of gravity of the section. Fig. 5 exhibited the manner in which this force of 475 tons operated to produce detrimental pressure on the outer fibre of the iron at M. The method of ascertaining this had occurred to him two years ago, when studying the strains upon iron arches, in cases where the curve of equilibrium had travelled away from the line of the centre of gravity. It depended upon this axiom, that whenever a prism of any elastic material was compressed by a force on some line not corresponding with its centre of gravity, flexure must take place, and that to such an extent as would remove the centre of resistance from its original situation until it was perpendicular beneath the force applied. Now, the force of compression on the prism, when applied over the centre of gravity of section G, Fig. 5, might be represented by the content of the solid intercepted between two parallel planes AE and CD. If the compressive force be removed from G to H, in the line of the arrows, the flexure which would ensue might be represented by a certain amount of angular motion of the plane AG M, around the centre of gravity G. By a property of the centre of gravity, any angular movement around G would generate solids of equal magnitude, G A B and GEF, on contrary sides of the centre, and these represented equal forces. Now if I, and K, were the centres of gravity of these solids, then, in order that the point H should become the centre of gravity of the solid F B C D, the solid AC DE into G H must be equal to (GABX GI)+(GEFx GK). When this condition was fulfilled flexure would cease, and H would be the true centre of resistance directly opposed to the compressive force. If ED be laid down from any scale of equal parts, as equal to the pressure in tons on the square inch, then LM would represent the additional pressure brought on the outside fibre of the iron, on account of the flexure, and AB would represent the diminution of pressure on the uppermost fibre.

Applying this to the Charing Cross Bridge, and calling—

a = the area of the whole section of

Thus, then, the square on pressure = E D=2.94 tons on the inch.

The pressure on the bottom fibre = E D+LM = 2·94+3·92 = 6.86 tons on the inch.

=

The pressure on the upper fibre AC-A B= 2.94-1.2= 1.74 ton on the inch.

Mr. HAWKSHAW,-President, thought that Mr. Phipps' observations on the Charing Cross Bridge were sound. It was clear that the diagonal strain on that bridge must be transmitted from one pin to another, along the top and bottom members of the bridge; and the nearer those strains passed along the line of the centre of gravity of those members the better. He did not however agree, that in boiler-plate bridges this desideratum was secured by the closeness of the rivets which connected the sides to the top and bottom. He believed the objection applied with more force to many of those bridges than to the Charing Cross Bridge. The girders of the Charing Cross Bridge were so designed, that the diagonal strain at the ends of the girders, if passed from pin to pin, through the vertical webs only, could produce no injurious effect, for the vertical webs at those points were made very strong; and the diagonal strains at the centre of the bridge were so small, that the vertical webs at that point also were stronger than necessary for the mere transmission of the vertical strains. Mr. Scott Russell had, in discussing this point, compared the Charing Cross Bridge with another bridge which was being erected over the new street in Southwark. But that bridge was constructed on totally different principles. It was in fact an arch, and Mr. Phipps' observations could not apply to it. It followed the same laws as any other arch, the weight being merely suspended from, instead of being placed upon, it. The bottom web was substituted for fixed abutments, and it also served to stiffen the roadway and to distribute the load. In that bridge the diagonal braces might be taken away, and still the bridge would stand. They were, however, of use

some

in assisting to distribute the load, and to provide, to extent, against flexure in the arch. For with loads unequally distributed, all iron arches were subject to flexure. This accounted for the small strain per ton which, it had been observed, was usually placed upon iron arches. If those arches could always be uniformly loaded, the whole would be put in compression; but light malleable and cast-iron arches, with heavy rolling loads, were subject to compression in some parts, and to extension in other parts; and thus it happened that a considerable portion of the cast-iron arches usually erected were not always available in resisting compression, and hence frequently not more than 1 ton to the square inch, having regard to their whole sectional area, was placed upon them. In some cast-iron bridges subjected to the rolling load of a railway, probably not one-half the section of the rib would be brought into compression.

May 5, 1863.

JOHN HAWKSHAW, President,

in the Chair.

The following Candidates were balloted for, and duly elected :JOHN BLOUNT and Captain VIRIATO DE MEDEIROS, as Members; EDWARD APPLETON, Captain FRANCIS JOHN BOLTON, and ROBERT TREFUSIS MALLET, as Associates.

No. 1,091.—“ American Iron Bridges." By ZERAH COLBURN. IN the United States, the first cost of the truss, or superstructure,' of a timber bridge of any given span is generally less than one half that of an iron bridge of the same strength. This fact will account for timber bridges having been so largely adopted in America. Iron was occasionally employed there as long ago as 1835, but it is only within the last ten or twelve years, that it has been used to any considerable extent. At the present time, it is generally admitted by American Engineers, that the original cheapness of timber structures does not compensate for their rapid decay, their frequent destruction by fire, and the constant repairs and watching which they demand.

Among the earliest iron bridges erected in that country were a few cast iron tubular arches, including one of 80 feet span. The design of these was due to Major Delafield, of the United States Engineers, who adopted them at about the time when somewhat similar arches were employed in France, by the late M. Polonceau, in the construction of the Pont du Carrousel, over the Seine. Major Delafield's arched ribs were elliptical in section, the transverse vertical axis being about four times the length of the conjugate axis. In 1858, an aqueduct bridge was erected at Washington by Captain Meigs, of the United States Engineers, who used for this work two arched ribs formed of water-pipes, through which the water flows, the pipes being circular in section, as that form encloses the greatest quantity of water with the least amount of iron. (Plate 20, Fig. 1.) The span of this bridge is 200 feet, the rise being 20 feet; and the width of the bridge over all is 28 feet. The pipes are 4 feet in diameter inside, and 1 inch thick. They are lined with staves of resinous pine, 3 inches thick, to prevent the freezing of the water. The pipes are not cast to the curve of

the arch, but are in straight lengths of about 12 feet, with flanged joints, faced in planes parallel to the corresponding radii of the arch. At the skewbacks, the ends of the pipes are faced to large conical bases admitting the water, and resting upon the masonry. There is no allowance for expansion, or contraction. The roadway is of timber, supported on spandrils formed of rolled wrought iron beams 9 inches deep, and weighing 90 lbs. per yard. The bridge was tested with the arched ribs filled with water, and with a load of 125 lbs. per square foot upon the roadway. The weight of each arched rib, when filled with water, is about 160 tons; that of the spandrils of each rib and of one-half of the roadway is about 30 tons; while the test load was equal to about 160 tons on each half of the roadway, making the whole weight about 350 tons on each rib. The thrust of one-half of this weight upon each abutment would be about 470 tons, and as there are 2383 square inches of sectional area of iron in the pipes, this would correspond to a strain of about 2 tons per square inch. This strain, while it includes that due to the weight of the water in the pipes, is exclusive of that due to its pressure. The pipes were proved to a pressure of 300 lbs. per square inch.

Major Delafield's and Captain Meigs' bridges are, so far as the Author is aware, the only iron arches yet completed in the United States; unless the application, in 1849, of a light cast iron arch to each side of a timber truss, of 133 feet span, on the Pennsylvania Central Railroad, may be regarded as another instance. A roadway bridge, with two cast iron arched spans of 185 feet each, to cross the Schuylkill River at Philadelphia, had been designed by Mr. Strickland Kneass, C.E., and was about being let to contract when the Author was last in America. The rise of these arches was to be 20 feet, and the whole width of the roadway and of the footways 42 feet. Each arch was to be formed of seven ribs segmental to a circle of 224 feet radius, and of a uniform depth of 3 feet 6 inches; the top and bottom flanges were to be 8 inches wide and 3 inches thick, the web 23 inches thick, and the whole sectional area of each rib 138 square inches. The thrust at the skewback of each rib, including that caused by the weight of he spandrils, cast iron floor-plates, 6 inches of gravel, a granite paving, and a load of 70 lbs. per square foot of the roadway, will be 217 tons, equal to 1.63 ton per square inch of the section of the rib. In addition to diagonal bracing of cruciform section, the ribs are to be tied together at fourteen points in their length, by round wrought iron rods, enclosed in cast iron distance pieces. The spandrils are to be upright posts, 4 feet apart from centre to centre, cruciform in section, and having each 28 square inches of cross sectional area. Some ornament is contemplated upon the bridge, but the violation of symmetry, involved in the adoption of an even