THE STEAM ENGINE. 



cylinder are closed by steam-tight covers, but proper openings 

 are provided for the alternate admission and escape of the steam. 



19. The cylinder is made of cast iron of adequate thickness and 

 strength. It is bored with the nicest precision, so that its inner 

 surface is truly cylindrical and of uniform diameter from end to end. 

 The piston is also made of iron, and its contact with the cylinder is 

 rendered steam-tight, either by a packing of hemp and soft rope, 

 called gasket, which fills a circular groove or channel surrounding 

 the piston, or by constructing the external rim of the piston of several 

 metallic segments, which are urged against the side of the cylinder 

 by springs which act upon them from the centre of the piston. 



A section of a packed piston is given in fig. 1 1 . The hollow 



groove containing tho 

 P ac ^ing is represented 

 at the sides next the 

 cylinder, and the top 

 is attached to the piston 

 by screws, by turning 

 which the packing is 

 compressed so as to be 

 I forced outwards against 

 the sides of the cylinder 

 until it is in steam-tight contact with them. 



20. Pistons which maintain steam-tight contact with the 

 cylinder without packing, and which are called metallic pistons, 



are of very various 

 construction, though 

 all of essentially the 

 same principle. One of 

 these is represented in 

 section in fig. 12, and 

 in plan in fig. 13, p. 

 21. A deep groove, 

 ^ square in its section, is 

 m formed around the pis- 

 ton, so that while the top and bottom form circles equal in 

 magnitude to that of the cylinder, the intermediate part of the 

 body forms a circle less than the former by the depth of the 

 groove. Let a ring of brass, cast iron, or cast steel, be made to 

 correspond in magnitude and form with this groove, and let it be 

 divided, as represented in fig. 13, into four segments c c c c, and 

 four corresponding angular pieces, D D D D. Let the groove which 

 surrounds the piston be filled by the four segments with the four 

 wedge-like angular pieces within them, and let the latter be 

 urged against the former by eight spiral springs, as represented 



Fig. 12, 



20 





