The origin of the riding cut off is most generally attributed to Gonzenbach.  His arrangement had two steam chests, the lower one provided with the ordinary slide valve of late cut off, and steam was cut off from this steam chest by the expansion valve covering the ports connecting with the upper steam chest.  This had the old disadvantage that all the steam in the lower chest expanded with that in the cylinder, at a consequent considerable loss.  This was further improved by causing the riding cut off to be upon the top of the main valve, instead of its chest, and resulted in a considerable reduction of the clearance space.

This is the simplest form, and is shown in Fig. 1.  The steam is supplied by a passage through the main valve which operates exactly as an ordinary slide valve would.  That is, the inside edges of the steam passage are the same as the ordinary valve, the additional piece on each end, if I may so term it, being merely to provide a passage for the steam which can be closed, instead of allowing the steam to pass the edge.  The eccentric of the main valve is fastened to the shaft to give the proper amount of lead, and the desired release and compression, and the expansion valve is operated by a separate eccentric fastened in line with or 180 deg. ahead of the crank.  When the piston, therefore, commences to move from the crank end to open the port, D, the expansion valve is forced by its eccentric in the opposite direction, and is closing the steam port and would have closed it before the piston reached quarter stroke, thus allowing the steam then in the cylinder to do work by expansion.  The eccentric operating this expansion valve may be set to close this steam port at any point in the stroke that is desired, the closing occurring when the expansion valve has covered the steam port.  Continuing the movements of the valves, the two would move together until one or the other reached its dead center, when the movements would be in opposite directions.

[Illustration:  Fig. 1.]

There are three ways of effecting the cut off in such engines, the main valve meanwhile being undisturbed, its eccentric fastened securely so as not to disturb the points of lead, release, and compression.  All that is required is to cause the edge of the expansion valve to cover the steam port earlier in the stroke, and this can be done, first, by increasing the angular advance of the cut off eccentric; second, by adding lap to the cut off valve; and third by changing the throw of the eccentric.  In all these instances the riding valve is caused to reach the edge of the steam port earlier in the stroke.  We will take first, as the simplest, those methods by which the lap of the cut off valve is increased.