and are moreover kept at
a high temperature by the steam. The balance piston is packed in the same
way as the main piston of the engine. Its cylinder, which is only a few
inches in length, is set on the top of the valve casing, and a trunk
projects upwards from its centre to enable the connecting link to rise up
in it to attain the necessary length.

[Illustration: Fig 52. CONNECTING ROD. Messrs. Bourne & Co.]

647. _Q._--What is the diameter of the piston rods and connecting rod?

_A._--The piston rods, which are two in number, are 3 inches diameter, and
12 feet 10 inches long over all. They were, however, found to be rather
small, and have since been made half an inch thicker. The connecting rod
consists of two rods, which are prolongations of the bolts that connect the
sides of the brass bushes which encircle the crank pin and cross head. The
connecting rod is shown in perspective in fig. 52. The rods composing it
are each 2-3/4 inches in diameter.

648. _Q._--Will you describe the configuration of the cross head.

_A._--The cross head, exhibited in fig. 53, is a round piece of iron like a
short shaft, with two unequal arms keyed upon it, the longer of which _b_
works the air pump, and the shorter _c_ works the feed pump. The piston
rods enter these arms at _a A._ The cross head is 8 inches diameter where
it is embraced by the connecting rod at _e_, and 7 inches diameter where
the air pump and feed pump arms are fixed on. The ends of the cross head _d
d_, for a length of 12 inches, are reduced to 3 inches diameter where they
fit into round holes in the centre of the guide blocks. Those blocks are of
cast iron 6 inches deep, 11 inches wide, and 14 inches long, and they are
formed with flanges 1 inch thick on the inner sides of the blocks. The
projection of the air pump lever from the centre of the cross head is 1
foot 9 inches, and it is bent 5-3/4 inches to one side to enable it to
engage the air pump rod. The eye of this arm is 6 inches broad and about 2
inches thick. At the part where one of the piston rods passes through it,
the arm is 8 inches deep and 6 inches wide; but the width thereafter
narrows to 3 inches, and finally to 2 inches; and the depth of the web of
the arm reduces from 8 inches at the piston rod, to 4 inches at the eye,
which receives the end of the air pump rod. The feed pump arm is only 3
inches thick, and has 9 inches of projection from the centre of the cross
head; but the eye attached to i