350 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



access to it. The pipe containing the smoke and gas is folded upon 

 itself several times, or is divided into several parts, sometimes hori- 

 zontal (Fig. 241), arid sometimes vertical an arrangement the object 

 of which is the increase of the heating surface so as to utilise as much 

 as possible of the heat obtained from the fuel. We see at once that 

 for this purpose the vertical arrangement is by far the most advan- 

 tageous, since the hot air in rising encounters nothing in its move- 

 ment but the sides of the horizontal pipes, while it remains during the 

 whole of its ascent in contact with the entire surface of the vertical 

 pipes. 



While the smoke and the gases are rising in the chimney, after 

 having given up to the heating chamber a great part of their heat, 



FIG. 241 Hot air heating apparatus. 



the air of the chamber thus heated rises through a central pipe 

 divided into several others. These open severally upon the various 

 floors and rooms of the building by apertures which may be more or 

 less closed at will. The intermixture of this air with that of the room 

 raises its temperature, but contributes nothing to the ventilation, 

 unless this be secured by independent means. The heating effect 

 of these apparatus may vary from 60 to 80 per cent. 



The air, when it reaches the room, is often at too high a temper- 

 ature. In order to obviate this it may be made on leaving the heat- 

 ing chamber to traverse a space to which the cold air has access by 

 apertures provided with regulators, and can be so managed as to bring 

 the mixture to a mean temperature say of 30 or 40 Cent. This method 



