'f'he only improver cot made by the moderns in vault ing, is the eylindro-cyl;nd•ic arches, which are made in opposite sides of a vault, and w hen regularly repeated, have a very beautiful ct.
In warenouses, which arc loaded with the greatest weights, and where the walls are placed at a remote dis tance, it becomes necessary to introduce many supports to the floors, w hide if constructed of timber arc liable to both fire and rot, and thereby exposed to sudden danger ; ci ery precaution should therefore be take. n to prevent the risk to v, hich they arc thus exposed ; at least as far as may appear eligi;3Ie in the profits arising from the articles to be deposited making a full compensation In. the additioaal expellee. The end will be fully answered by the intro( union of groins, which not only answer the same purpose as the flooring of timber work and the wooden posts which support it, but arc more durable, and a certain proof against both fire and rot. Though groins are only employed in the lower stories of buildings, owing to the great expellee which would be incurred by the gee .!t thickness of walling and the diminution of space w hich would be occasioned thereby, they may at all times be used in cellars and ground stories, without much additional labour or expenditure of materials.
It has been found that brick groins. rising from rec tangular piers, are inadequate to the weight they have to support, and incommodious to the turning of goods round the corners of the piers. An improvement has been suggested by Mr George Tapen, architect, in a small pamphlet lately published, where be iudiciou=iy recommends the piers to be constructed octagonally, and the square angles of the groin to be cut off equal to the breadth of the side of the octagonal piers. He gives a very satisfactory reason to this purpose, that the angles of groins built in the common way, forming a right angle, are hardly capable of sustaining themselves, and much less the load which is required to be supported, owing to the bricks being so much cut away at the angles in order to fit them thereto, that they have little or no lap upon each other. This scheme should certainly be car ried into practice wherever groins are applied to such uses.
In the construction of edifices fur dwelling, they ought always to be employed in cellars and other damp situa tions, and particularly in cellars which have paved apart ments above.
Groins for use only, may either be constructed of brick or stone, as the material of the one or the other may be more easily procured. If they arc only to be employed by way of proportion or decoration, their beauty depends on the generating figure of the sides, the regularity of the surface, and the acuteness or sharpness of the angles, which ought therefore to be rendered obtuse. In the best buildings, where durability and elegance are required at the same time, they may be constructed of wrought stone ; and where elegance is wanted at a small expence, of plaster, supported by timber-work.
Groins are constructed upon two different ways, ac cording as they are built of stone or brick, or constructed of timber-work, lathed and plastered over. In the for mer case, a timber centering is made to form the con cavity, and placed in order to support the groin during its erection. The centering consists of several ribs, dis posed at three or four feet distance, made to the size of the vault which has the greatest opening. These ribs
rest at their extremities upon beams supported by stan dards, and are boarded over without any regard to the transverse opening, which is afterwards formed by another set of ribs adapted thereto, and then boarded so as to meet the boarding of the first vault, which, if it be of considerable breadth, must have short ribs fixed upon its surface, in order to sustain the boarding of the trans verse opening, and thus the centering will be completed. It is obvious in forming the ribs for each vault, that the outer curve must be the arc of a circle or ellipse within the curve of the vault, and at a distance from it towards the axis equal to the thickness of the boarding. In the making of the ground centre, it will be necessary to find the place of the angles upon the boarding of the large vault, in order to ascertain the place of the ribs and board ing, of the transverse vault. This may lie done in three different methods. If two straight edges are placed vertically at the angles, and another straight edge or extended line be made to touch the surface of the board ing, and marked at all the points of contact, keeping this 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, as it would require the cross straight edge or line to be of infinite length, and the vertical ones infinitely high. A more eligible method, where there is room, is to fix two ribs in the transverse part, and direct a level straight edge upon the edges of these ribs, so that the end may come in contact with the boards, and mark the hoarding in this place ; lind a number of points in the same man ner as may be sufficient for the purpose, and curves being drawn through the points, will give the curves for fixing the ends of the ribs. But the best method is by the following geometrical problem.
Plate CXIX. Fig. 1. is a cylindrical groin ; No. I. the plan, No. 2. the elevation. Such groins as this fre quently occur in lobbies, halls, passages, antirooms, &c.
Fig. 2. is a cylindroido-cylindric groin ; No. 1. the plan, No. 2. the elevation. It is this kind of groining that generally occurs in collaring ; to construct which, a centering must be formed as at Fig. 2. No. 3, 4, 5, and 6. No. 3. shows the cylindroidal part, which is the widest opening, boarded over the whole length ; then in order to fix the transverse boarding, as at No. 2. the lines AB, CD, and EF, GH, No. 3. must be drawn upon the board ing for the wide opening, either by the former mechani cal means, or by the following geometrical process. No. 5. shows the end of this boarding. If the wide opening he of considerable dimensions, the shallow ribs, called jack-ribs, are fixed between the lines to the height of the cross-vault, and made less by the thickness of the boarding, so as to come within this thickness of the height of the groining. No. 4. shews the left hand part completely boarded in ; No. 5. the end of No. 3. No. 6. the end of No. 4., shewimg the face of one of the ribs of the centering, the thickness of the boarding, the jack-ribs over the boarding, and the posts and ends of the beams that support the whole.
Pnos. I. To find the mould or curve for determining the place of the angles upon the boarding of the cylin droid, in order to fix the jack-ribs and transverse border ing of the cylindric vault, No. 4.