Lap of the Slide Valve

—1. In a steam engine, the lap of the slide valve on the © steam side is the distance the outer or steam edge of the valve extends beyond, or laps over the outer or steam edge of the port when the valve is in its central position. It is given for the purpose of causing the engine to work expansively, by cutting off the admi ion of steam before the end of the stroke, and is called outside lap. 2. Also the lap which controls the exhaust and causes the pa age to the condenser to closed before the end of the stroke; the piston is then said to be cushioned by the elasticity of the confined vapor upon which it descends. This is called inside lap, and is the distance the inner or exhaust edge of the valve extends beyond or laps over the exhaust edge of the port when the valve is in its central position. Sometimes the inside lap is made zero or even neyalive.