176 



ON THE ECONOMICAL PRODUCTION OF 



Now, estimating the heat generated by the combustion of a 

 grain of coal at l°-634 per lb. of water, its absolute mechanical 

 value will amount to 1261-45 foot-pounds; hence, according 

 to Professor Thomson's formula, the work performed by any 

 perfect thermo-dynamic engine will, for each grain of coal 

 consumed, be represented by the equation, 



a 

 which applies, as before intimated, not only to air engines, 

 but also to those steam engines in which the principle of ex- 

 pansion is carried to the utmost extent, providing always that 

 no waste of power is allowed to take place in friction, and 

 that the entire heat of combustion of the coal is conveyed to 

 the boiler or air receiver. 



Professor Thomson was the first to point out the great 

 advantages to be anticipated from the air-engine, in con- 

 sequence of the extensive range of temperature which it may 

 be made to possess ; and in a paper communicated to the 

 Royal Society soon afterwards, I described a very simple 

 engine which fulfils the criterion of perfection according to 

 Professor Thomson's formula. This engine consists of three 

 parts, viz., a condensing air pump, a receiver, and an ex- 

 pansion cylinder; the pump forces atmospheric air into the 

 receiver, in the receiver its elasticity is increased by the 

 application of heat, and then the air enters the expansion 

 cylinder, of which the volume is to that of the pump as the 

 absolute temperature of the air in the receiver is to that of the 

 air entering it. The cylinder is furnished with expansion 

 gear to shut oflf the air, when the same quantity has been 

 expelled from the receiver as was forced into it by one stroke 

 of the pump. By this disposition the air is expelled from the 

 expansion cylinder at the atmospheric pressure, and at the 

 absolute temperature corresponding with b in Professor 

 Thomson's formula. 



As an example of tho above kind of air-engine, I will take 



