Polytechnic Association Proceedings. ggg 



the exhaust pipe. The steam passes through ports e' iu each end 

 of the valve into the induction ports of the cylinder, alternately as 

 they are opened by the motion of the valve, derived from an 

 eccentric in the usual manner. On the back of the valve, at either 

 end, is a slide, G, which can be made to cover the port at that end, 

 and these slides are each attached to one end of a piston, H, fitting 

 iu a small steam cylinder, bolted to the back of the valve, and so ad- 

 justed that when the port in one end of the valve is closed the other 

 is open. Upon steam being admitted to either end of the piston, H, 

 the piston is shot over, and the corresponding slide closed, to cut 

 off steam from that end of the main cylinder, while the port at the 

 other end of the main valve is opened, ready to admit steam to the 

 other side of the main piston when the valve shall arrive at the 

 proper position. 



It will be obseiTed that the cut-off slides, G, are always balanced 

 when moved. The one about to close having steam of equal pres- 

 sure upon each side, while the other one has been balanced by the 

 main valve riding past the end of the valve face on the cylinder, 

 thus admitting steam behind the slide, G. This condition obtains 

 duriug the whole stroke of the piston until the steam is cut off, 

 after which the cut-off slides, G, remain stationary relatively to 

 the main valve, until ready to cut off steam on the return stroke, 

 previously to which time they have been balanced by the over- 

 riding of the valve at the other end. These slides have, therefore, 

 literally no wear, and once fitted tight, they will remain so indefi- 

 nitely. The piston, H, in the small cylinder, is turned to fit, and 

 has no packing, neither have the rods stufiing boxes, as the pressure 

 is equal on both sides, except during the inappreciable time which 

 intervenes between the exhausting of the cylinder, I, and the 

 movement of the piston. The only tendency to wear in these 

 parts is due to the weight of the piston and rods, which is sup- 

 ported on large surfaces. In fact, after twenty months constant 

 use, none of these parts have worn sufficiently to obliterate the 

 tool marks upon the surfaces. 



Steam is admitted alternately to each end of the piston, H, at 

 every revolution of the engine, causing the cut-off slides to move 

 at every stroke, cutting off the steam at the point determined by 

 the governor. 



Fig, 2 shows a cross section of the cylinder, I, and its valve. 

 This valve is balanced by the plate, J, upon its back, and is operated 

 by a toe upon the rock shaft, L, carried upon the main valve, and 



[Inst.] 44 



