ACTION OF A PISTON. 



fact, is what is accomplished in steam engines as they arc practi- 

 cally worked. 



Tho direct and indirect powers of steam may also be easily 

 combined as well in the ascent as in the descent of the piston, 

 If we suppose the upper part of the tube, instead of being open to 

 the atmosphere, to communicate with a reservoir of water, to 

 which, like the bottom of the tube, a lamp or other source of heat 

 is applied, steam may be admitted above the piston p as well as 

 below it. Now, if such be the case, it is easy to imagine how 

 the piston can be at the same time affected by the direct and indi- 

 rect power of the steam. Thus, if we suppose that a vacuum has 

 been formed above it, by the condensation of steam, admitted from 

 the upper reservoir, while steam produced from the lower reser- 

 voir acts below it, the piston will be forced upwards by the com- 

 bined effect of the direct action of the steam below and the 

 indirect action of the condensed steam above, and when the pistoa 

 has been thus raised, we can imagine that while steam is admitted 

 above it from the upper reservoir, that which is below it may be 

 condensed, in which case it will be forced down by the combined 

 effect of the direct action of the steam above it and the indirect 

 action of the condensed steam below it, and it is evident that 

 such alternate action may be indefinitely continued. 



Such is the effect of the broad principle upon which all engines 

 of the class called condensing, or low-pressure engines, are 

 constructed. In their details there are numerous points of 

 great practical importance and of much interest in a mechanical 

 point of view. These arrangements, however, not affecting the 

 principle of steam, regarded in its most general sense, need not 

 here be further noticed. On a future occasion we shall explain 

 such of them as have the greatest popular interest. 



18. The apparatus by which the combustion of the fuel is 

 effected, and by which the heat evolved is transmitted to the 

 water to be evaporated, are furnaces and boilers of very various 

 forms and construction, according to the circumstances in which 

 they are applied, the one being adapted to the other, so that as 

 much of the heat shall arrive at the water as the circumstances of 

 their application permit. 



19. The quantity of water which would be evaporated, if all the 

 heat evolved in the combustion of a given weight of fuel could be 

 transmitted to the water, is the THEORETICAL EVAPORATING 

 POWER of the fuel, and the quantity of water actually evaporated 

 by it is the PRACTICAL EVAPORATING POWER. 



The theoretical evaporating power varies with the quality of the 

 fuel. A given weight of certain species of coal will evolve in 

 combustion a greater or less quantity of heat than other species. 



205 



