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given and withdrawn by degrees, particularly in the colder 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 
