CHAP, vi.] THE STEAM-ENGINE. 413 



little less than that of the cylinder, which are solidly bound together 

 as well as to the rod which passes through tliem. On their circumfer- 

 ence are situated grooves for holding the packing, that is, the part of 

 the .piston whose outside must glide easily, but perfectly air-tight, 

 upon the inner surface of the cylinder, so that the steam cannot pass 

 from one compartment to the other. The packing was formerly made 

 of hanks of hemp, which required often greasing, and even replacing, 

 on account of their rapid wear. For these, metallic packings have been 

 advantageously substituted, formed of portions of a ring pressed out 

 by springs inside, as in Fig. 288 ; and now even to these are pre- 

 ferred Eamsbottom's pistons, in which the body is composed of a 

 single plate, hollowed out for greater lightness, and surrounded by two 

 circles of soft cast-steel, fixed in two grooves round the outside 

 and forming a spring. The surface of these circles presses against 

 the sides of the cylinder, forming an excellent packing, which is 

 very simple, and very little expense to keep in repair. 



The Swedish pistons, Fig. 289, differ in no way from the preceding 

 except in the breadth of the bands, which is greater, and in their 

 composition, which is cast-iron hardened by a little tin. 



II. DISTRIBUTION OF THE STEAM. 



The piston and cylinder being so constructed and arranged, 

 it remains to be seen how the introduction and escape, in one word 

 the distribution of the steam, is effected. 



Consider Fig. 290, which gives a longitudinal section of a cylinder. 

 We see in a, a, near each end, the opening of a double conduit aa, a a', 

 made in the thickness of the side ; these are the openings by which 

 the steam comes alternately and works on one side and then on the 

 other of the piston. These are called the steam-ports. These two 

 open outwards on a well-polished surface, and between the two a 

 third opening E is seen, which serves to let the steam escape when it 

 has done its work, and which is called for that reason the exhaust- 

 port, c is the pipe by which the steam gains access to the open air 

 or to the condenser, where it parts with its elastic force. 



Now, by what contrivance is the distribution effected, consisting, 



