Зображення сторінки
PDF
ePub
[ocr errors]

1. The effects of change of temperature are acknowledged to be inconvenient where a frame, entirely constructed of metal, is tightly fitted into grooves, in which it is intended to slide; or even in rabbets to be opened by hinges. It is acknowledged also, that copper or brass frames are more liable to this objection than such as are formed of iron; the expansibility of these metals being as 95 and 89 to 60. But frames formed wholly of metal, and fitted into grooves, in which they are expected to slide, or move readily by hinges, form no part of the designs which have been submitted, nor will, I am persuaded, form a part of any good plan. The iron roofs which I have recommended are either wholly fixed, or the moveable sashes are laid on the rafters, and the styles of the one sash placed over those of the other alternately; or, they are iron frames fitted into wooden frames; in either of which cases their contraction and expansion will not affect their motion, nor occasion their twisting.

The breakage of glass has resulted chiefly from the twisting of long astragals, composed of broad narrow hoops of copper or brass, soldered or otherwise attached to a narrow moulding of the same metals or of lead, and not sufficiently strengthened by cross bars. This defect is to be remedied by making the rabbet and moulding stronger, and by lessening the distance between the bearing bars. In solid iron astragals, however, it cannot take place.

It can seldom happen that the mere expansion of iron in length can be the cause of the breakage of glass. Dr. Young' states the expansion of that metal to be of its length for each degree of eleva

1 Lect. li. Sir George Mackenzie, in the 2d vol. of the Edin. Hort. Trans. mentions the expansion of glass as occasioning its breakage; but as the expansibility of this material is still less than that of iron (Young), and as the panes are seldom more than eight inches square, and are bedded in putty, it is difficult to conceive any degree of expansion produced from our atmosphere capable of producing any practical effect. If according to Dalton the expansion of thin glass be nearly as that of iron, then astragals of that metal will be less likely to occasion breakage than wooden sashes.

tion of temperature: hence, supposing an elevation of fifty degrees, an astragal or rod of twenty feet in length would only be increased one inch and one third, or less than one line per foot, which could have no sensible effect.

As I propose in almost every case to use curved rafters, and astragals instead of front and sloping glass, that circumstance will have a tendency to direct what expansion there may take place outwards, and thus to prevent twisting.

In respect to copper, glass has certainly been broken in several cases from the too great length of astragals formed of that metal, which expands with every degree of heat to part; and hence the expansion in an astragal of twenty feet would be nearly two inches; which might do injury: but independently of this objection, the deleterious nature of the rust of this metal ought to exclude it from every description of hothouse.

2. The objections drawn from the conducting powers of metals, in respect to heat, are of a more serious nature; but though they cannot be entirely removed, they admit, in common with every other evil, of being greatly alleviated, if not of being rendered practically of no force. In greenhouses and such glass cases as do not require a high temperature during the winter months, this capacity of metals is of little consequence; but in stoves, in early forcing-houses, and in hot bed frames, its influence will be severely felt if not counteracted. The means of counteraction are painting, which lessens in a considerable degree the conducting as it increases the radiating powers of metals; where iron rafters are used, covering their inner edges with a slip of wood; and tinning iron astragals lessens in a slight degree the conducting power of that metal, and prevents its rusting. But the most complete mode is by applying an outer curtain, as before described. This remedy is simple, œconomical, and effectual. Where these pre

'The conducting powers of the metals are as under, viz. gold, silver, copper, iron, tiu, lead,

cautions are neglected or imperfectly adopted, the lost heat must be made good by increasing the supply from the flues: but I may repeat, as an important consideration, that it is always safer to preserve the heat already in the house, than to let it escape, and depend on the generation of more from the flues. I would therefore for known reasons strongly recommend the adoption of an outer or inner roofing in every description of stove or early forcing-house1.

M. A. Thouin2, the botanical professor to the Museum of Natural History at Paris, has, in as far as respects their conducting capacities, urged the objections to iron frames with more force and ingenuity than any English writer. He had adopted in the Jardin des Semis frames entirely of iron for some hotbeds and pits constructed there for raising tender seeds. He found the iron expand in summer, so as to become too light for the grooves, and in winter their conducting powers carried off great part of the heat. As the frames are of very clumsy manufacture, and the mass of iron large in proportion to the volume of heated air within; and as in some of the pits there is no flue to supply the heat as it is withdrawn, and no outer curtain to retain it in any of them, the effect must undoubtedly be considerable. The remedy is only to be found in a complete outer curtain, or in a better original construction. The astragals alone should have been of iron, not the styles and rails; and by having them attached to an inner frame, such as will be afterwards described, the effects of contraction and expansion would not have been felt as an injury, and all the advantages of iron frames (which M. Thouin allows to be great) retained. It is obvious that the larger the volume of heated air, the

In the Niederlandische Hesperides, by Commelines, an old Dutch book on the culture of oranges and their preservation during winter in cellars, greenhouses, &c. the author mentions an amateur who bestowed great attention and expense on their culture, but whose trees never looked so well as those of a less opulent neighbour. On inquiry, he found his gardener used as many baskets of turf in one year, as his poor friend did in three. The obvious conclusion which Commelines draws from this circumstance is not without application in our own times. • Essai sur l'Exposition et la Division méthodique de l'Economie rurale, &c. Paris, 1814.

longer time it will require to cool; and consequently the conducting power of an iron roof will be injurious inversely as the size of the house. As the conducting power would from the greater cold of the atmosphere be greatest in the night-time, it may be considered that an outer curtain would counteract it sufficiently for all practical purposes; and it appears to me, no person who consulted taste (even while he might study utility) would ever put the expense of an outer, curtain, or some additional loads of fuel, in competition with the great advantages of metallic houses in point of admission of light, durability, elegance, and susceptibility of any shape and magnitude.

3. The objections taken from the rusting of metallic substances, and especially of iron and copper, if one may judge from some houses recently erected of the latter material, are not without force. In respect to iron, the cause of rust is to be found in bad workmanship; and is to be remedied by the important precaution of first fitting all the iron work together at the foundry or manufactory, and then taking it to pieces, heating it nearly to a red heat, and applying a coat of cool tar or common paint. Engineers have found that iron so heated will not rust for many years. Heating copper and brass previously to painting these metals, has little or no effect on them; but even if it had the deleterious nature of the rust of these metals-the risk of their exhibiting some rusty spots even under the best treatment-the certainty that they must in the end, in common with all metals, decay through oxidation-these things, coupled with the circumstances of some families having been nearly poisoned by the verdigrise dropping from copper astragals on grapes and cucumbers, ought to prevent the use of that metal in hothouses in any form, or under any disguise whatever.

Tin and lead, but especially pewter, do not rust, or their oxidation is so slow, as that, speaking practically, it amounts to nothing. As, in addition to this, they are worse conductors of heat than iron, their use as

Fulton in Treatise on Canals.

mouldings to iron rabbets, or for coating over solid iron astragals, is of considerable importance.

4. The conducting powers of metals in respect to electricity have been urged against their adoption in hothouses; but this objection hardly requires answering. Where they are entirely of iron they may be considered as one large conductor; where partially, a proper conductor may be applied. If this objection were tenable, what would have become of iron foundries? Besides, there are houses entirely of iron which have been in existence from five to fifteen years, without sustaining or occasioning any injury'.

These are the ostensible objections; but the real cause why metallic houses are so seldom erected has but little connexion with them. From the comparative difficulties of their execution, the regular hothouse builders do not like to meddle with them; and it is not likely, speaking generally, that either architects, builders, or tradesmen in full employ will take the trouble, and go to the expense of introducing an article, or perfecting a construction, from which they could gain nothing more than what they already possess-namely, a sufficient business. If the professional aid of a regular architect or hothouse builder is not called in by the party about to erect a house, the design and execution are left to the gardener, who generally is obliged to have recourse to the carpenter of his employer, and of course wood is his material. These circumstances would materially increase the expense of metallic houses to such private gentlemen as thought of attempting their adoption; and to the same cause has conspired the use of the more expensive metals, such as copper, brass, tin, or other compounds, instead of the cheaper and more durable article, iron. As œconomy is always an object pursued by every one, the first cost has also retarded their introduction, though this fancied œconomy will, in this instance at least, be found to end in its opposite.

1 Mr. Blackburne's vinery near Preston, a conservatory at Windsor, at East Sheen, &c. See the table in PL, VIII.

« НазадПродовжити »