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LX. Some Account of the Methods of laying the Founda- 

 tions of Brkkes, &c. By the Author of ^\ Some Account 

 of the different Theories of Art lies," &c. in the Fhiloso- 

 phical Magazine for December 1811. 



The practice of laying the foundations of bridges in deep 

 water, cenerally adopted by the ancients ?.nd moderns, has 

 been to lav the piers d,y, either by lurnnig the ^vater mto 

 a new course teniporarilv, or by the erection ot a cotter- 

 dam round the site of the pier ; so continuous, as to pre- 

 vent the water interferinir with the works after it had been 

 once pumped out*. They have also practised another 

 method somewhat more ingenious. The emperor Claudius 

 practised it in erecting the port of Ostia ; Dragnet Keys, 

 in erecting the mosque in the sea at Constantinople; and 

 Sir Samuel Bcntham is said lately to have introduced it in- 

 10 this country, in the construction of some works at bheer- 

 ness. A strons grating of wood- work covered with plank- 

 ing at once forms a floating raft, and the floor upon which 

 the stone pier is to be erected : the pier is composed ot 

 stones well secured together and rendered by cement water- 

 tight: the whole body is made to float upon the water 

 until it has advanced in height, so, that it it were sunk it 

 should be above low-water mark, or higher, as might be 

 found expedient: this levity is obtained either by the 

 assistance of vessels to which the raft is attached by ropes; 

 or by the pier beinj; worked up with sufiicient vacuities to 

 render it specifically liirhter than an equal bulk of water: 

 the pier is then sunk" ehher by letting the water into the 

 vacuities, or by loosening the ropes (as the case may be), 

 the bed of the river being previously prepared by machines 

 of the description of ballast-heavers that it may gro""'! 

 level: should the pier not ground level it is raised by 

 pumping out the water from the vacuities, or by means ot 

 machinery in the vessels, and the operation 1= performed 

 until it grounds to the satisfaction ot the architect. 



Mr. Labelve in the erection of Westminster bridge, pro- 

 bablv conceiving that he had improved upon this latter 

 method, erected the piers of that structure in caissons or 

 water-tight boxes ; the bulk of the box producing a mass, 



• A cofferdam is a double inclosure of timber conshting of piles driven 

 ,lose together, and .tren^hened at intervals bv larger p.les ^°d lu.nzontal 

 p.ece.: the space between the inclosures i. f.Ucd w.th l'^'.'^'^^'"'' '>"J' ^^ 

 other material., to serve tl>e double purpose of g.v.ng ^'K^^ to he lam 

 and excluding the water; and i. an increased expense of about onc-th.rd 

 oJ that of erecting the piers. .u„.,„u 



Vol. 39. No. 170. June 1812. D d though 



