HEATING. 333 



materially affected by the admission of the air necessary 

 to the growth and healthy state of the plants.' AYe are 

 not aware of its having been employed as a primary 

 '.sourcp of heat, except in the case of Dr. Anderson's pa- 

 tent hot-house, in which heated air was kept, bottled 

 up, as it were, in separate chambers ; an arrangement 

 too irregular and unmanageable to be of much utility 

 in our variable climate. 



Vegetable substances in a state of fermentation 

 evolve a considerable quantity of caloric, and are much 

 employed to produce bottom heat in hotbeds, pine-ap- 

 ple, or melon pit^. In a few instances they have been 

 applied to warm the atmosphere of vineries and peach- 

 houses, in which, however, they have been found to be 

 but an indifferent substitute for the other means already 

 explained. 



In the management of artificial heat, a considerable 

 degree 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 therefore 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 vegetation be continued uninterrupt- 

 edly through foliation, inflorescence, and fructification. 

 He will cause the temperature to increase by day and 

 decrease by night, to rise in summer and fall in w^inter. 

 He will, in short, imitate, as much as possible, the na- 

 tural and varying 'influence of the sun. 



It IS scarcely necessary to say that a Fahrenheit 

 thermometer is. an indispensable instrument to the gar- 

 dener, not only in the forcing-house, but in every 

 29 



X 



