Chap. VIII.

BRITAIN.

507

Suspension Bridges, of iron, do not appear to have been introduced before the year 1741,when one was built across the Tees for the use of the miners; its length was 70 feet, andwas suspended above the level of the river at a height of GO feet : Scamozzi “ Del IdeaArchi,” published in 1615, conveys some notion of these structures, but their true princi-ples were not made known till they obtained the attention of Bernouilli. Hutchinson, inhis “ Antiquities of Durham,” gives the following account of the winch bridge : —“ It is sus-pended on iron chains, and stretched from rock to rock, over a chasm 60 feet deep ; itswidth is 2 feet, with a hand-rail on one side, and it is planked in such a manner, that thetraveller experiences all the tremulous motion of the chains.”

When Mr. Telford was called upon to report upon the practicability of forming abridge at Runcorn , he commenced a series of experiments upon the tension of iron, andfrom that period engineers seem to have studied the principles of suspension bridges. Theseimportant experiments occasioned iron to be more universally introduced into construction ;and we now find that the skill of the carpenter is not always required to cover in a build-ing with timber.

Mr. Telford commenced his experiments by proving what force would pull asunderlengthwise pieces of iron from inches to of an inch in •diameter. The experiments

t i

Fig. 511.

MACHINE FOR PROVING THE STRENGTH OF IRON.

were made upon those of the largest diameter, by means of an excellent hydrostatic machine,and on those of the smaller by attaching weights perpendicularly, and repeating them atvarious times.

He then made several experiments upon different diameters, from ^ to of an inchdrawn horizontally, and with different degrees of curvature; and this was performedbetween points 900, 225, 140, and 139 feet 6 inches apart, and was repeated 200 times. Inthe experiments made upon ^ of an inch and under, the wire was drawn over pulleys; some-times both ends were fixed, and sometimes one end only, the other having weights attachedperpendicularly to show the effects when compared with those loaded upon the curvedpart of the wire; these last were disposed at J, and ^ divisions of the distance over whichit was stretched. These experiments being completed, it was ascertained what blow' wouldbreak the wire when stretched nearly horizontally and at different curvatures, which wasdone by dropping weights from a given height. The several wires were weighed, andthe weight of 100 feet in length of each noted.

The result of the experiments was, that a bar of good malleable charcoal-iron 1 inchsquare w’ill suspend 27 tons, and that an iron wire, ^ of an inch in diameter, 200 feetin length, weighing 3 pounds 3 ounces, will suspend 700 pounds; and that the latter, witha curvature or versed sine of 3 ' 0 part of the chord line, will support ^ of the weight sus-pended perpendicularly, when disposed equally at and $ its length, and with a curva-ture of of the chord, it w'ill bear £ of the aforesaid perpendicular weight disposed in asimilar way. A w’ire of an inch in diameter, drawn very tight between points, 31 feet6 inches apart, resisted the impulse of 20 pounds weight, falling from a height of 7 feet9 inches.

A bar of good English malleable iron 1 inch square w'ill suspend from 27 to 30 tonsbefore it breaks, and will bear from 15 to 16 tons before its length is at all extended.With a curvature of ^ of the length, malleable iron, besides its own weight, sustained £ ofwhat broke it perpendicularly. An inch bar would therefore bear ^ of 15 tons withoutderanging its parts ; but it is better in practice to assume that an inch square in sectionshould only bear 4 tons.

Galashiel Bridge was constructed in the year 1816, at a cost of 40/., and is 112 feet inlength ; it is suspended from iron wires of very small diameter.

Peebles, over the Tweed, called King’s Meadow Bridge, was constructed in 1816, for the