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HISTORY OF ENGINEERING.

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the work, so that each point of the curve of the bridge shall feel the precise degree ofvertical pressure which theory shall assign to it, and unless this difficulty can be got over,all the reasoning concerning curves of equilibration will be of no use. The engineers willbe the best judges how far it can be removed, and it will no doubt occur to them, that thismatter is much easier to manage in the case of stone bridges.

Dr. Charles Hutton , of the Royal Military Academy at Woolwich, states it as his opinionthat all the small frames or parts ought to be so connected together, at least vertically, asthat the whole may act as one frame of iron, which can only be destroyed by crushing itsparts. For by this means the pressure and strain will be taken oft' from every particulararch or course of voussoirs, and from every single voussoir or frame, and distributeduniformly throughout the whole mass. Hence it will happen that any particular partwhich may by chance be damaged or be weaker than the rest will be relieved and preventedfrom fracture ; or if broken, prevented from dropping out and drawing other parts after itwhich may be next to it, either above or on the sides of it. By this means also the effectof any partial or local pressure, or stroke or shock, whether vertical or horizontal, will bedistributed over or among a great number of the adjacent parts, and so break and divert theeffect from the immediate places of action. By this means also will be obviated any dan-gerous effects arising from the continual expansion or contraction of the metal, by thevarying temperature of the atmosphere, in consequence of which the bridge will altogetherin one mass, in a small and insensible degree, keep perpetually and silently rising andsinking, as the arch lengthens by the expansion, or shortens by the contraction of the metal.This unity of mass will be accomplished by connecting the several courses of arch piecestogether vertically, or the lower courses to the next above them, and also by placing thepieces together in such a way as to break joint, after the manner of common or wallmasonry, and that, perhaps, in the longitudinal and transverse joints as well as the verticalones.

Mr. Attwood , of Sloane Street , states that, according to the plan of the bridge, the interiorcurve is a circular arc, and the whole of the iron-work between the arc and exterior ter-mination on the road is divided into sections of a wedge-like form, the sides of which beingprolonged unite in the centre of the circular arch. The sides of the sections considered asplane surfaces, which are projected into the aforesaid lines, are placed contiguous after themanner in which blocks of stone are disposed which form the arches of a stone bridge ;but in the plan of the iron bridge, the wedge-like form of the sections, instead of beingconfined to the arch adjacent to the interior curve, as in the case of stone bridges, is ex-tended throughout the whole structure as far as the road. The ties and fastenings appliedto prevent the sections from changing their places in the direction of the sides of thewedges coincide with the lines in which the surfaces of the contiguous sections are united.For these reasons I suppose that the whole iron-work is to be considered as consisting ofsections, which act as wedges by their pressure and gravity. It may also be observed, thatif the iron-work above the immediate arch be divided by circular arcs or lines of division,drawn from centres situated in the vertical line which bisects the entire arc, the said cir-cular arcs will divide the whole of the iron-works into separate arches, the sections ofwhich may be adjusted so as to form so many distinct arches of equilibration. Thesearches when united will become a single arch of equilibration.

Colonel Twiss , of Jfoolwich, states, that every part of a bridge, whether formed in theshape of a wedge, or laid on in horizontal joints, such as the walls and other loading usuallyerected upon the arches of stone bridges, should be all considered as acting by gravity, andevery heavy body so situated, being prevented from falling vertically, will produce a lateralpressure, varying according to circumstances ; in considering the strength of arches, I nevercan suppose the whole to act as one frame, which can only be destroyed by crushing itsparts.

Mr. William Jessop , of Xewarh, says, respecting the practicability of constructing an archof iron of 600 feet span, and the weight that such a material is capable of sustaining beforeit will crush with the pressure, I am inclined to believe that those who may critically in-vestigate this matter will remove all doubts on these heads. I can easily conceive that acube of Portland stone, of 6 inches for instance, would require no great weight to crush it ;that such a cube of granite or marble, though much stronger, might also be crushed; butI feel it difficult to form any conception of a wei^it that would crush such a cube of iron,and though a similar arch of stone would have more surface in contact, yet as its com-parative weight (allowing for the difference of specific gravity) would be nearly in pro-portion to the surface of its section, its advantages on the one hand, of having more pointsof bearing, will, in my opinion, be much inferior to the advantage, which, on the otherhand, will be derived from the greater hardness and tenacity of the materials, and the greatdecrease of absolute weight.

Presuming then, that there should be no doubt on this part of the subject, it will next berequired that all its parts should contribute their due proportion of resistance to thepressure, and that when so constructed, every part should have a disposition to remain at