The St. John’s Wood Railway, which starts from the Baker Street Station of the Metropolitan Railway, is in tunnel throughout, and is a series of stiff gradients, culminating with the stiffest of all at its St. John’s Wood end. The line is 2¾ miles long, and its total rise in this length will be 255 feet, but the elevations are very unequally distributed. Starting from Baker Street Station, it proceeds for a short distance on a level, and then it rises 1 in 90 and 1 in 44 to the Regent’s Canal. From the canal the line descends slightly, and then at three quarters of a mile from Baker Street will commence an ascent of 1 in 60, or at the rate of 88 feet in the mile, for 660 yards. Then follows an incline of the same length of 1 in 150 (35 feet in the mile), then for 440 yards nearly level, except, just for a few yards, 1 in 80. At one mile and 1,320 yards from Baker Street commences a gradient of 1 in 27, or 196 feet in the mile for a length of 1,320 yards. Half way up the gradient will be a station, but the steepness of the gradient will be diminished for about 200 feet to 1 in 250, or 21 feet in the mile. Mr. John Fowler, the President of the Institution of Civil Engineers, is engineer of the Metropolitan, the Metropolitan District, and the St. John’s Wood Railway Companies. The construction of an extension of this last-named line to Hampstead has been authorised. The longest tunnel in Europe _over_ land and over water is the Britannia Tubular Bridge built across the Menai Straits, parallel to and some mile and a-quarter from Telford’s beautiful Suspension Bridge opened for road traffic in 1829. It is 1,834 feet 9 inches long, and in fact consists of two independent wrought iron tubes, each placed alongside of the other. There are four spans, two of 460 feet each, and two of 230—that is, the tubes rest upon two abutments and three towers of masonry—at an elevation of 100 feet above high water mark. The tower called the Britannia Tower is built upon a solid rock that projects above high water nearly in the centre of the Channel. The summit of this tower is 130 feet higher than the level of the railway in the tubes. The total weight of iron in the tubes is 9,360 tons, each tube of 460 feet weighs 1,587 tons, each of 230 feet weighs 753 tons, but these weights of iron are greatly in excess of what would be put in tubular bridges of like spans at the present time; and for a length but little exceeding a third of a mile, one tube, and not two, would be considered more than sufficient for all traffic, in both directions, that could be conveyed through it. The tubular bridge across the Conway River, forty-five miles from Chester, consists of two tubes placed alongside each other, each is 400 feet long and 1,180 tons. The combined cost of the two bridges, Britannia and Conway, is always set down at a million sterling. Now-a-days they would be constructed for about half that amount.