SECT. VII.] 
STEAM ENGINES. 
215 
extremely variable on this point, but the mean appears to be not far distant from 
the rule. For leather on iron the friction is greater, the average approaching to 
one-fifth of the pressure. 1 When the pressure on the piston draws the rod, a 
thickness not more than four-tenths of that which is required when the rod is 
pushed will be sufficient. 
It will be evident enough that the central part of the thickness of the piston 
adds little to the steadiness of its motion, though it increases the friction; hence 
that construction of a piston has the advantage which renders the upper and lower 
part a b tight, without putting a like stress on the intermediate ones at A. 
464. The common position is a double cone of wood (Fig. 19.) having two 
Fig. 19. Fig. 20. 
bands of strong leather fastened round it with nails or hoops. The joints in the 
leather are not seamed, but closed as accurately as possible, and not put opposite 
to one another. 
465. If the parts be made of metal, a cylinder of brass should be turned to 
fit the barrel or the cylinder it is to move in, (Fig. 20.) so that it will slide freely 
without sensible resistance. Then an upper and lower plate, made of sufficient 
thickness for the piston to be of the depth the diameter requires, confines two 
cupped leathers C C, with the edges cut to an angle of about 45°. 
Both these pistons have the advantage of the friction being at the upper and 
lower edges; and bevelling the edges of the leather causes the force of the fluid 
to spread it against the surface of the pump barrel. This mode of bevelling the 
leathers seems to have been first used by Mr. Smeaton for a fire engine bucket, 
and the general principle of construction was first applied in his air pump in 1752. 
Mr. Bramah applied it to the various parts of his presses, and found its advantage 
in the application of high pressures. 
1 Belidor, who seems to have first applied the solid piston, gives the proportions so that the 
thickness is nearly equal to the diameter: the friction, with such a proportion, must be greatly 
increased, as it must be in every part air-tight. His plates in another place show the thickness 
somewhat less than one-third. Architect. Hydraulique, vol. ii. p. 117 and 223.