No. 4.] 



STABLE VENTILATIOX 



121 



conditions exist, the functions of the tubes would also be 

 reversed. 



To establish and maintain inward currents through one 

 and outward currents through the other, it has been recom- 

 mended not to build the dividing partition nearer than eigh- 

 teen inches to the face of the ceiling. Others advise for the 

 same purpose that a board set at an angle be put into the 

 bottom of one tube and one into the top of the other, their 

 position being such as to ofter resistance to the circulation 

 of a current of air in the opposite direction from what is 

 desired. 



To insure at all times the desired action of a shaft or tube, 

 either as inlet or outlet, cowls are sometimes attached to the 

 upper end. There are two vari- 

 eties, the fixed and movable. 

 The principles of action vary 

 according to the pattern. Some 

 are so constructed as to produce 

 an upward circulation by the 

 Archimedean screw principle, 

 the motor force being a mechan- 

 ism which is operated by the 

 wind. In other varieties the 

 force of the wind is so directed 

 across the open end or side that 

 air is either driven through the 

 tube into the building or is as- 

 pirated out of it. So far as I 

 have observed, none are abso- 

 lutely positive in their action. 

 The stationary variety has the 

 advantage over the movable 



kind in that it is entirely automatic, acting with the wind in 

 any direction, and is less lial)lc to get out of order than any 

 movable pattern (Fig. 4). 



In stable ventilation draughts are best prevented, and the 

 incoming currents best distributed, by having numerous 

 small openings as inlets and outlets, rather than one or two 

 large ones for that purpose. 



I have considered somewhat at length the construction, 



Fig. 4 — Stationarv outlet cowl. 



