SECT. VII.] 
STEAM ENGINES. 
205 
in the joint as a hinge. The leather is strengthened by a metal plate on each side, 
the lower one less, and the upper one larger than the valve aperture. It must 
open to an angle of about 30°, to allow a free passage equal to its aperture, and the 
box should be one and a half times the diameter of the valve aperture. 
Its chief application in the steam engine is for the valve between the condenser 
and air pump, called the foot valve, and for the blow valve; but on account of the 
heat of the water, it is necessary to use metal instead of leather, and to grind the 
parts to fit. 
The foot valve G, Fig. 1 and Fig. 4. Plate v. is sometimes suspended by a 
hinge joint to the upper side of the passage, and falls against an inclined seat, the 
inclination being as much as to cause the weight of the valve to close it, and not 
more. 
437. A double clack valve consists of two semicircular valves, and is used 
for pump buckets : the construction of this is similar to the single clack valve, and 
the valves must rise to the same angle. They have the advantage of being more 
convenient than single ones for large pistons. The air pump bucket in a steam 
engine is furnished with metal valves of this species. See Plate v. p, Figs. 1 and 4. 
To afford a greater quantity of passage with less resistance to open the valves, a 
kind of pyramidical valve, consisting of four triangular pieces, is sometimes used ; 
but the construction is complex, and without corresponding advantages. 
438. A flat metal plate has frequently been recommended for a valve, par 
ticularly for a safety valve: it requires a guide sufficient to keep it in its proper 
seat; which may be most effectively done by a spindle sliding in holes in cross 
bars, above and below the valve. The diameter of the box should be to that of the 
valve as 3:2; and the parts should be ground together with emery, till they fit 
steam-tight. Its advantage as a safety valve is supposed to consist in its being less 
liable to stick fast, and with this opinion I perfectly agree : in other respects it 
differs little from the conical valve. 
439. The conical steam valve is a plate of metal, with its edge bevelled to fit 
into a conical seat: it is sometimes called a puppet or T-valve. The steam valves 
of Watt’s engines were at first made of this kind. In this valve the diameter of the 
box should be to the greater diameter of the valve as 3 : 2 ; and it should rise not 
less than one-fourth of its greatest diameter when quite open ; but both these pro 
portions must be increased if the valve be out of the centre of the box. These 
valves and seats are often made of brass ; but gun metal is better, the plate and 
the seat for it being of the same metal. They are turned to fit as nearly as 
possible ; and afterwards the one is ground into the other with fine emery powder, 
till it accurately fits the seat. 
The best angle for the valve to fit in its seat is 45°, for the pressure is ha-