484 



THE STEAM-ENGINE. 



cock, instead of being moved constantly in the same direction, as in the case 

 of the common four- way cock, will require to be moved alternately in opposite 

 directions. 



PISTONS. 



The office of a piston being to divide a cylinder into two compartments by 

 a moveable partition which shall obstruct the passage of any fluid from one 

 compartment to the other, it is evident that the two conditions which such an 

 instrument ought to fulfil are first, that the contact of its sides with the sur- 

 face of the cylinder shall be so close and tight throughout its entire play that 

 no steam or other fluid can pass between them ; secondly, that it shall be so free 

 from friction, notwithstanding this necessary tightness, that it shall not absorb 

 any injurious quantity of the moving power. 



Since, however accurately the surfaces of the piston and cylinder may 

 be constructed, there will always be in practice more or less imperfection of 

 form, it is evident that the contact of the surface of the piston with the cylin- 

 der throughout the stroke can only be maintained by giving to the circumfer- 

 ence of the piston sufficient elasticity to accommodate itself to such inequali- 

 ties of form. The substance, whatever it may be, used for this purpose, and 

 by which the piston is surrounded, is called packing. 



In steam-pistons the material used for packing must be such as is capable 

 of resisting the united effects of heat and moisture. Hence leather and other 

 animal substances are inapplicable. 



The packing used for steam-pistons is therefore of two kinds, vegetable pack- 

 ing, usually hemp, or metallic packing. 



The bottom of the common hemp-packed piston is a circular plate just so 

 much less in diameter than the cylinder as is sufficient to allow its free motion 

 in ascending and descending. A little above its lowest point this plate begins 

 gradually to diminish in thickness, until its diameter is reduced to from one to 

 two inches less than that of the cylinder, leaving therefore around it a hollow 

 space, as represented in fig. 46. The cover of the piston is a plate similarly 



Fig. 46. 



formed, being in like manner gradually reduced in thickness downward, so as 

 to correspond with the lower plate. In the hollow space which thus surrounds 

 the piston a packing of unspun hemp or soft rope, called gasket, is introduced 

 by winding it round the piston so as to render it an even and compact mass. 

 When the space is thus filled up, the top of the piston is attached to the bot- 

 tom by screws. The curved form of the space within which the hempen 

 packing is confined is such that, when the screws are tightened, that part ot 

 the packing which is nearest to the top and bottom of the piston is forced 

 against the cylinder so as to produce upon the two parallel rings as much 

 pressure as is necessary to render it steam-tight. When by use the packing 

 is worn down so as to produce leakage, the cover of the cylinder must be re- 

 moved, and the screws connecting the top and bottom of the piston tightened; 



