WESTMINSTER BRIDGE was built according to the designs, and under the superintendence of, a Swiss architect, named Labeyle, who, in its construction, adopted the practice of forming the foundations by means of caisson; instead of driving piles into the ground, or making use of according to the general custom. This he effected in the following manner :—Large timber cases, consisting of a strong platform, or raft, surrounded by sides, were made water-tight, and floated on the surface of the Thames to such places as the piers of the intended bridge were eventually to occupy. In these, when firmly moored in proper position, the masonry of the piers was commenced : the caissons gradually sinking in the water as the masonry advanced, until at length they finally settled on the bed of the river the sides of the caissons were then removed, to be used else where, and the piers, which were built of Portland stone in blocks, each of which was at least a ton in weight, and many of them weighing as much as five tons, were carried up to the springing of the arches. These blocks of stone were cemented together with Dutch tarras, besides being con nected with each other by iron cramps fastened with lead.
The building of this bridge occupied nearly twelve years, owing to the plan that had been adopted in laying the foun dation ; in consequence of whieh, one of the piers sunk so much, as seriously to endanger the stability of the structure, and it was found necessary to put another in its place ; this circumstance greatly enhanced the cost of construction, and the various sums expended amounted to £3S9,500. It appears that materials of this bridge, to the value of £40,000, are constantly under water. The caisson on which the first pier was erected, contained 150 loads of timber, on which were laid 3,000 cubic feet, or nearly three tons of solid stone.
Westminster Bridge is 1,220 feet long, and is 41 feet wide, including the foot-path tor passengers on either side of the carriageway; it consists id 13 large, and two small arches, 14 piers, and two abutments. The arches spring at about 2 feet above low-water mark, they are all semi-circular. The centre arch is 76 feet span, and the others decrease on either side by 4 feet, so that the arches near the banks of the river are only 52 feet span. There are, besides, the two small arches each of 25 feet span.
The piers are 70 feet in length, and terminate in east waters, pointing up and down the stream. Those support the centre arch are each 17 feet wide, and every successive pier is diminished a foot in breadth, so as to leave a water-way in the clear of 870 feet.
There was formerly a handsome stone balustrade, which formed the parapet of the bridge. on either side, and over every pier there was a bay or recess, covered in with a kind of half cupola ; while over the central arch, a rectangular space increased the width of the bridge by means of a pro jection towards the water.
The soffits of the different arches in this bridge, were carefully turned in Portland-stone. Over this, another arch was formed of I'urbick-stoue, well bonded with the preced ing, and so arranged as to make the thickness of the double arch about the haunches four times that at the crown, The spandrels were constructed with barrel and intermediate arches, so as to maintain the edifice in equilibrio.
When Old London Bridge was taken down, and a more uninterrupted outfall was thus afforded to the stream, the bed of the Thames became considerably deepened, and the foun dations of the different bridges which had been previously constructed, were thereby more or less affected. Westmin ster Bridge suffered particularly from that cause ; for, owing to the very insecure substructure on which it was founded, its piers have been gradually giving way for years ; and, in con sequence of the inequality of yielding which has taken place, the entire bridge is now in a deplorably dilapidated condition. It has been deemed absolutely necessary to prop up several of the arches by centerings, to remove the weight over others, and in order to prevent a too manifest appearance of the unsightly breaks which have taken place in the roadway, the stone balustrade has been replaced by a temporary wooden fence or parapet.—To our thinking, the repairs which it has lately undergone, were effected in a most injudicious manner. In the first place, the system of propping up some of the arches, and allowing others to sink freely, was highly preju dicial to the stability of the bridge ; and indeed one of the arches was only partially supported, so that the remaining portion separated from it, and sunk several inches, to the imminent peril both of its real and apparent efficiency. Again, the uselessness of facing the piers with new stone, at a time when the entire demolition of the structure became inevitable, must be evident to every one who will give the subject but a passing consideration.
As, however, this bridge is to be taken down very shortly, and a handsome modern structure will ere long supply its place, we do not think it necessary to say more on this sub ject at the present time.
Next in point of date among the stone bridges of the Thames, is that of Blackfriars, which was commenced in the year 1760, and finished in 1770, by Robert Mylne, a Scotch engineer, who had just before returned from Rome, where he had been pursuing his studies, and where he had earned a well-deserved reputation ; having received a silver medal from the Roman Academy, which he deposited, together with several coins of George the Second, in the tbundations of the bridge.