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 bodily in a direction parallel to the stroke of the piston the point of cut-off of the steam would be the same at both ends of the cylinder, but owing to the sloping position which the rod necessarily occupies, the point of cut-off is later on the “out stroke,” when the piston is moving towards the crank axle, than on the “instroke,” when it is moving away from the axle. For example, the “valve card” of a certain locomotive shows that when in “full gear” the piston moved 18¾ ins. on the out stroke (the total stroke being 24 ins.) before steam was cut-off, but on the return or inward stroke the cut-off occurred when the piston had travelled only 16¼ ins.

Systems of Valve-Setting. It is possible to set the valves on three different systems, each being dependent upon one of the three variables which affect the steam distribution. These are— 1. Equalizing the lead at the front and back. 2. Equalizing the port opening at front and back. 3. Equalizing the point of cut-off at the front and back. The third of these, which involves unequal laps, is rarely adopted in locomotives and need not be further considered. Of the two others the system of equalizing the leads is more usual, as this gives a more nearly equal compression of the steam at the two ends of the stroke.

Bumping Marks and Clearance. If the piston were allowed to travel far enough at each end of