It will be noted that there is but one edge of this valve that is required to do any work, and that is to close the valve.  The eccentrics are so placed that the passage in the main valve is opened long before the main valve itself is ready to admit steam to the cylinder, so that only the outer edges are the ones to be considered, and it will be readily seen that the two valves traveling in opposite directions, any lap added to the working edge of the cut off valve will cause it to reach the edge and therefore close the port earlier than it would if there was less lap.  And we might carry it to the extreme that we could add lap enough that the steam passage would not be opened at all.

In Fig. 2 is shown the method by which this is accomplished, in what is called Meyer’s valve, and such as is used in the Kendall & Roberts engine.  We have only one point to look after, the cut off, so we can add all the lap we wish without disturbing anything else.  In this engine the lap is changed by hand by means of a little hand wheel on a stem that extends out of the rear of the steam chest.  The valve is in two sections, and when it is desired to cut off earlier, the hand wheel is turned in such a direction that the right and left hand screws controlling the cut off valve move one valve portion back and the other forward, which would, if they were one valve and they should be so considered, have the effect of lengthening them, or adding lap to them.  The result would be that the riding valve would reach the edge of the steam port earlier in the stroke, bringing about an earlier cut off.  If the cut off is desired to be later, the hand wheel is so turned that the right and left hand screws will bring the valve sections nearer together, thus practically taking off lap.  Now this may be done by hand or it may be done by the action of a governor.

[Illustration:  Fig. 2.]

In the latter case the governor at each change of load turns the right and left hand screws to add or take away lap, as the load demands an earlier or later cut off; in other cases the governor moves a rack in mesh with a gear by which the valve sections are brought closer together or are separated.  The difficulty with the case where the hand wheel is turned by hand is that the cut off is fixed where you leave it, and governing can only be at the throttle.  For this reason anywhere near full boiler pressure would not be obtained in the cylinder of the engine.  If the load was a constant one, and the cut off could be fixed at about one-third, causing the throttle to open its widest, very good results would be obtained, but there is no margin left for governing.