156 APPLICATION OF PEINCIPLES. 



earth and the air of their houses, and in their pits for 

 vegetables, they cover the surface of the ground with 

 the same oiled paper, by which means vapour is effec- 

 tually intercepted, and the air preserved from exces- 

 sive moisture. 



In forcing-houses, ventilation is thought to be 

 required at the time when the fruit is ripening, for 

 the purpose of increasing the perspiration of the 

 plants, and, consequently, of assisting in the elabora- 

 tion of the secretions to which fruit owes its flavour ; 

 but, even for this, its utility is perhaps overrated, 

 and it is quite certain that it may be easily carried to 

 excess; for if it is so powerful as to injure the leaves 

 by over-drying the air, an effect the reverse of what 

 was intended will be produced ; that is to say, the 

 quality of the fruit will be deteriorated (64, 75). 

 Upon this subject Mr. Knight has made the following 

 observations : — " A less humid atmosphere is more 

 advantageous to fruits of all kinds, when the period 

 of their maturity approaches, than in the earlier 

 stages of their growth ; and such an increase of ven- 

 tilation at this period, as will give the requisite 

 degree of dryness to the air within the house, is 

 highly beneficial, provided it be not increased to such 

 an extent as to reduce the temperature of the house 

 much below the degree in which the fruit had pre- 

 viously grown, and thus retard its progress to matu- 

 rity. The good effect of opening a peach-house, by 

 taking off the lights of its roof during the period of 

 the last swelling of the fruit,» appears to have led 

 many gardeners to overrate greatly the beneficial 



