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HISTORY OF ENGINEERING.

Book I.

The Thames Tunnel — Having described some of the stone, iron, and suspension bridgesconstructed over the rivers of Great Britaiu, our attention is drawn to a method of passingunder the current, so as not to interrupt navigation: this has beenaccomplished by Sir Mark Isambard Brunei, a native of Trance, aftersixteen years’ constant labour and attention ; the Thames having brokenin upon the work three several times.

Before it was commenced, and as early as the year 1798, Mr.

Dodd proposed a communication between Gravesend and the Essexcoast by a passage under the Thames; another at Rotherhithe wasprojected by Mr. Chapman in 180*1, and three years afterwards ashaft, 11 feet in diameter, was sunk at a distance of 315 feet from theriver. Mr. Trevithick , who thoroughly understood the difficultiesof such an undertaking, w*as then engaged to drive a way under thenavigable part of the Thames , and after great labour he formed a driftway, 5 feet in height, 2 feet 6 inches in breadth at top, and 3 feet atbottom, for a distance of 1046 feet under the river. Early in the year1808 the water broke in upon the workmen, although there was atleast a thickness of 25 feet between them and the bed of the river;

this, perhaps, might have been prevented, if precautions had been

taken to line the way with brick as the work proceeded. After this

event a company was formed to carry out the plans which were pro-

jected by Mr. Brunei in 1823: a shaft was afterwards sunk, by firstdriving 24 piles, with a shoulder projecting on the side of each, withinthe circle intended to receive it; on these was laid the timber curb,which also partly rested on one of iron; 48 wrought-iron bolts,

2 inches in diameter, passed upwards through this to a height of40 feet, or to the top of the intended shaft. The curb or rather towerwas built upon it with bricks laid in cement, and as the work pro-ceeded it was bound together by 26 circular timber hoops £ inchthick : when the brickwork was completed, on the top was placedanother wooden curb, through which the long iron bolts passed, andtheir ends being formed into screws, the whole was by nuts held inone entire mass. The construction of this brick curb was completed inthree weeks, and in seven or eight days, when the cement had hardened,

16 of the piles on which it rested were driven by pairs opposite to eachother, ^ inch at a time, and then the whole gradually sunk, carryingwith it the other 8 piles; after this effect was produced, the 16 pileswere drawn out by opening the ground at the back ; the whole weightof the brick shaft, which was 910 tons, then rested on the 8 piles, andthese were eventually drawn, when a bed of gravel was arrived at fora foundation. Mr. Brunei commenced this work on the 1st of Janu-ary, 1826, and by the 27th of April in the following year 540 feetof the tunnel were completed. On the 18th of May the river brokein and filled it. After this irruption only 50 feet was advancedin 1827, when, upon a second irruption, the work was totally aban-doned. In 1835 the government made some advances of money, and anew shield was provided, a masterpiece of ingenuity and contrivance,executed by Messrs. Rennie, which was fixed March, 1836; the workwas resumed and continued until the 11th of June, when the wateragain broke in, and retarded any further progress for six weeks;after which it was continued until completed, in 1842, without anyserious interruptions.

There is a double tunnel throughout the whole distance between theshores of Rotherhithe and ^Yapping. through a blue tenacious clay.

On the Rotherhithe side, at 150 feet from the river, is a shaft 50 feet indiameter, 42 feet in height, and 3 feet in thickness; this was built onthe surface, and the ground excavated afterwards, the earth and waterbeing drawn out at the top by means of a steam-engine. It was sunkin its place bodily, and passed through a bed of gravel and sand 26 feetdeep, causing great difficulty in the first attempts: when this 50 feetshaft had descended 65 feet, it was followed by another from the lowerlevel, of 25 feet diameter, which was carried down 80 feet for the pur-pose of effectually draining the works.

The tunnel was commenced at a depth of 63 feet, with a fall of 2 feet 3 inches in every100 feet. The part excavated was 38 feet in width, and 22 feet 6 inches in height, havinga sectional area of 850 feet; the bottom at the deepest part of the river was laid 76 feetbelow high water. The excavation was carried on by means of the shield, which was