THE STEAM-ENGINK. 493 



THE STEAM-ENGINE, 



(FOURTH LECTURE.) 



THE machinery which has been explained, consisting of the cylinder with 

 its passages and valves, the piston-rod, parallel motion, beam, connecting-rod 

 and crank, together with the condenser, air-pump, and other appendages, having 

 no source of moving power in themselves, must be regarded as mere instru- 

 ments by which the mechanical effect developed by the furnace and the boiler 

 is transmitted to the working point and so modified as to be adapted to the 

 uses to which the machine is applied. The boiler is at once a magazine in 

 which the moving power is stored in sufficient quantity to supply the demands 

 of the engine and an apparatus in which that power is fabricated. The me- 

 chanical effect evolved in the conversion of water into steam by heat, is the 

 process by which the power of the steam-engine is produced, and space is 

 provided in the boiler, capacious enough to contain as much steam as is neces- 

 sary for the engine, besides a sufficient quantity of water to continue that supply 

 undiminished, notwithstanding the constant drafts made upon it by the cylin- 

 der : even the water itself, from the evaporation of which the mechanical 

 power is produced, ought to be regarded as an instrument by which the effect 

 of the heat of the combustible is rendered mechanically efficient, inasmuch as 

 the same heat, applied not only to other liquids but even to solids, would like- 

 wise be productive of mechanical effects. The boiler and its furnace are 

 therefore parts of the steam-engine, the construction and operation of which 

 are entitled to especial attention. fc 



COAL, the combustible almost universally used in steam-engines, is a sub- 

 stance, the principal constituents of which are carbon and hydrogen, occasional- 

 ly mixed with sulphur in a small proportion, and earthy incombustible matter. 

 In different sorts of coal the proportions of these constituents vary, but in coal 

 of good quality about three quarters of the whole weight of the combustible is 

 carbon. 



When carbon is heated to a temperature of about 700 in an atmosphere of 



