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 ba- 



