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. 
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 ata, and is 
pressed down the inclined plane towards the grate c, by an 
