ADMISSION OP 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 



