round the
cylinder, measuring mid-way 1-1/4 inch deep, by about 3/4 inch thick, and
the greatest depth of the finger for moving the valve is about 1 inch. The
depth of the lever for moving the valve shaft is 2 inches at the broad, and
1-1/4 inch at the narrow end. The internal breadth of the mortice in which
the valve finger moves is 5/16 inch, and its external depth is 1-3/4 inch,
which leaves three eighths of an inch as the thickness of metal round the
hole; and the breadth, measuring in the direction of the hole, is 1-1/2
inch. The valve rod is three fourths of an inch in diameter, and the
mortice is connected to the valve rod by a socket 1 inch long, and 1-1/8
inch diameter, through which a small cutter passes. A continuation of the
rod, eleven sixteenths of an inch diameter, passes upward from the mortice,
and works through an eye, which serves the purpose of a guide. In addition
to the guide afforded to the segment by the ascending tail, it is guided at
the ends upon the columns of the framing by means of thin semicircular
brasses, 4 inches deep, passing round the columns, and attached to the
segment by two 3/8 inch bolts at each end, passing through projecting
feathers upon the brasses and segment, three eighths of an inch in
thickness. The curvature of the segment is such as to correspond with the
arc swept from the centre of the trunnion to the centre of the valve lever
pin when the valve is at half stroke as a radius; and the operation of the
segment is to prevent the valve from being affected by the oscillation of
the cylinder, but the same action, would be obtained by the employment of a
smaller eccentric with more lead. In some engines the segment is not formed
in a single piece, but of two curved blades, with blocks interposed at the
ends, which may be filed down a little, to enable the sides of the slot to
be brought nearer, as the metal wears away.

631. _Q._--What kind of plummer blocks are used for the paddle shaft
bearings?

_A._--The paddle shaft plummer blocks are altogether of brass, and are
formed in much the same manner as the cap of the piston rod, only that the
sole is flat, as in ordinary plummer blocks, and is fitted between
projecting lugs of the framing, to prevent side motion. In the bearings
fitted on this plan, however, the upper brass will generally acquire a good
deal of play after some amount of wear. The bolts are worked slack in the
holes, though accurately fitted at first; and it a