no 



CAUSES AND PREVENTION OF DISEASE. 



what objectionable of itself and is especially important as being 

 a reliable guide as to the prevalence of certain organic impuri- 

 ties which are more directly and positively harmful. 



Natural forces. — The factors that operate in natural ventila- 

 tion are: 



I St. The force of the wind. 



2nd. Weight of air, as varied by its temperature. 



3rd. Diffusion of gases in obedience to a natural law. 



FIG. 41. VENTILATION. (PAIGE.) 

 Outlet with too many angles. Cow receiving inlet air from over a 



manure pile. 



Wind. — The force of the wind is probably the most im- 

 portant one of these factors, and must always be taken into con- 

 sideration in planning ventilation or in mathematical estimates 

 of the amount of air needed. It is of course irregular, but vari- 

 ations can be made in the ventilating facilities to compensate 

 for this. A fundamental point in using this factor is to see that 

 the incoming air does not pass over or through any contaminat- 

 ing source, e. g.. over a manure pile, and it is very important 

 that the general plan of construction should be well considered. 



Temperature and zccif^ht. — The second factor, difference in 

 weight between the lighter warm, and heavier cold air, is not. so 

 important in natural as in artificial ventilation, and yet it is a 

 factor of consideral)lc importance. The heat which warms the 

 air in the lower levels is that which comes from the bodies of 

 the confined animals, as it is radiated from the surface, or warmed 

 in passing through tlic lungs. 



