264 EFFECTS OF VENTILATION. 



provision at all for ventilation ; and we have the direct testimony 

 of Mr. Knight, as to the advantage of the practice to many 

 cases to which it has been commonly applied. 



" It may be objected," says Knight, " that plants do not 

 thrive, and that the skins of grapes are thick, and that other 

 fruits are without flavor, in crowded forcing-houses. But in 

 these, it is probably light, rather than a more rapid change of air, 

 that is wanting ; for in a forcing-house, which I have long devoted 

 to experiments, I employ but very little fire heat, and never give 

 air till the grapes are fully ripe, in the hottest and brightest 

 weather, further than is just necessary to prevent the leaves 

 from being destroyed by excess of heat. Yet this mode of 

 treatment does not at all lessen the flavor of the fruit, nor ren- 

 der the skins of the grapes thick. On the contrary, their skins 

 are always moist, remarkably thin, and very similar to those 

 grapes which have ripened in the open air." [Hort. Trans.] 



We have experienced the same results, as those recorded by 

 Mr. Knight, under similar treatment, and that too under a more 

 powerful sun. We have pursued this method of giving a very 

 limited supply of air, on an extensive scale, in some large 

 graperies in Maryland, and under glass of the very worst possi- 

 ble description. Yet, during one of the hottest summers which 

 had been experienced for some years, these vines grew beyond 

 anything we had ever seen, without any indication of injury by 

 the sun's burning rays. The lower surfaces of the houses, how- 

 ever, were kept moist, by frequent sprinkling with water during 

 the day. Many large houses in England are never aired, 

 except, perhaps, a few apertures at the top of the house, which 

 are left open, night and day, during the summer. But in all 

 cases within our knowledge, water is abundantly supplied to the 

 atmosphere from the floors of the house. 



3. The philosophy of this method is easily perceived. The 

 under surface of the glass is continually covered with a deposi- 

 tion of the evaporated moisture, which intercepts the calorific 

 rays, and prevents them from being concentrated on the leaves, 

 from which cause the leaves are scorched and burned ; the 

 atmosphere, at the same time, undergoing comparatively little 

 change, or admixture with the external air. 



