they avo fastened by running a little melted load into the interatices J"
the angles two large screws arc inserted into the styles of the lights to fasten them
“ Air is admitted by sliding the front sashes, and by ventilators
in the back wall, which are made to swing on
pivots, and open by means of long iron rods, ha'ang
holes to fix on pins driven into the wall, so as to regulate
tlie proportion of air at plcasui'e. To prevent
these ventilators from being unsightly, a square piece ot
treUis-work is placed over the opening inside the house._
(P a x to n ’s M ag azin e o f B o ta n y and G ardening, vol. n.
p. 84.) The sash-bars {fig s. 612, 613.) are fitted with
grooves for tlie glass to fit in, that the putty may not
sheU off and admit the wet. “ The house is heated by
four fires, and the flue-covcrs arc dished for the advantage
of steaming.” (Ib id .) “ The walks are 3^ ft. wide, and
are composed of bars of wood 4 in. broad, and 1^ m. tliick, nailed
4 in. apart upon strong bearers.” ( Ib id .)
Su b se ct. 3. D e ta ih o f the C onstruction o f the glazed P a r t o f Hothouses.
2078. T h e glazed cover may either be wholly fixed, wholly movable, or partake of
both modes. Each of these varieties may be considered in respect to component pai-ts
and materials. „ , ^ .
2079. F ix e d roofs arc cither fonned of a series of bars of iron or wood, proceeding
at once from the front parapet to the back wall, or from the base to the centre ; or they
may be composed of sashes, placed beside each other, or between ratters,_ as m common
lean-to houses. Roofs of this fixed kind have been approved of by Kmght for vines;
by Beattie of Scone, for peaches; and by some cidtivators for the culture of pines and
palms; but, except for the latter purpose, the general experience of gardeners is (in
om- opinion, vciy justly) against them. It is to be observed, that in aU cases of fixed
roofs, shutters for ventilation are fonncd in the parapet, and in the upper part of the
back wall immediately under the roof. Economy in first cost, and less breakage of
glass aftcrwai'ds, are the chief arguments in their favour : the latter advantage, however,
is generally denied, it being improper glazing rather than the moving of the sashes,
which occasions the breakage of glass.
2080. T h e m ate ria l o f fix e d roofs is generally iron, as being less bulky in proportion
to the strength required, most durable, and admitting, in the case of curvilinear roofs,
a curvature to be formed at less expense than it could be of timber. In_ these roofs, in
general, no other bars or opaque bearers are required than those for receiving the glass ;
and hence thcir simplicity and unity with regai-d to component parts, and the equal
degree of transparency in every pait of the surface.
2081. M ovab le roofs arc either composed of sashes 6 ft. or 8 ft. long, and 3 ft. or 4 ft.
wide, which slide over each other and are moved by cords and puUeys ; or they are
fomed of sashes which open outwards at their lower extremety, and have hinges at the
upper part. Sometimes the lower sashes are made movable, while the upper part _ of
the roof is fixed, as in the case of domes, or acuminated roofs, the upper part of which
rises in the manner of a skylight.
2082. T he m aterials o f movable roofs avc most commonly timber ; bnt frequently also
timber and iron, or timber and copper joined together. Tlms, cast-iron and -wi'ought-
iron rafters are frequently used ; and in these ai-e placed sashes with styles and rails of
timber, and bars of copper, and of cast or wrought iron.
2083. A rrangem ents f o r opening the sashes o f hothouses are of the utmost importance,
as the health of the plants depends in a great measure on the facilities possessed by the
gardener for admitting and changing the air of the house ; but as there are various
modes of opening the sashes of hothouses, the details will be given under the head of
ventilating hothouses. (See Subscct. 10. p. 618.)
2084. Arrangements fo r covering the roofs o f hothouses by hoards, canvas, or mats, to be
lifted or rolled up or down, might be easily conrtived, and advantageously used ; but,
except in pits and low hothouses, they are not thought worth attending to \ it being
considered better to gain the admittance of all the light possible, than to lose it for the
sake of a little economy in fuel.
2085. T h e p illa rs , o r props, w hich are placed on the p a ra p e t to support the rafters,
whether of timber or iron, are generally formed of the same thickness as the rafters,
because similar sashes are placed between them.
2086. In te rio r uprights, to support wide roofs, are almost always of iron, either
wrought-metal or small cast-fron columns, sometimes forming intersecting arches, or
treillaged capitals, or connecting imposts for training creepers.
2087. T he w all-p late , o r coping o f the p arap et, is sometimes a plate of timber, sometimes
of stone, and occasionally of cast-iron. Wherever upright glass is not employed, ■
it must, of necessity, fonn also the guttering for the water of the roof ; and, at the same
time, for the water which condenses on the glazed inside of the house.
2088. Objections to m etallic roofs. In general, it may he observed, that, tiU la te ly ,
gardeners had a prejudice against metallic roofs. We shall here, as briefly as possible,_
enumerate thcir objections ; which are, expense, irast, breakage of glass, abduction of
heat, and attraction of electricity.
Expense. Metallic houses ai-e, in general, rather more expensive than wooden ones ;
but they admit more light, and are more durable and elegant.
B u s t. That all ordinaiy metals ai-e liable to rust is undeniable. This objection cannot
be got rid of. The reply is, balance against it the advantages of light and durability ;
and take into consideration, that cai'cful painting will, in a gi'eat degree, prevent it.
Knight observes, if one third of the sum requisite to keep a wooden roof properly painted
be expended upon an iron roof, no injury will ever be sustained from the liability of that
to suffer fi'om rust. (H o rt. T ra n s ., vol. v. p. 231.)
B reakag e o f glass. This is altogether denied, as respects cast or wi'ought iron, at
least ; and if applicable at all, can only be so to copper or compound metallic roofs,
wlierc wcalcncss produces a bending of the sash ; or where corrosion or unequal expansion
of improper mixtures of metals, as iron cased with copper, occasions a twisting of
the bar. Cast-iron or solid wrought-iron frames have never been known to occasion the
breakage of more glass than wood. The expansibility of copper is greater than that ol
brass, and that of brass greater than the expansion of iron, iu the proportion of 95, 89,
60. (Y o u n g ’s L e e t.) Consequently, copper is above one third part more likely to break
glass than iron ; bnt, when it is considered that a rod of copper expands only one hundred
thousandth part of its length with every degree of heat; and that iron only expands
the one hundred and sixty-six thousand six hundred and sixty-sixth part, the practical
effects of onr climate on these metals can never amount to a sum equal to the breakage
Abduction o f heat. The power of metals to conduct heat is an objection which, like
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