396 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



modern steam-engine. An alternate rectilinear motion is obtained by 

 the action of the elastic force of steam in a completely closed cylinder ; 

 which action ceases immediately that the steam is condensed by cooling. 

 The motion being obtained, all that is required more is to apply it to 

 a useful purpose by transforming in a thousand ways, according to the 

 requirements of the manufacturer, or the use to which it is to be put ; 

 whether, for example, it is required for great power or great speed, or 

 speed and power combined. The machinery which carries out this 

 transformation is a third element which we must study in order to 

 complete the description of the steam-engine, which thus includes 



The steam generator or boiler. 



The driving and distributing machinery or prime mover. 



The machinery for transmission. 

 We will now study in detail each of these parts of the engine. 



III. -THE BOILER, OR STEAM GENERATOR. 



The forms of boilers now adopted are so numerous that we cannot 

 attempt even to enumerate them all ; it will be amply sufficient for 

 present purposes to explain in what the principal systems resemble 

 each other and in what they differ. But before pointing out this we 

 must describe rather more particularly an example of one of them. 

 We will take the boiler most commonly adopted in manufactories 

 where stationary engines are employed, that is, engines erected 

 and fixed in the place where they are to work. Fig. 274 gives an 

 exterior view of one. We will explain the interior arrangements. 



On the upper part of the brickwork rests a large wrought-iron 

 vessel of a cylindrical form throughout its whole length, and having 

 a hemispherical termination at either end. This is the body of the 

 boiler, the chamber which contains the greater part of the water to 

 be vaporized. Figs. 275 and 276 the one a transverse section, the 

 other a longitudinal one show it at c. 



Below the principal body are two, sometimes three, long cylin- 

 drical tubes B B, which communicate with it by short tubes. 



These heaters, completely filled with water, are directly exposed to 

 the furnace, whose flames play upon their outer surface, and it is 



