250 ART OF BUILDING. 



elements of Plane Geometry, in the following manner. 

 Heat moves in straight lines, perpendicular to the hot 

 body or fire, therefore, the heat from the ends of the fire 

 will strike the jambs at the same angle, on one side, as 

 the jambs and back make, on the other side ; the angle 

 will be just 45 degrees, or a mitre. This is the angle with 

 which a ray of heat strikes, and as it is a law of heat 

 that it is reflected, or will fly off, at the same angle with 

 which it strikes, that will be 45 degrees more ; and of 

 course, the angle between the striking ray and the re- 

 flected ray will be 90 degrees, or two mitres ; and as no 

 other angle but 135 degrees will give these proportions, 

 this must be the true flare or angle. A less angle, how- 

 ever, may be used according to the size of the room and 

 the nature of the fire, whether for coal or wood ; for a 

 small room and when coal is to be used, a less angle will 

 answer. The back of the chimney should be brought 

 within four or five inches of the mantel bar, or breast of 

 the fire-place, for the following reasons. 



It is known that the smoke ascends in a chimney in 

 consequence of the air therein being rarefied or warmed, 

 which renders it lighter, and it therefore ascends carrying 

 the smoke with it ; this warm air is replaced by the colder 

 air of the room ; by this means a current is produced, and 

 the smaller the opening (to a certain point) the greater 

 the draught or current. For the same reason the flue 

 should be placed as directly over the fire-place as possible, 

 and should be straight and smooth the whole extent; the 

 flue should be about 14 inches wide. The most common 

 material for fire-places is brick and jambs of sandstone, 

 marble or soap-stone (steatite) is now used extensively ; 

 this is far preferable to sandstone or dark colored marbles, 

 as it is of a light color, takes a tolerable polish, and be- 

 comes hard and durable by exposure to heat. Light 

 colored, polished jambs, are the best, because they absorb 

 less heat and reflect more than dark colored and rough 

 substances. 



It sometimes happens that chimneys constructed in the 

 above manner, will, notwithstanding, smoke, particularly 

 if the top should be overtopped by some other building, 

 as in that case eddies' of air are produced which will 

 drive the smoke down chimney ; in which case the 



