194 
VENTILATION OF FORCING-HOUSES. 
serving it, the sap, which during sunshine, will drop from the quill 
without intermission, will he found nearly to cease, by the mere cir¬ 
cumstance of a cloud obscuring the sun for only a few minutes. 
Had the individuals who first introduced the methods of admitting 
air only through the front of houses, once given the subject proper 
consideration, they certainly would not have left the subject so much 
exposed to just complaint. If we send a man into the woods, to 
strip the hark from oaks at the proper season, although it may he 
the first day he ever did any thing of the kind, he is soon able to 
tell us, that the bark will not quit the trunk, should the wind change 
from south to north-east. There is no doubt, hut the stems of trees 
and plants under glass, are much more susceptible of changes, than 
those grown in the open air. We must not look upon the tender fo¬ 
liage of plants, as the only seat of sensibily, hut consider all parts of 
the tree liable to he similarly affected. 
No method of giving air surpasses that of opening the lights on 
the roof; no sooner is one of these lights opened, than a stream of 
heated air rushes out, and is as forcibly met by a current of cold, 
endeavouring to enter and occupy the lower part of the house; the 
commotion occasioned by the ingress and egress, cause the cold and 
hot airs to intermix, and a proper medium of heat is the result. It is 
the very worst piece of philosophy to admit strong currents of cold 
air in front, to drive out the hot air through apertures in the back 
of a forcing-house, for this will always have a very injurious effect on 
vegetation, and be attended with a great loss of suitable heat There 
is no occasion to ever admit air through the front, if the construc¬ 
tion will allow of a proper convenience for its entrance through the 
roof. If a person took a lighted lamp, and held it in the lower parts 
of the house, and another individual opened a top sash, the vibration 
of the flame would convince them, that every part of the house was 
affected by the entrance of the cold air at the top. 
No tree or plant appears to do well when acted upon by a current 
of air, this increases the difficulty of providing a sufficient quantity 
of underwood in many large woods. 
All medical men prohibit their patients from subjecting them¬ 
selves to the current of a window, and the strongest constitution can¬ 
not bear this long without injury. I venture to state, that no per¬ 
son can, at this season of the year, admit air in the front of his hot- 
house, without reducing the lower part below the desired degree of 
warmth, for the cold air is sure to compel the heat to escape, and 
will hold the place previously occupied by it; and although the top 
or back wall apertures may be contracted, so long as any portion 
