HISTORY OF ENGINEERING.

Book I.

«HG

as described in the drawings; the minor transverse and diagonal braces arc to be composedof whole timbers only.

“ The whole of these braces are to be firmly united together with wrought-iron straps andties at their ends, in order that their dams may be firm and immovable in every part; thereis to be a tunnel, not less than 3 feet square, placed towards the centre of the dam, and atthe level of low water mark of spring tides, it is to be secured with a proper sluice workedby machinery, as shown in the drawings, in order that it may be raised or lowered atpleasure ; this tunnel is to be properly secured on all sides to the piles, and to have itsjoints well caulked, so that it may remain perfectly water-tight. There will be two of thesedams.

“ Pier Cofferdams. — These dams are to be of an elliptical form, of the size and dimensionsas shown inthe drawings; they are to be composed entirely of the best Dantzic, Memel, Riga ,or Stettin fir, all the piles to be composed of whole timbers, and none to be less than 12£inches square when in the work, the main dam to have two rows of piles, not less than 5 feetasunder, and when driven to be not less than 5 feet above high water mark of spring tides,according to the Trinity House standard ; they are to be properly secured and connected

PLAN OF COFFERDAM, LONDON BRJDGE.

together with three rows of double whole timber waleings, having the best wrought-ironscrew bolts, 2 inches diameter, passing through them at every 10 feet, and well secured attheir ends with wrought-iron plates and nuts, and wooden cleats as before described ; thereis also to be a third or outer row of whole timber piles, not less than 6 feet from the secondrow ; the heads of this row of piles are not to extend higher than the level of half tides, thatis, seven feet below high water mark of spring tides, according to the Trinity Housestandard, only that there are to be two piles at such distances as shown in the drawings, tothe full height of the inner rows; this row of piles is to be connected together with threerows of double whole timber waleings, not less than 12| inches square, and to be secured tothe main dam with long wrought-iron screw bolts, 2 inches diameter, with plates, cleats, andnuts as before described, passing through three rows of piles at each row of waleings; at thecentre of everv space between the bolts belonging to the two main rows of piles, the twoouter rows of piles are to be driven not less than 8 feet below the lowest part of thefoundation, and the inner row not less than 10 feet, that is, the square part of the piles areto be of this depth besides the shoes; the spaces between the three rows of piles are to beproperly filled with tough well-beaten clay as before described, the whole of the exteriorjoints being previously well caulked and covered with pitch. There are also to be propersets of cross and diagonal half-timber braces to connect the rows of piles together, and thewhole dam is to be braced longitudinally and transversely with double and single wholetimber braces, struts, and wrought-iron straps and ties; at the upper, middle, and lowertiers of waleings, as shown in the drawings, there is to be a tunnel at one end of the dam,3 feet square, secured by a sluice worked by proper machinery, as shown in the drawings,in order that the sluice may be raised and lowered when required, the above tunnel to be