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