HABITATIONS 277 



those from loam or sandy clay, and those from marl or 

 lime clay. Fire bricks are made from clay containing 

 neither lime nor magnesia. 



The colour of a brick depends upon the degree to which 

 it is burned, and the amount of iron and lime it contains. 

 Its average size is 9 inches long, by 4| inches broad, and 

 2 inches thick. Its average weight is 5 lbs., and it can 

 hold about sixteen ounces of water : it is therefore very 

 porous. 



The thickness of a wall is described as consisting of so 

 many bricks, for example a one brick wall is a nine inch 

 wall. Bricks, as we have seen, are very porous, and air 

 can under very little pressure be driven through them. 

 There are special non-porous bricks, known as Blue Staf- 

 fordshire, which are excessively hard, and owing to their 

 being practically non - porous 

 are largely used for stable 

 flooring. 



Bricks are laid in courses, 

 and mortar should be used not YJBSiS 



only between the courses, but ^'S 83 — Vitefied Damp p:oof 



•' . . Couise. 



also between the vertical joints, 



so that no two bricks should touch each other. The 



care with which mortar is applied is a good test of the 



thoroughness of the work. Mortar is made from one part 



of quicklime and three parts of clean sharp sand. Sand 



from the sea shore makes a bad mortar, and leaves a damp 



wall. 



Roofing. — The material used in roofing may be tiles, 

 slate, thatch, or galvanized iron ; the latter is very durable, 

 but is a hot or cold cover depending on the season ; slates 

 make a good water-tight roof, while the tile is perhaps too 

 brittle, though extensively used in farm buildings. Thatch 

 is the warmest roof that can be supplied, but is obviously 

 dangerous. 



The eaves of the roof should project beyond the walls so 

 as to save them from rain, and should also be provided 

 with a good gutter for carrying off the water. It is poor 



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