318 FORCING GARDEN. 



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 disadvan- 

 tage that, by sliding 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 movable, the gardener, with a little attention, may 

 correct in a great measure any inequality in this respect. 

 Sliding sashes require a depth of rafter which greatly aug- 

 ments the shade in oblique sunshine, an evil which cannot 

 easily be obviated. With fixed roofs, and more especially 

 those 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 having 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. 



The quantity of air to be admitted from time to time 

 must vary with the season, the temperature required to be 

 kept up, and the kinds of plants cultivated. It should be 

 given and withdrawn by degrees, particularly in the colder 

 portions of the year. The sashes or ventilators, for in- 

 stance, may be partially open by eight A. M., top air being 

 given before front air ; full air may be employed about ten : 

 a reduction should take place before three P. M., and the 

 whole should be closed between four and five in the after- 

 noon. In summer less caution is necessary, as in many 

 cases the external air differs little in temperature from that 

 within the house. Most commonly air is given only dur- 

 ing the day, and is excluded at night, with perhaps an 

 increase of fire-heat. Judicious horticulturists will some* 

 times reverse this process. Knowing, for example, that in 



