In a line with 1-32d and under 1-3d is .113 and .073; therefore .113×72-8.136 inches the piston is from the end of the stroke, when the exhausting-port before the piston is shut, and .073X72-5.256 inches the piston is from the end of the stroke, when the exhausting-port behind. it is open.

Table by which to ascertain the amount of lap necessary on the steam side of a slide-valve to cut the steam off at various fractional parts of

the stroke.

To cut the steam off, after the piston has passed through 11

of its stroke. Multiply the given stroke of the valve by

.323 .289 .250 .204

.177 .144

.354 and the product is the lap of the valve in terms of the stroke.

Example.-Required the lap necessary to cut the steam off at the end of five-sixths of the stroke, the stroke of the valve being twelve inches, and without lead. .204X12=2.448 inches.

As lead is not taken into account because of different quantities being required to different applications of the steam-engine, subtract from the lap half the lead; the remainder is the lap required. Thus, suppose the lead equal .25÷-2 .125 and 2.448-.125=2.323 inches, the lap with one-fourth inch of lead as given.

CHAPTER IX.

HOW TO SET A SLIDE VALVE.

IT is first necessary to find the two centres for the cross-head. A, in Fig. 27, is the rim of the fly-wheel, and B, a piece of wood of a length to reach from the floor or foundation to about the height of the centre of the shaft.

On the guides A, Fig. 28, draw a line a, at or a trifle beyond the travel point of the cross-head. Measure off a short distance (2, 3, 4 or 5 in.) on the guide from line a, and draw the line b. Place the cross-head as near on the centre as can be done with the eye, and turn the fly wheel, the crank-pin travelling up till the cross-head

arrives flush with line b, as shown, B being the cross-head. Then with the stick resting on the floor or foundation, and against side of fly-wheel, make a mark a, Fig. 27, as shown. Now return the cross-head to the centre, revolving the fly-wheel in an opposite direction to that in which it was first moved, till the pin passes the centre and travels down, and the cross-head has again arrived at line b as shown in Fig. 28, the stick B remaining in the same position, another line, may now be drawn flush with the end of the stick.

The centre between a and c may now be found and drawn, as b. Now, when the line b is brought flush with the end of the stick by turning the wheel, the cross-head will be on the centre. A similar operation for the other centre will also ascertain it.

The length of the eccentric rod is next in order, and should be lengthened or shortened as the case may be, till the valve opens one port about or nearly as far as the other.

While doing this the position of the eccentric makes no difference, and may, in fact, be fastened anywhere.

Having approximated the correct length of the rod, place the engine on the centre (either). The direction in which the engine is intended to run being known, the position of the eccentric must be ascertained. If the engine has a

rocker-shaft, the full part or belly of the eccentric will follow the crank-pin; if there is no rocker-shaft, the full part of the eccentric will lead the crank-pin, being in either case nearly at right angles with the pin. The engine being on the centre, as in Fig. 29, and having no

B

B'

Fig. 29.

rocker-shaft, we will suppose it is to run in the direction of the arrow. As the eccentric in this case leads the pin, it will be in the position of A.

Now adjust it till the required lead appears at a, fasten it, and try the lead when the engine is on the other centre.

Whatever difference there is in the lead, lengthen or shorten the rod till the lead is equally divided on both ends, not moving the eccentric till this is accomplished, and when it is, shift the eccentric to the crank if there is too much lead, and from it, if there is not enough.

The centre of the eccentric b, will then be the lap and lead from the centre line c c.

If the engine was to run in the opposite direc

tion, the eccentric would occupy the position of B. That the position A is the correct one, is evident from inspection, for imagining the pin p to commence and travel in the direction of the arrow, it is evident that the eccentric A will drive the valve from left to right, opening the port a as required.

If we connect the valve-stem to eccentric B, and suppose the pin to still travel in the direction of the arrow, the eccentric would drive the valve from right to left, and close the port a, and it is plain that B is not in the correct position for the direction of the arrow.

However, if the engine is to travel in the opposite direction, it at once becomes evident that B is in the correct position for that direction. In either case, the valve occupies the same position when the pin p is on the centre.

Locomotive engineers find in this fact an explanation for setting a slipped eccentric by throwing the reverse lever in the opposite direction to the slipped eccentric, marking the valve-stem, throwing the lever in the motion for the slipped eccentric, and shifting the eccentric till the mark on the valve-stem reappears in the same relative position as when it was made, as the valve occupies the same position when the engine is on the centre, whether the engine runs forward or backward.

By examining Fig. 30 it will be seen that the