METHODS OF VENTILATION. 



275 



or with that perfection in horticulture at which it is our duty to 

 aim ; inasmuch as the revelations of science are against it, as 

 has already been shown, and practice has hitherto given no evi- 

 dence to prove it beneficial to tender plants. 



3. In large and lofty structures, and especially in dome-shaped 

 houses, the management of the atmosphere becomes a matter 

 of much more importance than in small houses. During mild 

 and temperate weather, things may go on very well, as at such 

 times the external air may be allowed to circulate through the 

 house with greater impunity ; but during the heat of summer, 

 and the cold of winter, the atmosphere is much more difficult to 

 equalize. With a frosty air externally, and the temperature at the 

 surface of the earth down to zero, it is impossible to maintain a 

 proper degree of temperature, in all parts of the house, without 

 positive injury to those plants that may be growing, or have 

 their branches extended into the upper regions of the house. In 

 fact, without the precaution of covering, or some such expe- 

 dient, mischief is absolutely unavoidable. What has already 

 been said, upon the nature and properties of air, will sufficiently 

 explain the cause ; and, although it has been repeatedly asserted 

 by theorists, that one part of a house being heated by radiation, 

 from a body radiating heat, the equalizing law of nature will 

 heat all parts of the house to the same temperature, and as 

 speedily, too, yet we must enter our decided protest against 

 the practical correctness of such a statement ; at least, in our 

 own practice, we have never found it so, under any circumstance, 

 or by any system of heating. And hence, whatever the natural 

 law of equality may be, the practical effects cannot be mistaken, 

 or disputed, as far as regards hot-houses. We know that heated 

 bodies tend to an equality of temperature ; but, as has been 

 already observed, air, of all other bodies, possesses peculiar 

 properties in this respect in regard to heat, and in nothing is this 

 peculiarity more strikingly illustrated than in the case under 

 consideration. 



4. With regard to the motion of the atmosphere in a hot- 

 house, we know that the greater the difference between the tem- 

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