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 a, and is 

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



