Copy 1, Volume 3
Pantologia. A new cyclopaedia comprehending a complete series of essays, treatises, and systems, alphabetically arranged; with a general dictionary of arts, sciences and words ... illustrated with engravings, those on history being from original drawings by Edwards and others ... / by John Mason Good, Olinthus Gregory, Newton Bosworth.
- Date:
- 1813
Licence: Public Domain Mark
Credit: Pantologia. A new cyclopaedia comprehending a complete series of essays, treatises, and systems, alphabetically arranged; with a general dictionary of arts, sciences and words ... illustrated with engravings, those on history being from original drawings by Edwards and others ... / by John Mason Good, Olinthus Gregory, Newton Bosworth. Source: Wellcome Collection.
24/690
![vertical Ne parallel to eaca other, at the dis+ tance of 5,6, 7, or 8 fect, like the trusses, or main couples of a roof; and bridging joists laid across them. In smaller works these are laid sparingly, but of considerable ‘scantling, and are boarded over; but for great arches, a bridging joist is laid for ev ery course of arch-stones, with blockings be- tween, to keep them at their proper distances. The stones. are not laid immediately .on these joists, but beams of soft wood are laid along each joist, en which the stone is. laid.. These beams are afterwards cut-out with the chisel, in order to separate the centre from the ring of stones, which must now ‘support each other by their mutual abutment. The centre is distinguishable into two parts ALLB (fig. 7. Pl. 25.), and. LDL, which are pretty inde- pendent of each other, or, at, least act separately. The horizontal srRETCHER LL cuts the semicircle ADB half way between the spring and the crown of the arch; the arches AL, and LD being 45° each. This stretcher is divided in the sarne pro- portion in the points G and H; that.is GH is one half of LL, and LG, HL are each one fourth of LL nearly. Each end is - -supported by two STRUTS Kl, GI, which rest below on a sole or BED pro- perly supported, The interval between the heads of the struts GI, HK is filled up by the sTRAINING BEAM GH, abutting in a proper manner on the struts. (See CARPENTRY). The extremities L, L, are united in like manner by butting joints, with the heads ofthe outerstruts. ‘The ARCH MOULDS AP, BP, are connected with the struts by cross pieces, PQ, which we shall call BRIDLEs, which come inwards on each side of the struts (being double), and are bolted to them. This may be ealled the lower part. of the frame. The. upper part consists of the king-post. DR, supported. on each side by the two struts or braces ML, ON, mortised into the post, and also mortised, into the stretcher at. the points L, N, where it is sup- ported by the struts below, The arches LD, LD are connected with the struts by the bridles PQ, in the same manner as below. There is_a great propriety ia many ve of this arrangement. ‘The lower parts. or haunches ofthe arch ] press very lightly on the centres. Each archstone is lying on an inclined plane, and tends to slide down only with its relative weight; that ‘is, its weight is to its tendency to slide down the joint/as radius to the sine of elevation of the joint. Now it is only by this tendency to slide down the joint that they press on the centering, which in every part of the arch is perpendicular tothe joint: but the pressure on the joint, arising from this eae is much less than this, by reason of the friction of the joints. A block of dry freestone wiil not slide down at all 3.and, therefore, will not press.on the centering, if the joint be not elevated 35 degrees at least. But,the archstones are not faid in this manner, by sliding them down along down (their slope, till they touch the blocks on which they “are to rest ; so that,.in laying the archstones, we are by Rs means, allowed to make the great Aedastion from their weight just now mentioned, and which Mr. Couplet. prescribes (Mem, Acad. Sciences, 1729)... But, there is an- other cause which diminishes the pressure on, the centres; each block slides. down the planks on which itis faid, and presses, on the block below it, inthe direction of the tangent. to the arch. This eres is transmitted through this ‘block, ° in the same direction, to the next, and through it to the third, &c. In this manner it is plain. that, as the arch advances, there is a tangential pressure on the lower archstones, which diminishes their pressure on the frame, and, if sufficiently great, might even push them away from it. Mr, Couplet bas given an analysis of this pressure, and shews, that in 4 semicircular arch of uniform thickness, none of the archstones below 30° press on the frames. But he, without saying so, calculates ‘on the supposi+ tion that the blocks descend along the circumfe- rence of this frame, in the same manner as if it were perfectly smooth. As this is far from, being the case, and as the obstructions are to.the last degree various and irregular, it is quite useless. to institute any calculation on the subject. A little reflection will convince the reader, pat in this ‘case the obstruction arising from friction must be taken into account, and that 1 must not be taken into account in estimating the pressure of each successive course of stones as they are laid. It is enough that we see that the pressure of the lower courses of archstunes on the frame is diminished. Mr. Couplet says, that the whoiepressure of a semi- pircalar arch is butgths of its weight; but it is much reater for the eee just now given, From a! ae of circumstances which it would be tedious to relate, we believe that the centres car- ry, at least, 2ds of the weight of the arch before the keystone isset, In elliptic al and lower pitched | circular arches, the proportion is still greater, It seems reasonable enough, therefore, in dis- posing the framing, to direct the main support to the upper mass of the arch which presses most on the frame. We shall also derive another advan tage from this construction. The struts which carry the king-post spring from those points of the stretcher where it rests on the str uts below: thus the stretcher, on which all depends, bears. no: transverse. strains. It is. stretched by the strut above it, and it is compressed ina small degree between the strats below it, at least by the outer ones. Mr. Pitot proposes the straining beam GH as a lateral support to the stretcher, which may, therefore, be of two pieces: but although it aug= ments the strength it ‘does not seem necessary. The stretcher is ; abundantly carried by the strap, which should suspend it from the king- -post. The great use of the straining piece is to give a firm abutment to the inner struts, without allowing any lateral strain on*the stretcher. This centre gives a very distinct view of the of; fices of all the parts. It is the simplest.that can be in its principle, because all the essential parts are subjected to one kind of strain. The stretcher LL is the only exception, and its extension is ra- ther a collateral circumstance than a step in the general support. Precisely the same mode of construction may be adopted for the centre of an elliptical arch: but it would be unjust to recom- mend the method,generally without pointing out its defects. We must obsery e, then, that the la- teral strains which are brougbt upon the upper struts by the bridles, certainly transmit to them part of the weight of the archstones on the curves, ‘The space between. the curves and ML might ad. vantageously be trussed, But, the causing the middle bridle to reach down to the stretcher seems’ to secure the upper struts from all risk of bending It has another very palpable defect. If.the piers. should yield to the load, and the feet of the. centre able strain on the stretcher, tending. to break it](https://iiif.wellcomecollection.org/image/b29330208_0003_0024.jp2/full/800%2C/0/default.jpg)
