92 VETERINARY HYGIENE 



and expanded air it will pass out, and fresh, cool air will flow in 

 to take its place. 



The passage of air into and from a building is not solely de- 

 pendent upon the degree to which the inside air is heated. Wind 

 acting from the outside is a most powerful ventilating force, and 

 its action is two-fold. It drives into a building through any suit- 

 able opening, mixes with and dilutes the vitiated air and forces 

 it on and out through openings on the other or lee side. This is 

 called the perflating action of the wind. The second power, and 

 one of great importance, is the aspirating effect which is set up when 

 wind passes across or through any opening. If wind passes in at 

 one window and out at another on the opposite side it draws air from 

 all points as it passes. Similarly, when it blows across an opening 

 in the roof it aspirates air out of the building and so sets up other 

 currents which flow in to fill the space formerly occupied by that 

 which has escaped. As the atmosphere is very rarely still in this 

 country there will nearly always be a certain movement of air into 

 and from a building due to the action of the wind. A condition of 

 absolute calm, however, does not necessarily imply that no air 

 change will take place. As has been shown, so long as the temper- 

 ature in a building is higher than that outside there will be a constant 

 outward flow of hot air and an inward flow of cool air in exactly the 

 same manner, though at a slower rate. 



There is little difficulty in ventilating any animal house, even 

 when not artificially heated, on either a cold, frosty day with com- 

 plete absence of wind or on a warm day with an appreciable wind. 

 The difficult days are those that are neither cold nor windy. 



From what has been said, it will be clear that every system of 

 ventilation is composed of two parts, provision for the escape of 

 foul air and provision for the inlet of fresh air. The inflow of 

 fresh air, especially in the absence of wind, depends largely upon 

 the rate at which the foul air passes out. Each half of the system 

 is dependent upon the other for the efficiency of the whole, and 

 a potentially good inlet system is often restricted in efficacy by 

 the inadequacy of the outlet. 



SIZE OF INLETS AND OUTLETS. Wind blowing at the rate of 

 3 miles per hour, which is practically a calm, will introduce into a 

 building 110 cubic feet of air per hour for every square inch of inlet. 

 Very rarely does the movement of air remain at such a low speed for 

 any length of time, as the average velocity of the wind in this country 

 is approximately 15 miles per hour. Provision for the inlet and 

 outlet of air should be such as will permit satisfactory ventilation 

 when the wind is scarcely perceptible. At least it is necessary 



