CHIMNEYS AND FIRE-PLACES 51 



by corbelling, so as to produce a gradually diminishing passage from 

 the dimensions of the fire-place down to those of the flue. The centre 

 of the flue, immediately above this diminishing part, should be vertically 

 above the centre of the fire-place. 



Chimney Shaft. — When the breast reaches the ceiling of the highest 

 room, it is reduced in cross- sectional dimensions until it is only wide 

 enough to contain the flues. This reduced part is known as the chimney- 

 shaft. Where the chimney-shaft passes through the roof- covering, it 

 will require to be "flashed" (see Chap. VI., p. 64, Figs. 65 and 66). 



Chimney-pots or Cans. — Chimney-pots of earthenware are often placed 

 over flues to prevent the formation of wind eddies, which would be 

 caused by the flat surface of the top of the chimney shaft. The top of 

 the chimney cans should not be lower than the highest part of the build- 

 ing, in order to prevent down-draughts. 



Precautions against Fire. — No wooden plugs or slips should be built 

 into walls near flues. It should also be remembered that iron brackets 

 or spikes, driven into the walls of breasts, may conduct sufficient heat 

 to the woodwork which they support, to ignite it. 



A suitable method of supporting a mantel-piece is shown in Fig. 214. 

 Any nails, or supporting spikes, are driven into the thick brickwork 

 below the mantel-piece, care being taken that no metal comes near to 

 the interior of the flue. 



Building the Flues, etc. — Chimney breasts, even in stone buildings, are 

 generally built of good hard bricks in preference to stone, which is 

 a difficult material to work with for this purpose, and is generally less 

 satisfactory than brick. In the case of a stone building, however, that 

 part of the chimney shaft which is above the roof covering may be built 

 of stone for the sake of appearance. 



The external walls of chimney breasts should be 9 inches thick, 

 rather than 4-^ inches. The provision of 9-inch external walls within the 

 building reduces the risk of woodwork near the breast becoming ignited. 

 It is also advantageous to have a 9-inch wall round the chimney shaft, 

 because that part which is above the roof-covering is exposed to the 

 weather, and a 4^-inch wall is apt to be breached, ?n which event cold 

 air is admitted to the flue causing it to smoke. 



Notwithstanding the above disadvantages the external walls of both 

 breast and chimney shaft are, in ordinary buildings, usually made only 

 4£ inches thick, for the sake of eoonomy. In this case, the part above 



