THE INDUSTRY OF ALL NATIONS.

the bridge to the line of the road, and the absence of an equal diffusion of theload, and consequent strains, has led the curve to he disregarded of any purposeother than determining the direction of the courses of the stones, which shouldbe at right angles to the strain. The courses in a vertical wall, it is obvious, shouldhe horizontal.

In the illustration above, the neutral axis remains the same after the beam isconverted into the arch; and as the application of material at the lower partof the arch would he near the neutral axis, and of no utility, it is manifestthat the strength should be increased, as in the beam, by increasing its depth orrise. In parallel, concentric arches, the lower meets no strain at the centre; andthe upper, none at the ends. It is necessary, however, to adapt the arch to ahorizontal road-way, either by direct suspension, as in the aqueduct bridge overthe Calder in Scotland, and the Trenton bridge in New Jersey; or by a system ofties and braces forming a truss, affording a mutual support to the arch and roadstring-pieces, as in the Upper Schuylkill bridge, which was designed and built byL. Wernway, and has a single span of 340 feet.

In the consideration of first principles alone, we have found the arch to he thebest disposition of material; and also that a benefit may be derived from the supportsof the road-way being employed as braces to the arch. As the strength of a structureis the strength of its weakest part, and an excess of strength in any other part ismade worse than useless by the injurious weight of the surplus material, it is ne-cessary to connect the systems where a part of one may be useful to the other,and may he employed without danger of infringing the independence of either.

If one half of an arch be removed, and replaced by a vertical wall of sufficientstability to resist the thrust of the remaining half arch, there will he no othersupport needed, and it will stand as firmly as before; showing that while thepiers sustain the whole weight of the archacting vertically in the direction of gravity,there is no direct vertical force acting at thecrown. This alteration in the direction ofthe strain from the vertical to the horizontal,will obtain in any arrangement of parts. Thecurve taken by a string, freely suspended atits ends, shows the direction of the forcesthroughout its length, and the lowest pointin the curve is obviously under the action ofhorizontal forces alone.

When the arch rises above the neutral axis,therefore, there is only a horizontal strainoperating at the crown, which can be bestopposed by mere area of cross section; andthe braces and ties adapted to a vertical strain are there of no use. The heels ofthe arches support the weight, acting more or less angularly as the centre ofgravity is distant. If the heels of the adjacent arches butted, or were worked toge-ther, there would be no necessity for arguing the advantage to be gained by tying thecrowns together. Notwithstanding the action is precisely similar to that of a tie

compression, relieved at the heels in the arch truss, by the bracing which transfersa portion of the strain, and converts it into a tensile force operating in the direc-tion of the chords, and in some cases almost entirely relieving the abutment ofthe thrust.

Mr. Haupt, an eminent engineer in extensive practice, states, in a valuablework recently published by the Appletons on this subject, that when a straightbridge settles, the quickest curvature is nearest the abutment; and that he hasfound, in examining a large number of bridges, the joints of the braces near theabutments were invariably compressed and tight, whilst near the centres therewere no symptoms of crushing, and an occasional imperfection in fitting wouldallow the admission of a knife-blade.

The trussing also permits the application of the important principle of counter-bracing which we have before adverted to; and which, by a system of keying tothe braces, or screwing to the iron ties, may compress the arch, and indeed thewhole structure, as if by the application of a load; so that the load itself on itspassage relieves the counterbracing to the extent of the artificial load, and is pre-vented from exercising any lifting motion upon the opposite part of the arch ortruss.

The conclusions to which we are led by these considerations, which apply moreparticularly to wooden bridges, are:—1, the arch, assisted by trussing at thehaunches, is the strongest method of disposing of the material; 2, the strain atthe crown is horizontal, and best met by direct section; 3, the strain at the piersis vertical, and renders the use of ties and braces, as in a truss, most advantageous.

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Allen's self-supporting , web, truss bridge. The model is said to be a correctrepresentation on a scale of half an inch to the foot, of a bridge of 190 feet span.It is a light, slender affair, weighing, probably, about 30 pounds, yet it is said tobe able to sustain three tons. Experiments of this kind, as has already beenshown, are of but little worth; and the value of this model for experimental pur-poses is not increased by making up the body of the chords in pieces—the resultswould have been materially modified, however, had the butting joints been intro-duced by dividing the pieces longitudinally in the chords taking a tensile strain.The entire thrust of the arch is transferred by the braces to the upper chord whichoperates as a tie-rod. The reduction of the section of the arch and increase ofvertical timber, by lessening the length of the panels towards the centre of thespan, is in opposition to leading principles.

James Scott , of New Lisbon , Conn., exhibits a model of a combined truss andarch bridge, of which he is the inventor and patentee. The braces and counterbracesare similar, and starting in the same plane, spring over one another in crossing.The adjoining sets meet in mortices in the vertical posts or ties, which are halfsundered to admit them, and are wedged up by keys.

Bollman's Bridge. —It is a suspension bridge, in which the return chains orstays, and the anchors are replaced by a hollow cast-iron stretcher extendingfrom pier to pier. The vertical posts are also of cast-iron, and, by the bot-tom of each being suspended from both pier-heads, the sections are supportedindependently of each other. If a weight he placed anywhere but at the centreof a beam supported at both ends, it is unequally distributed; yet in this bridge,

rod of a roof, and the fact apparent to one least in the habitof considering such matters, that the strain is met in a directline, and the whole construction much relieved by such anarrangement, we frequently find the arches of a bridge madeeach complete in itself, and neither assisting nor benefit-ing by its neighbor. We have seen that where there is atortional or revolving strain about a neutral axis, as in an ordinary beam or straighttruss, the depth is the most important element, as the strength increases in propor-tion to its square. In the equilibriated arch every particle of matter is in a state of

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the ties from a post next one of the piers have the same sectional area, althoughthe one to the adjoining pier-head carries nearly the whole weight, and the otherextends at a very acute and inefficient angle over the remaining portion of the