1849. 1 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



219 



with the construction of the tuhes, by an engineer resident on tlie 

 spot, has just been published by Messrs. Chapman and Hall, to 

 whom we are indebted for the accompanying engravings and de- 

 scriptive account. 



Origin and Necessity of the Bridge. — In 1844 the Chester and 

 Holyhead Railway Company was formed, and it was soon perceived 

 by their engineer that the grand difficulty of the line was, how to 

 carry it over the estuary of tlie Conway and the Menai Straits. 

 As it is impossible to make use of the diain suspension bridge for 

 the passage of heavy railway trains, its flexibility rendering it 

 wholly unfit for cases where a stiff inflexible roadway is required, 

 some other mode of accomplishing the object was necessary; and 

 the difficulty in the way of this arose from the following causes: — 

 At both places there is a considerable traffic, carried on by vessels 

 of large size, to avoid any interference with which, it was reipiisite 

 that, in the building of the bridges, no scaffolding or centering 

 should be used, since that, if employed, would of course obstruct 

 the passage. In the ease of the Menai Straits, Mr. Stephenson 

 met this requirement by a design for a bridge, of two cast-iron 

 arches, the span of each to be 450 feet; the lieight of tlie crown 

 of the arch 100 feet, and of the springing 50 feet, from the water; 

 the use of centering being with great ingenuity dispensed with, by 

 connecting by tie-rods the half-arclies (m each side of the centre 

 pier with each other. The site intended for this was tliat on which 

 the present bridge is being erected. But the first dilhculty having 

 been surmounted, another and far more serious one presented 

 itself. The Commissioners of the Admiralty, on this design being 

 submitted to them for approval, insisted on a height of 100 feet 

 above the water, not merely at the crown of the arch, but also close 

 to the piers; thus giving but two alternatives — to retain the ardied 

 form, but increase its height by 50 feet; or, relinquishing the arched 

 form, to construct on some entirely new method, a beam, which 

 should depend for its stability simply on the strength of its parts; the 

 first i-equiromeut, moreover, rendering it necessary that it should 

 be either constructed in its ultimate position on a suspending 

 scaffolding, or else lifted entire, and at once, into its place, after 

 having been put together elsewhere. Tlie latter alternative was 

 the one chosen by Mr. Stephenson. His proposition of a tube or 

 girder, -1.60 feet in clear length, strong enough not only to carry 

 a railway train, but to bear its own weight, was received by the 

 public, on its first announcement, with almost universal incre- 

 dnlity. However, though the public doubted, the railway com- 

 pany had confidence in their engineer; and his labours and inves- 

 tigations have resulted in the present tubular bridges, whicli, from 

 their stupendous magnitude, the singularity of their form, and the 

 gigantic nature of the opei'ations by which entire bridges of such 

 unexampled weight are transported and raised into their position, 

 have excited more interest, both in the scientific world and the 

 public, than any other engineering works of the pi-esent day. We 

 have said that the arch and the chain bridge being unavailable, 

 Mr. Steplienson was driven to adopt tlie third possible form — that 

 of a beam; and we would impress on our readers that these tubes 

 are nothing but gigantic beams; they derive no strength from any 

 transmission of horizontal pressure to the abutments, as is derived 

 by the arch; nor from any mode of suspension, as in the chain 



leaving the end of the bridge, passing close under the Anglesea 

 column. The shores are of the same precipitous and shelving 

 cliaracter at both places, but the stream is wider here than at the 

 suspension bridge, being about 1,100 feet across at high water. It 

 is divided nearly exactly in the middle by the Britannia Rock, 

 which at high water is covered to a depth of 10 feet. The rise 

 and fall of the tide is ordinarily 20 feet, and its velocity very 

 great, often as much as 8| miles an hour. It is from the Britannia 

 Rock that the bridge takes its name, the centre pier being based 

 upon it. It and the Anglesea shore consist of chlorite schist, a 

 very hard and intractable kind of rock, worked with great diffi- 

 culty: from this and the circumstance that no cofferdam was 

 used, and therefore few hours only could be consecutively spent 

 on the rock, some months were passed in laying tlie bottom course 

 of the tower. It was commenced in May 1W4B, the first stone 

 being laid without ceremony by Frank Korster, Esq., acting en- 

 gineer of the portion of the railway between Conway and Holy- 

 head, and of the masonry, scaffoldings, &c. of the Britannia 

 Bridge. 



Const runt io7>. — The stone of which the towers are built is a hard 

 carboniferous limestone, or marble, called Anglesea marble; it 

 abounds in fossils, and is capable of receiving a very high polish. 

 It is obtained from quarries expressly opened for the purpose on 

 the sea shore at I'enmon, at the northern extremity of the island, 

 where it occurs in great abundance and in convenient strata of 

 every thickness, from 3 feet or 4 feet downwards. The stones are 

 split off with great dexterity by iron wedges, and wrought into 

 shape with heavy steel picks. Some of the stones in the work are 

 no less than 20 feet in length, and others weigli from 12 to 11 tons. 

 A great poi'tion of the interior masonry, however, is built of red 

 sandstone, from Runcorn, in Cheshire. This is a very soft stone, 

 and easily worked, but at the same time very durable, especially 

 when not exposed. The stones in the towers are all left with a 

 rough or quarry face, except at the angles and in the recesses and 

 the entablature at the top. This circumstance, coupled with 

 their immense size and height, gives the towers a truly noble ap- 

 pearance. 



The approaches to the bridge are ornamented on each side by 

 collossal statues of lions, in the Egyptian style. They are each 

 composed of eleven pieces of limestone, and, although in a couch- 

 ant attitude, are 12 feet in height. Their lengtli is 25 feet, and 

 their weight about 30 tons. Being associated wiili so many other 

 large objects, their real size is not apparent. They were sculp- 

 tured by Mr. Thomas, of the new Houses of Parliament. It was 

 once contemplated to surmount the centre tower with a figure of 

 Britannia, in stone, 60 feet in height, by the same artist; but the 

 idea for the present is abandoned. The designs for the masonry, 

 both for this and the Conway, were furnished by F. Thompson, 

 Esq., of London. When the whole structure is completed, it will 

 consist of two immense wrought-iron tunnels or tubes, each con- 

 siderably upwards of a quarter of a mile in length, placed side by 

 side, through which the up and down trains respectively will pass. 

 The ends of these tubes rest on abutments, the intermediate por- 

 tions being supported across tlie Straits by three massive and lofty 

 stone towers. The centre tower, as has been just observed, stands 



bridge, but resist incumbent pressure on exactly the same princi- 

 ples as the short plank does by which the village brook is crossed. 

 But their form, and the method of employing the material of 

 which they are composed, is so novel and beautiful, and so very 

 different to those of a simple beam or girder, that we would 

 willingly draw the attention of our readers to a few of these 

 points before proceeding to a detailed description of the tubes 

 themselves. 



Description of the Work.— The particular spot at which the 

 Britannia Bridge crosses the Menai Straits is exactly a mile 

 nearer to Carnarvon than the suspension bridge; the railway, after 



on a rock, which is covered by the tide at high water. The side 

 towers stand on the opposite shores, each at a clear distance of 

 460 feet from the centre tower. The abutments are situated inland 

 at a distance of 230 feet from the side towers. The Britannia 

 tower is 62 feet by 52 ft. 5 in. at the base; it has a gentle taper, so 

 that where the tubes enter it is 55 feet by 45 ft. 5 in. Its total 

 height from the bottom of the foundations will be, when com- 

 pleted, nearly 230 feet; it contains 148,625 cubic feet of limestone, 

 and 144,625 of sandstone, weighing very nearly 20,000 tons, ami 

 there are 387 tons of cast-iron built into 'it in the shape of beams 

 and girders. The land towers are each 62 feet by 52 ft. 5 in. at 



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