SECT. VII.] 
STEAM ENGINES. 
209 
449. The chief object of attention in setting out a, slide, is to shorten its 
motion as much as possible, so as not to reduce the area of the passages. The 
area of the rubbing surface can scarcely be estimated at less than 8 times that of 
the passages, which will be about sVth of the area of the cylinder, (art. 154.) hence 
is-ths = the pressure; and taking the maximum pressure to be double the mean 
pressure, and the friction being supposed ith of the pressure, it will be 2 2 oths of the 
moving force, and it will be, in a short cylinder in action, about ith of the length 
of the stroke ; whence the loss amounts to about eVnd part of the power of the 
engine. In long cylinders the ratio will be less. 
450. The cylindrical slide of metal, like a piston in a tube, was applied by 
Edelcrantz to the safety valve, but such a slide would obviously either be subject 
to stick fast, or allow steam to escape, as it would bear neither wear nor corrosion. 
Woolf’s slide for regulating the quantity of steam passing an aperture is of the 
same kind, and seems to have no useful application whatever. 1 The attempt has 
been made to apply the metallic piston as a slide, and there is no doubt that it 
may be used both for that purpose and for the back of a flat slide: the object 
must be to construct it so as to be tight, and wear equally when applied to a 
cylinder. The advantages of such a slide I have endeavoured to show in Plates iv. 
and vi. 2 
The Box Slide, now much in use, is shown in Plate v. Fig. 6. 
451. Seaward’s slides. A great improvement has been recently made in 
the construction and application of slides, by Mr. Samuel Seaward, who took out 
a patent in 1835. On each of the nosles, or passages to the cylinder, and within 
the steam pipe, is attached a smooth face of cast iron or other metal, having the 
smooth part standing from the cylinder. Upon these faces two flat slide valves 
or shutters are caused to move either conjointly or separately, by means of stalks 
or spills, each being furnished with a knuckle-joint at its upper or lower end, 
where it is attached to the stalk so freely, as to easily move from off the face, 
independent of the spill or stalk. These slide-valves or shutters will effectually 
prevent the steam from entering the cylinder, since the pressure of the steam from 
the boiler will keep them close to the faces upon which they move. For the exit 
1 Philosophical Magazine, vol. xvii. p. 164. 
2 In Fig. 4. Plate iv. I have shown a mode of construction for the piston slide, which -would 
possess some advantages. A ring, cylindrical on the outside and conical in the inside, may he 
cut into two or more parts, with lap joints, and these parts may be expanded by the pressure of 
the steam on a conical part made to fit the interior of the ring; on the opposite side there should 
be a plate ground to fit the surface of the ring, and between this plate and the bottom of the cone 
an elastic packing of hemp should be inserted, and the whole held together by nuts upon the 
piston rod. The steam apertures should be divided so that no single aperture should exceed one- 
eighth of the circumference. 
2 D