VENTILATION 75 



explained by Fig. 15, the star indicates where the current 

 strikes, the impact destroying half its velocity. Any bend 

 in an inlet or outlet tube must be a curve as in Pig. 16. 



Apart from these considerations dust and dirt accumu- 

 late in tubes, in spite of wire gauze and other contrivances 

 being placed over the opening. If of great length these 

 shafts cannot be cleaned out, and should they possess right 

 angled bends it is impossible to deal with them. 



The shape of a tube is important ; a circular shaft gives 

 the largest area within the least circumference. Further, 

 there is less friction in a circular than in a square tube. 



As an inlet for large stables a shaft must be absolutely 

 condemned, it can never be made sufficiently large to 

 supply the requirements. In a private stable, on the 



Fig. 15. Fig. 16. 



contrary, it may be made a useful inlet for a small number 

 of horses, where more perfect means do not exist. 



If long tubes are used as extraction shafts the air in 

 them becomes rapidly cooled, and instead of ascending it 

 falls unless a good exhaust cowl exists. Further, the 

 cooling of the tube may cause a double current to be 

 established, warm air passing up one side and cool air 

 down the other. 



The practical deductions are obvious, extraction tubes 

 cannot be too short, nor can they be kept too warm. If 

 they happen to be carried near a chimney it will greatly 

 assist ; if in a loft they may be surrounded by forage, and 

 thus the cooling influence of the outside air avoided. 



A tube with a central division has been made to act both 

 as an inlet and outlet (Fig. 17). There is an upward 

 current on one side and a downward on the other. 



Muir designed a shaft with four partitions, the external 



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