266 EFFECTS OF VENTILATION. 



produce dryness, which may also prove injurious under certain 

 circumstances. 



4. When the external air is admitted into a glazed house, 

 below the temperature of the air it contains, the heated moist 

 air rushes out at the upper ventilators; or, if it cannot find 

 egress, it is quickly condensed upon the cold surface, against 

 which it is forced to ascend ; the latter rapidly abstracts from 

 the plants, etc., a part of their moisture, and thus gives a shock 

 to their constitution which cannot fail to be injurious. 



This abstraction of moisture is in proportion to the rapidity 

 of motion in the air. But it is not merely dryness that is thus 

 produced, or such a lowering of the temperature as the ther- 

 mometer suspended in the house may indicate. The rapid evapo- 

 ration that takes place, upon the admission of the air, produces a 

 degree of cold upon the surface of the leaves, and of the pots in 

 which they grow, as well as all other bodies around them, of 

 which our instruments give no indication. To counteract these 

 mischievous effects, many contrivances have been proposed, in 

 order to insure the introduction of fresh air, warm and loaded 

 with moisture ; such as compelling the fresh air to enter a house, 

 after passing through pipes moderately heated, or over hot-water 

 pipes surrounded by a damp atmosphere, which have been proved 

 decidedly advantageous, and to which we will subsequently 

 refer. 



If ventilation is merely employed for the purpose of purifying 

 the air, . >,, for carrying off extraneous gases and vapors that 

 may be generated by artificial heat, it should be introduced, by 

 all means, with great caution ; and some expedient should be 

 adopted for supplying it with moisture, as well as to warm it 

 slightly on its passage inwards, more especially in cold, frosty, 

 or windy weather. 



If it is only introduced for the purpose of lowering the tem- 

 perature, as in mild and genial spring and summer weather, it 

 may be admitted without any such precaution ; and the freedom 

 of admission should be in proportion as the external and inter- 

 nal temperatures approach each other in equality. 



In hot, sultry weather, air should be sparingly admitted, as 



