DOMESTIC ECONOMY OF FUEL. i 99 



keep a steam engine in an ordinary house to pump in the warmed air. 

 These open fireplaces would then, however, be wasting the spare heat 

 which each fireplace sends up its own chimney; but, on the other 

 hand, very much smaller fires would be needed, to keep the rooms 

 warm, than when the rooms are not supplied with fresh warmed air. 

 Theoretically, however, it can be shown that if we. are prepared to 

 give up open fireplaces, and arrange our houses on the plan of having 

 flues which would draw off the air from near the floors of our rooms, 

 and which would also warm fresh air, heated from a central fire, to be 

 constantly admitted near the ceilings, and if the climate were such as 

 to make us desire to have the system in continuous operation, such a 

 system would probably be by far more economical of fuel than open 

 fireplaces, because the fuel used could then be made to do its full duty. 

 The variations of our climate and the low price of fuel, which have 

 hitherto prevailed, have prevented such systematic arrangements from 

 being adopted in this country. 



The plan of carrying the heat from the fire to the air to be warmed 

 by means of hot-water pipes affords also a very economical method of 

 warming air, because the best-constructed hot-water apparatus will 

 enable the full heating value to be got out of the fuel. Fuel may be 

 consumed to far greater advantage in a close furnace than in any open 

 grate, because the admission of air for the combustion of the fuel can 

 be regulated to any required extent. The heating surface of the boiler 

 may also be so arranged as to absorb a very large proportion of the 

 heat generated by the fire. 



But in deciding on the amount of heat in hot-water pipes which is 

 most favorable to economy, the following considerations occur : At 

 least twice the quantity of air which is strictly necessary by theory 

 passes through the fire in the best-constructed furnaces. In an ordi- 

 nary grate this consumption is enormously increased. Each part of 

 oxygen supplied by the air and necessary for combustion is accom- 

 panied by four parts of nitrogen, which is of no value for combustion. 

 Consequently, if twice as much oxygen passes through the fire as is 

 strictly necessary, we have one part which combines with carbon and 

 produces combustion, and nine parts which, being inert, must act, in 

 the first place, to lower the temperature of the fire, and, secondly, to 

 carry a larger amount of unutilized heat up the chimney. Moreover, 

 when water is heated sufficiently to generate steam, each particle of 

 water converted into steam absorbs or makes latent 960 Fahr. of tem- 

 perature. In experiments on the evaporation of water, the tempera- 

 ture of the gases passing off in the chimney was ascertained to vary 

 from 430 to 530, diminishing to 415 at the top of a flue 35 feet high, 

 with the dampers open ; and about 380 at the bottom of the flue with 

 the dampers closed. With a boiler of which the temperature of the 

 water is maintained at 200 without evaporation, the temperature of 

 the flue need not exceed from 230 to 240. 



