316 FORCING GARDEN. 



and is generally covered by a shed. For the most 

 part it is constructed so that the upper part of its arch 

 shall be on a level with the top of the flue ; but where 

 a considerable heat is required, as in pine-apple stoves, 

 it is found preferable to sink the furnace, in order to 

 produce a nech or rise of about a foot and a half in 

 height, which moderates the intensity of the heat on its 

 first entrance, and, by increasing the draught, causes 

 the fire to burn freely. The size of the furnace must be 

 regulated by the kind of fuel employed. Where coke 

 or charcoal is used, it may be about eighteen inches 

 square ; but where small coal, turf, or peat is to be 

 burned, it should be two feet, or even two and a half, 

 square, by two feet in height. A la'rge furnace insures 

 the long continuance of the fire, a fact which in practice 

 has received too little attention. To resist the effects 

 of heat, the interior -should be lined with- fire-brick. 

 The roof should be strongly arched^ The door may be 

 about a foot square, and when it is double,^ as it ought 

 always to be, the outer half should ha a little larger 

 than the inner. The grate is of- the same breadth as 

 the door, and may extend'about two-thirds of the length 

 of the furnace. The ash-pit is equally wide, and from 

 fifteen to eighteen inches deep ; it is furnished with a 

 ventilator in 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 mean?, 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 pos- 

 sessed of valuable qualities. A vertical section of it 



