434 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



available elastic force the energy contained in the fuel, such as coal. 

 Heat is the agent of this transformation; it passes from the fire to 

 the substance which forms the heating surface of the boiler, and is 

 communicated from the iron to the water, the temperature of 

 which it elevates, causing and maintaining ebullition, and continu- 

 ously supplying the steam space with a gaseous and elastic mass at 

 a pressure necessary for the work to be done. The fire, the grate, the 

 ash-pit, the Hues and chimney, the heaters and body of the boiler, 

 valves and safety apparatus, pressure gauges, and water and steam 

 gauges such is the generator of the engine with its accessories. 



Secondly, the steam being produced, the engine properly so called, 

 is composed oF machinery for motion, of receivers of the energy, and of 

 apparatus for its distribution, having for their object, the production 

 of alternating rectilinear motion. The cylinder, the steam chest, the 

 slide valve, the condenser, are the principal structures in this part of 

 the engine. It forms the driving machinery. 



Lastly, the motion once produced under its first form, it is necessary 

 to transform it, and render it fit for the work which is required, and 

 this is most often in the form of a continuous circular motion. The 

 connecting rods, cranks, beams, slides, are the pieces ordinarily 

 employed in this part of the engine to which the name of transmitting 

 machinery is applied, The fly-wheel and the governors have a 

 special object, that of keeping the velocity of working within definite 

 limits. 



These different functions being well understood, and the apparatus 

 connected with them being clearly conceived, at least in their 

 principal arrangements, we can next proceed to the examination of 

 the different types of engines which have been invented since 

 the first use of steam, and which are now used in great numbers in 

 manufacturing industries, on railways and in steamships, and even 

 in agriculture. 



Before reviewing these types, and showing the steam at work in 

 the many services it renders to civilization, we must be allowed, not 

 a digression, because it concerns something essential, a short ex- 

 planation of certain terms and expressions frequently employed in 

 speaking of engines and estimating their power. 



The power of an engine does not depend only on the pressure per 

 square inch of the steam which moves it. This is merely an element. 



