SECT. VII.] 



STEAM ENGINES. 



213 



will remain nearly the same, and the cone will be more equally pressed into its 

 socket. 



459. DOUBLE-PASSAGED COCKS. By far the most simple method in practice 

 would be to use two double-passaged cocks : the apparent simplicity of one cock 

 involves more trouble and care, and after all is not so good as two small ones. 

 Two are as easily moved as one when the movements are simultaneous, and more 

 conveniently managed where the steam is to be cut off. 



460. PLATE OR FLAT VALVES. The general nature of these valves may readily 

 be conceived by imagining two flat plates to be ground to fit one another, and 

 one to turn on an axis passing through the other plate ; the plates being both 

 pierced with apertures which coincide in one position of the moveable plate, and 

 are all closed in another position. Valves of this kind, made of hard steel, were 

 resorted to by Perkins for high pressure steam. When accurately made and 

 applied, so that the pressure is tolerably equal on the moving plate, they might 

 be useful. They admit of reducing the quantity of motion to open them in a 

 considerable degree, but not without dividing the passage into small apertures. 



461. REGULATOR. The steam valve is called a regulator in the atmospheric 

 engine ; it is a kind of rotary plate valve, but it is formed wholly on one side of 

 the axis, and hence is more difficult to make work air-tight. Its construction, 

 as designed by Smeaton, is shown in the annexed figure, 



FIG. 17. 



where A B is the under side of the aperture, and C D a section, with the plate P 

 which covers it, and which is turned by a handle applied at E. 



OF PISTONS. 



462. The great desideratum in a piston is, that it should admit of no leakage, 

 and have as little friction as is consistent with this indispensable quality. 



