VENTILATION 5 



important if the desideratum be a free circulation of air without 

 cold draught. Many practical men agree that the ordinary 

 leverage system, as applied to thousands of greenhouses, is not 

 a perfect system, whatever may be said for it as a labour-saving 

 appliance. In practice it too often means that you must open 

 all or none, and there are often occasions where, in a hothouse, 

 one open ventilator in the centre is all that is necessary. Top 

 vents without inferior lower vents are unsound. They are the 

 natural outlets for the hot air, but not for the admission of fresh 

 air. It is the nature of hot air to rise ; the hottest part of 

 the house is up in its apex. The admission of fresh air must 

 be provided for low down in the structure, the most convenient 

 place being in the sustaining walls. With quite small vents 

 there, the ingress of air can be controlled, and it then mingles 

 with and helps to circulate the warmer atmosphere, in Nature's 

 own approved way. 



The handling of ventilators is very important, and, as a 

 rule, it is always done, as it should be done, under the direction 

 of the foreman or some other responsible hand. Imagine for 

 a moment that it is April. A cold north-easter is blowing 

 outside, which penetrates to the marrow even though the sun 

 is brilliantly shining. The hothouse is crowded with thousands 

 of seedlings, delicate and fragile as a gossamer. The sun runs 

 the temperature up to an abnormal and unhealthy height, 

 and there remains nothing for it but to temper it to the seedlings 

 by an admission of fresh air. It needs the brain behind the 

 hand to do this with the maximum amount of benefit to the 

 plants and the least possible chance of doing harm. The 

 experienced man knows which way the wind is blowing, and 

 this decides him as to which vents to open and how wide. He 

 opens those on the lee side, and if a free ingress of air is needed 

 he reaches it by degrees. He appreciates the fact that the 

 tender foliage is laden with heated moisture of the same tempera- 

 ture as the house, and that a too sudden douche of cold air 

 might easily chill and blight it. So, when we insist upon the 

 importance of leaving the ventilation of the houses in skilled 

 and experienced hands, we feel we are on very safe ground 

 indeed. 



