at Gateshead, is brought to the central station by the magnificen bridge now under notice.

The Tyne bridge has two piers at the margin of the river, and four others in the stream itself; besides minor piers to support the land arches. These piers are of massive masonry. The distance from pier to pier is about 124 feet, and this determines the span of the arches. At a height of about 90 feet above the level of high water runs a level bridge for carriages, horses, and pedestrians; and at a further height of about 25 feet above this roadway runs the railway itself. The entire height of the masonry and iron work, from the bed of the river to the parapet of the railway, exceeds 130 feet. The whole length of the structure, from the high ground of Gateshead to the high ground of Newcastle, is nearly 1400 feet. There are nearly 5000 tons of iron-work in the structure. The masonry in and over the river has cost more than £100,000; the masonry and brickwork in fixing the land arches, about an equal sum; while the iron-work has cost a still larger sum.

The Tweed bridge has been opened for traffic in the autumn of 1850, on the occasion of her Majesty's journey to Scotland. It is one of the finest and largest railway viaducts ever constructed. The old bridge over the Tweed was built in the reigns of James I. and Charles I.; it is nearly a thousand feet in length, and consists of 15 arches. When a junction became desirable between the North British and the various English railways, a viaduct over the Tweed was necessary; and Mr. Robert Stephenson planned the structure which has been lately opened. In order that the traffic might be accommodated, a temporary timber viaduct was built, to be used while the permanent viaduct was being constructed. This timber viaduct was itself a clever and even elegant structure; it was 1200 feet in length, and contained nearly 300,000 cubic feet of timber. The permanent stone viaduct consists of 28 circular arches, each 61 feet span, springing from lofty piers 8 feet broad. The total length is 2160 feet; and the greatest height, from the bed of the river to the parapet, is 126 feet. One half of the length is over the river; the other half is over the low shore on the south bank. The breadth between the parapets is 24 feet. The land portion of the viaduct has a graceful curve towards the east, on a half-mile radius. The roadway is on an incline, ascending towards Berwick. There are 1,250,000 cubic feet of masonry; and 2,500,000 bricks. Southward of the viaduct there is an embankment more than half a mile in length, in some places 60 feet high, and containing 700,000 cubic yards of earthwork; so that the entire lofty structure to connect the English with the Scotch railway is very little short of a mile in length. The cost of the viaduct has been about £200,000.

The Britannia Tubular Bridge:-But we have now to speak of a triumph of engineering which throws the Tyne and Tweed bridges -beautiful as they are quite into the shade. It is a felicitous lot for one engineer, that his name should be associated so prominently with three such works-all in progress at the same time.

It is a remarkable instance of the progressive advancement which marks the present age in all that regards mechanical invention, that the Britannia tubular bridge is intended to supply the place of-we

may also say supersede-one of the finest bridges in the kingdom; and that the railway of which the tubular bridge forms a part, is in like manner a substitute for one of the finest mail-coach roads ever constructed. The road from London to Holy head has been for ages regarded as the highway from the British metropolis to Dublin; and the late Mr. Telford was applied to by the Government, about thirty years ago, to devise the best means for perfecting this route. He did so; and the London and Holyhead mail-coach road, with its beautiful suspension bridges over the river Conway and over the Menai Strait, was the result. This noble road is now almost abandoned from end to end-at least for its primary purpose; since the Irish mails are now conveyed by railway, and Telford's route is used only for minor purposes.

When Chester became a centre of railway communication, a few years ago, it was considered that a through route to Holyhead would be more conveniently established from that point than from Shrewsbury, which lies in the route of Telford's road. Accordingly the Chester and Holyhead Railway was constructed; and in its course, both the Conway and the Menai had to be crossed. Then came the question whether the suspension bridges would serve to carry the railway; and if not, what other means should be adopted. Many eminent engineers thought that Telford's bridges would suffice, provided trains proceeded over them at a slow speed, and were drawn over them by horses instead of locomotives, to lessen the weight; and we believe that such an opinion is still entertained, in many quarters entitled to credit. But in the golden days of 1845-golden in the visions of railway speculators-all such economical sugges tions were slighted: both the Government and the public called aloud for something grander, and the railway magnates were not slow in responding to the call. Mr. Robert Stephenson, the engineer of the railway, taxed his ingenuity to devise the best mode of carrying his railway over the Menai (the Conway being obviously a more simple affair, from its much smaller width). The Admiralty required, as many vessels of large size are accustomed to traverse the Menai Strait, that the new bridge, of whatever kind it might be, should be constructed without the aid of that assemblage of large timbers which is called centering. Mr. Stephenson selected a spot in the Menai Strait, about a mile southward of Telford's Suspension Bridge, and where the width of the stream is about 1100 feet at high water. At this spot there is a rock called the Britannia rock near the middle of the Strait, the surface of which is about ten feet above the surface of low water, and ten feet below the surface of high water: so that it is above the water during about twelve hours out of the twenty-four. He made a design for a bridge of two cast-iron arches, with a central pier on the Britannia rock; and he laid his plans so that he could have built the bridge without centering, by employing tie-rods to connect the half arches on each side of the central pier. But here a new difficulty occurred. The Admiralty had required that the arches should have a clear height of 100 feet above high water; and Mr. Stephenson planned his bridge accordingly, with a height of 50 feet at the springing or bottom of the

arch. The Admiralty, however, now demanded that the height of 100 feet should be maintained along the whole length of the bridge -a condition which could not be obtained without increasing the height of the crown or centre of the arch to 150 feet. This would have been incompatible with the general level of the railway. Hereupon it was determined to plan a straight or level bridge carrying a train either upon or within it.

And here it is necessary to observe caution in attaching the name of any particular engineer to this wonderful work, in respect to the originating and carrying out of the principle of a tubular bridge. It is a painful incident in the history, that Mr. Stephenson and Mr. Fairbairn now put forth claims which are mutually irreconcilable, in regard to the honour which justly accrues from the noble enterprise. Both cannot be right; but we shall refrain from entering into the controversy, or into the paper-war which has marked it. The public have an earnest wish to give honour to both the claimants to speak of Stephenson and Fairbairn in the same kind of honourable union as they would speak of Le Verrier and Adams. Suffice it for our present object to say, that both those gentlemen, and others who were subordinate to them, have worked out results which claim the admiration of all who can appreciate the nature of the difficulties to be overcome. As soon as it was decided that the bridge should be a hollow beam or tube, through which a train might be propelled, it had to be determined in what way such a tube might be made strong enough for the intended purpose. Should its section be circular, or oval, or square? Should it be of wrought or cast-iron? Should it be of equal thickness on all sides? The Railway Company supplied the funds for an extensive series of experiments, by which many remarkable and important points were elicited. The engineers were perfectly satisfied with the results, and a plan was arranged on the following basis:-that there should be two tubes across the Menai Strait, side by side, one for the up train and one for the down; that a lofty supporting tower should be constructed on the Britannia rock; that two other towers should be built, close to the Anglesea and Caernarvonshire shores respectively; that strong abutments should connect the tubes with the railway on either shore; and that each tube should consist of four lengths, viz. from the Caernarvonshire or east abutment to the east tower, from thence to the Britannia tower, from thence to the west tower, and from thence to the west or Anglesea abutment. A plan similar to this in all its chief details was adopted for the Conway Bridge, but smaller in dimensions; and both plans have now (November, 1850,) arrived at almost entire completion. In the following descriptive details we shall speak exclusively of the Britannia bridge, except where the Conway bridge is especially mentioned.

First, then, for the Britannia tower. The Britannia rock is formed of chlorite schist, a very hard stone, difficult of working; and as the rock is dry only for a few hours at a time, the labour and difficulty of forming the foundation of the stupendous tower were very great. The tower is built of hard carboniferous limestone, obtained from the Penmon quarries on Anglesea island; the stones were quarried

with iron wedges, and worked into form with heavy steel picks; some of the stones are 20 feet in length, and others weigh from 12 to 14 tons. The stones are all left with a rough or quarry face, except at the angles, the recesses, and the entablature. A great portion of the interior masonry of the tower is formed of Cheshire red sandstone. The total height of the tower from the foundation is 230 feet, nearly 30 feet higher than the Monument on Fish Street Hill, and 200 feet above high water. Its width and depth at the base are 62 feet by 52; but at the height where the tubes rest on or rather enter the tower, these dimensions diminish to 55 feet by 45. The tower contains 148,625 cubic feet of limestone, and 144,625 of sandstone, weighing together nearly 20,000 tons; while 387 tons of cast-iron are built into it in the form of beams and girders.

The east and west, or Caernarvonshire and Anglesea towers, are similar in genèral construction to the vast Britannia tower, but somewhat smaller; they stand at a clear distance of 460 feet from the Britannia tower. Each tower measures 62 feet by 52 at the base, tapering to 55 feet by 32 at the level of the bottom of the tubes. The height is 190 feet above high water. There are 210 tons of cast-iron beams and girders worked into each tower.

The east and west, or Caernarvonshire and Anglesea abutments, are situated inland, at a distance of 230 feet from the east and west towers respectively, and are constructed of massive masonry. They are ornamented by colossal figures of lions, in the Egyptian couchant style; these lions are each composed of eleven pieces of limestone ; they are 25 feet long, 12 feet high, and weigh about 30 tons each. They were sculptured by Mr. Thomas, who is employed on the stonecarving for the new Houses of Parliament. It was part of the gigantic plan to have a colossal figure of Britannia, 60 feet high, on the central tower; but this has not yet been executed, and the Company have no funds to bestow upon it.

But the tubes are the most remarkable feature in the bridge. Each portion of tube, between two adjacent towers, is in fact a hollow girder, strong enough to bear its own weight and to bear a laden railway train in addition; and it was a necessary condition of its construction, that it should be either constructed in its ultimate position on a suspended scaffolding, or else lifted entire into its place, after having been put together elsewhere. Each tube is a girder, too, in this respect-that it derives no strength from any transmission of horizontal pressure to the abutments, such as is derived by the arch; nor from any mode of suspension, such as in a chain bridge; but it resists incumbent pressure in the same way as any rigid plank, beam, or girder. These girders or tubes are quadrangular in section; they are hollow from end to end; and their roofs and floors are each formed of a row of smaller square tubes; for it has been found that greater strength is attained by this arrangement, than by forming the same weight of metal into a solid sheet or plate. Each line of tube, the up and the down, is upwards of a quarter of a mile in length; the ends rest in the abutments, and the intermediate portions rest on the three towers, or rather pass through square openings in them. Although they thus form two continuous

tubes, they consist in fact of eight pieces, four to each tube, which are joined end to end at the piers. The height of the tubes is 30 feet at the Britannia tower, and diminishes to about 23 feet at the abutments: the upper surface being slightly arched, but the lower horizontal. The clear internal height varies from about 19 feet to 26. The external width is nearly 15 feet, and the internal about. 14. The sides, top, and bottom are all formed of wrought-iron plates, varying from 6 to 12 feet in length, from 21 to 28 inches in width, and from ths to ths of an inch in thickness. The plates (some of which weigh nearly 7 cwts. each) are laid lengthwise in the top and bottom, but vertically in the sides of the tube. The largest plates are in the bottom, where they are arranged in a double layer. The plates are joined together by rivets; and are stiffened and strengthened at the joints by T-shaped iron, both inside and out, which form vertical bars up the sides, at distances of two feet apart. The connexion of the top and bottom with the sides is made more substantial by triangular gusset-pieces,' rivetted in at the corners. The rivets in the entire structure are almost incredibly numerous; they are placed four inches apart in the top and bottom, and three inches apart in the sides. They are rather more than an inch in diameter, and were driven red-hot into the rivet-holes, which holes were made by a powerful machine that punched out forty holes in a minute. The whole bridge contains nearly 2,000,000 of these rivets. The square cells, tubes, or flues, which form the top and bottom, are 14 in number: viz. 8 in the top, measuring 21 inches high by 21 wide; and 6 in the bottom, 21 inches high by 28 wide. The vertical sides of these cells are strongly connected to the plates of the top and bottom with L-shaped bars of wroughtiron. The two tubes contain 65 miles in length of T and L iron. The whole weight of wrought iron in the tubes is about 3200 tons.

The mode of constructing these tubes was not the least remarkable part of the operations. The short tubes (those between the abutments and the side towers) were constructed on platforms at their ultimate level; but the long tubes (those between the side towers and the Britannia tower) were constructed on floating platforms on the Caernarvonshire beach. The scaffolding for building the towers and the short tubes was among the finest ever yet formed. It consisted of whole balks' of timber, logs from 12 to 16 inches square, and some of them as much as 60 feet long; they were fastened together without nails, so as to be afterwards available without injury for other purposes. This beautifully formed seaffolding beneath the short tubes was about 100 feet in height; and around the Britannia tower it rose to a height of 250 feet. span between the abutments and the side towers is 230 feet; but the short tubes are each 242 feet long, to allow space for resting on their supports. In like manner the span between the Britannia tower and the side towers is 460 feet; but the long tubes are 472 feet, to furnish supports at the ends.

The

The long tubes, it has been stated, were constructed on the Caernarvonshire beach. The platforms for this purpose were made of whole balks of timber, and extended nearly half a mile along the