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Fig. 3. Is the section of what Adanson considers the best form of a greenhouse. The glass being perpendicular, admits the sun's rays in the winter months: it is also less obnoxious to frozen dew, or snow, than sloping houses. In summer the plants are supposed to be placed open air.

in the

Fig. 4. Is Adanson's section of a stove or house for general purposes in the neighbourhood of Paris. The ground plan may either be oblong, elliptical, or a tetragon or polygon, similar to the glass. The coincidence of this and some other diagrams given by Adanson, with the semidome proposed by Sir George Mackenzie, is remarkable; and the fact of such houses being common in Holland in Adanson's time, proves that there is but little new at least in this branch of horticulture. These two diagrams may be dated 1760.

Fig. 5. Shows the slope proposed by Nicolas Facio de Douillier for a fruit wall in latitude 52°. I have shown this wall as placed on arches; but those erected at Belvoir Castle, under Facio's direction, were on banks of earth, as indicated by the dotted lines a, b, c, in this figure. I shall afterwards describe a mode of erecting such walls with a moderate quantity of materials, and of fitting a curtain to them, so as to render them in some degree a medium between a hotwall and a glass case. Facio's plan is dated 1699.

Fig. 6. Is a design for a Dutch vinery, for the earliest forcing season. The slope of the back wall and that of the glass are nearly alike, a circumstance noted by Adanson in the quotation given from him, and worthy of imitation. The large flues and shed behind of temperate air, to be exchanged with that in the glass case in severe weather, when the external air cannot be admitted, show a degree of care and science beyond what is generally bestowed or discovered in this country. This diagram is nearly a fac simile of one given in the Encyclopédie Méthodique. It may be dated 1730.

Fig. 7. Shows the slope of Mr. Knight's vinery,

Fig. 8. The slope of Mr. Knight's peach-house.

Fig. 9. The slope recommended by Miller and the Rev. T. Wil

kinson for general purposes, and which seems to be more generally adopted than any other by Mr. Aiton, in the houses constructed under his direction at Kew, Kensington, &c.

Fig. 10. Is the slope for cucumber frames and pits, adopted by Mons. Thouin in the Jardin des Semis at Paris', and by Mr. Knight in Herefordshire?.

Fig. 11. Is a vertical section of the glass semidome proposed by Sir George Mackenzie, to be afterwards described.

a. Situation of the flue which is led round under the glass.

b. The trellis.

c. The back wall.

Fig. 12. Is a section of a house designed by Mr. Braddick of the London Horticultural Society, and erected at Mr. Palmer's at Kingston, Surrey. The idea is avowedly taken from Sir George Mackenzie's semidome. Both plan and section are parts of an ellipsis. The sashes open on the principle recommended by Adanson and Mr. Knight, and adopted in the London sky-lights.

a.

Is the front flue and walk, over which is a wire trellis

under the glass for vines.

b & c. Temporary trellises for peaches, till the vines cover the

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Much additional interest has recently attached to the subject of glass roofs, from the plan proposed by Sir GEORGE MACKENZIE; the universal principle of which, together with its elegant appearance, has very naturally fascinated a number of ingenious horticulturists. In 1812 Sir George built a house at Coul, his seat in the Highlands of Scotland, for the production of grapes and peaches, which he describes in the Transactions of the Caledonian Horticultural Society as

' Essai sur l'Exposition et la Division méthodique de l'Economie rurale, &c. Paris, 1814. 2 London Hort. Trans. vol. i.

on an improved and œconomical construction'; but which he afterwards discovered to be of an opposite description; and with that candour and openness to conviction which is or ought to be the case with

The general form of the house was not different from that of common oblong houses, with sloping glass. The improvements consisted in having no front or end glass; the sloping glass fixed with a trellis under it for vines, and with transverse trellises for peaches. These transverse trellises were formed in the original plan of open work in the usual way; but in any fu ture house built on this principle, transverse brick walls were intended to be substituted, as better adapted to retain the heat. The object gained was nearly double the extent of trellis, and a greater command of temperature; from which of course was expected more fruit, at less expense. It strikes me as somewhat remarkable, that this plan found its way into the Transactions of a society composed chiefly of practical men; since any gardener could have predicted its failure; and it does not appear to merit being recorded, from any general reasoning calculated to excite new ideas. Besides, cross trellises had been already tried in Scotland, at Sir John Stewart's, near Lanark.

I hope I shall not be suspected of enlarging on this subject from any adverse feeling towards Sir G. Mackenzie, for whom I have a very high respect. I state the fact entirely for the sake of proving to my readers, that though the Transactions of these societies abound in valuable communications, yet that every plan introduced, however plausibly recommended, must not be implicitly followed, or considered as at once setting at nought old practices; nor every result, however interesting or partially known, considered as a new discovery. Having adduced an instance in support of the former, I shall prolong this note by adding one to illustrate the latter proposition.

Fruit trees are apt very frequently to become too luxuriant in branches and foliage, and to produce but few blossoms. It is, therefore, a most desirable object to counteract this tendency; and in the papers of both societies the results of various attempts are given, attended with different degrees of success; to almost each of which the inexperienced reader would be apt to award the palm due to original discovery. One gardener lays bare their roots during the winter season; another during the summer months; and both find more blossom produced the following year. Another cuts off part of the main roots; a fourth saws the main stems of his trees half through; a fifth bends down the extremities of the branches, or puts a cincture of wire round them where they issue from the main stem or leader. Each of these modes, as well as several others, will be attended more or less with the desired effect; and to render any one of them known to such as are ignorant of the whole, though it be of importance, need not be mistaken for originality. The fact is, the success of all the modes depends on one simple principle known and practised before the time of Vitruvius or Virgil, and consists in preventing the main body of the cortical sap, while in action in the branches, from returning to reinvigorate the roots. It has always been known to physiologists and to many scientific gar

every one who attempts to devise or introduce improvement, and which may be said to be characteristic of Sir George Mackenzie, he was the first to notice the mistake he had fallen into1.

The failure or disapprobation of this plan would, no doubt, lead the active and philosophic mind of its author to take a more scientific view of the subject; and, in as far as respects the principle of perpendicularity to the sun's rays, he seems to have been particularly happy in reaping the fruit of all his contemporaries in that department of construction. For, by going one step further than they have done, he has hit on the ultimatum of the principle.

Every reader who is in the habit of seeing the London Horticultural Society's Transactions will be aware that I allude to a paper in their second volume, entitled “On the form which the glass of a forcing-house ought to have, in order to receive the greatest possible quantity of rays from the sun: by Sir George Mackenzie, bart. F.R.S. read August 1, 1815."

In this paper Sir George observes, that he does not presume that any of the members of the London Horticultural Society are ignorant of the solution of so simple a problem as " what that figure is which will receive the greatest possible quantity of the sun's rays at all times of the day, and at all seasons of the year." "That form," he then

observes, "is to be found in the sphere; and it is the segment of a globe which I propose for the glass, when it is desired to receive into a forcing-house the greatest possible quantity of light. The segment is one fourth, or a semidome, which I consider as sufficient, though to

deners, and is best performed by cutting off a circle of bark round each branch when the tree is in blossom. Buffon tried this practice both on fruit and forest trees, in the year 1733, with great success, and has given an account of his experiments in the Mémoires de l'Acadé mie des Sciences, A. D. 1788. See also a general history and rationale of the practice in the Journal Physico-Economique, communicated by Mons. Suriray Delarue, 1803; Darwin's Phytologia, vol. i. p. 393; and Botanic Garden, vol. i. canto 4; and various other authors, as Hales, Grew, Adanson, &c.

1

Introductory remarks to the paper on the Semidome. London HortTrans. vol. ii.

catch the sun at all times during summer the segment would have to correspond with the greatest circle which the sun describes."

The plan and elevation given are next described.

The plan is a semicircle, the diameter a back wall. The elevation a semidome of glass, placed on a low front wall. The entrance is in the back wall, or rather in a sort of porch to a recess in that wall, against which recess the trellis is placed. Air is proposed to be admitted by openings in the front wall, and at the top of the back wall; or, it is added in a postscript, " by making the glass semidome in two parts, and placing it on rollers, in the manner of an observatory dome, the whole might be moved with great ease and safety, so as to expose the plants in the interior to the direct influence of the sun, &c."

The author of the article "Horticulture" in the Edinburgh Encyclopædia has introduced this plan among the recent improvements in hothouses, of which he has a very favourable idea; and adds, “ Mr. Knight, we understand, highly approves of this invention, and is of opinion that it will answer every purpose better than any form hitherto contrived."

Whilst I cordially agree with the writer of the article alluded to, as to the elegance of the plan, and consider it in many respects a desirable addition to horticultural architecture, I certainly must dissent from the opinion stated (doubtless from good authority) to be that of Mr. Knight. So far from thinking it fit for every purpose, I am persuaded that it cannot be so successfully and œconomically applied to the forcing department as the best plans now in use. Whether my reasons are well grounded, must be left for the reader and future experience to determine. There can be little harm in submitting them if the plan is really "better than any form hitherto contrived," while if to some its advantages appear doubtful, discussion may tend to remove those doubts.

Figs. 1 and 2, (PL. II.) may be considered as an outline of the ground plan and elevation; and Fig. 11 of PL. I. as a section of this design. Though mere dimension has little to do with the properties of any

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