342 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



at pleasure in front of the fire, and are advantageous substitutes 

 for bellows. 



The warming of an apartment by means of a fireplace is accom- 

 plished not only by radiation, but also by convection : that is to say, 



FIG. 234. An ordinary modern fireplace. 



by the transport of warmed particles of air, which rise towards the 

 ceiling, and are replaced by the cold air, which is constantly drawn 

 in from outside, In this way there arises a natural ventilation, 

 generally more than sufficient from a hygienic point of view ; but 



FIG. 235. Modern fireplace with movable blower; 



another result is that the fresh air, being cold, has to be perpetually 

 heated by the fire, the warmth of which is thus disadvantageously 

 used. Moreover, since the chimney contains a great quantity of 



