570
HISTORY OF ENGINEERING.
Book I.
pleted; but it was not until the year 1818, that it was proceeded with under the directionof Mr. Thomas Telford . The proposed entrance at Berkeley Pill being considered toomuch exposed to the south-west winds, it was determined that one should be formedunder a projecting headland called Sharpness Point, where, restrained by a rocky shore oneach side, the river channel was the only passage for the flux and reflux of the tide, thussecuring deep water free from sand and mud banks, to which the flat shores of the Severnare subject.
An entrance basin and extensive piers of masonry were constructed, and by a large tide-lock the canal was raised to the same level as that of the Gloucester basin. Near the Sharp-ness Point, the canal is carried for the distance of a mile along the face of a very steep bankof rock marl; the outer bank being formed of and secured by a sea-wall.
In addition to the sea-lock, which admitted vessels of 300 tons burthen, another wasconstructed for barges of 70 or 80 tons. It is high water at spring tides at this entranceat 7 o’clock, and the depth upon the lock-sills is from 26 to 28 feet, at neap tides from14 to 16 feet. The length of this canal is about 16 miles, its depth 18 feet; it is suppliedwith water from the river Frome.
Grand Junction Canal, one of the principal engineering works of Mr. William Jessop , formsan important link in the system of inland navigation, by which the capital is connected withthe iron and coal fields of the midland counties. Within a mile of its commencement, itrises 37 feet to its first summit; this summit level is 4£ miles long, and has a tunnel 2045yards in length, through which the canal enters the marlstone ridge of the lias formation;it then falls 60 feet into the vallc-y of the Nene, and continues on a level for 13 miles toGayton, from which place there is a branch to Northampton. On this level the famousBlisworth tunnel is situated, 3080 yards in length, which was completed in March, 1805.Its internal width is 16^ feet, the depth below the water-line to the inverted arch 7 feet, andthe softite or crown of the arch is 11 feet above the same line. The side walls are segmentsof a circle of 20 feet radius, the top arch one of 8 feet radius; the side and top walls are2 bricks thick, and the bottom or inverted arch 1^ bricks. The mortar was composed of1 bushel of Southam lime (blue lias) to 3 of good sand: 6 inches under the water-line oneach side of the tunnel, slide rails of fir 5 inches square, to keep the barges off the wallsarc fixed by pieces of oak let into the wall below them ; these rails project 9 inches fromthe wall, and at every 9 feet a chock of wood is fixed upon the rail for the bargemen to settheir pole against. This tunnel was contracted for at 151. 13s. for every running yard, thesoil being a hard blue clay, interspersed with two or three thin rocks. Sufficient headingshaving been driven at the company’s expense several years before, the same contractors werepaid lO.^tf. per cube yard for excavating the deep cutting at one end of this tunnel, andlid. per cube yard for the other. The expenses of driving the above headings was 36s.to 42s. 6d. per yard run; nineteen tunnel pits, some 60 feet in depth, were sunk for theuse of the tunnel, which cost above 30s. including steining, for every yard in depth.
The canal next falls into the valley of the Ouse, where its level is 172 feet below thatof the Braunston summit. Following the line of the Ouse, it passes close to the town ofStony Stratford, whence a branch goes off to Buckingham; it then continues to Lin-ford Magna, Fenny Stratford, and Leighton Buzzard, through the sands and gault clayof the green sandstone formation, and rises to the summit level at Tring by a lockage of192 feet: it attains this level near Marsworth, 51 miles distance from Braunston. Thesummit level, 3] miles in length, is carried through a chalk cutting of 30 feet in depth, andpassing by Berkhamstead and Hemel Ilempsted, the canal falls into the valley of theColne, and continues by King’s Langley and Harcfield to Uxbridge. Near King’s Langleyis a deep cutting, on a level of the canal, 127 feet below the Tring summit. From Uxbridgeit passes through a fiat district, and after crossing the river Cran and Brent, terminates inthe river Thames at Brentford. The total cost of this great undertaking was 2,000,000/.sterling: its entire length from Braunston to Brentford is 90 miles, the width of the canalat bottom is 28 feet, at the water surface 42 feet, and its depth 4 feet 6 inches: the lengthof the lock chambers is 80 feet, and their breadth 14 feet 6 inches.
The reservoirs which supply the canal at