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total proportion does not amount to 1-114th part of the entire contents. In regard to light, therefore, this may be considered as perfect a roof as is practicable consistently with having the panes of glass of a moderate size. On comparing the columns of expense, repairs and durability, it will also be found the most œconomical in every other respect.

No. 2. The same as No. 1; but glazed in the ridge and furrow

manner.

No. 3. Iron rafters every six feet, and light iron frames placed on them, to be raised by levers as in Fig. 4, PL. V.

No. 4. Sashes four feet four inches broad, iron styles one inch and a quarter, and iron astragals, the styles of the one sash placed over those of the other, and screwed together, so that when finished the glass will be in two planes. The front sashes open, but the roof is fixed. This is an excellent plan for narrow houses, and in œconomy and durability comes nearest to No. 1. I have erected here a greenhouse, thirty feet long by ten feet broad, nearly on this plan.

No. 5. The same general plan as No. 4, but the astragals formed of wood.

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No. 6. Wrought iron rafters, uprights, imposts, &c. the front sashes to open, and a part of the upper sashes to slide down; the whole nearly as in a specimen lately erected, in copper and iron, at the Union Nursery, King's Road, by Mr. Jorden of Birmingham.

No. 7. Cast iron standards, rafters, and imposts; the sashes with wooden frames, with an inner frame of iron, and filled in with wrought iron astragals. The rafters one inch and half broad, styles and rails two inches. Astragals three fourths of an inch. There are twelve rafters and twenty-four sloping sashes, which slide past each other, and eleven front sashes, which open outwards. This may be reckoned the most improved method at present in use for erecting common houses, and is the style I some time ago adopted in an extensive range at Dan y parc already mentioned.

No. 8. Constructed entirely of timber;-wooden uprights, imposts,

rafters, styles, and rails. Rafters three inches broad and fourteen inches deep. The styles and rails two and half to three inches broad. The front lights sliding past each other. The whole according to the most recently erected wooden houses, as at Kensington Gardens, Kew Woodlands, Chiswick, Wanstead House, and even the curved wooden house erected by Mr. Palmer at Kingston.

On examining the table, it will be found that this style is by far the most expensive in respect to painting, and other annual repairs; and that its comparative durability is the least of the whole.

I am aware that tables of this description are liable to many objections, being generally exaggerated, so as to show the advantages of some one plan in preference to all others. I may however appeal to every experienced person in behalf of the moderation of that here presented. The first style, No. 1, admits of no dispute, as any reader may calculate the space darkened; and in respect to No. 8, or the wooden roofs, I am sure that every one will allow that if I had stated one half as the proportion rendered opaque by the massive rafters and clumsy sashes, it would not have been too much. Had I calculated the ends of all these houses, in connection with the roof, the proportion would have been much greater in favour of No. I. and against No. 8; for no form or plan is so well calculated to admit light as the quarter dome ends.

In most of the designs given in PL. V. as well as for the hemispherical and campanulated house, I have recommended an outer curtain or canvass roofing. In respect to the benefits to be derived from adopting either an outer or inner curtain, I have never had any doubts; and so far back as 1804 I made several attempts by constructing models, and fitting up a house for public inspection at Edinburgh, at my own expense, as well as by publishing on the subject, to introduce them. From various causes, however, but chiefly from the force of prejudice, which in Scotland is particularly strong, they have been but very partially adopted. One reason, in as far as respects England, has been the want of conviction in the minds of gardeners that so slight a covering as a piece of thin canvass would have much effect;

but since Dr. Wells has published his valuable experiments on dew, the inutility of such a covering can no longer be pleaded as an excuse. The grand defects in this branch of horticultural œconomy have proceeded in general from inattention to the important fact illustrated by Professor Leslie, Count Rumford, Dr. Hoffman, and various others, "that radiating heat is subject in all cases to the optical laws which govern the reflection and refraction of light'." Hence the importance of placing a screen at a small distance from a heated body to reflect back the rays of heat which proceed from it in straight lines. The effect of such a screen, formed of a ragged cambric handkerchief, one yard square, placed horizontally over the surface of a grass plot, and so as to be six inches above it, but open on all sides, was to preserve a temperature from eight to eleven degrees higher than that of the grass fully exposed to the sky: " a difference," the doctor observes," sufficiently great to explain the utility of a very slight shelter to plants, in averting or lessening injury from cold, on a still and severe night2.

From all that I have been able to observe, much greater injury is sustained by plants from being overheated in the night-time, or from accidents happening to flues at that season, than by cold alone. To retain, by a double roofing, either of glass, canvass, boards, or otherwise, the heat obtained during the day, appears therefore a much safer mode than to trust to powerful fires, to be kept burning during the night, to supply the heat constantly radiating from the glass at that season, and lost in the atmosphere. Even in point of œconomy the double roofings deserve to be adopted in many situations, since they would both save fuel and nightly attendance. They are very generally used on the continent, and especially in the northern parts of Germany and Russia, where double windows are often adopted for the same purpose.

Young's Nat. Phil. lect. li. p. 673.

An Essay on Dew, and several Appearances connected with it: by W. C. Wells, M. D. F.R.S. London, 1814.

Independently of the safety and œconomy of these outer roofings, there is another advantage attending their use, which is by no means inconsiderable: this is, their preventing the inner surface of the glass from being covered with moisture or condensed exhalations, (as some gardeners call that appearance,) and which, by dropping all over the house, is so prejudicial to the plants. Some imagine that these dewdrops are formed chiefly from putrid effluvia exhaled from the plants, and that the only method of preventing them is to allow the foul air to escape or condense, by some contrivance in the upper part of the roof'. But the truth is, such drops are formed in an empty house in equal quantities as in one full of plants, under certain circumstances, viz. the cooling of the air of the house after having been heated to a degree considerably above that of the atmosphere.

With respect to the detail of construction of a canvass outer roofing, for the campanulated and globular houses, it should be formed in gores, so arranged as to wind up on rollers, which may be concealed under the projecting coping of the front or basement wall; and stout wires or iron rods bent to the same curve as the astragals of the roof are to be fixed, one end in the stone coping, and the other in the apex of the cone; and supported from the astragals at intervals, so as not to be less than ten inches from the glass. Then, by having slight rods or wires, like those used for fastening stair-carpets, attached across the gores, at the distance of every three or four feet, when drawn up by a cord and pulleys these cross rods would bear upon the long rods or ribs, and the rotundity of the roof would keep them so tight as to prevent their being disturbed by the wind.

If doubts are entertained on this head, the following contrivance may be adopted. Let each gore have a cord twice its length fixed at its narrow end, so that when the whole are drawn up in the evening, there will be as many long cords hanging down from the summit of the roof as there are gores. Half the cords, taking them alternately,

! Appendix to Abercrombie's Practical Gardener, 2d edition, 1817.

are then to be laid athwart the gores, and firmly tied to a hook, or to one of the rollers under the coping. The other half are also to be laid athwart and tied, but in a contrary direction, and so as to form the roof into lozenges, in the manner corn ricks are thatched in some districts. This plan would effectually prevent the gores from being ruffled by any wind whatever.

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Where the roof is moveable, in the manner represented in the seetion PL. VII. Fig. 3, of course no wire or rod for the gores to rest on necessary, the line for opening the sashes answering every purpose. In longitudinal houses, such as those in PL. V. nothing can be more simple than their construction; and in all cases their expense is so moderate as to be no object. The trouble of working them twice a day will be reckoned an objection; but if they preserve from ten to fifteen degrees of heat, which would otherwise escape from the house and have to be made up by a larger supply of fuel, this will amply compensate for a little additional trouble.

As most of the foregoing improvements depend on the adoption of iron or other metals instead of wood, and as some eminent gardeners! do not consider that there have yet been sufficient trials of the former materials to justify the rejection of the latter, I shall here notice the objections commonly urged against metallic sashes and rafters. These objections have in almost every case proceeded either from the badness of the workmanship, a mere dislike to innovation, or from the use of improper metals; and they may be resolved into—

1. Breakage of glass, from the contraction and expansion of metallic sashes.

2. The conducting powers of metals, by which they carry off the heat.

3. Their liability to rust.

4. Their attraction of the electricity.

1 Abercrombie, and Mean in Abercrombie's Practical Gardener, App.-also Nicol in Villa Garden Directory, &c.

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