[Footnote 1:  The reader is referred to an excellent little treatise, entitled “The Slide Valve” (Messrs. Percival Marshall and Co., 26 Poppin’s Court, Fleet Street, E.C.  Price 6d.), for a full explanation of the scientific principles of the slide valve.]

The Connecting Rod.—­The length of this from centre to centre of the pins on which it works should be established as follows:—­Slip over the piston rod a disc of card 1/32 inch thick.  Then pass the rod through the gland and assemble the crosshead and fork on its end, and assemble the guides round the crosshead foot.  Turn the crank pin full forward, pull the piston rod out as far as it will come, measure the distance between pin centres very carefully, and transfer it to a piece of paper.

The rod consists of a straight central bar and two rectangular halved ends.  The ends should be cut out of brass and carefully squared.  Through their exact centres drill 1/8-inch holes, and cut the pieces squarely in two across these holes.  The sawed faces should be filed down to a good fit and soldered together.  Now drill holes of the size of the pins, using what remains of the holes first made to guide the drill.  The bolt holes are drilled next, and finally the holes for lubrication and those to take the rods.  Then lay the two ends down on the piece of paper, so that their pinholes are centred on the centre marks, and the holes for the rod are turned towards one another.  Cut off a piece of steel rod of the proper length and unsolder the ends.  The rod pieces must then be assembled on the rod, and with it be centred on the paper and held in position while the parts are soldered together.

OTHER DETAILS.

Adjusting the Guides.—­Put the connecting rod in place on its pins, and revolve the crank until the guides have taken up that position which allows the crosshead to move freely.  Then mark off the holes for the guide holding-down screws, and drill and tap them.

Packings.—­The glands and piston should be packed with asbestos string.  Don’t be afraid of packing too tightly, as the tendency is for packing to get slacker in use.  The rear end of the cylinder should be bevelled off slightly inside, to allow the packed piston to enter easily.

Joints.—­The cylinder head and valve chest joints should be made with stout brown paper soaked in oil or smeared with red lead.  All screw holes should be cut cleanly through the paper, and give plenty of room for the screws.

[Illustration:  Fig. 66.-Vertical section of force pump driven by engine.]

When making a joint, tighten up the screws in rotation, a little at a time so as not to put undue strain on any screw.  Wait an hour or two, and go round with the screw-driver again.

Lubrication.—­When the engine is first put under steam, lubrication should be very liberal, to assure the parts “settling down” without undue wear.