steam the boss is hollowed out true to
accommodate a _gland_, G^1, which is threaded on the rod and screwed
up against the boss; the internal space between them being filled with
packing. Steam from the boiler enters the steam-chest, and would have
access to both sides of the piston simultaneously through the
steam-ways, W W, were it not for the

SLIDE-VALVE,

a hollow box open at the bottom, and long enough for its edges to cover
both steam-ways at once. Between W W is E, the passage for the exhaust
steam to escape by. The edges of the slide-valve are perfectly flat, as
is the face over which the valve moves, so that no steam may pass under
the edges. In our illustration the piston has just begun to move towards
the right. Steam enters by the left steam-way, which the valve is just
commencing to uncover. As the piston moves, the valve moves in the same
direction until the port is fully uncovered, when it begins to move back
again; and just before the piston has finished its stroke the steam-way
on the right begins to open. The steam-way on the left is now in
communication with the exhaust port E, so that the steam that has done
its duty is released and pressed from the cylinder by the piston.
_Reciprocation_ is this backward and forward motion of the piston: hence
the term "reciprocating" engines. The linear motion of the piston rod is
converted into rotatory motion by the connecting rod and crank.

[Illustration: FIG. 22.--Perspective section of cylinder.]

The use of a crank appears to be so obvious a method of producing this
conversion that it is interesting to learn that, when James Watt
produced his "rotative engine" in 1780 he was unable to use the crank
because it had already been patented by one Matthew Wasborough. Watt was
not easily daunted, however, and within a twelvemonth had himself
patented five other devices for obtaining rotatory motion from a piston
rod. Before passing on, it may be mentioned that Watt was the father of
the modern--that is, the high-pressure--steam-engine; and that, owing to
the imperfection of the existing machinery, the difficulties he had to
overcome were enormous. On one occasion he congratulated himself because
one of his steam-cylinders was only three-eighths of an inch out of
truth in the bore. Nowadays a good firm would reject a cylinder 1/500 of
an inch out of truth; and in small petrol-engines 1/5000 of an inch is
sometimes the greatest "limit of error" allowed.

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