426

HISTORY OF ENGINEERING.

Book I.

it was loaded with 700 tons ; after casing it round with strong piles, to prevent a further 4slipping of the gravel, it was taken down to the level of low water, and rebuilt; to lightenthe work which rested upon it, arches were constructed in the spandrills.

Bridge over the Taaf near Llantrissart, in South Wales, was commenced in 1746 byWilliam Edwards, a country mason; it consisted of three arches, and was admirablyexecuted; two years and a half after its completion, it was carried away by a flood, and asthe builder had engaged to maintain it for seven years, he commenced rebuilding it. Thesecond bridge was of one arch, the span or chord of which was 140 feet, its height 35 feet,the segment being that of a circle of 175 feet; the breadth is 15 feet, and the thickness ofthe arch 2 feet 6 inches. When completed, with the exception of the parapets, thethrust was so great that it pressed in the haunches, and the middle of the bridge sprungup, when the key-stones were forced out. This second misfortune did not quell thecourage of our maison, and he undertook his task a third time.

In each of the haunches he introduced three cylindrical holes through the whole thick-ness, and by this means so reduced the weight that no farther danger was anticipated. Theholes or cylinders rise above each other, ascending with the form of the arch ; the diameterof the lowest is 9 feet, the second 6, and the uppermost 3 feet; this was finished in theyear 1755; the chord line and versed sine were the same as those of the second bridgewhich fell four years before; besides the cylindrical holes, the spaces between them wereleft hollow, and filled up with charcoal.

The fame of this latter work spread far and wide, and Edwards was employed to buildthe bridge at Usk in Monmouthshire , another of three arches over the Tawy, and Pont arTawy, over the same river, ten miles above Swansea; this had but one arch, its chord being80 feet; over each haunch was one cylindrical hole.

Bettws Bridge , in Caermarthenshire, consisting of one arch 45 feet span.

Londonvery Bridge, in the same county, with one arch 84 feet in the span, and onecylinder over the haunches.

Wychbree Bridge over the Tawy , 2 miles above Morriston, which has one arch, 95 feetspan, 20 feet in height, with two cylinders over the haunches to relieve them.

Aberaven Bridge, in Glamorganshire , consisting of one arch, 70 feet span, 15 feet high,but without any cylindrical holes.

Glasbury Bridge, near Hay in Brecknockshire , over the Wye, which consisted of fivearches.

Edwards, born in 1719, was the youngest son of a farmer residing in Glamorgan-shire , and some of his first works in the art of construction were the stone walls for theinclosure of the lands on which he was employed; they were so well built, that theyattracted considerable attention. An opportunity being afforded him of seeing the toolsused by the regular masons, he procured some of them, and commenced squaring the stonefor building a workshop, which was so well executed, that it met with the approbationof all who saw it, and it is said that he there introduced the properties of the arch. Hisfirst bridges, consisting of only one arch, were evidently too high, which he avoided byflattening the arch, as he became better acquainted with the subject; he proceeded, however,with caution and judgment, having no other guide than his own experience, and he atlength learnt that where the abutments are prevented from spreading, arches of little rise areperfectly secure.

As the Castle of Caerphilly was in the parish in which he lived, it is more than probablehe derived all his knowledge of construction from that ruin, all his masonry, and themanner of hewing and dressing the stone, being so similar to what he doubtless admiredthere.

The science of bridge-building had already considerably advanced, and various ideaswere then published on the form of arches, in which the merits of the circular, elliptical,and cycloidal curves were discussed. ^ The elliptical seems to have been considered as pos-sessing particular advantages for forming the intrados of an arch, as at its springing thecurvature was more considerable, and it rose more perpendicularly, affording an extensiveand commodious opening for vessels to pass; and towards the summit or crown the curv-ature decreased, so as to form almost a horizontal line, which was the best adapted for theroadway; on this account it was probably preferred for some of the bridges afterwardsconstructed. The cause of the stability of an arch was not at this time sufficiently under-stood, nor had the engineers given attention to the subject of increasing or decreasing thedepth of the voussoirs, or what constituted the absolute weight to be provided for in theabutments: it was not then the practice to consider, that for an arch of equilibration it wasnecessary to have the curvature of the intrados different from that of the extrados; theprinciples of the catenarian had not been applied, but in the course of a few years it wasgenerally adopted by our most celebrated builders; and the laws which nature has laiddown prescribe the form to be given to the extrados of an arch intended to be lasting.T>i\ David Gregory had, in the Philosophical Transactions for 1697, stated, “that none butthe catenary was the figure of a true legitimate arch, and that when an arch of any other