422

HISTORY OF ENGINEERING.

Book I.

m

near enough, I believe, to let a man know what he is about when he either designs orerects one.

“ In respect to arches, their strength doth not so much depend ontheir shape, as the weight they bear being well adapted to them; for \P~fa.

although it must be granted that catenarian arches, if considered inde-pendent of any weight but their own, are the strongest of all others, yetif the weight on their crowns is not proportionable to the height theyrise, they may be said to be weaker than a semicircle, or arches of anyother form, when their weight and abutments are well proportioned.”

Westminster Bridge was the commencement of an entirely new systemfor laying foundations in deep water, as well as the construction ofcentres for arches required to span great rivers. An act of parliamentfor its erection was passed in 1736, and another fixing its site, two yearsafterwards. The first design was a curious wooden structure, by Mr.

James King, resting on stone piers.

The two large middle piers were contracted for soon after, and thewooden superstructure was to have been placed upon them and com-pleted in a year after all the piers were finished, for the sum of 28,0007.

Mr. Charles Labelye, a native of Switzerland , presented to the com-missioners, in 1738, a design for a stone bridge, which was approved of,and preferred to many others, and it was resolved that the piers shouldbe erected as high as the springing of the arches, viz. about a foot abovethe level of low water mark, and over these broad piers, smaller ones ofsolid stone, each to be the width of the bridge in length.

The design for the stone bridge consisted of thirteen semicircular gf^ garches, springing about a foot above the level of low water mark ; themiddle arch 76 feet span, and the others decreasing in width equally oneach side by 4 feet, the two next the middle arch being 72 feet wide,and so on to the least of the large arches, which are 52 feet wide. Thetwo small ones nearest the abutments are 25 feet wide.

The length of every pier is 70 feet from point to point, and each endterminates with a salient angle.' The two middle piers are 17 feetwide at the springing' of the arches, and the others decrease in breadthequally on each side by 1 loot, the two next the largest being 16 feet . ,,

wide, and so on to the two least of each side, which are 12 feet wide at flk \ s

the springing of the arches. =

The piers are 4 feet wider at their foundations than at their top, andeach of them is laid on a strong grating of timber, the same shape as thepier, about 80 feet long, 28 feet wide, and 2 feet thick.

The depths and heights of every pier are different, and none of theirfoundations are at a less depth than 5 feet under the bed of the river,nor at a greater than 14 feet. This difference of depth is occasionedby the nature of the ground, for although all the piers and abutmentsare laid on a bed of gravel, which on boring was found harder as thedepth increased, it was still more difficult to penetrate on the Surreyside than on the Westminster.

In July, 1738, Labelye, with a company of London masons, arrived atthe Isle of Portland, and commenced quarrying and working into properscantling all the stone necessary for the two largest piers. About thesame time the carpenters began to make and erect on the Surreyshore twelve frames of timber,. supported in a vertical position,parallel one to another, and kept in their proper places by short pilesdriven into the ground. These frames reached about 2 feet above thecommon high water mark, and were braced together so as to be keptupright and steady, till the caissoon should be formed and finished onthe top of them. The ground sills or bottom pieces of these frameswere made cylindrical, that by taking the braces away they might serveas so many axes or centre pieces, round which the frames could revolveall together, for the lowering or launching of the finished caissoons athigh water without danger of racking or straining.

In September the engine for driving the piles, which was contrived byMr. James Vauloiie, a watchmaker, was completed, and on the 13th ofthat month the first pile was driven.

The piles were of fir, 13 or 14 inches square, and 34 feet in length,shod with iron: round their tops were thick iron hoops, which weretaken off after driving, and used again ; they were driven 13 or 14 feetbelow the surface of the bed of the river, and were placed about 7 feet asunder, in lines

Fig. 426.