Chap. VIII.

BRITAIN.

445

entire foundations completed on the 1st of March following; the masonry was then carriedup to the springing of the arches.

The first arch turned having now stood the entire winter, the wedges were struck2 inches back on each side, and the crown lowered | of an inch ; the wedges werethen driven back 4 inches more on the following day, when the crown of the arch sankanother half inch. On the third day they were driven back 6 inches, when the crown ofthe whole arch was clear, and shortly after the wedges were entirely driven back, when thesoffite of the arch was accurately examined, and found to have preserved its form entire,although it had lowered inch. By this time the centres of all the other arches wereplaced, and the masonry considerably advanced : in 1829 and 1830 the centres of the middle,fourth, and fifth arches were shifted back, and when released of their load, the middle archsank 2£ inches, the fourth 2^ inches, and the fifth Ijj inch.

The centre arch is 152 feet span, and rises 29 feet 6 inches above Trinity high watermark; the arches on either side span 140 feet, and rise 27 feet 6 inches above the sameline, and the abutment arches span 130 feet each, and rise above the same line 24 feet

6 inches.* The entire water-way being G92 feet, the total length of the bridge 1005 feet, itswidth from out to out 56 feet, and its height above low water GO feet. The two centrepiers are 24 feet in thickness, and the two others 2 feet less.

So important a national work requires us to enter more into particulars, and to give thespecification and forms of the contract, which are perfect models of their kind, and will serveas such for similar undertakings.

“ Cofferdams for the abutments .— These dams are to be of a circular form, and of the sizeand dimensions in every respect as shown in the drawings exhibited; they are to be com-posed entirely of the best Memcl, Dantzic, Riga, or Stettin , fir timber. The piles are to becomposed entirely of whole timbers, that is, none to be less than 12^ inches square whenmeasured in the work, and as much larger as the timber will allow ; the main part of thedam is to be composed of two rows of piles, not less than 5 feet apart, the inner row to bedriven not less than 10 feet, and the outer row not less than 8 feet below the lowest partsof the foundations, that is, the square parts of the piles are to be driven to tins depth besidesthe shoes, and when driven, both rows are not to be less than 5 feet above high water markof spring tides, according to the Trinity House standard. These tw o row's of piles are tobe properly secured together with three row's of double waleing, not less than 12J inchessquare each ; these three rows of waleing are to be secured together with the best wrought-iron screw bolts, 2 inches diameter, passing through each set of waleings at every 10 feet,and having their ends secured with the best wrought-iron plates, not less than 10 indieslong by 8 inches wide, and 1 inch thick, and wrought-iron screw nuts 5 inches square, and2 inches thick, and wooden cleats not less than 12J inches square and 3 feet long, the wholeto be well fitted and screwed up to their bearings. There is to be a third or outer row ofpiles, to be composed entirely of whole timbers as before, and to be not less than 6 feetasunder from the main rows of piles ; these piles are to be driven not less than 8 feet belowthe lowest part of the foundation, that is, the square part of the piles are to be of this depthbesides the shoes, and their heads are not to extend higher than the level of half-tides, or

7 feet below high water mark of spring tides, according to the Trinity House standard,except that there are to be two piles at such distance as are shown in the drawings, to beof the full height of the inner rows. This outer row of piles is to be connected togetherwith not less than three rows of double whole timber waleings, as before described, and tobe connected to the main dam with long w T rought-iron screw-bolts, 2 inches diameter, passingthrough the three rows of piles at each row of waleings, and at the centre of every spacebetween the bolts belonging to the inner row’s of piles, and having their ends secured withcleats ami nuts as before described, and well fitted and screwed up to their bearings : thewhole of the three rows of piles above described are to be planed and straightened on theedges, and the joints of the tw-o inner rows are to be caulked and covered over with pitch,so as to be perfectly water-tight; they are also to be braced firmly together by transverseand diagonal braces, composed of half-timbers firmly fitted and spiked together, and havingtheir ends supported by cleats where required ; the whole of the piles are to be properlyhooped and shod with the best wrought-iron, and the hoops and shoes to average 56 poundsweight each. The spaces between the three rows of piles are to be well fitted with thebest tough well-beaten clay, thoroughly puddled, so as to be impervious to water ; the spacebetween the two inner rows of piles is to be filled with the above clay to the level of notless than 2 feet above high water of spring tides, according to the Trinity House standard,and the space between the outer and middle row of piles is only to be filled with clay up tothe level of 2 feet below the upper waleing of the second row of piles, and to inclinedownwards to the heads of the outer row of piles; besides the diagonal and transversebraces above described, there is to be a series of main, transverse, and longitudinal bracingsfor the interior of the dam ; at the level of each row of waleings these transverse and longi-tudinal braces are to be composed entirely of double whole timbers, firmly scarfed, bolted,and strapped together with the best wrought iron bolts and straps, and supported by piles,