■22 



On Climate, $c. 



earth is regulated by the cold of the upper regions, so in a 

 house the point of deposition is governed by the temperature 

 of the glass with which it is in contact. In a well ventilated 

 hot-house, by watering the floor in summer, we may bring the 

 dew-point within four or five degrees of the temperature of 

 the air, and the glass will be perfectly free from moisture ; 

 by closing the ventilators we shall probably raise the heat 

 10 or 15 degrees, but the degree of saturation will remain 

 nearly the same, and a copious dew will quickly form upon 

 the glass, and will shortly run down in streams. A process 

 of distillation is thus established, which prevents the vapour 

 from attaining the full elasticity of the temperature. 



This action is beneficial within certain limits, and at par- 

 ticular seasons of the year, but when the external air is very 

 cold, or radiation proceeds very rapidly, it may become ex- 

 cessive and prejudicial. It is a well known fact, but one 

 which I believe has never yet been properly explained, that 

 by attempting to keep up in a hot-house the same degree of 

 heat at night as during the day, the plants become scorched ; 

 from what has been premised it will be evident that this is 

 owing to the low temperature of the glass, and the consequent 

 low dew-point in the house, which occasions a degree of 

 dryness which quickly exhausts the juices. 



Much of this evil might be prevented by such simple and 

 cheap means as an external covering of mats or canvass. 



The heat of the glass of a hot-house at night cannot exceed 

 the mean of the external and internal air, and taking these 

 at 80° and 40°, 20 degrees of dryness are kept up in the 

 interior, or a degree of saturation not exceeding 528. To 

 this in a clear night we may add at least 6° for the effects 



