t on the opposite side of the cross head for
the reception of the other piston rod is of the same length as that part of
the air pump arm which one of the piston rods passes through. The piston
rods have strong nuts on each side of each of these arms to attach them to
the arms, and also to enable the length of the piston rods to be suitably
adjusted, to leave equal clearance between the piston and each end of the
cylinder at the termination of the stroke.

[Illustration: Fig. 53.

CROSS HEAD AND PUMP ARMS. Messrs. Bourne & Co.]

649. _Q._--Will you recapitulate the main particulars of the air pump?

_A._--The air pump is made of brass 12-1/2 inches diameter and 42 inches
stroke, and the metal of the barrel is 9/16ths of an inch thick. The air
pump bucket is a solid piston of brass, 6-1/2 inches deep at the edge, and
7 inches deep at the eye; and in the edge three grooves are turned to hold
water which answers the purpose of packing. The inlet and outlet valves of
the air pump consist of brass plates 1/2 inch with strong feathers across
them, and in each plate there are six grated perforations covered by india
rubber discs 7 inches in diameter. These six perforations afford
collectively an area for the passage of the water equal to the area of the
pump. The air pump rod is of brass, 2-1/2 inches diameter.

650. _Q._--What are the constructive peculiarities of the discs and crank
pin?

_A._--The discs, which are 64 inches diameter, are formed of cast iron, and
are 2-1/2 inches thick in the body, and 5 inches broad at the rim. The
crank shaft is 8-1/2 inches diameter, and the central boss of the disc
which receives the shaft measures 10 inches through the eye, and the metal
of the eye is 3 inches thick. In the part of the disc opposite to the crank
pin, the web is thickened to 10 inches for nearly the whole semicircle,
with the view of making that side of the disc heavier than the other side;
and when the engine is stopped, the gravitation of this heavy side raises
the crank pin to the highest point it can attain, whereby it is placed in
mid stroke, and cannot rest with the piston rods and connecting rod in a
horizontal line. The crank pin is 8-1/2 inches diameter, and the length of
the bearing or rubbing part of it is 16 inches. It is secured at the ends
to the discs by flanges 18 inches diameter, and 2 inches thick. These
flanges are indented into thickened parts of the discs, and are each
attached to its correspond