formed with a tongue piece, with an indented
plate behind the cut; and the cut is oblique to prevent a ridge forming in
the cylinder. The total thickness of the piston is 5-1/2 inches. The piston
rods are formed with conical ends for fitting into the piston, but are
coned the reverse way as in locomotives, and are secured in the piston by
nuts on the ends of the rods, these nuts being provided with ratchets to
prevent them from unscrewing accidentally.

646. _Q._--What species of slide valve is employed?

_A._--The ordinary three ported valve, and it is set on the top of the
cylinder. The cylinder ports are 4-1/2 inches broad by 24 inches long; and
to relieve the valve from the great friction due to the pressure on so
large a surface, a balance piston is placed over the back of the valve, to
which it is connected by a strong link; and the upward pressure on this
piston being nearly the same as the downward pressure on the valve, it
follows that the friction is extinguished, and the valve can be moved with
great case with one hand. The balance piston is 21 inches in diameter. In
the original construction of this balance piston two faults were committed.
The passage communicating between the condenser and the top of the balance
piston was too small, and the pins at the ends of the link connecting the
valve and balance piston were formed with an inadequate amount of bearing
surface. It followed from this misproportion that the balance piston, being
adjusted to take off nearly the whole of the pressure, lifted the valve off
the face at the beginning of each stroke. For the escape of the steam into
the eduction passage momentarily impaired the vacuum subsisting there, and
owing to the smallness of the passage leading to the space above the
balance piston, the vacuum subsisting in that space could not be impaired
with equal rapidity. The balance piston, therefore, rose by the upward
pressure upon it momentarily predominating over the downward pressure on
the valve; but this fault was corrected by enlarging the communicating
passage between the top of the balance piston and the eduction pipe. The
smallness of the pins at the ends of the link connecting the valve and
balance piston, caused the surfaces to cut into one another, and to wear
very rapidly, and the pins and eyes in this situation should be large in
diameter, and as long as they can be got, as they are not so easily
lubricated as the other bearings about the engine,