ARTIFICIAL HEAT. 



301 



the door to regulate the admission of air. In practice the 

 furnace, and especially the ash-pit, should be kept clear of 

 ashes; as by this means, coals of an inferior quality may 

 be burnt with ease. 



The following figure (Fig. 22) represents a longitudinal 

 section of the common garden furnace. It is surrounded 

 by a double wall to prevent the escape of heat. 



Fig. 22. 



Mr. Witty has invented a furnace, which is possessed 

 of valuable qualities. A vertical section of it is given in 



Fig. 23. 



Fig. 23. The fuel is supplied by the door at , and is 

 pressed down the inclined plane towards the grate c, by an 



