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In placing before our readers Mr. Ellet's plan for a suspension bridge for the Schuylkill, at Philadelphia, we beg to congratulate them upon the prospect of a re-introduction of this mode of bridge building. We have frequently commented upon the importance of this subject, and it may be remembered, that when introducing Mr. Ellet's pamphlet, on Wire Bridges, that we suggested the adoption of this form of structure for the Croton Acqueduct at Harlem river. Mr. Ellet, long since proposed a suspension bridge, but as no definite plan has yet been agreed upon, we think it not too late to take him up. We repeat what we then said, that a failure of the high bridge in even a small portion would cost more than a complete Wire Bridge which would answer in every particular. In these days of economy and retrenchment, it becomes our Water Commissioners to look well before they leap.

SUSPENSION BRIDGES. PLAN OF THE WIRE SUSPENSION BRIDGE ABOUT TO BE CONSTRUCTED ACROSS THE SCHUYLKILL, AT PHILA

DELPHIA. By Charles Ellet, Jr., Civil Engineer.

The wire suspension bridge represented in the annexed engraving, was designed in compliance with an invitation of the Commissioners of the city and county of Philadelphia; and is intended to succeed the Fairmount bridge, which was destroyed by fire in the summer of 1838. The plan was approved by the Board, who have since advertised for proposals for the erection of the work; and it is understood to be their intention to urge its immediate completion.

Without adverting to the merits of this plan, as a particular application of the principle of suspension; the fact that such a bridge is about to be constructed, cannot but be a matter of interest to the profession, as an auspice of the introduction into this country of an improvement which has deserved and acquired the most abundant success abroad.

Suspension bridges present many claims for public attention, which have been fully recognized wherever they have been fairly submitted to the test of experience. Nearly all the principal rivers of the cultivated portions of Europe flow through densely populated districts, in which the importance of an unembarrassed communication with the opposite shores, has been long appreciated; and bridges were, consequently established at an early period, wherever the interest was sufficient to justify the expense of their erection. But nevertheless, as soon as one or two suspension bridges had been successfully accomplished, it was found that there were numerous places where similar works were greatly needed, and where the abilities of the inhabitants had yet proved to be unequal to the charge required for their erection and maintenance, by the application of any system of construction previously devised, which might thenceforth participate in the advantages held out by the new method. In the course of very few years, great numbers were accordingly erected; and it soon became manifest that the anticipated objections to the recent innovation were either entirely fanciful, or such as could be readily removed.

Popular opinion had foreseen inconvenience from the supposed instability of the platform; but experience taught, that when properly built, the motion in an arch of 300 feet opening, or more, did not exceed that which has place in ordinary wooden bridges; and theory at the same time demonstrated that the oscillations produced by an equal disturbing force diminish as the length of the span is augmented.

The apprehension of a rupture of the cables, was dissipated before the proofs which were furnished by science and experiment;-proofs which show that of all the applications of building materials to architectural purposes, none admitted of greater certainty than that of iron, and especially of iron wire, to the support of bridges; that the tension to which the metal was exposed, is susceptible of the most rigorous determination, and its strength, of the most accurate preparatory trials.

At the same time that these positive objections were relieved, the comparative value of the system was made equally apparent. The unrivalled beauty, and even gracefulness, of those examples which had been constructed, gave them a preference for positions where ornament was essential and elegance a merit. Their susceptibility of being applied to livers where the width of span must necessarily be very great, rendered them peculiarly appropriate for situations in which it was requisite, for the preservation of the navigation, or other object, to avoid obstructing the water way by the establishment of too many piers in the channel. They possessed, withall, the merit of durability, to an extent which will render them monumental; and to these advantages was superadded a claim not unfre

* The span of the Freibourg bridge, built by M. Chaley, is 889 English feet, and its flooring is 167 feet above the surface of the Sarine. Another bridge in Switzerland, by the same officer, over the valley of Gottron, has a span of 152 metres (499 feet,) and is elevated 340 feet above the bottom of the valley. Ships pass under the flooring of the Menai bridge under full sail.

quently esteemed as still more important: viz. that the first outlay neces sary for their construction, will generally be less than that for any other description of bridges. There are exceptions to this assertion, but where they obtain, the stream must be very inconsiderable, and such as will permit the adoption of the wooden bridge requiring the simplest possible combination of the materials.

With these claims-which in fact cover the whole ground on which a preference could be founded-the system of suspension prevailed throughout Europe; and was immediately applied to the most important streams, in the heart of populous cities, and on various lines of railroads. Arches were commenced and successfully accomplished, from two to three times as wide as had ever before been attempted; and the practice of constructing bridges of any other description, was nearly superseded by the favor yielded to a system which appeared to combine every possible advantage, with a susceptibility for universal application, and which could scarcely be opposed by one solid objection.

The rivers of the United States, and especially those west of the Alle. ghany, present a broad field for the introduction and development of this improvement. In general, these streams are remarkably wide, and subject to freshets of great height and power; they are navigable for steam boats during a certain portion of the year, and sustain a commerce of the highest value to the country; considerations which may be regarded as so many objections to the obstructing of their channels by piers, independently of the expense which the erection of such works, in these circumstances necessarily involves. Besides, the sparseness of the population in most of the States, and the deficiency of funds incident to that condition of things, renders it important to fix on plans which will at the same time require the smallest annual charge for maintenance, and the smallest capital for origi nal construction.

These conditions are best satisfied by the pendant principle; and its introduction into this country must therefore be regarded as a valuable contribution to the means of perfecting our great and numerous lines of intercommunication.

The engraving of the bridge intended to be constructed across the Schuylkill offers a very faint idea of the appearance presented by such a structure, when tastefully designed and viewed in place. It will, however, serve to convey an impression of the general appearance of suspension bridges, to those who have not possessed the opportunity of witnessing some of the fine specimens of the art which have of late years been erected in Europe.

This bridge consists of one principal opening 400 feet wide, and two lateral stone arches of 65 feet. Upon the piers which sustain the thrust of these lateral arches are to be erected 4 isolated columns for the support of the wire cables which uphold the platform. There are four of these cables, each of which is about 600 feet in length, from 4 to 5 inches in diameter, and composed of about 900 strands of iron wire. The wire is the of

a foot in diameter; and each strand is covered with a coat of durable varnish before it is put in the cable.

The cables pass over the summits of the granite columns, where they bear on a moveable saddle, resting on rollers of cast iron, which are placed between the saddle and the upper surface of the capital. These rollers have a slight play, and the object of introducing them is to prevent the columns from being disturbed by movements of the cables, whether caused by the dilatation of the wire consequent on atmospherical changes of temperature, or the small oscillations and vibratory movements of the platform.

The ends of the cables descend through appertures in the masonry, and are securely anchored in the abutments of the littoral arches; passages are formed for the easy access of the superintendant to these fastenings, in order to admit, if necessary, of the removal of a cable, or the application of fresh paint or varnish to the iron as often as it may be judged expedient.

The width of the flooring of the bridge is 26 feet; of which space 18 feet in the centre, is intended for a carriage way, and 4 feet on each side, next to the parapets, for foot ways. The foot ways are raised 10 inches above the level of the carriage way. The dimensions of the cables are assigned with a view to the support of the bridge when the whole area of the platform is occupied by people; and it is assumed, in the calculation, that the number who can conveniently stand upon it is equal to 3500 men of ordinary stature. The weight of this number of people, estimated at 150 pounds per man, is 234 tons. The weight of the bridge, or of that portion of it which is suspended between the columns, is computed at 214 tons; and consequently the whole weight on the cables, where the platform supports a load of this amount, will be 448 tons; from which will result a tension at the points of support the summits of the columns-of 720 tons. The ultimate tenacity of the wire which will be used, may be safely estimated at three fifths of a ton for each strand; so that the aggregate strength of all the wires in the four cables-which contain 3600 strand-will be 2160 tons; or three times the tension to which they will be exposed when the flooring is occupied by 3500 men.

The bridge is to be sustained by 4 granite columns standing on bases 9 feet square; which are obviously too light to be depended on for the support of any material part of the tension; and it is consequently essential that the direction of [the cables, on the opposite sides of the columns should be such, that the resultant may fall as nearly as possible in the centre of their bases. By this arrangement the horizontal components of the tension neutralize each other, and each column is subjected only to a pressure in the direction of its axis, of about 225 tons, or half the weight of the bridge and its load. The stability of the columns is consequently greatly increased by the weight which they sustain; and it would be much more difficult to overthrow them when crossed by the cables and receiving the pressure due to the load which they support, than when isolated.

The platform is one foot higher in the centre than at the abutments; and

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