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i . e surface of the groined centre, on which the stone orar ch is to be turned.

f r frames or trusses which support the boarding aregently called ribs; and the short ribs which are fixed„ boarding, and made to range with the whole ribs, are

binder the word Stone-Bridge, &c., the theory andsection of arches will be described; in the presenttill l We P ro P ose t0 show how the avchstones are supportedj e '"he arch is completed; and the most commodious and^expensive manner in which this can be accomplished.

0 f „" e proper construction of such supports, or the best modec gaming the centres for large works, has always beenatt 81 . ed so important a subject, that it has occupied theo n . °n, and exercised the talents, of the most eminentS'neers and architects. The principal object to be kept

a jo * eVV * s > to dx the various parts of the centering in suchof anr) er, as to support, without change of shape, the weightMi i mat erials that are to come upon them, throughout thet 0 e Progress of the work, from the springing of the archb 6etl e fixing of the key-stone. This object has not alwaysof ..efficiently attended to by the professional men, eitherb een Is > °r of other countries; for in many instances it hascj 0(1 asc ertained that the centres of bridges, from the injudi-8b a 8 Principles of their construction, have changed theirCo . considerably, or entirely failed before the arch wasCt c faP le te > and in consequence of change of shape only, thefro ^ 8 fiuht upon them have varied, both in form and strength,fifist• 6 * ntent >°n of the engineer. In the large works ofha Vft ,nd ’ however, erected in Great Britain, our best engineerspo r t c ° ns tructed their centres on principles calculated to sup-riic,|,f! ei 'y weight, and resist every strain to which they

. ‘‘Th 6 ex P° sed ‘

in ; ts qualities of a good centre,” says Tredgold, “ consistof ^ B eing a sufficient support for the weight or pressuretbj 0l] , arch-stones, without any sensible change of formHip a °v° u t the progress of the work, from the springing ofof fa . 11 to the fixing of the key-stone. It should be capable•day fa® easily and safely removed, and designed so that itffa e erected at a comparatively small expense.”

8t rUct centre of a large vault, as that of a bridge, is con-!ST° f trusses disposed equidistantly in vertical parallelJJay ’ and boarded over so that the convexity of the boardingThe a- nc *de with the intended internal intrados of the arch,eight f ance of the ribs may be disposed at from three toMigfat a ccording to the strength of the boarding and[°r e Vo ° the arch. In very large works, a bridging is laid^1

: ^ep .7 course of arch-stones, with blockings between, to

le m at rpcrnlai* dictannpa Tlio rimr-otrinon nnf

a iw a y“‘ eta at regular distances. The ring-stones do noti'll be/ 681 u P on these bridgings ; planks being sometimes^eparatjt? 611 ’ that they may be cut away afterwards, to0 as Cer . , le centre and the intrados from each other, in orderait tiag es ain whether there are any settlements, to repair the^Vhei4 P ut the arch in a state of equilibrium.

> cte(1 . e river is not navigable, the trusses may be con-‘«icultv Wlth a fi eam a t the bottom: in this case, there is no5 ^ form fi° rms f° r the trusses of roofs with tie-beams,e »tre. , e grand or principal part of the truss for the

h>u.

Wg e when the river is navigable, the centre requirestha^ 11 °P en ' n g as is consistent with its strength, in' s inter VeSSe ' s 111 a y P ass un der it! an d as the horizontalJ ch ^ u Pted, this disposition of the timbers will require„ ^ the a ^ ' n the carpenter.

tfa V ’§abl e r *fa er over which a bridge is to be built be notfa at it .J ^ manner of constructing the centre is so easy,Miere ^ unnec essary to give any examples here ;the river is navigable, instead of the horizontal2 N

tie, a number of ties are disposed around the polygon, formingthe interior part of the centre ; but as in many practical casesthe most judicious and well-skilled theorist might be deceivedas to the equilibrium of the arch to be supported, or thepoints in which it has the most tendency to fall, it would bevery difficult to say what are ties and what are Strutts; andeven if the true pressure of the arch could be ascertained,the knowledge of this alone would not be sufficient; for thesame parts of the vaults, in the process of execution, varytheir pressure in every succeeding additional part, and whatwas a tie at one time, becomes sometimes a strutt; while aStrutt, on the contrary, will become a tie, either in building,or at the completion of the vault. This ought to be wellconsidered ; and where the pressure is doubtful, or any ofthe lengths of timber forming the centre are ascertainedto be in the two different states above mentioned, suchtimbers should be made to act in either case.

Though the timbers upon which the vault immediatelyrests, cannot be supported transversely throughout, the otherpieces, which support the arch from the several pressingpoints, may all be made to act, by a judicious arrangement,in the direction of their lengths. The abutting joints, whichare pressed, will be sufficiently resisted, when their shouldersare made perpendicular to the direction of their force, andwith the small tenon; but if the timbers are drawn in adirection of their length, the joints ought to be strapped

The beauty of every truss is to have as few quadrilateralsas possible. All the openings should be triangles : the inter-section of the timber should be as direct as possible. Obliquedirections exert prodigious strains, which require timbersof very large sections to withstand them, and press upon theabutments so much as to make the whole truss sag by thecompression of the intermediate joggles.

If proper attention be paid to these circumstances, andthe bearings of the timbers be well ascertained, a centre,constructed upon such principles, must answer its intendedpurpose, provided a proper estimate be taken of the com-municating forces during the execution of the vault, and thecentre be well secured at its abutment.

A centre for the arch of a bridge over a navigable river,may either be accomplished with one centre around theinterior of the entire arch, supported between the piers ; or,if the span of the arch will admit, the aperture may be sub-divided into two or more apertures, by one or more supporters,each consisting of one or more posts of wood, braced togetherwhen necessary; these supporters, together with the sidesof the stone piers, support the centre of the aperture, onwhich the stone arch is to be erected over the whole. By thismode, the centering is much more simple in its constructionand requires fewer timbers, and these of smaller scantlingsthan when made in one centre.

If a centre be truly constructed, every point of the vaul*to be built ought to be supported, without giving any trans-verse strain to the incumbent part of the centre: but this i?impracticable ; for, as it would require a multiplicity of jointsit would, from the shrinking of the timber, be less sufficientthan if composed of few pieces, supporting only a certainnumber of points disposed at judicious distances, leaving theintervals to be supported by timbers in which the super-incumbent part of the arch might act transversely, but stillpresenting such a resistance, as not to be materially bent orput out of form by the load of the arch above.

By these precautions, the centre will be constructed so asnot to yield, or give way, though the load should vary duringthe erection of the arch, and will stand as firm as if thewhole had been constructed out of a single solid: the onlything to be attended to, as before observed, being to make