PLATE XVII. 
The Plan of a Green House ; which differs in its internal disposition, from those in common use 
in this particular ; that an arch of brick is introduced, in place of the wooden stage that commonly 
supports the plants, by which it is intended to exclude from the house, a quantity of noxious air, 
generated under the stage, by the constant accumulation of dead leaves, and earth brought down 
from the pots, by the water passing through them, after watering. Opening the sashes, in houses 
covered with dead roofs, will not be sufficient to dissipate it, as it is screened (in a very great degree) 
from the action of the external air, by the stage and plants, and is in itself denser than the external 
air, and consequently cannot ascend ; therefore would require a particular current, directed under 
the stage to remove it. Warm humid air (which is constantly circulating) is well known to acce¬ 
lerate the growth of plants, and swell fruit surprisingly : but cold stagnant humid air is also well 
known to be extremely injurious to them : without recurring to philosophical experiments, one 
instance can be mentioned familiar to every gardener of observation, which is, that if after watering, 
the house be shut for three or four days (even two), they will assume a yellow, sickly appearance, and 
drop their leaves, (particularly at any time between October and March, when the coldness of the 
weather does not require fires;) and if continued longer, their existence as plants will be endangered. 
1 he excluding such a quantity of noxious air is not the only improvement proposed by this dispo¬ 
sition, for as the volume of air the house contains will then be considerably less, so it will require 
less fuel to warm it, and consequently be less expensive. 
The fire place could not be shewn in the plan without confusing it, but the dotted lines proceeding 
from under the stairs on the right hand, express the different revolutions of the flue. For farther 
explanation see Plate XIX. 
