VENTILATION. 



203 



1. By the direct action of a current of air from outside, 

 entering by the windows or other openings, and thus 

 imparting movement to the air inside, which by passing 

 out through various forms of escape, allows fresh air to enter. 

 This method of ventilation is utilised when advantage is taken 

 of the direction of prevailing winds, and on board ship, when 

 wind-sails are employed. 



2. By the action of a current of outside air indirectly 

 imparting movement in an outward direction to air inside the 

 building, as we may see when wind blows across an open 



\ 



Fig. 13- 



Fig. 14. 



window, open door, top of chimney (Fig. 13), or other opening 

 in the stable. 



3. By convection, that is to say, by the formation of currents 

 of air inside the building due to the heating of the air from 

 the presence of the animals. This air becomes lighter in pro- 

 portion to its increase of temperature, and consequently rises. 

 The movement thus imparted to the body of air within the 

 stable, tends to cause a portion of it to escape through con- 

 venient openings, and thus facilitates the entrance of fresh 

 air from outside. In this process, we must not lose sight of 

 the fact that the carbonic acid given off by the lungs and to 

 a small extent by the skin, is heavier than air, and there- 

 fore has a tendency to become accumulated near the floor. 

 Also, horses breathe downwards ; hence the advisability of 

 securing ventilation on or near the floor, as well as higher up. 



4. By the diffusion of gases, which is the property one gas 



