The bridges of Westminster and Black friars, over the river Thames, at London, are among the finest structures of the kind in Europe. The former is 1220 feet long, and 44 feet wide, having a commo dious broad foot-path on each side for passengers. It consists of thirteen large and two small arches, fourteen interme diate piers, and two abutments. The length of each abutment is 76 feet ; the opening of each of the smaller arches is 25 feet ; the span of the first of the large arches at each end is 52 feet, of the next 56 feet, and so on, increasing by four feet at a time to the centre arch, the span of which is 76 feet. The two piers of the middle arch are 17 feet wide ; and the others decrease equally on each side, by Due foot at a time, every pier terminating with a salient right angle against either stream. The arches are semi-circular, and spring from about the height of two feet above low water. The breadth of the river in this place is about 1220 feet, and the water-way through the bridge amounts to 870 feet. The bridge was be gun in 1738, and opened for passengers in 1750, at a neat expense of 218,8001. It is constructed of the best materials, and in a neat and elegant taste ; hut the arch es are too small in proportion to the quantity of masonry.
Blackfriars bridge, nearly opposite to the centre of the city of London, was be gun in 1760, and completed in ten years and three quarters, at a neat expense of 152,8401. It is an exceedingly light and elegant structure ; but, unfortunately, the materials do not seem to be of the best kind. as many of the stones in the niers are decayed. The bridge consists of nine large, handsome, and nearly elliptical arches ; the centre arch is 100 feet wide, and the four arches on each side, reck oning towards the shores, decrease gra dually, being 98, 93, 83, and 70 feet re spectively, leaving a water-way of 788 feet. The whole length from wharf to wharf is 995 feet, the breadth of the car riage-way 26 feet, and that of the raised foot-way on each side 7 feet. The upper surface of the bridge is a portion of a ve ry large circle, which forms an elegant fi gure, and admits of cons enient passage over it. On each pier there is a recess or balcony, with two Ionic columns and pilastres, which stand on a circular pro jection of the pier above high-water mark. The bridge is rounded off at each extremity to the right and left, in the form of a quadrant of a circle, rendering the access commodious and agreeable. This edifice must be regarded as a fine specimen of Mr. Milne's ingenuity and judgment, though the method of con struction has never been made public.
Wooden bridges now demand our at tention. The simplest case of these edi
fices is that in which the road-way is laid over beams placed horizontally, and sup ported at each end by piers or posts. This method, however, is deficient in strength and width of opening ; it is, therefore, necessary, in all works of any magnitude, to apply the principles of trussing, as used in roofs and arches. Wooden bridges of this kind are stiff frames of carpentry, in which, by a pro per disposition, beams are put so as to stand in place of solid bodies, as large as the spaces which the beams inclose ; and thus two or three, or more, of these are set in a hutment with each other, like mighty arch stones. At Schaffhausen, in Switzerland, where the Rhine flows with great rapidity, several stone bridges had been destroyed, when, in 1754, Gruben hamm offered to throw a wooden bridge of a single arch across the river, which is nearly 390 feet wide The magistrates, however, required that it should consist of two arches, and that he should, for that purpose, employ the middle pier of the last stone bridge, which would divide the new one into two unequal arches of 172 and 193 feet span. The carpenter did so, but contrived to leave it a matter of doubt, whether the bridge is at all supported by the middle pier. It was erected on a plan nearly similar to the Wittengen bridge, at the expense of about 8,0001. sterling. Travellers inform us, that it shook if a man passed over it ; yet waggons, heavi ly laden, also went over it without danger. This curious bridge was burnt by the French when they evacuated Schaffhau sen, in April, 1799.
Iron bridges are the exclusive invention of British artists. The first that has been erected on a large scale is that over the river Severn, at Coalbrook Dale, in Shropshire. This bridge is composed of five ribs, and each rib of three concen tric arcs, connected together by radiating pieces. The interior are forms a com plete semi-circle, but the others extend only to the tills under the road-way. These arcs pass through an upright frame of iron at each end, which serves as a guide ; and the small space in the haunch es between the frames and the outer arc is filled in with a ring of about seven feet diameter. Upon the top of the ribs are laid cast iron plates, which sustain the road-way. The arch of this bridge is 100 feet 6 inches in span ; the interior ring is cast in two pieces, each piece being about 70 feet in length. It was construct ed in the year 1779, by Mr. Abraham Darby, iron•master at Coalbrook Dale, and must be considered as a very bold ef fort in the first instance of adopting a new material. The total weight of the metal is 3781 tons.