SuBSECT. 1. O fth e F rm d p le s o f D e sig n in P lant-honses.
2054. T o ascertain the princip les o f action, it is always necessaiy to begin by considering
tho end in view. The object or end of plant-houses is to form habitations for vegetables,
and either for such exotic plants as wUl not grow in the open air of the country
where the habitation is to be erected, or for such indigenous or acclimated plants as it
is desired to force or excite into a state of vegetation, or accelerate their maturation
at extraordinary seasons. The foi-mor description are generally denominated greenhouses
or botanic stoves, in wliich the object is to imitate the native climate and
soil of the plants cultivated ; tho latter comprehend forcing-houscs and culinaiy stoves,
ill which the object is, in tho first case, to form an exciting climate and soil, on general
principles ; and in tho second, to imitate particular climates. Tho chief agents of vegetable
life and growth ai-e heat, light, air, soil, and water; and tho merit of artificial
climates consists in the perfection with which those arc supplied.
2066. Such heat as is required in addition to that of the sun is most generally produced
by tbe ignition of carbonaceous materials, wliich heat tho air of the house, either directly,
when hot embers of wood are left in a furnace or stove, placed within tho house, as m
Sweden and Russia ; mediately, as when smoke and heated air, from or passing through
ignited fuel, is made to circulate in flues ; or indirectly, when ignited fuel is applied to
boil water, and the hot vapour, or the wator itself, is impelled through tubes of metal or
other conductors, either to heat the air of the house at once, as in most cases, or to
heat masses of brickwork, sand, gravel, rubble, or earth, tan, or even water (H o rt.
T ra m ., vol. iii.), wliich materials may afterwards give out the heat so acquired slowly to
the atmosphere of tho house. But heat is also occasionally suppUed from fermenting
vegetable substances, as dung, tan, leaves, weeds, &c., applied either beneath or around
tho whole or a part of'the house, or placed in a body within it.
2056. I n p a rtic u la r situations heat may he obtained fr o m anomalous sources, as in Iceland,
Toplitz, and Matlock, from hot springs; and perhaps in some eases, especially in
coal districts, from a basement composed of certam compounds of sulphiu' and iron, &c.
Dr. Anderson (T re a tis e on the P a te n t H othouse) proposed to preserve the supei-fluous
heat generated by the sun in clear days, and to retain it in reservoirs placed under,
above, or at ono side of tlie house, re-admitting it as wanted to keep up the temperature;
but the plan, though ingenious and philosophical, required too much nicety of execution,
and the clear days in this country arc too few to admit of adopting it as a substitute for
heating by ignition. Heat must not only be produced in hothouses, but its waste avoided,
by foi-ming as large a portion of the cover as possible of materialB through which it
escapes with difficulty, as far as tills is consistent with other objects. Hence, in certain
classes of houses, the side to the north is formed of opaque and non-conducting materials.
2057. L ig h t is admitted by constructing the roof, or cover, of transparent matter, as
oiled paper, talc, or glass (the last being found much the best material), joined to as small
a proportion of opaque substances, as timber or metal, as is found consistent with the
strength requisite to bear the weight of the glass, resist tho accidents of weather,
All plants require perpendicular light, but some, as mauy succulents and others, which
throw out, or are allowed to radiate thoir branches, require the dii'cct influence of light
on all sides; others natm-ally, as creepers or climbers, or artiflcially, when rendered
creepers or climbers by tho art of training on walls or trellises, require direct light on
ono side only ; and hence it is, that, for certain purposes of culture, hothouses answer
perfectly well when the transparent covering foms only a segment of their transverse
section, provided that sogmoiit meets the sun’s rays at a large angle the gi-cator portion
of the growing season. This, of course, is subject to limitations and variations accordmg
to circumstances, and has given rise to a groat variety in tho external foms of hothouses,
and the angles of their roofs. It decides, however, the necessity of placing all houses
whoso envelope is not entirely transparent, with their glazed side to the somli.
2058. T he introd uction and management o f lig h t is the most im p o rta n t p o in t to attend to
in ihe construetion o f hothouses. Every gardener knows, that plants will not only not
thrive without abundance of light, but wiU not tln-ive unless they receive its direct
influence by being placed near to, or at no great distance from, the glass. The cause of
this last fact has never boon satisfactorily explained. (S ow e rb y on L ig h t a n d Colours,
1816.) It seems probable that the glass acting in some degree like tho teiangular
prism, partially decomposes or deranges the order of the rays. It is an important
fact, also, that light in nature is always accompanied by heat; and, therefore, it
shoMd not only bo an object to admit the sun’s direct rays in clear weather, when ho is
visible, but even when the rays are reftactod and deranged hy clouds and vapours, when
he is invisible. , . . , „
2069. T he theory o f the transm ission o f lig h t through transp arent bodies is derived trom
a well known law 'in optics, that the influence of the sun’s rays on any sm-faco, both in
respect to light and heat, is directly as the sine of the sun’s altitude; or, in other words.
directly as his perpendicularity to that surface. If the surface is transparent, the number
of rays which pass through the substance is governed by the samo laws. Ihus, it
1000 rays fall perpendicularly upon a surface of the best crown glass, the whole will
pass through, except about a fortieth part, wliich the impunties of even the finest
ciTstaJ, aocSrding to Bouguer, will exclude ; but if these rays M at an incidental angle
of 76° 299 rays, according to tho same author, will be reflected. The incidental angle,
it will’be recoUectod, is that contained between the plane of the fallhig or impinging
ray, and a pei-pendicular to the surface on which it falls. „ „. , , ,
2060 T h e benefit derived fro m the sun’s infiuence on the roofs o f hothouses depends, as
far as respects fom of surface, entirely on this principle. Boerhaave applied it to
houses for preserving plants'through the winter, and of course required that the glass
surface should be perpendicular to the sun’s rays at the shortest day, when most heat and
litrht were required. MiUer (D ie t. art. S u n ) applied it to plant-stoves, and prefers two
afeles in te r o o f; one, as the upright glass, to meet the winter’s sun nearly at right
angles and the other, as the sloping glass, to meet him at an angle of 45 for summer
use and “ the better to admit the sun’s rays in spring and autumn. Wiikmson
(H o rt. T ra n s ., vol. i. p. 161.) prefers this angle (46°) in all houses, as do most gardeners
probably from habit; bnt Knight prefers, in forcing-houses at least, such a slope of root
? ShaU he at right angles to the sun’s rays at whatever season it is intended to ripen tlic
fruit. Ill one of tho examples given (H o rt. T ra n s ., vol. i. p. 99.), his object was to
produce a large and highly flavoured crop, rather than a very early crop of grapes ; and
he accordingly flxcd upon such a slope of roof as that the sun s rays might be perpendicular
to it about the beginning of July, the period about winch he the crop to
ripen Tho slope required to effect this purpose, m latitude 62°, he found to fom
an ante of 34° ivith tho plane of the horizon. In the application of the same princip e
to the peaeh-house (H o rt. T ra m ., vol. i. p. 206.), in order to ripen the fe lt aboiit mic -
summer, the roof was made to form an angle with the honzon of 28 . Both these
houses, Knight assures us, produced abundant crops perfectly ripened.
2061. A s d a ta to determine the angles o f glass rao/s, the following are laid down by
Wiikmson. The angle contained between the back waU of the fm c ing -lw use, and tho
inclined plane of the glass roof, always equals tho sun’s altitude, when his rays fall perpendicularly
on that plane, provided that the inclination of the plane to the h?"?™ ^
at an angle not less than 28° 2', nor greater than 75°.^ Within the above limits, the
sun’s rays are perpendicular twice in the year, once m going to, and once m rcturmng
from, the tropic. Hence, thou, having determined in what season wc wish to have he
most powerful effects from the sun, we may construct our houses accordmgly, by the
following rule ;-„Make the angle contained between the back wall of tho house and its
roof equal to the complement of latitude of the place, less or more tho suns decimation
for that day on which we wish his rays to fall perpendicularly. From the vernal to the
autumnal equinox, the declination is to bo added, and the contraiy. Thus, to apply
these principles to the slope of roof recommended by Knight, for ripening grapes m
July, say at London, wc have
Latitude of London - - ~
Sun’s declination on the 21st July - 17 31
33° 58', or-34° nearly.
Wilkinson adds, that “ as wo want the genial wai-mth of the sun most in spring,
therefore, for general purposes, that construction would perhaps be best which givra
the greatest quantity of pei-pendlcular rays then. If the mclmation wcie 45 , the sun s
rays would to perpendicular about April 6th and September 4th. And as the rays
would vary very littlo from the perpendicular for several days before and after the
Cth of April and September 4th, the loss of rays arising from reflection would, as appears
from the aimoxod table, be nearly a minimum. Even at the ivmter solstice, tho foss by
the obliquity of the angle of incidence would be only two m 1000 more than when the
rays fall perpendicularly, as appeai-s by Bouguor’s Table of Rays reflected from Glass.
Of 1000 incidental rays when the angle of incidence is
- " --- " ■ - 34 are reflected.
27
25
25
87° 30' 584 are reflected. 75° 299 arc reflected. 40°
85 543 70 222 30
82 30 474 65 157 20
80 412 60 112 10
77 30 356 50 57 1
. 25”
( H o a rt.
r t T ra n s ., vol. i. p. 164.)
When, in addition to this, it is considered, that the slope of 45° is the least that will
effectually drain tho wator from tho intervals between the lapping over oi the paMS
of glass, that angle appears to us, as Wiikmson suggests, decidedly the best slope foi
general purposes.