412 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



one atmosphere, instantly, or at least in , great degree, disappears, 

 so that the chamber of the cylinder where it has just been working 

 is itself nearly reduced to a vacuum, and the steam introduced on 

 the other side has then no more to overcome than the resistance 

 of the piston itself. 



The various arrangements invented to conduct the steam in this 

 way, first into the cylinder on either side of the piston, and after- 

 wards into the open air, or into a condenser, for taking away, as 

 soon as done with, its elastic force, constitute what is known as the 

 distribution of the steam; and we will now see what are the principal 

 systems employed for this purpose. 



FIG. 288. Spring piston. FIG. '289. Swedish piston. 



First let us speak of the cylinder, which is the most essential part 

 of the whole of the driving machinery. 



It is commonly (Fig. 290) a cast-iron box, the inside of which, per- 

 fectly cylindrical, has been turned and bored with the greatest care 

 one of the ends is sometimes cast, sometimes firmly bolted on like the 

 other end, so that one of the two at least may be entirely removed, in 

 order to admit of the introduction of the piston. 



One of the ends gives passage to the piston-rod, and the opening 

 which allows this is provided with a stuffing-box, in order that 

 the rod in its movement may not permit any escape of steam from 

 the cylinder. 



The piston itself is constructed in several different ways ; 

 most commonly it is formed of two metal pktes, of a diameter a 



