421. Q.—­Will you explain the means that are adopted to regulate the admission of steam to the cylinders?

A.—­In locomotives, the admission of the steam from the boiler to the cylinders is regulated by a valve called the regulator, which is generally placed immediately above the internal fire box, and is connected with two copper pipes;—­one conducting steam from the highest point of the dome down to it, and the other conducting the steam that has passed through it along the boiler to the upper part of the smoke box.  Regulators may be divided into two sorts, viz., those with, sliding valves and steam ports, and those with conical valves and seats, of which the latter kind are the best.  The former kind have for the most part consisted of a circular valve and face, with radial apertures, the valve resembling the outstretched wings of a butterfly, and being made to revolve on its central pivot by connecting links between its outer edges, or by its central spindle.  In some of Stephenson’s engines the regulator consists of a slide valve covering a port on the top of the valve chests.  A rod passes from this valve through the smoke box below the boiler, and by means of a lever parallel to the starting lever, is brought up to the engineer’s reach.  Cocks were at first used as regulators, but were given up, as they were found liable to stick fast.  A gridiron slide valve has been used by Stephenson, which consists of a perforated square moving upon a face with an equal number of holes.  This plan of a valve gives, with a small movement, a large area of opening.  In Bury’s engines a sort of conical plug is used, which is withdrawn by turning the handle in front of the fire box:  a spiral grove of a very large pitch is made in the valve spindle, in which fits a pin fixed to the boiler, and by turning the spindle an end motion is given to it, which either shuts or opens the steam passage according to the direction in which it is turned.  The best regulator would probably be a valve of the equilibrium description, such as is used in the Cornish engine:  there would be no friction in such a regulator, and it could be opened or shut with a small amount of force.  Such valves, indeed, are now sometimes employed for regulators in locomotives.

CHAPTER VIII.

CONSTRUCTIVE DETAILS OF ENGINES.

PUMPING ENGINES.

422. Q.—­Will you explain the course of procedure in the erection of a pumping engine, such as Boulton and Watt introduced into Cornwall?