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 Ath of the area of the cylinder, (art. 154.) hence 

 &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 Aths 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 ^Vnd 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. 



4 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 be 

 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. 



2o 



