ADMISSION OF AIR. 317 
In a few instances they have been applied to warm the 
atmosphere of viheries 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. 
: Jt is scarcely necessary to say that a Fohrantisit 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 
