the piston another piece of
tubing, fitting the first telescopically. Provided that the fit is true
enough to prevent the escape of steam, while not so close as to set up
excessive friction, a packing behind the piston is not needed; but should
serious leakage be anticipated, a packing of thick felt or cloth, held up
by a washer and nuts on the gudgeon G, will make things secure. Similarly
for the built-up piston valve P may be substituted a piece of close-fitting
brass rod with diameter reduced, except at the ends, by filing or turning,
to allow the passage of steam.

CONSTRUCTION.

[Illustration: FIG. 51.--Elevation of simple reciprocating steam engine.]

The bed is made of wood, preferably oak, into the parts of which linseed
oil is well rubbed before they are screwed together, to prevent the entry
of water. A longitudinal groove is sawn in the top of the bed, as indicated
by the dotted line in Fig. 51, to give room for the connecting rod in its
lowest position, and a cross groove is scooped in line with the crank shaft
to accommodate the lower part of the crank disc and the big end of the rod.
(If the wing W under the cylinder is screwed to the side of the bed,
instead of passing through it, as shown, a slight cutting away of the edge
will give the necessary clearance in both cases. )

[Illustration: FIG. 52.--Plan of simple reciprocating steam engine.]

The cylinder and valve tube A should be flattened by filing and rubbing on
emery cloth, so that they may bed snugly against one another and give a
good holding surface for the solder. A steam port, S P, should next be
bored in each, and the "burr" of the edges cleaned off carefully so as not
to obstruct valve or piston in the slightest degree. "Tin" the contact
surfaces thinly, and after laying valve tube and cylinder in line, with the
portholes corresponding exactly, bind them tightly together with a turn or
two of wire, or hold them lightly in a vice, while the solder is made to
run again with the aid of a spirit lamp. If it seems necessary, run a
little extra solder along the joint, both sides, and at the ends.

The valve, if built up, consists of a central rod, threaded at the rear
end, four washers which fit the tube, and a central spacing-piece. The
forward washer is soldered to the rod. Behind this is placed a felt
packing. Then come in order the central spacing-piece, with a washer
soldered to each end, a second packing, and a fourth washer. The series