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figure, yet I shall in examining the semidome adopt those given by its author. The diameter or length of the ground plan is thirty feet, and its radius, or height and width, fifteen feet. The trellis being placed against the back wall, is, of course, the greatest vertical section contained in the figure. Now, if the sun had an equally great effect upon the earth's surface, during every moment which he appears above our horizon; and if his influence on plants in a glass case were equally great when they are ten or fifteen feet, as when they are two or three feet from the glass, then would this design be unexceptionable, as far as respected the attainment of one object in view. But we are taught by experience that the horizontal rays of the rising and setting sun, from their obliquity, as well as from the mist and vapours through which they have to pass, have but little effect; and that the most vivid of his rays which at any hour of the day pervade a window or a hothouse, become so divergent a few feet within the glass as to produce attenuation in young plants, and to lose great part of their influence in ripening fruits. As therefore a large portion of the glass in this design depends for its effect on the morning and evening sun, and as great part of the trellis will be from five to fifteen feet from the glass, its disadvantages will be at once sufficiently obvious.to practical men.—It will receive but few and weak rays in the morning and evening, and what enters the glass in the middle part of the day will be too far from the trees.

On inspecting Fig. 1. the elevation of the trellis, it will appear that there is a circumferential portion of it, k, k, k, which at all events is not far distant from the glass; and a superficial observer would conclude that on this part of the house every advantage of the sun's rays would be obtained. But though this semiring, as it may be called, be nearer the open air or reflected light, than any other portion of the trellis, yet, from the figure of the house, it will receive least benefit from the sun's rays, for the following reasons:

1. As every line falling perpendicularly on the surface of a circle must be parallel to its axis; consequently, on whatever part of the

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semidome the sun's rays fall perpendicular to the glass, they must tend to the centre of the house, which lies of course at an equal distance from every part of the glass at the bottom of the back wall, viz. at n in Fig. I and 2. Therefore his influence on any other part of the trellis must always be inversely as the distance of that part from the centre.

2. As the quantity of heat and light produced by the sun's rays is as the squares of the sines of the angles of their elevation, it follows that his minimum of influence for each day is at his greatest declination; that is, at morning and evening. Therefore, when this circumferential portion of the trellis does enjoy the direct influence of the sun, it enjoys his least possible direct influence.

3. The maximum of the sun's influence will be at midday, when he is perpendicular to the central part of the glass dome (as at I, Fig. 1 and 2). · But when his rays are most perpendicular to his centre, they are most oblique to his sides. Therefore, when the sun has most power in communicating his influence, the circumferential part of the trellis has the least means of receiving it.

Ocular demonstration may be obtained to the same effect by inspecting the dotted lines in the figures. If a, b, c, in both diagrams represent the sun's path in the heavens on any particular day,―say on the 22d of June,-then a will represent sunrise, b midday, and c

sunset.

Hence the sun having advanced eighteen degrees in his diurnal course, or to E, his rays fall perpendicularly on the glass at e. At

e 2 the angle of incidence is 8o, at e 3 it is 7o, and at e 4 it is 10°; consequently, according to Bouguer's table of rays, at each of these places 25 rays in every 1000 are reflected.

Advanced 36 degrees, or to F, his rays fall perpendicularly at ƒ: at ƒ 2 the angle of incidence is 6o, at ƒ 3, 12o, at ƒ 4, 17o, and at each of these points the same proportion of rays as above is reflected.

Advanced 72 degrees, or to H, his rays fall perpendicularly at h. At h 2 the angle of incidence is 9o, at h 3, 19°, and at h 4, 29°: at

the two former points 25, and at the latter 27, rays in 1000 are reflected.

Advanced 90 degrees, or to his meridian at I, his rays fall perpendicularly at b. At i 2 and 3, 25 rays, and at i 4, 27 rays, in 1000 are reflected.

In this way an accurate estimate may be formed of the proportionate number of the sun's rays which fall on different parts of the trellis; and the absolute force of these rays may be similarly estimated from the dotted lines answering to the same letters in Fig. 1, which indicate the sun's altitude; leaving always out of the question atmospherical, dioptrical, and other accidental effects'. The conclusion is obviously what I first stated on general grounds; that the circumference of the trellis will not have sufficient solar influence, and that the central part will be too far from the glass, the intermediate spaces partaking of course of the two extremes. Therefore it does not appear to me the best plan hitherto devised for ripening fruit.

The radius of the semidome being fifteen feet, the contents of the trellis will be 353.42 feet; and of the semidome of glass 706.84 feet. Now in a common oblong house, with the trellis about eleven inches from the glass, a superficies of trellis is obtainable equal to that of the glass. Here it is considerably under one half of the glass's surface; so that in respect to expense of glass roofing, the difference between a common house and the semidome is as two to one against the latter.

The mass of air to be heated by one fire will be 353.858 cubic feet;

1 As many persons are not practically aware of the astonishing difference between the effects of the sun when perpendicular, and when oblique, to a transparent surface, I would suggest the simple experiment of placing a pane of glass between that luminary and the eye, and giving it different degrees of obliquity. By observing his effects also at different hours of the day in the large bell glasses used for protecting cucumbers, some idea may be formed of his operation on a dome. Divide one of these glasses vertically by a board covered with white paper. Wash this paper with muriate of silver and expose it to the south, similarly to the back wall of the semidome. The deoxidizing rays will take effect, and after some weeks that part of the paper on which the sun has had most influence will be the blackest.

giving 353.42 feet of trellis. But a house forty feet by twelve, and twelve high, contains 2880 cubic feet also to be heated by one moderate fire, and gives 660 superficial feet of trellis. Here the calculation is still greater against this design: so that on the whole it does not seem the best plan hitherto devised in point of œconomy.

Something would be gained for this figure by imagining the trellis of the same form as the glass, and placed eleven or twelve inches within it. The contents of the trellis would be increased, and the vaulted canopy of fruit and foliage in the ripening season would be most elegant; but one half of the trellis would be in shade great part of the morning, and the other half great part of the afternoon. At midday no part would be under shade; yet then the obliquity of the sun's rays to the sides of the dome would exclude a considerable circumferential space next the back wall from their direct effects.

Joined to a vertical trellis, there might be a horizontal one, which would double the extent of space for training. But the objections to a vertical trellis apply with equal force to a horizontal one; and unless mere quantity of fruit, without regard to quality, were the object, this plan is not to be recommended.

Probably the best form of a trellis would be that of a flattened semidome, or segment of an oblate spheroid, whose boundary lines should coincide with those of the glass, and whose lesser axis of the elliptic base should not exceed two thirds of the axis of the dome. Fig. 3, PL. II. is a horizontal section of such a trellis. When, however, we compare, in imagination, the sun at midday perpendicular to one spot of a glass semidome, and the sun's rays at midday passing perpendicularly through every pane of a common sloping roof, properly adjusted to the ripening season, and exercising their full influence on the crop of fruit spread out immediately under the glass, the difference is striking, and decisively against the semidome as a forcing-house.

As fruits to be ripened in perfection must be kept very near the glass, a much better form than a semidome is a segment of a circle, frequently employed by the Dutch, and of which an instance may be

seen in a vinery built by the late Mr. Hope of East Sheen1. A portion of an ellipsis, also in use among the Dutch, is still better, and has recently been adopted by Mr. Palmer at his villa at Kingston', as suggested by an active member of the Horticultural Society, Mr. Braddick of Thames Ditton'.

Fig. 4, PL. II. is an outline of the ground plan of Mr. Hope's vinery; and Fig. 5 of that of Mr. Palmer;—both from memory.

But though the semidome cannot be adopted as a forcing-house without considerable disadvantages, yet it may be used in some cases even for maturing fruits; and as its appearance is most elegant, and it admits of a happy combination of lightness with strength in the construction; it may be considered, with the improvements of which it is susceptible, as a most valuable acquisition to the horticultural architecture of this country. For greenhouses, conservatories, and every description of botanical hothouse, it is indeed peculiarly applicable.

As an improvement, I should propose in all cases to acuminate its apex, the better to throw off the rain; and in some cases to spread out its base (thus giving it a campanulate form), in order to admit of small plants being placed close under the glass.

In most cases, however, I should prefer an entire and detached dome, or acuminated solid of revolution of glass on all sides, to a semidome, as a more elegant single object, and as admitting light in every direction. A large conservatory on this plan attached to the mansion, or a greenhouse placed in the centre of a small lawn in a flower-garden or shrubbery, would be more elegant, as well as more congenial to the plants, than the present square or triangular shed-like buildings `placed against walls.

On a moderate scale-say twenty or thirty feet diameter-they are admirably adapted for receiving exotic Floras; as for the Erica, New

! Surrey.

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