CHAPTER XVII.

THE VALVE-MOTION 199

The Locomotive Slide-valve.--Invention and Application of the Slide-valve.--Description of the Slide-valve.-- Primitive Slide-valve.--Outside Lap.--Some Effects of Lap.--Inside Lap.--The Extent of Lap usually adopted.-- First Application of Lap.--The Allen Valve.--Advantages of the Allen Valve.--Case where the Allen Valve proved its Value.--Inside Clearance.--Lead.--Operation of the Steam in the Cylinders.--Back Pressure in the Cylinders.--Effect of too Much Inside Lap.--Running into a Hill.--Compression.--Definition of an Eccentric.-- Early Application of the Eccentric.--Relative Motion of Piston and Crank, Slide-valve, and Eccentrics.--Attempts to Abolish the Crank.--Valve Movement.--Effect of Lap on the Eccentric’s Position.--Angular Advance of Eccentrics.--Angularity of Connecting-rod.--Effect on the Valve-motion of Connecting-rod Angularity.--Aids to the Study of Valve-motion.--Events of the Piston Stroke.--What Happens Inside the Cylinders when an Engine is Reversed.--Events of the Stroke in Reversed Motion.-- Purpose of Relief-valve on Dry Pipe.--Using Reverse-motion as a Brake.