OF ATMOSPHERICAL TEMPERATURE. 145 



the amount would be increased to 120 grains in the 

 same time, if the dew-point were to remain station- 

 ary, and the temperature were to rise to 80° ; or, in 

 other words, if the saturation of the air were to fall 

 to -726. {Hort. Trans, vi. 20.) It is well known that 

 the effect of maintaining a very high temperature in 

 hot-houses at night, during winter, is frequently 

 to cause the leaves to wither and turn brown, as if 

 scorched or burnt ; and this is apparently owing to 

 the dryness of the air, in consequence of the above 

 causes. 



It is evident that the mode of preventing this dry- 

 ing of the air by the cold surface of a glass roof will 

 be, either by raising the temperature of the glass, 

 which can only be effected by drawing a covering of 

 some kind over our houses at night, so as to intercept 

 radiation, or by double glass sashes ; or else by 

 keeping the temperature of the air of the house as 

 low as possible, consistently with the safety of the 

 plants, and so diminishing the difference between the 

 temperature of the external and internal air. 



A bad system of ventilation is another cause of the 

 loss of vapour in the atmosphere of glazed houses, 

 to which reference will be made in the succeeding 

 chapter. 



It is, in all appearance, to the attention that, since 

 the appearance of Mr. Daniell's paper, in 1824, upon 

 this subject, has been paid to the atmospherical mois- 

 ture of glazed houses, that the great superiority of 

 modem gardeners over those of the last generation is 

 mainly to be ascribed ; there are, however, traces of 



7 



