72 



HEALTH AND DISEASE 



factory. The same may be said of a somewhat similar arrangement, the 

 double tube: a large one for the outside, and a small one passing through 

 it. In this plan the larger tube is intended to act as an inlet, and the 

 smaller one as the outlet. But it is very commonly found that the result 

 is a down-draught always in full action, while the outlet either has the 

 opposite to the intended effect, contributing to the down-draught, or other- 

 wise does not act at all. 



It must be obvious that the 

 force and direction of the wind 

 must always be powerful factors 

 in ventilation, and one difficulty 

 which is not easily overcome arises 



\ \ 



Fig. 464. Direction taken by Air-currents with the 

 Windward Windows open 



from the frequent changes which 

 they undergo, at one time a super- 

 abundance of air being driven 

 forcibly into the building, while at 



another a scarcely calculable quantity will pass in. To meet this diffi- 

 culty, to some extent, the author of Veterinary Hygiene is in favour of 

 openings being made on opposite sides of the building; and he insists 

 that to get the full benefit of such an arrangement the buildings must 

 not be more than from 25 feet to 30 feet wide. He found that a 

 current entering through an inlet will cause the air in the stable to set in 

 towards it in a direction more or less at right angles; and if the velocity of 



the incoming current is great, it 

 may pass out again at the opposite 

 opening before it has properly mixed 

 with the air in the stable. The 

 results of the experiments made by 

 the author of Veterinary Hygiene 

 will be best understood with the 

 aid of the accompanying diagrams, 

 which are taken from his work. 



In the first illustration the wind 



is supposed to enter at a window which is opened to windward (fig. 464). 

 Soon after entering, the current is described as spreading out fan-shaped 

 and passing towards the ground; and in the case of a powerful current 

 it may be measured 18 or more feet from the point of entry, but under 

 ordinary conditions its speed rapidly decreases owing to the pressure of 

 the air in the stable at about 6 or 8 feet from the inlet. The current 

 is further described as striking the ground on the opposite side of the 

 stable, much of it escaping by the leeward side of the ridge, or by the 



Fig. 465. Direction taken by Air-currents with Wind- 

 ward and Leeward Windows open 



