Зображення сторінки
PDF
ePub

cular to half the glass in the early meridian, he would at the same time be perpendicular to half the wall or arch behind the trellis. It is in some degree foreign to the present work to introduce the subject of garden walls; but I may be allowed to remark, that by fixing a proper trellis to the zig-zag wall, so as to form one line or plane with the general front of the masonry, this style of walling would be much better adapted for training fruit-trees in gardens than that in common use, whilst the cost would be little more than one third of a solid fourteen-inch wall. But of this plus ultra.

I have now to give the detail of what I consider to be the best mode of opening the sashes of glass houses, referred to above, in the description of Fig. 4.

Raising the sashes I consider with Mr. Knight as greatly superior to arrangements in which they slide over or pass each other; for the part of the house beneath or next the double glass is deprived of half its usual quantity of sun and light; and besides, in this way the plants can never enjoy the benefit of a shower. In many pineries, and other wide houses, the upper sashes let down, and the under ones draw up; and when this is done, the consequence is, that in the middle part of the roof over the pit the glass is double, and the plants are darkened, whilst the sun's rays are only admitted directly where they are useless, viz. to the path at the back and front of the house.

Fig. 2, PL. VII. represents a rafter, on which are placed five sashes of equal dimensions, the one overlapping the other, and each hinged at the upper angle.

a. a.

are levers which may be attached in various ways to a central bar, or to one of the side bars in each sash, or

to each side bar if the sash is broad.

a. b. c. The circle in which the point, a, of this lever moves. It will be sufficiently evident that by attaching a cord to the end of any one lever, and passing that cord over a pulley attached to the under edge of the rafter, the sash may be raised after the manner of common sky-lights, as suggested by Adanson's diagram, Fig. 3, PL. I,

but this would be a clumsy mode for so many sashes, and would occupy too much time. The problem therefore is, how to move the whole so as to elevate and retain them at any required angle?

This is to be solved by passing a cord, wire, or chain, d, through the ends of all the levers, with stops on the cords at e, e, e, in order that when the cord is pulled by hand or otherwise at f, the sashes 1, 2, 3, 4, may be raised to g, i. e. to the angle of sash 5 before that sash is moved. The moment that sash moves, then the whole range are in the same plane or angle, g, and may be further elevated to any degree; as for example to 63o, represented by h, for the greatest altitude of the sun near London; or to i, the perpendicular for the admission of rain. When the sashes are to be let down, the upper one, 5, will shut first, and the rest in order; while the stop k will remain at a, k, and the cord will rest in a sort of parabolic curve between the ends e, e, of the levers. The power at ƒ may be a screw wheel, working in a pinion having a small cylinder on which the cord winds up, as represented by Fig. 4. The levers and stops may be arranged after the manner of Fig. 5; the stop a in that figure being represented with a small screw to attach it to any part of the cord at pleasure; and by which means, if thought desirable, variations may be made in the opening of the sashes. For instance, by fixing the stop of e, 5, at a, k, sashes 1 and 5, which are in the most sheltered parts of the roof, may be raised in severe weather for the admission of a little air, and the rest, which are in the most exposed part, kept shut to exclude the storm.

Fig. 1 is merely a variation of the same mode, in which the sashes are hinged, or turn on pivots, at or near their centres. Of course this mode will not answer where vines or other trees are trained near (that is within two feet of) the glass; but for lofty botanic houses (such as that delineated in PL. IV.) it is to be preferred to any other; as, from one half of each sash nearly balancing the other, little power is required to elevate them; while their pivots are still so far removed from the centre of each sash towards its upper angle, as to lower them by

F

their own gravity, and hold them down. This effect is further aided by the weights shown depending from the extremities of the cords in the apex of the section in PL. IV.

For Mr. Angerstein's greenhouse, the large conservatory at Stockholm, or those immense stoves erected at Schönbrun; or for such as I have heard the Prince Regent intends to erect at Brighton, and, as Sir Joseph Banks predicts, will hereafter be erected generally in this country to admit the full growth of exotic trees; this plan is peculiarly adapted. Indeed there seems hardly any other way that such houses could be so arranged as to give the trees in their earlier years, and during their progress from the floor to the roof, any chance of growing to that height in a healthy state; for, admitting them to be regularly supplied with light, heat, air, and water; yet that tree could hardly support itself long, which had attained the height of 50 feet, without deriving the advantages, which Mr. Knight has so ably proved are derivable, from wind. Sir Joseph suggests that such houses may be heated by steam; and I may add, that on the boilers used for this purpose, machinery connected with a thermometer on Mr. Kewley's plan, might be contrived to open and shut the sashes of these immense roofs almost instantaneously'.

An objection will be urged against circular or curved roofs, from the greater expense of all curved work, compared with that in which straight lines prevail. This would be an argument of considerable force, if it were proposed to have either the rafters or astragals of timber; and if, in addition to this, uprights, imposts, sliding sashes, &c. were necessary. But as great part of these roofs are composed of astragals only, without rafters, the objection is of trifling weight. The expense of such roofs as Figs. 3, 5, 6, 7, and 8, PL. V. will be little more than if they were of one declination or inclined plane. The whole business of iron roofs and sashes, and particularly of iron astra

1

1 A fine specimen of what may be done with cast iron rafters, in curved roofs, may be seen in Brown and Co.'s chain cable manufactory, Isle of Dogs.

gals, will be materially simplified and improved by the introduction of solid iron astragals. These I have prevailed on an eminent iron, master to attempt, by drawing rods of iron through suitable moulds; and after repeated trials, at considerable expense, he has at last succeeded in producing an article, which, if the expression be not too high for the subject, will form a new æra in sash-making. Hitherto metallic astragals have been formed of two or more pieces, the moulding and rabbet apart, and the latter let into a groove in the former, or in some instances only soldered to it. To bend such astragals to a curved line is with some sorts impossible, and with every description must evidently lessen their strength; but with a solid body the case is materially different: by heating a solid iron astragal it may be bent to any shape whatever, and yet retain all its original tenacity; and if the convex side of the curve is placed uppermost, as in the case of these roofs, it is evident the astragal will be much stronger than if retained in a straight direction.

But the grand advantage of metallic astragals and iron rafters in glass roofs, is the increase thereby obtained of transparent surface. No person intending to construct a house, and who values the quality of fruit, ought ever to hesitate as to their adoption. If mere quantity is the object, a common wooden house will answer every purpose; but where the expense of a glass case is incurred, it will always be found the wisest œconomy to have one, all other circumstances being equal, which admits the greatest possible quantity of light.

That the operation of different constructions may be judged of in this respect, I shall here estimate the quantity of transparent surface in six different descriptions of roof, taking the roof of Fig. 1, PL. V. as the most transparent; and the best constructed wooden houses; such, for example, as those of Kensington Gardens and Kew, as the minimum of my scale.

In each of these examples I imagine the house to be forty feet long, and twelve feet wide, with a front wall one foot high, the front glass two feet (where front glass is used), and the back wall fifteen feet

high. The parallelograms, 1 to 8, PL, VIII. represent the total contents of such roofs; and the part covered with dark lines the proportion of each, which is opaque from the profile or breadth of the astragals, rafters, &c. while that covered by dotted lines represents the additional space rendered obscure by the shadows of these parts on the glass; taking the medium of each day's shadow. The section opposite each parallelogram represents the style of sash and roof; the price in the first column of prices is that of the sashes glazed and completed per foot; in the second the combined expense of the sashes, rafters, and supports, per foot; and the third column, the price per foot at which the part entirely transparent stands the purchaser.

Taking these parallelograms in another point of view, we may imagine each of them to represent a peach, grown in the sort of house the roof of which the figure represents. The black part of each parallelogram, or peach, may be supposed to represent insipidity or tartness, and the white part aroma or flavour. This will give a sensible notion of the difference in quality of the fruit produced by the different houses. The parallelogram

No. 1. represents the contents of the glass roof of a house of the dimensions above detailed, and which is formed of solid iron astragals, tinned and placed eight inches apart, curved to the circumference of a circle whose radius is the width of the house, and with about one third part of the upper part of each astragal straight, as in Fig. 3, PL. V. In the longitudinal part of this house there will be sixty astragals, each eighteen feet six inches long and five eighths of an inch wide, supported by two horizontal bars half an inch wide, and which bars extend the whole lengt of the house. The space darkened by these astragals and cross bars does not amount to five superficial feet. As the total contents of the space to be lighted (i. e. of a plane of a breadth equal to the hypothenuse of a right-angled triangle, whose base and sides are as the width and height, and whose length is that of the house) is 680 feet, this gives of opaque space 1-136th part of the whole. Add one fifth to the opaque space for the shadows of the astragals, and the

« НазадПродовжити »