THE STEAM ENGINE. 



Fig. 14. 



the diameter of the piston rod. Above this hole is a cup, in 

 which, around the piston, is placed a stuffing of hemp or tow, 

 which is saturated with oil or melted tallow. This collar of 



hemp is pressed down by another 

 piece, also perforated with a hole 

 through which the piston rod 

 plays, and which is screwed down 

 on the said collar of hemp. 



The piston-rod, by this con- 

 trivance, being moved with the 

 same alternate motion, and tho 

 same force as the piston itself, 

 can be made to impart that fore 3 

 to any suitable piece of mecha- 

 nism outside the cylinder, wit'i 

 which it may be put in connection. 



24. Since the ends of the cylinder are closed by metallic covers, 

 in the manner explained above, the openings for the exit and 

 entrance of the steam at the ends, are placed, not in the covers, 

 but in the sides, at points in immediate contiguity with the 

 covers. These openings are governed by contrivances of various 

 forms, and variously denominated COCKS, VALVES, and SLIDES. 



25. Let two openings be imagined to be provided at each end 

 of the cylinder, one leading from the boiler, and the other for the 

 escape of the steam. Let stop-cocks, or valves, or sliding 

 shutters, be adapted to these openings, so that they can be closed 

 or opened by acting upon the handles of the cock valve or slide, 

 and let these handles be supposed to be put in such connection 

 with the piston-rod that when the piston arrives at either end of 

 the cylinder the handles are driven by the rod, so as to open the 

 passage which admits steam to the end of the cylinder at which 

 the piston has arrived, and to close the passage which is provided 

 for its escape, and, on the contrary, to open the passage for the 

 escape of the steam from the other end of the cylinder, and to 

 close the passage for its admission from the boiler. By this 

 means the piston, being acted upon by the steam at the end at 

 which it has arrived, and, being relieved from the action of the 

 steam on the other side of it, will be driven to the other end of the 

 cylinder where the piston-rod will again act upon the handles of 

 the cocks, valves, or slides, so as to reverse the flow of the steam, 

 allowing that which has just impelled the piston to escape, and 

 introducing steam from the boiler to the end of the cylinder 

 at which the piston has just arrived. In this way the piston will 

 be driven back to the other end of the cylinder, and so on alter- 

 nately from end to end. 



22 





