revolved. If one port is more
exposed than the other, adjust by means of the eccentric screws till a
balance is obtained. Should the ports still not be fully uncovered, the
throw of the eccentric is too small, and you must either make a new
eccentric or reduce the width of the valve. (The second course has the
disadvantage of reducing the expansive working of the steam.) Excess
movement, on the other hand, implies too great an eccentric throw.

Setting the Eccentric.--Turn the crank full forward, so that a line
through the crank pin and shaft centres is parallel to the bed. Holding it
in this position, revolve the eccentric (the screw of which should be
slackened off sufficiently to allow the eccentric to move stiffly) round
the shaft in a clockwise direction, until it is in that position below the
shaft at which the front steam port just begins to show. Then tighten up
the eccentric lock screw.[1]

[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