42 CONSTRUCTION AND MANAGEMENT OF SMALL GASWORKS. 



of T ^ inch at the base. A larger chimney, say 2 fest square 

 in section, will not give half that pull, because eddies take 

 place in the corners, and check or interfere with the 

 draught. These may be prevented by building in a round 

 drain pipe or piece of broken retort at the base, and 

 another at the top. One of the best chimneys I have 

 met with was 35 feet high, and 18 inches square in 

 section, and had the corners filled up for a space of 

 about 3 feet high at the base, and an 1 8-inch drain 

 pipe built in at the top. 



Sometimes the fire-brick lining is continued only up to 

 half the height, but this is a mistake, because fire-brick is a 

 better non-conductor of heat than red brick. The unlined 

 portion of the chimney is of very little value as a means of 

 increasing the draught, because the products of com- 

 bustion are cooled or reduced in temperature. A chimney 

 of absolutely non-conducting material, that enables the 

 products of combustion to maintain their temperature till 

 they escape at the top, will give the best results. In one 

 case, an unlined chimney cracked at the top, and 10 feet 

 had to be taken off; and the shortened chimney actually 

 gave a rather better pull. 



It is an advantage to weather the top of the chimney, not 

 to leave it flat or horizontal, say at an angle of 30* from 

 the inside edge outwards. If the chimney is 18 inches 

 square, an iron cap with a round hole 18 inches in diameter, 

 and sloping downwards at 30 towards the outer edge, may 

 be provided. 



A separate and independent outlet to each setting is pre- 

 ferable, for several reasons, and the cost need not exceed that 

 of a single chimney and main flue. Each setting can be 

 worked independently and under unvariable conditions, and 

 the chimney can be bodily taken down, if desired, without 



