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HISTORY OF ENGINEERING.
Book I.
when called upon to shorten the course of a river, or improve its channel, he directed hisattention to the causes which had produced the results he was required to amend. Thiseminent engineer died the lGth of September, 1792, to whom we are indebted for manyimprovements in air and water-mills; before his examination into the force of water, ourmachinery was in a very rude state; he called forth the inquiries from which the presentgeneration are now reaping advantage.
Bristol Bridge —When Smeaton was consulted upon the rebuilding of this bridge in1762, he observed, that no limit was yet fixed for the span of arches, or the proportion oftheir rise, since the widest and flattest that had been attempted upon right principles hadsucceeded as well as the narrowest and highest, provided the abutments were good, and thestone and cement of a firm texture.
One of the peculiarities in this engineer’s designs was that of perforating the spandrillsof his arches by a circular opening, and also,employing segmental or elliptical arches inpreference to semicircular. The skill with which he managed his centres cannot be toohighly admired ; the strength is always greatest where most required, and there is no un-necessary quantity of timber made use of. Of the various bridges that arc mentioned in hisreports are those of Perth, Stoneham Creek, Glasgow , Dumbarton, Old London, Ilowick,Aberdeen, Edinburgh, Coldstream, Newcastle, Hexham, Berwick, Banff , Dumballock,Braan, Altgran, Bewlic, Conon , Sutton, Walton, Harraton, Carlton Ferry, and Montrose,and others, several of which he entirely rebuilt. The utmost span given to his arches was7.5 feet, the width of the piers being made about a fifth ; the same proportions we shallfind at Perth bridge, which he built about the time that Blackfriars was completed.
Bridge over the River Tag, at Perth _In 1763 Smeaton was consulted by the justices of
the peace for the county on the practicability of building this bridge ; and, after the necessarysurveys, he gave a design for one of stone, having seven principal arches, extending 605 feet9 inches, and in the whole length 893 feet. The width is 22 feet in the clear between theparapets; the span of the two outer arches 70 feet, the next 72 feet 10^ inches, the next74 feet 6 inches, and the centre arch 75 feet, leaving a water-way of 509 feet 9 inches. Thepiers of the middle arch were in width 17 feet, the other four isolated piers each 16 feet.An estimate accompanied the design, supposing the foundations to be laid 8 feet under thebottom of the deepest part of the river, which was based upon the information he hadreceived, that there was a stratum of hard gravel 8 feet below the bed of the river, uponwhich the piers were to be placed, without either piles or grating. The river at its ebb hadnot more than 2 feet depth of water, and at ordinary spring-tides did not rise more than8 feet above this mark, and he considered that a dam capable of keeping out the water 4 feetabove its low state would enable the men to work nine or ten successive days between eachspring-tide; and that a dam of this height was not only less expensive in its construction, butalso less likely to be injured, than another raised high enough to keep out the spring-tides.A sluice made in this dam let the water in and out, and it was so formed, that a clear spaceof 16 feet was left round the pier for the convenience of the workmen. The dam was ellip-tical, the better to resist the tides and floods.
Cofferdams, materials in, and cost. £ s. d.
26 gage piles, of 10 feet long, at 10s. - - 13 0 O
2328 feet supl. of plank piling, 9£ feet long, at Is. 2d. - 135 16 0
122 feet cube of timber in strong pieces for supporting the
pile heads, at Ss. - - - - - -1860
Extra work to sluice for letting in and out the water - 2 10 0
Timber work in one cofferdam - - - -
To pile shoes for 26 gage piles, at Is. - - -16 0
To plank pile shoeing 245 running, at 6d. - - 6 2 6
To 25 bolts for the string pieces, at 2s. - - 2 10 0
To extra iron work about the shuttle, and contingencies - 2 0 0
Iron work about one cofferdam
£ s. d.
169 12 0
11 18 6
A cofferdam complete - - - - - - 181106
A cofferdam complete - - - - - - 181106
The materials for the first pier are supposed to be of half value towards
each succeeding pier, which will therefore be No. 6, at 90/. 15s. 3 d. - 544 11 6
726 2 0
Cofferdams for the whole