SECT, vii.] STEAM ENGINES. 211 



it then, however, becomes so very difficult to fit, that its use has not been con- 

 tinued ; and this difficulty must always exist in a valve with two seats, otherwise 

 it is easy to simplify Belidor's valve. l 



A species of slide to revolve on an axis was designed by Bettancourt for a double 

 engine : such a slide would not, however, keep in order for any length of time, 

 and does not appear to have been used. * 



453. COCKS are so well known as to need no description ; and on a small 

 scale they are certainly the best adapted for opening and closing pipes of any 

 thing that has yet been proposed. They do not answer so well when they are in 

 constant action ; but even then it is doubtful whether or not they are inferior to 

 other methods, and much depends on their being properly constructed. For a 

 single or common cock the plug should be nearly cylindrical where it has to be 

 exposed to much pressure. The common reduction of the diameter is about one- 

 sixth of the length. 



454. For various purposes a double passage cock is useful, and in some cases 

 one with a triple passage may be required ; but the one most commonly applied 

 to the steam passages of steam engines is of the kind called the four-way cock, 

 and is in fact a rotary slide. Of these we have to consider two kinds : the common 

 one, the application of which was suggested by Leupold, art. 12. and applied by 

 Trevithick ; and Bramah's improved one. 



455. A FOUR-WAY COCK, by its motion round its axis, opens a communication 

 alternately from the boiler and condenser, to the top and bottom of the cylinder 

 of a steam engine; Fig. 1. Plate iv. The simplicity of its action in some degree 

 compensates for its friction, but there is the disadvantage of part of the steam 

 being lost in the pipes at each stroke. Its form should be nearly cylindrical, 

 otherwise its friction and tendency to wear unequally will be increased. When 

 it is ground to fit truly, the pressure of the steam tends to keep the surfaces in 

 contact, and to wear the cavity into an elliptical shape ; hence it is soon necessary 

 to grind it to fit again. 



456. The cock applied in this manner does not admit of the steam being cut 

 off at any portion of the stroke without the use of other valves ; but by dividing 

 the spaces, so that the solid part on each side of the aperture by which the steam 

 passes to the condenser is double the aperture, the cock may be moved at twice, 

 so as to cut off the steam at the first movement, and leave the passage to the 

 condenser open till the second. See Fig. 6 and 7. Plate vi. The cock must move 

 back and forward in this case, but it will be obvious that the disposition of the 

 surfaces is such as will prevent the wear being so destructive as it is in the 

 common form. 



1 Architect. Hydraulique, torn. ii. p. 220. 2 Prony, Architect. Hydraulique, torn. i. p. 572. 



