ON THE VENTILATION OF FORCING-HOUSES. 225 



weather, further than is just necessary to prevent the leaves being 

 destroyed by excess of heat. Yet this mode of treatment does not at all 

 lessen the flavour of the fruit, nor render the skins of the grapes thick ; 

 on the contrary, their skins are always most remarkably thin, and very 

 similar to those of grapes which have ripened in the open air. It is 

 always my wish to see the temperature of this house, in the middle of 

 every bright day in summer, as high as 90 ; and, after the leaves of the 

 plants have become dry, I do not object to ten or fifteen degrees higher. 

 In the following night the temperature sometimes falls as low as 50; and 

 so far am I from thinking such change of temperature injurious, I am well 

 satisfied that it is generally beneficial. 



Plants, it is true, thrive well, and many species of fruits acquire their 

 greatest state of perfection in some situations within the tropics, where 

 the temperature, in the shade, does not vary in the day and night more 

 then seven or eight degrees ; but in these climates the plant is exposed 

 during the day to the full blaze of a tropical sun, and early in the night 

 it is regularly drenched with heavy-wetting dews ; and consequently it is 

 very differently circumstanced in the day and in the night, though the 

 temperature of the air in the shade at both periods may be very nearly 

 the same. If the thermometer, under the above-mentioned circum- 

 stances, were to be exposed, as the plant is, to the sun, it would probably 

 indicate, in the middle of the day, a temperature little below that of 

 boiling water. In the forcing-house so much light and heat are repelled 

 by the glass and wood-work of the roof, that the degree of heat to which 

 the leaves are subjected does not greatly exceed that indicated by the 

 shaded thermometer ; and, by excess of ventilation, I have several times 

 found the temperature of forcing-houses in the gardens of some of my 

 friends reduced so nearly to that of the external air in the middle of a 

 bright, but not very warm day, that the progress towards maturity of 

 the fruit was certainly rather retarded by the shade than accelerated by 

 the protection of the glass roof. During the night the loss, as far as 

 related to time, was probably redeemed by the flues ; but the fruit thus 

 ripened during the night never rivals in flavour that which is chiefly 

 ripened by confined solar heat. This kind of heat can also be made to 

 operate in every moderately bright day without incurring either expense 

 or increased trouble ; for any observant gardener will soon discover 

 precisely to what extent air may be confined in differently constructed 

 forcing-houses in every different state of the atmosphere and weather, 

 and thus guard in his absence, for a short time, against all danger of 

 injury to the foliage of his trees ; at the same time that these may be 



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