iming of the boiler, when it
occurs, is very injurious to the cylinders and valves of locomotives,
especially if the water be sandy, as the grit carried over by the steam
wears the rubbing surfaces rapidly away. The face of the cylinder on which
the valve works is raised a little above the metal around it, both to
facilitate the operation of forming the face and with the view of enabling
any foreign substance deposited on the face to be pushed aside by the valve
into the less elevated part, where it may lie without occasioning any
further disturbance. The valve casing is sometimes cast upon the cylinder,
and it is generally covered with a door which may be removed to permit the
inspection of the faces. In some valve casings the top as well as the back
is removable, which admits of the valve and valve bridle being removed with
greater facility. A cock is placed at each end of locomotive cylinders, to
allow the water to be discharged which accumulates in the cylinder from
priming or condensation; and the four cocks of the two cylinders are
usually connected together, so that by turning a handle the whole are
opened at once. In Stephenson's engines, however, with variable expansion,
there is but one cock provided for this purpose, which is on the bottom of
the valve chest.

509. _Q._--What kind of piston is used in locomotives?

_A._--The variety of pistons employed in locomotives is very great, and
sometimes even the more complicated kinds are found to work very
satisfactorily; but, in general, those pistons which consist of a single
ring and tongue piece, or of two single rings set one above the other, so
as to break joint, are preferable to those which consist of many pieces. In
Stephenson's pistons the screws were at one time liable to work slack, and
the springs to break.

510. _Q._--Will you explain the connection of the piston rod with the
connecting rod?

_A._--The piston rods of all engines are now generally either case hardened
very deeply, or are made of steel; and in locomotive engines the diameter
of the piston rod is about one seventh of the diameter of the cylinder, and
it is formed of tilted steel. The cone of the piston rod, by which it is
attached to the piston, is turned the reverse way to that which is adopted
in common engines, with the view of making the cutter more accessible from
the bottom of the cylinder, which is made to come off like a door. The top
of the piston rod is secured with a cutter