BRICKWORK. 



103 



flues might be used to adjust the draught ; but this would be relying too 

 much upon the workmen. It often happens, if there is no check, that during 

 the night the heat of the ovens will be neglected, and suffered to fall below the 

 proper temperature ; and then, to make up, the dampers are opened and the 

 furnaces forced, to the deterioration of the retorts, the waste of fuel, and the 

 production of inferior gas. When there is an air-opening into the shaft this 

 cannot be done, and there will therefore be less danger from such care- 

 lessness. 



With this precaution, the chimney may with advantage be built seventy 

 feet high even for a small number of retorts ; but the height of the shaft must 

 always be regulated by the description of property surrounding or in the 

 vicinity of the works. 



The foundation of a structure bearing a great weight upon a small surface 

 must be carefully attended to. If a good natural bottom is not to be obtained, 

 an artificial foundation must be made, either by concreting or driving piles ; 

 the former is generally sufficient. At the spot upon which the structure is to 

 be raised, the different strata immediately beneath the surface must be ex- 

 amined by an instrument called a " searcher," being a kind of auger, with 

 which the earth may be bored, until some definite stratum is reached : by a 

 reference to the depth of this bed, some idea of the extent of excavation is 

 arrived at. In ah 1 cases the " made earth" must be removed. If the stratum 

 immediately beneath is of clay, gravel, chalk, or other firm bed, and found to 

 be sufficiently thick for a solid bearing, the excavation may be finished and 

 the foundation laid in. In the neighbourhood of London the substratum is 

 generally made earth, beneath which, at variable depths, not often exceeding 

 twelve feet, a good bottom is usually met with ; when, to save brickwork, 

 concrete may be thrown in, until only enough depth is left from the surface 

 to cover the footings of the chimney, as shown in Fig. 23. 



Fig. 23. 



