(51 



be about eight inches square, starting near the floor and running out through 

 the roof. It is usually more convenient to place these outlet shafts beside the 

 walls, in which case, after passing through the ceiling, they may follow the 

 slope of the roof to the peak, where they can be given outlet. In any case, the 

 top of the shaft should be higher than the peak of the ro(^. The inside of the 







"^^^^^7:^=5:^^^^^^^^^^^^^^^ 



Pig. 18. — Method of admitting fresh air. 



shaft should be perfectly smooth, and there should be no sudden turns in its 

 course, A right-angled turn will render an outlet shaft practically useless. 



The second method takes in the fresh air at, or near, the floor, and the foul 

 air escapes through shafts which open at the ceiling, and run straight up through 

 the roof. As in the other case, the top of the shaft should be higher than the 

 peak of the roof. Fig. 18 shows how the inlet is constructed. The drawing shews 

 the outside structure made of concrete, but it may be made of wood if desired. 



