64 VETEEINAEY HYGIENE 



also be met, viz., stagnant air, though such a thing is 

 rarely met with in Great Britain, where the average annual 

 movement is from six to twelve miles an hour. 



Besides its perflating power, the wind also produces an 

 aspirating effect. This is caused by currents setting in at 

 right angles to the path of the moving body, to replace 

 the partial vacuum produced. The draught up a chimney 

 is one example of this, and advantage has been taken 

 of its aspirating effect, to cause the wind to produce 

 movements of air in a tube for the purpose of ventila- 

 tion. 



It is the aspirating action of the wind which is utilized 

 in the numerous patterns of cowl which are employed 

 either for chimney or ventilating purposes. 



Masses of air of unequal temperature are not a satisfactory 

 method of ventilation in veterinary hygiene, as the heated 

 air inside a building only arises from the bodies of the 

 animals, and is therefore vitiated. Heated air expands and 

 by so doing partly escapes, an attempt to restore the 

 balance is met by colder air coming in ; and so the process 

 is continued, cool air entering and warm air escaping. The 

 greater the difference between the temperature of the 

 external and internal airs, the greater the movement ; the 

 less the difference the smaller the action. 



It is quite possible to roughly determine the purity of 

 stable air with the thermometer. In the depth of winter a 

 difference of five degrees between the inside and outside 

 airs may be associated with considerable impurity, while 

 one degree difference may be indicative of great purity. 

 The explanation lies in the fact that the heated air is 

 derived from the bodies of the animals present. As a 

 means of natural ventilation this process of unequal weights 

 of air does not occupy a strong position ; with artificial 

 ventilation where heating is employed, it may be of the 

 utmost value. 



In summarising the three forces acting in natural venti- 

 lation there is no difficulty in recognising the wind as the 

 chief agent, next comes diffusion, while columns of air of 



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