Ventilation. 49 



are efficient, not only for removing contami- 

 nated air, but also for promoting draughts 

 which will have a cooling effect on horses 

 during hot weather. They are of special use 

 in a calm, or when there is a light wind aft, 

 the velocity of which is about equal to that 

 of the steamer ; or at anchor when no wind 

 is blowing, in which case during tropical 

 heat it might be advisable to keep the ship 

 moving, so as to promote the circulation of 

 air through her. It is evident that the 

 smaller the area of the openings in the 

 side of a ship, the more useful will be the 

 action of these fans. Of course they cannot 

 produce a good result unless they are kept 

 revolving at a high rate of speed. In a 

 poor class of ship, the transference of power 

 from the engines to the fans might diminish 



