2-1/4 inches broad, 1 inch thick,
and is capable of being raised or lowered 25 inches in all. In the open
part of the link is a brass block, which, by raising or lowering the link,
takes either the position in which it is represented at the centre of the
link, or a position at either end of it. Through the hole in the brass
block a pin passes to attach the brass to the end of a lever fixed on the
valve shaft; so that whatever motion is imparted to the brass block is
communicated to the valve through the medium