In war, houses which are loaded with the greatest weights, and where the ‘), ails are placed at a remote distance, it becomes necessary to introduce many supports to the these, it' constructed of timber, being liable to accidents from and to rot, are oinsequently exposed to sudden danger ; to prevent which, every preeantion should be taken, at least as far as may iqqi.ar to he warrantable from the profits to arise from the articles to be deposited. This will be fully accom by th.• inttoduednm of groins, which not only answer the same purpose as the limning of timber-work with its wooden supporters, lint are more durable, and proof against tire and rot. Though groins are only employed in the low cr stories of buildings, on account, of the great expense and loss of space w hick would be occasioned by the requisite thickness of the walling; y et they may at al] times be used ill cellars and ground stories, without much additional labour or expen diture of materials.
It having been finnid that brick groins, rising from rect angular piers, are inadequate to the weight they have to support, and are incommodious to the turning of goods round thy: corners the piers, it will be found convenient to employ' octagonal piers, and to cut off the square angles of the groin, equal to the breadth of the side of the piers. This mode of co»struction is decidedly preferable to that in which square piers aro used ; for the angles of the groins, built in the common way as they form a right angle, are hardly capable of sustaining themselves, much less the load required to be sinpo•ted, ow hug to the bricks being so much cut away at the angles. in order to lit them thereto and to each other, so that they have little or HO lap. This scheme should certainly be carried into pram ice, wherever groins are to such uses.
In the construction of edifices finr dwelling, they ought always to be employ ed in cellars, and other damp situations, particularly where there are paned apartments above.
Groins for use only, may be indifferently constructed of brick or stone, as one or littler material may be most easily procured.
If employ cd by way of proporthin or decoration, their beauty depends on the generating figures of the sides, the regularity of the surface, and the acuteness or sharpness of the angles, which should not thereil ire be (Mutinied. In the best, buildings, a here durability and elegance are equally required, they limy be constructed of wrought stone ; and Where elegance is wanted at a small expense, of plaster, sup ported by timber-work.
Groins are consequently constructed in two different ways, according as they are built of stone or brick, or formed of timber-work, lathed and plastered. In the former case, a timber centering is made to form the conctvity. at d to sup port the groin during its erection. The ecntering e011sistS 4,1 several disposed at three or four feet the size ,1• the vault, which has die gruate.t opening: The extremities of these ribs rest on beains supported by standards, and are boarded over without any rigard to the transverse openings, which are afterwards formed by another set ribs adapted thereto, and then boarded so as to meet the boarding of the first vault, which if of considerable breadth, !Mist have short ribs fixed upon its surface, ill order to shorten the bear ing of the boarding of the transverse openings; and thus the centering will be completed. It is out hulls, that in form
ing the ribs for each vault, the miter curve must be the are of a circle or ellipsis within the curve of the vault. and dis tanced from it towards the axis equal to the thickness of the boarding. In making the groined centre, it will be necessary to Mid the place of the angles on the boarding of the huge vault, in order to ascertain the place of the ribs and boarding of the transverse vault; this may be done by three different methods. First, let two straight edges be placed vertically at the angles, and a third straight edge, or an extended line, be made to touch the surthee of the and marked at all the points of contact, keeping the latter straight edge or line always upon the edges of the two vertical straight edges The defect of this method is, that the place of the angles at the bottom can never be found, since it would require the cross straight edge or line to be of infinite and the vertical ones of infinite height. A more eli?dhle method, therefore, where there is room, is, secondly, to fix two in the transverse part, and direct a level straleht edge upor their so that the end may come in contact with the boards, and mark the hoarding in this place; find a sufficient number points for the purpose., in the same manner, and draw curves through the points, which will give the curves for the (lid of the filling-in ribs, otherwise called jack ribs, hr ,ruins to be finished with plaster, the angle.ribs mist be lint fixed, then straight longitudinal piece.; parallel to the axis of the groin pixel, either flush with the under sides of the angle-ribs. or their under sides a little below- those of the angle-ribs. so as to admit of their being nailed together; this is the most eligible method of constructing plaster groins.
There is :mintier mode, by forming curved ribs, in planes perpendicular to the axis of each simple vault : hut here, as the curve of these ribs must he the same as that of the eylin drit of each simple vault, the waste of timber w ill be very great; though not if the ribs sue constructed in straight pieces. \Vhatever mode is adopted in the formation of plaster groins, the under sides of the ribs ninst alway s range ill the intended surface of each simple vault. These con structions will be more clearly understood in the following explanations.
flute I, (CENTERING FOR GROINS...) Fiyure 1, No. 1. A plan of the widest of the groin, first boarded the wind,• length without interruption ; then the cross vaults are iarded ; the two cross openings upon the left hand appear as finished, ready to receive the masonry or brickwork, while that on the right exhibits the ribs without the boarding.
No. 2. The elevation of the widest aperture. showing the edges of the ribs of the transverse openings fixed upon the surface of the boarding of the longit ndinal opening ; this also shows the height of the jack-ribs, which will give their length also.
No, 3. Shows the elevation of the transverse apertures, as completely finished,