76 feet in span, and the rest decrease on each side equally by 4 feet. That of Blackfriars, begun in 1760, and completed in 1770, is another imposing structure, consisting of nine elliptical arches, the middle one of which has a span of not less than 100 feet, with a breadth of 43. The arches are low in pro'portion to their span; but as the bridge is built to accommodate the navigation of the river, the upper surface of the whole forms a portion of a very large circle, and appears a gentle swelling ground under foot all the way. Waterloo Bridge, begun in 1811, and completed in 1817, is another of those costly erections which modern enterprise has thrown across the Thames. It consists of nine arches, all of one span and height, so as to form a level roadway; and this road is continued on either side the river, and supported on a number of brick arches. The length of the stonework or bridge is 1242 feet, the brickwork of forty arches on the Surrey side is 1250 feet, and of that on the Strand side 400 feet. Each stone arch has a span of 120 feet, and the height of the road above the river is 50 feet. The New London Bridge, begun in 1825, and opened in 1831, is the last of the stone structures over the Thames which we shall notice. It consists of five arches the centre one having a span of 152 feet, the two on each side a span of 140 feet, and the abutment arches a span of 130 feet. The rise of the middle arch is 32 feet, that of the abutment arch falls to 25 feet. The extreme length of this structure is 950, and its breadth 55 feet. With the exception of the packings, it is constructed of granite of the finest description and workmanship; and including the approaches, is said to have cost about two millions sterling.

But it is not in the neighbourhood of the metropolis alone that we find structures of this class evincing both boldness and ingenuity of design. The Severn, the Ouse, the Tees, the Tyne, and other English rivers, are spanned by stone arches, some of which are of surprising dimensions. The centre arch of the bridge in Blenheim Park is 101 feet span; a single arch across the Tees at Worstone is 109 feet; and one across the Taaf, in Wales, is not less than 140 feet. But these are inferior to some which recent railway requirements have called into existence. Thus Victoria Bridge, over the Wear, at a place called Penchar, and on the line of the Durham Junction Railway, consists of four main arches, and of six smaller ones to support the roadway. Of the main arches, that beneath which the river flows is 160 feet span; the north arch 144 feet; and each of the end ones 100 feet. The total height of the structure, from the foundation to the top of the parapet, is 156 feet; the clear width of roadway 21 feet; and the length 820 feet.

Scotland can also boast of some very handsome bridges; chiefly, however, of modern erection. The Tay at Perth, for example, is crossed by one of nine arches, the largest of which has a span of 77 feet; the Tweed, at Coldstream, by one of five

**27

arches: the North Esk, nr live arches, each with a span of 78 near Montrose, by one of seven; the Tweed, at Kelso, by one feet the Spey, y, near Gordon Castle, by one of four arches, the largest having a span of 95 feet; and the Tay, at Dunkeld, by one of seven

is perhaps the, the largest having a span of 90 feet which

that feet;

is perhaps the finest structure of the kind in Scotland. Of Scote tish bridges having a large span and height, we may mention the Dee Burn, near Aberdeen, whose span is 130 the Dee, near Kirkcudbright, having a span of 118 feet; the North Bridge of Edinburgh, consisting of three large and two small arches the large ones having each a span of 72 feet, and a height of 65; the Peas Bridge, thrown across a deep dingle on the Edinburgh and Berwick road, consists of four arches, each 55 feet span, and 124 feet in height; and the Dean Bridge, near Edinburgh, the main body of which is com posed of four arches, each 90 feet span, and from the bed of the river to the level of the roadway 106 feet high. pomeo of Jisa2

ARCHED AND SUSPENSION IRON BRIDGES.

Within the last half century, iron has been frequently subs stituted for stone in the construction of large arches; so that, instead of a solid vault of masonry, we have now a light, but substantial framework of metal. The first iron bridge of any importance constructed in Britain was one near Coalbrookdale, on the Severn the invention of Mr Obiah Darby. It was com pleted in 1779, and consists of a single arch 101 feet span, with a rise of 45. Since then, a vast number of such structures have been erected some of them nearly triple the span and elevation of their humble parent.

In 1796, an iron bridge was erected at Buildwas, near Coalbrookdale, under the direction of Mr Telford, the span of which is 130 feet, and its rise only 17 feet. In the same year Mr Bur-2 don completed the celebrated iron arch across the Wear at Sunderland, the span of which is 240 feet, and its elevation above low water fully 105 feet. This elevation consists of seventy-five feet of stone pier, and thirty of a rise in the arch-thus allowingd large vessels to pass under without striking their masts! In 1803, an elegant iron bridge, somewhat resembling that of Sun-s derland, was erected at Staines, having a span of 180 feet, and a rise of only 16. Since then, a number of others have been constructed of 80, 90, and 100 feet span, some rising so little in their curve as 5 feet. But all these works have been far exceeded in extent and importance by those built over the Thames. Vauxhall Bridge, which was completed in 1816, consists of nine arches of cast-iron, each having a span of 78 feet, and 11 and 12 feet rise; and that of Southwark of three arches, each 240 feet span, and only 24 feet rise! In most of these structures the iron arch rests on stone piers and abutments; but in some the piers are also of iron, thus giving to the whole an appearance

of lightness and elegance which it is impossible to equal erections.ga & ddiw 969290916 9 10 900 99 7690hains; these 9dSuspension bridges are such chains being supported on piers or pillars, and stretched across the chasm or water-course over which it is designed to form a passage. From the chains a platform for the roadway is suspended by means of a series of equidistant vertical rods. Peru, China, and other remote regions, bridges this on principle, though of an extremely rude and perishable co construction, and forming a most unstable and oscillating path, seem to have existed anterior to anything which is at present known of the history of those countries, but the introduction of SuspensionBridges into civilised states, at least on a large scale, and of a substantial fabric, is of very recent date. By far the most stupendous in Britain, and indeed in the world, th295 date of its erection, 1826, is that constructed by Mr Telford across the Menai Strait, to connect the island of Anglesea with the mainland of Wales.

highest span for central arch between the

containing about 1200 wires, the united strength of which could support three times the weight of two rows of loaded wagons extending over its whole length. The wires are simply laid together, and bound in a cylindrical form by means of annealed wire wound round them at distances of two or three feet; and the whole is painted white, both for preservation, and that the least tendency to rust might be detected at once. But iron, even in its most approved form of wire cable, has been surpassed by the employment of steel; for we learn from the Register of Arts, that in 1832 a bridge of this material was erected over the Danube, near Vienna. The span of this structure is 234 feet, with a rise of only 15. A saving of one-half in the total weight is calculated to have been effected by the use of steel instead of iron; and the strength is also said to be much greater.

TUNNELS.

A tunnel is merely a hole or passage cut through a hill, in order to carry a canal, a road, or a railway in an advantageous course, and is generally resorted to as a means of lessening the expense which would ensue if an open cutting were made. Since railways began to be spread like a network over our island, tunnels have become exceedingly common, varying from 100 yards to more than a mile in length, and being cut through all sorts of material, from the hardest basalt to the softest quicksand. In the present state of engineering, the question is not difficulty but cost-the price being sometimes so enormous as £150 a-yard, or even, as in the case of the Thames Tunnel, which is a double one, £1200 a-yard! Passing by all the tunnels, great and small, which are to be met with on almost every railway line, we shall only notice that carried under the Thames, which is justly regarded as one of the greatest of modern accomplishments.

The object of this undertaking, it may be premised, is to afford a free communication between both sides of the river without impeding the navigation, as a bridge or any kind of ferry would undoubtedly do. It seems that, in 1804, an attempt was made to construct a tunnel from Rotherhithe to Limehouse; and although a driftway was carried under the river to the extent of 923 feet, and within 150 feet of the opposite shore, the work was abandoned, owing to difficulties that had occurred, and which the engineer declared at the time to be insurmountable. The plan of a tunnel under the river was, however, always looked upon as a matter of deep interest and great importance; and when Sir I. Brunel, in 1824, exhibited his plan for constructing one with a double and capacious roadway, it was not only well received, but liberally supported. The spot selected for the formation of the work in question-namely, from Rotherhithe to Wapping-was considered to be the most de sirable, not only as regarded the traffic in the immediate neigh

Coluna dada 20 diel, but also as related to the neighbouring bourhood of the tunnel, but also as counties. An act of parliament having been obtained on the 24th of June 1824, and £180,000 having been raised by means of shares, Sir I. Brunel, the engineer, began his operations in March 1825. His first object was to prepare a shaft of descent, which he did by forming one 50 feet in diameter, at 150 feet from the Rotherhithe bank of the river. This he effected by constructing a brickwork cylinder of the above diameter, 42 feet in height, and 3 feet thick. Over this he set up an engine for pumping out the water, and for raising the earth taken out from within the cylinder, which he then sunk en masse into the ground, after the manner that well-shafts are sometimes sunk. When the 50 feet shaft was sunk to the depth of 65 feet, another shaft of 25 feet diameter was sunk still lower for the purpose of drainage. At 63 feet, the tunnel commences dipping 24 feet per 100 feet. The excavation was 38 feet wide, and 22 feet high; and the bottom, at the deepest part of the river, is 76 feet below high water. The excavation was effected by a shield composed of twelve frames close together, having each three cells above one another, for the miners-making in all room for thirty-six workmen. Its mode of operation, which was truly ingenious, was somewhat as follows:-The front was placed against the earth to be removed, and the workmen having removed one board, excavated the earth behind it to the depth directed, and placed the board against the new surface exposed. The board was then in advance of the cell, and was kept in its place by props; and having thus proceeded with all the boards, each cell was advanced by two screws, one at its head and the other at its foot, which, resting against the finished brickwork, and turned, impelled it forward into the vacant space. The other set of divisions then advanced. As the miners worked at one end of the cell, so the bricklayers formed at the other the top, sides, and bottom, the superincumbent earth of the top being still held up by the shield till the bricklayers had finished; and following the shield, in each

archway was a stage for the assistance of the men in the

upper cells. On the 1st of January 1826, this shield was placed at the bottom of the shaft, ready for the reception of the

excavators. The

double archway of the tunnel was then proceeded with; but on the 25th of the same month, the stratum of clay through which

31