CHAP. IL] THE ART OF WARMING. 351 



of heating may be as injurious to health as stoves are and for the same 

 reasons. The air they supply is dry, and the carbonic oxide and other 

 deleterious gases may pass through the metallic pipes, and, finally 

 evenness of temperature is far from being secured by this system. 



II. HOT WATEK AND STEAM HEATING APPARATUS- 

 HEATING BY GAS. 



IN these last respects heating by the circulation of hot water is pre- 

 ferable. The temperature of the heated air is in this way more 

 moderate and regular, and it is easy to account for these advantages. 



The hot water boiler is fixed, as in the apparatus already described, 

 in the lowest floor of the building. The fire is applied directly to 

 heat the water of a boiler, C. This water, expanding by the elevation 

 of temperature, rises by reason of its diminution of density through a 

 vertical pipe which leads to the uppermost story. There it commu- 

 nicates its heat to the water of a reservoir or cistern, D, which 

 acts as a kind of stove, since the heat from it radiates throughout the 

 room and warms it. 



A pipe descends from this reservoir to a similar one, F, on the floor 

 immediately below, and so on till the water is returned to whence 

 it started, and having grown cold by this incessant change, is heated 

 again in the boiler, and there recommences the same movement. 



It is easy to perceive that this circulation is continuous, that it 

 begins, so to speak, as soon as ever the fire is lighted, and that it attains 

 its maximum velocity as soon as the maximum difference between 

 the temperature and density of the water in the boiler and in the 

 uppermost reservoir is reached. The pipes which carry the water 

 assist also in warming the rooms. 



The uppermost reservoir is provided with an opening by which 

 the water may be renewed, and from which the air disengaged by the 

 heat or the steam arising from too great an elevation of temperature 

 may escape. 



Bonnemain, an architect of the 18th century, was the inventor of 

 hot water heating, and his plan is still followed without material 

 modifications. The water in this system has usually a temperature 

 less than 100 Cent., and hence such apparatus are called low 

 pressure apparatus. 



