Chap. VIII.
BRITAIN.
317
below the zero on the scale; which gave the total lift of the tide 24 feet 6 inches; hut onOctober 18, 1841, the tide rose one foot higher.
The excavations for the several works are carried down one foot below the level of lowwater spring tides, and rest upon a bed of Hints, which cap the chalk.
The first tier of columns, 15 feet long, is placed upon stone bases, which rest on brickpiers 7 feet 6 inches square ; and the other columns were fixed by means of cast-ironcylinders 6 feet in diameter, formedof segmental plates, firmly boltedtogether. When the first cylinderhad been forced into the mud, otherswere placed upon them, and securedby iron bolts, thus forming a speciesof coffer-dam.
To base some of the columns, itwas found necessary to have cylin-ders 7 feet in diameter, supportedwith pieces of timber; when thesecylinders were above high watermark, others, 6 feet in diameter, weremade use of.
For the foundations of the Thead, the outer cylinders or coffer-dams were not used, guides beingsubstituted for them, formed byplacing timbers on the land, boltedto the fender piles, and so placedas to enclose a tier of three cylin-ders ; across these timbers plankswere laid down and nailed, so thatthere was a square space throughwhich the cylinders could sink; theywere guided above at the levelof high water by a ring of wroughtiron, held by four guy chains se-cured to the fender piles; to themetal ring were attached four ironrollers, which enabled the cylinder to slide freely through it, by which means they wereplaced very correctly.
The cylinder plates are five-eighths of an inch in thickness, and when placed, they wereweighted with five or ten tons of stone, according to the resistance presented.
After the several cylinders were sunk to their required depth upon the solid chalk, afloor was formed of two courses of dry brick, and a thickness of 18 or 24 inches of brick-work was brought up in Roman cement, with two courses of plain tiles, also in cement;this was done for the purpose of keeping out the spring water, which, in some of the found-ations, was found to rise in considerable quantities ; it was led up through a pipe 6 inchesin diameter, bedded upon the dry courses below, to the mouth of which drains were formed,from where the water was most abundant. The water was pumped out by this means, andkept below the level of the work as it proceeded. When the bottom w'as found sound thepipe was filled with concrete, formed of Thames sand and Roman cement, and a blank flanchsecured over the top.
After the cement foundation was completed, a cast-iron cross, with a wrought-ironholding down bolt through it, was bedded on the work ; the rest of the brickwork was car-ried up in mortar, composed of blue lias and puzzolana in equal proportions, with two anda half measures of clean river sand, and iron hoops were laid between the several coursesto hind the whole together.
The iron bolt was frequently plummed upright as the work proceeded, and a space ofsome inches was left around it to afford facility for its adjustment; after which it was filledup with concrete.
Upon this brickwork was laid a circular base of Bramley Fall stone, bored to pass overthe bolt; and being properly bedded, the columns were lowered and placed upon it. Eachis in one casting, 26 feet long, 4 feet in diameter at bottom, and 3 feet at top ; one column,which rather exceeded 1‘ inches in thickness, weighed 10 tons, the others averaged about9.^ tons each. They are held together at top by cast-iron cross-bracing frames, fitted andbolted between the caps. The three girders which rest on the first three columns, and thoseof the T head, arc cast to one section ; six of them are 54 feet 9 inches long, and the three,next the T head 55 feet 9 inches long.
The Doric entablature which surrounds the pier and forms the casing to the externalgirders and parapet is 7 feet in height, and rises 2 feet 9 inches above the platform.
t ig. 322. TERRACE PIER SECTION.