THE STEAM-ENGINE. 475 



be such that the aperture produced by it shall be equal to the aperture which 

 it stops. This will be effected if the angle through which it rises be about 30. 

 The valve by which the air and water collected in the bottom of the air- 

 pump are admitted to pass through the air-pump piston is a double clack, con- 

 sisting of two semicircular plates, having the hinges on the diameters of these 

 semicircles, as represented in fig. 27. 



Fig. 27. 



Of the valves which are opened by a motion perpendicular to their seat, the 

 most simple is a flat metallic plate, made larger than the orifice which it is 

 intended to stop, and ground so as to rest in steam-tight contact with the sur- 

 face surrounding the aperture. Such a valve is usually guided in its perpen- ' 

 dicular motion by a spindle passing through its centre, and sliding in holes 

 made in cross bars extending above and below the seat of the valve. 



The conical steam-valves, which have been already described, usually called 

 spindle-valves, are the most common of this class. The best angle to be given 

 to the conical seat is found in practice to be 45. With a less inclination the 

 valve has a tendency to be fastened in its seat, and a greater inclination would 

 cause the top of the valve to occupy unnecessary space in the valve-box. The 

 area, or transverse section of the valve-box, should be rather more than double 

 the magnitude of the upper surface of the valve, in order to allow a sufficiently 

 free passage for the steam, and the play of the valve should be such as to allow 

 it to rise from its seat to a height not less than one fourth of the diameter of its 

 upper surface. 



The valves coming under this class are sometimes formed as spheres or 

 hemispheres resting in a conical seat, and in such cases they are generally 

 closed by their own weight, and opened by the pressure of the fluid which 

 passes through them. 



One of the advantages attending the use of slides, compared with the other 

 form of valves, is the simplicity with which the same slide may be made to 

 govern several passages, so that a single motion with a slide may perform the 

 office of two or more motions imparted to independent valves. 



In most modern engines the passage of the steam to and from the cylinder 

 is governed by slides of various forms, some of which we> shall now explain. 



In figs. 28 and 29, is represented a slide-valve contrived by Mr. Murray of 

 Leeds. A B is a steam-tight case attached to the side of the cylinder ; E F 

 is a rod, which receives an alternate motion, upward and downward, from the 

 eccentric, or from whatever other part of the engine is intended to move the 

 slide. This rod, passing through a stuffing-box, moves the slide G upward 

 and downward. S is the mouth of the steam-pipe coming from the boiler ; T 

 is the mouth of a tube or pipe leading to the condenser ; H is a passage ' 

 leading to the top, and I to the bottom, of the cylinder. In the position of the \ 

 slide represented in fig. 28, the steam coming from the boiler through S passes ( 

 through the space H to the top of the cylinder, while the steam from the bottom 

 of the cylinder passes through the space I into the tube T, and goes to the 

 condenser. When the rod E F is raised to the position represented in fig. 29, 

 then the passage H is thrown into communication with the tube T, while the 



