MECHANICS AND USEFUL ARTS. 15 



article, a series of observations on the working of the bridge is given 

 with some minuteness, especially those with reference to the expan- 

 sion and contraction of the iron, all of which were perfectly sat- 

 isfactory. 



THE BRITANNIA TUBULAR BRIDGE. 



IN describing this great triumph of modern art, it will be at once the 

 easiest and the clearest mode of proceeding to divide our description 

 into four parts. 1. The principle upon which the bridge is construct- 

 ed. . The mode of construction. 3. The floating of the tubes. 

 4. The manner in which they were subsequently raised. And, first, 

 as to the principle of the construction. In constructing a railway 

 from Chester to Holyhead, the great difficulty to be surmounted was 

 to discover a means of transporting the trains across the Menai Strait, 

 between Caernarvon and the island of Anglesey. The point selected 

 for crossing is as narrow as could be found, but is exposed to tre- 

 mendous gales of wind. The Admiralty insisted that the bridge 

 must be sufficiently high above the water to allow of vessels passing 

 under it freely, 100 feet being the space required, and they also for- 

 bade that any scaffolding or centring should be used. After much 

 deliberation, Mr. Stephenson selected the plan of two cast-iron arches, 

 which were to be made to balance each other in the centre, in a man- 

 ner that has been pronounced, by a high authority, " one of the most 

 beautiful structures ever invented." But the Admiralty vetoed this, 

 as not leaving the requisite space except at the centre of the arches. 

 Mr. Stephenson then resorted to the present plan, whose principle is 

 to have the trains pass through long, low, straight, hollow tubes, 

 one for the up trains and one for the down ones, composed of wrought- 

 iron " boiler plates," firmly riveted together. The tubes he decided 

 to have oval or elliptical in shape, to turn aside the force of the winds, 

 and with the ends resting on abutments of masonry. To complete 

 his plan he had three intermediate towers between the abutments, one 

 to be constructed at high water-mark on each side of the strait, and 

 the third, no less than 210 feet in height, to be erected near the mid- 

 dle of the stream, on a small rock. The four lengths of each of the 

 twin tubes he proposed to have as follows: from the Caernarvon 

 abutment to the tower at high-water mark, 274 feet; from the latter 

 tower to the central Britannia tower, 472 feet ; from the central tower 

 to that at high-water-mark, on the Anglesey shore, 472 feet; and 

 thence to the Anglesey abutment, 274 feet, giving for each line of 

 tubes a total length of 1,492 feet. 



Having formed his plan, Mr. Stephenson recommended that ex- 

 periment's should be made to test the proper strength of the various 

 parts, the shape, &c., which was done, and the results we give * 

 in another article. It is sufficient to remark here, that it was found 

 that the tubes should be stronger at the top than at the bottom, and 

 that the shape should be rectangular. It was also determined that the 

 four shortest galleries, each 230 feet long, should be at once con- 

 structed upon scaffolds, in the positions in which they were to remain ; 



' * See page 81. 



