ADMISSION OF AIR. 317 



In a few instances they have been applied to warm the 

 atmosphere of vineries and peach-houses, in which, how 

 ever, they have been found to be but an indifferent substi 

 tute for the other means already explained. 



In the management of artificial heat, a considerable de 

 gree of caution is required. All the operations of nature 

 are gradual; and in forcing, it is well to follow these as 

 the safest examples. The judicious gardener will there 

 fore apply his heat very gradually at first ; he will increase 

 it by degrees for several weeks, and, in particular, he will 

 guard against any sudden decrease of warmth, as nothing 

 is more necessary to success than that the course of vege 

 tation be continued uninterruptedly through foliation, inflo 

 rescence, and fructification. He will cause the tempe 

 rature to increase by day and decrease by night, to rise 

 in summer and fall in winter. He will, in short, imitate, 

 as much as possible, the natural and varying influence of 

 the sun. 



It is scarcely necessary to say that a Fahrenheit ther 

 mometer is an indispensable instrument to the gardener, 

 not only in the forcing-house, but in every department. 

 Six s Registering thermometer is very convenient for point 

 ing out the extreme temperatures during night or day. 



The admission of Air. The deteriorating influence 

 which all living plants are supposed to exert on the atmos 

 phere must operate with tenfold force in a glazed house, 

 where the proportion of air to vegetable substance is infi 

 nitely smaller than under the open sky, and where the cor 

 rective agitations of the wind, and the changes of tempera 

 ture, are much less perceptibly felt. The respiration of 

 plants, and the exhalations of putrescent vegetables, re 

 quire a constant circulation of the aerial fluid, and this is 

 maintained by means of movable sashes, and ventilators in 



