334 FORCING GARDEN. 



department. Six's Registering Thermometer is very- 

 convenient for pointing 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- 

 atmosphere must operate with tenfold force in, a glazed 

 house, where the proportion of air to vegetable sub- 

 stance is infinitely smaller than under the open sky, and 

 where the corrective agitations of the wind, and the 

 changes of temperature, are much less perceptibly felt. 

 The respiration of plants, and the exhalations of putre- 

 scent vegetables, require a constant circulation of the 

 aeriaL fluid, and this is maintained by- means of mov- 

 able sashes, and ventilators in the roof of the house. 

 Of these, sashes seem preferable, as less apt to produce 

 currents of cold air, which are always injurious to 

 vegetation. It is, indeed,- a -disadvantage that, hy slid- 

 ing down over one another, they diminish the influx of 

 light. In winter, however, when light, from its scarcity 

 in our high latitude, is most valuable, they are seldom 

 drawn down to any extent; and, by having all the sashes 

 m.ovable, the gardener, with a little attention, may 

 correct in a great measure a-ny inequality in this'.re- 

 spfect. Sliding^ sashes require a depth of rafter >hich 

 greatly augments the, shade in oblique sunshine,' an 

 evil which cannot easily be obviated. With fixed roofs, 

 and more especially th6se which are curvilinear (to be 

 immediately described), numerous ventilators are the 

 only means by which a proper circulation of air can be 

 obtained. Some very intelligent gardeners prefer hav- 

 ing all the sloping sashes fixed, and ventilating chiefly 

 by means of large windows at each end of the house, 

 aided by small ventilators in front. ' 



