ARTIFICIAL HEAT. 315 



nace. The bricks in the side walls are placed on their 

 edges, and the top covering is of tile an inch and a half 

 in thickness. In districts where sandstone flag abounds, 

 the covers are often formed of that material. Horti- 

 cultural writers have recommended that flues should 

 be about eighteen inches deep, and of nearly equal 

 breadth ; but to obtain the greatest quantity of heat, it 

 clearly appears, from the experiments of Mr. Stevenson 

 {Cal. Hort. Mem., i. 143), that, where possible, the 

 breadth should be nearly double the depth. It is ad- 

 vantageous to detach flues as much as possible from the 

 walls of the building which encloses them, in order that 

 the heat may be communicated to the air only. For- 

 merly they were often built, sometimes one above an- 

 other, with only one side exposed, a practice which, as 

 it occasioned great waste of heat from conduction, has 

 been generally abandoned. When it is necessary to 

 lead one flue above another, or to make it return upon 

 itself, spaces should be left between them, to allow the 

 free passage of caloric from every side. 



AYith a view to economy' qf fuel, can-flues and cast- 

 iron cylinders have been proposed, and occasionally 

 adopted, but their use has not hitherto become general. 

 The arrangement of the flues must depeUd upon the 

 nature of the house ; it may, however, be remarked 

 generally, that, as heated air has a tendency to ascend, 

 they should be placed as near as can conveniently be 

 done to the front of the house, where, of course, the 

 sloping roof is lowest. It is likewise important that 

 the flue should be introduced, and exert its greatest 

 influence, at that part of the structure which is most 

 exposed to any refrigerating cause. 



The furnace is most properly situate behind the house. 



