chamber; c c, mouths of the cold-air di-ain ; d, cold- nlr drain; e, the point at which the
hot air enters the house ; / , a pit heated by hot air. The admission of hot air into the
house and pit is regulated by means of valves, so that the one may be heated independently
of the other, or both may be heated at once if required, i? is a vertical valve
for the admission of the extenial air, tho cold-air drain (d ) only returning the cold air
from the house to be reheated by the stove; whereas in the original house at Polmaise
tliere was only one drain for both the external and interaal cold air, and the cold air
occasionally was supposed to rise tlu'ough the gratings at c c, and to chill the house;
h is the ash-pit.
S u b s e c t . 10. O n the V e n tila tio n o f Plant-houses.
2150. V e n tila tio n as ap p lied to plant-houses has long been a subject of great difficulty
to gardeners, as the opinions of vegetable physiologists and of practical men seemed to
be in dfrect opposition respecting it. The late Mr. Knight, who was considered the
most able vegetable physiologist of his time, had asserted that “ change of air was not
cither necessary or beneficial to plants, except to a very limited extent,” and all the other
writers on the subject followed in liis train, till at last the general opinion was that a
veiy small supply of air was sufficient for any plant structure, so far as the growth of
the plants was concerned, provided the air of the house were tolerably pure; but where
the house was heated by smoke flues, or by fermenting stable dung, it was thought that the
air might become charged with sulphureous and other noxious gases, and in such cases
a mixture of fresh air might be necessary. It was also thought advisable, in conservatories
and frames, where a considerable portion of earth was exposed, to let off the
moist vapom's arising from it ; though the most common purpose for •which ventilation
was applied, was simply to lower the temperature, when tho house felt too hot, by the
admission of the atmospheric air.
2151. T h e e v il effects o f the sudden admission o f cool a ir in to a heated hothouse were
very great. The usual mode of lowering the temperature was by opening the sashes in
front, and those in the upper part of the roof at the same lime, and thus placing the
plants in a thorough draught. In addition to this, it was found that “ when the external
air enters a hothouse in which the air is at a high temperature, it rushes in with considerable
velocity, driving out by the pvcssiu'c of the atmosphere the hot and vaporous
air by which the plants are surrounded, and becoming heated and charged with moisture
at tlie expense of the earth in the pots, aud the foliage of the plants.” (L ou d o n ’s
H o rtic id tu ris t, p. 85.)
2152. The d ra in o f moisture h y th e escape (/Aeafoi/mV is much greater than is generally
imagined; but some idea of it may be formed from the following considerations. “ The
capacity of air for moisture, that is to say, the quantity of water which a cubic foot of air
will hoid in in'visible solution, depends upon its temperature, and increases with it in a
rapid ratio. It is doubled between 44° and 66°. The consequence is, that every cubic
foot of air which escapes at the latter temperature carries off with it twice as much
moisture as it brought in. Wliere the difference of temperature is greater, the drain
becomes greater also : air entering at 44°, and escaping at 80°, caiTies off three times as
much as it brought; escaping at 90°, four times.” (R og ers in G a rd . M a g ., vol. x\n.
p. 282.)
2153. T he w ritin g s o f L ie b ig having proved the important fact that plants derive a
considerable portion of their food from the air, and that consequently a regular supply
of fresh air is as necessary to the health of plants as a regular supply of fresh water,
threw quite a new light on the subject of ventilation; and it was now understood why
plants deprived of fi*esh air became debilitated, lost their leaves, and finally died; and,
in short, a distinction was made between mere ventilation, or the letting out of impure
vapours, and what Dr. Lindley very properly calls aeration, or the admission of cuiTents
of fresh air at a temperatui-e suitable to that already existing in the house.
2154. “ T h e importance o f a e ra tio n ," Dr. Lindlcy observes, “cannot be over estimated.
It is the one thing which now requires to be secured, in order to render our artificial
climates natural. A man’s reason, indeed, must tell him that a plant condemned to pass
its life in a still atmosphere, is like nothing so much as a criminal set fast in an everlasting
pillory. In order to secure motion in the vegetable kingdom, ciin-ents of air are made
to do the work of the muscles, limbs, and volition of animals. It is not at all improbable
that, in addition to the mechanical effect of motion in assisting the propulsion of the
sap, it may be important that the stratum of air in contact with the leaves of plants
should be incessantly shifted, in order to enable them to procure an adequate supply of
food; for we find that water in motion feeds them better than that which is stagnant.
Leaves are continually extracting from the air the veiy minute quantity of cai-bonic acid
which it contains. When the air moves quickly over their surface, fresh supplies of that
food are incessantly presented to it, and the operation of abstraction may be facilitated ;
while, on the contrary, if the air is stagnant, the absorption of carbonic acid may be veiy
much slower. Just as would be the case if a great sponge filled with milk were to be placed
in mere contact with a man’s mouth: ho would be a long time in sucking out its contents
if the sponge were immovable; but he would soon possess himself of the milk, if
the surface of the sponge were continually shifting.” (G a rd e n e rs’ C hronicle, io r 1846,
^ 2155. V a rio u s modes o f ae ration have been practised, some of them even before the
full importance of the operation was understood; the object being at first merely to
imitate the natural action ofthe atmospheric air in keeping the leaves in motion. _ One
of the earliest and simplest plans, obser\'CS Dr. Lindley, in the elaborate article in the
G ardeners’ C hronicle which has been afready quoted, “was that practised by Mr.Knight
himself; and this consisted in passing pipes, open at each end, through the heating
materials of a hotbed, one end being in tbe interior of the frame, and the otlier exposed
to the open air; ” and in tliis way Mr. Knight “ succeeded in constantly renewing the
atmosphere of the frame, and in keeping the leaves in motion, with, as he tells us, the
liappiest effect.” Among various other modes of aeration which are mentioned by Dr.
Lindley, is that of Mr. Williams of Pitmaston, who “keeps the south end of his melon
frame open to the outward air night and day, except that it is covered over with a
screen of fly wire painted black, and continued in the inclination of the roof. This
screen receives the rays of the sun from 10 a . m. to 3 p. 2>t. all the summer long; it
becomes heated to 8 0 ° or 100° , and consequently heats the afr that passes between its
interstices. By raising the sashes at the back, a very powerful cun-ent of afr is established,
though the thermometer ranges from 80 ° to 90 ° below the leaves in a sunny day; and,
in short, the atmosphere is as hot as is experienced in the southern parts ot Italy, with
almost as much ventilation as if the plants were growing in the open air.” (Ib id . and
J o u rn . o fth e H o rt. Soc., vol. i. p. 4 3 .) Another plan mentioned by Dr. Lindley is that
contrived by Mr. Leafs gardener at Strcatham; and this plan “ consists in passing a
zinc pipe, thickly perforated with small holes, from end to end of the vineiy, and exactly
beneath the range of hot-water pipes which heat the stmcture. In the outer wall, communicating
with this perforated pipe by means of a broad funnel, a register valve is
fixed, by which the admission of afr can be regulated with the utmost nicety, or the
supply may be shut off altogether. Tliis valve is fixed a little below the level of the
perforated pipe. The action of this contrivance was evident enough, from the motion
communicated to the foliage of the vines; and its effects were apparent in the unusually
healthy and vigorous appearance they bore until the period of the fruit ripening.”
(G a rd . C hron. for 1846, p. 268.) Tenn’s mode of heating (see § 2148.) was very
efficacious in keeping the afr in motion; but its fault was, that it did not supply a
current of fresh air. The Polmaise system, on the other hand, derives its prmcipal advantage
from its affording the plants a constant supply of wai-med/rcs/i air (sec § 2149.) ;
and this is probably the nearest approach to the atmosphere of tropical climates that
can be obtained in this country.
2156. I n greenhouses and other houses where the heat maintained is not much g^-eater
than that of the atmospheric air dm-ing the middle of a fine day, all that is required is
to supply some means of opening the windows simultaneously, so as to afford a supply of
air to eveiy part of the house at the same time; and various modes have been devised
for doing this with the least possible trouble to the gardener, some of which have been
already described ; but the best of which is probably that iu constant use in the Royal
Gardens at Frogmore.
2157. T h e mode o f ve ntilating a t Frogm ore has been kindly supplied to us by the engineer,
Mr. Jones. F ig . 646. is a perspective view of the house, in which a is an iron shaft
running the wholc length of the house, and working in brass chains or beai-ings.
Fixed on this shaft, and opposite the centre of each front light, is a brass pinion, working
into a toothed quadrant attached to the bottom rail of each light. The shaft being
turned round by means of the handle (6), the quadrants arc, of course, either throini out,
or drawn in, as the handle is moved backward or forward ; and the whole of the lights,
each moving on its centre at top, are simultaneously opened or closed to the extent required
; tho smallest or widest opening being given with equal case at the pleasure of
the gardener. F ig . 647. is a section of the same house, in which the handle and quadrant
of the apparatus for opening and closing tlie front lights are shown at a h. In the back
wall under each intermediate top light is a ventilating frame, shown iu the section at
c ; and above these frames, outside the house, are a con-esponding number of open-work
gratings, marked d in fig s. 646. and 647. Flues or chambers in the wall are shown at e in
Jig . 647 ; and these flues are the length of the ventilating frames; the doors of which open
into the flues at bottom, while the gratings cover their openings at top. When it is
desired thoroughly to ventilate the house, and the front lights arc opened, it is only
necessary to turn the screw or worm ( f ) , which is connected by the rod (gr) to the lever ( li) ;
the lever being fixed on the end of a wrought-iron shaft, running the whole length of the
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