EXPLANATION OF THE PLATES. 379 



it, v v, Are the exhaustion slide facings or back plates, the apertures through which 

 correspond, in like manner, with those of the nozles 3 and 4. 



The four back plates r, s, t, v, are distinct from the casings, and are fixed in the 

 boxes or recesses m, m", n, n of the slide casings, by folding wedges or keys, so that 

 r r, s s have their backs against the cylinder, while 1 1, v v present their faces to the 

 same, their backs being wedged to projections or flanches adapted thereto, in the 

 recesses n, n". 



To insure perfect joints, a plain caulking space is left at the bottom and back of 

 each slide-face, while the edges, which fit the casing at the face, have dovetail 

 portions removed so as to permit the insertion of soft or elastic packing, which, 

 with that beneath and at the back, completely surrounds the aperture and renders 

 all tight. 



Facility is afforded of removing either back plate when requisite, by driving back 

 the folding wedges or keys, which keep them firmly in place, this dovetail form of 

 the edge permitting the facings to be readily detached from the packing after being 

 pressed by the wedges. 



w, #,y, z, Are four slide valves, of rectangular figure, receiving their motion from 

 rods or spills passing through the stuffing boxes 5, 6, 7, 8. 



The valves w, x are the steam slides, having their backs always presented to the 

 steam from the boiler, which maintains them in close contact with the faces of the 

 back plates r r, s s. 



The valves w, x, it will be seen, are connected to their rods or spills by knuckle 

 joints, so as to permit perfect freedom of motion off their facings, independently of 

 the rods. By this contrivance, water carried over from the boilers into the cylinder, 

 is forced back by the piston into the steam pipe ti, till drawn off by cocks fitted at 

 the bottom for such purpose ; thus obviating the risk of accident from its accumulation, 

 and dispensing at the same time with the necessity for escape or discharge valves. 



The backs of the exhaustion valves y, 2, being alternately submitted to the pres- 

 sure of the steam on the descent and ascent of the piston, while their faces are pre- 

 sented to the condenser, it will be observed that perfect contact of the slides with 

 their back plates 1 1, v v will ensue. 



Thus the four slides w, x, y, z are caused to operate without packing, and with- 

 out being submitted to greater friction than that due to the simple pressure of the 

 steam. 



