VENTILATION 



63 



the most favourable circumstances, viz., where the air is 

 obtained through cracks and crannies in doors and windows, 

 diffusion is a slow process, and by itself can never be 

 trusted for the purpose of renovating vitiated air. 



Diffusion merely causes the various gases present in a 

 building to mix. Eoscoe showed that carbonic acid experi- 

 mentally evolved in a room was reduced to one-half in 

 ninety minutes by this process. This is not sufficiently 

 rapid for the purpose of ventilation, and further, it leaves 

 the solid particles of the air quite unaffected. 



Winds are the great power in natural ventilation ; they 

 mainly act by perflation, viz., setting masses of air in 

 motion, and as a means of flushing and renewing the air. 



Fig. 1. — Transverse section of stable. Two horses are placed between 

 opposite windows, heads to outer walls. The arrows indicate the direction 

 of the wind. 



there is no natural force comparable with them ; for 

 example, blowing at the rate of three miles an hour (which 

 is little more than perceptible) through a ventilator one 

 square foot in size, 15,840 cubic feet of air will pass per 

 hour, the amount, in fact, demanded for the larger 

 herbivora. Analyses of stable air show how rapidly by 

 free perflation organic impurities may be removed ; in half 

 an hour the COg of respiration may be reduced from '6 to "1 

 per 1,000. 



The chief difficulty in dealing with the wind as a ventila- 

 ting agent is in regulating its velocity. With high velocities 

 it causes a draught so that regulation of windows and 

 ventilators becomes necessary. The opposite extreme may 



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