The line ef g h i, Fig. 3. in Plate CCCXI. extended to the north wall, slims the declivity of the ground on which the pine-stoves are placed. Nearly opposite to the door in the back wall of the pine-stove, there is a door in the gar den wall, leading to the pine-shed, where the plants are kept in time of shifting. In front of the pine•stoves, it may be noticed, are situated the general forcing-pit, the melon-pits, and the cucumber frames.
Before leaving the subject of glazed houses, we may no tice some improvements which have of late years been pro posed, 239. Mr Knight remarks, that where sunshine and na tural heat do not abound, the form which admits the great est quantity of light through the least breadth of glass, and which affords the greatest regular heat with the least ex penditure of fuel, must be the best. It is e%ident that the sun's rays ought to fall as perpendicularly as possible on the glass roof ; because the quantity of light which glances off without entering the house, must be inversely propor tionate to the degree of obliquity with which it strikes upon the surface of the glass. Mr Knight made many experi ments to ascertain by what elevation of the roof the great est quantity of light can be made to pass through it ; and he found that in latitude 52°, tin; best angle of elevation is 34°. But it cannot be denied, that the rays of the sun will fall, in a directly perpendicular direction, on this inclined plane, only t Wiee in the year, and then for only very short spaces of time : at all other periods, they must fall in an in clined direction, and never perpendicular to the plane of the glass. Without expecting, therefore, that the rays will ever fall precisely perpendicular upon it oftener than twice in the year, it is of importance that they should do so as much as possible, during those periods when the influence of the sun is most desired. Mr Knight, (in Hort. Trans. Lond. vol. i. p. 100.) and the Rev. Mr Wilkinson (same volume, p. 162.) do not agree as to the proper inclination of the glass-roof: instead of 34°, proposed by the former, the latter would have the angle 45°. It seems unnecessary to detail the reasons assigned by either writer.
240. It has been remarked by Sir George Mackenzie, that if a form for the glass roof can be found, such that the rays will be perpendicular to some part of it during the entire period of the sun's shining, not on two days, hut on every clay of the year, that form must be considere'd the best. This form is to he found in the sphere ; and he proposes the quarter segment of a globe, or a semi-dome ; though, to catch the sun at all times, the segment would have to correspond with the greatest segment of the circle which the sun describes. He does not propose to bring each pane of glass into the form of a small segment of a sphere ; this would not only be expensive, but unnecessary. The, size of a glazed house of this kind, can scarcely, with pro priety, exceed a radius of fifteen feet, that is, thirty feet of length for training. The plan, elevation, and section of a
vinery, constructed on the principles thus suggested by this ingenious and scientific horticulturist, have been published by the London Horticultural Society, in the second volume of their Transactions ; and in Plate CCCXII. we have given these, with considerable improvements since made by the author. It has been found, that the frame for the glass-roof may easily he formed of ribs of hammered iron ; each rib consisting of three slips of iron, such as shewn at full size at Fig. 4. The ribs are fixed in an iron plate at the bottom. The distance between them at the base, is about fifteen incites ; and when the gores c ont•act to half that width, every alternate rib may stop. The word gore, we may remark, is that commonly used for a slip of any material, so cut, as when joined to others, to form a globe or any round figure. The frame-work might also be made of wood ; but the wrought iron is not only much cheaper at first, but far more durable. The under frames may be about thirteen feet high ; they are rivetted into an iron ring at top, and made fast all round to the coping and upright vvall. Iron rods may be placed for supports at x, x, x, x, Fig. 3. if thought necessary. The width of the planes at the bottom is about a foot, Liminishing to six inches at the second set of ribs ; when they begin again at one foot, and contract upwards to four inches. Air is admitted by sliding shutters, which may be glazed, if thought proper, in the parapet wall in front ; and also by wooden shutters, moving on pivots, and opened or shut by means of colds along the back wall ; and by windows in the pediment roof. The glass-roof itself is immoveable ; but the upper part of it may be made into moveable sashes, if required, by forming a sufficient number of ribs with grooves, and fixing stay rods on the under sashes, to receive the upper ones when let down ; and Sir George Mackenzie mentions, that, front vie•ving the structure of the roof of Short's old observatory at Edinburgh, he is convinced that the glass semidon c might be soak in t‘‘ o parts, and placed on rollers, so as to expose, at pleasure, eve!y plant in the interior to the direct influence of the sun. If it is wished at times, to defend the plants from the sun, a gore of canvas may he so contrived as to cover one-half of the glass. The general appearance of such a house (as seen in the elevation, Plate CCCXII. Fig. I.) is doubtless highly elegant ; and it seems pretty evident, that several such houses, tastefully disposed in a garden, would have a much finer effect than one great range, although the latter must necessarily be more econo mical. Mr Knight, we understand, highly approves of this invention, and is of opinion that it will answer every pur pose, better than arty form hitherto contrived.