434
HISTORY OF ENGINEERING.
Book I.
On the upper rim iron studs were screwed, over which an iron chain could be cast whenthe caissoon was moored, a pile being driven above and below bridge for this purpose.
All the joints of the planking and sides, and the cross and upper planking of the bottomwere grooved about J inch in depth ; and a lath 1} inches in breadth, and ^ inch thick, waslet into it with tar; and over the joints was nailed astrip of thick flannel covered with tar, 3 inches inbreadth upon the outer edge.
During the excavation, the floods carried away thegravel under the caissoon, the fourth pier from thesouth abutment settled about 18 inches; in manyplaces, the gravel was removed to the depth of 30inches, and several of the stones were driven out ofthe cutwater of the pier.
This damage was remedied by enclosing the se-veral foundations with a row of sheet piling, whichprevented the gravel from further washing fromunder the base of the piers; and then filling up thecavities below the bed of the river with rough rubblestones, and the inside of the piles with smallerquarry rubble and clean sand, in the proportions of2 of stone to 1 of sand.
To underpin the piers effectually, it was necessaryto make use of a diving machine, the principal partof which was an air-chest, 3 feet 6 inches in length,
4 feet 6 inches in depth, and 2 feet in width, pro-vided with a copper air-pump which threw in agallon of air at a stroke; it was sunk with theproper tackle, by attaching 16 pigs of lead to it.
A board was put across for a man to sit on, withanother for his feet, and a pane of glass inserted attop to admit the light.
'The pump was formed of a copper cylinder 10inches in diameter, and 12 inches high; wired attop, and a flanch at bottom 1J inch broad, by whichit was screwed down to the top of the air-chest; thecopper is described as being the thickness of a half-penny. The sides of the air-chest had 2£ plank forthe ends and bottoms, and 1^ inches for the sides.
Kelso Bridge , over the Tweed , consists of fivearches, each 72 feet span, with a versed sine of 20feet 9 inches; the diameter of the circle of curvatureat the vertex being 114 feet.
This beautiful work, completed by Mr. Rennieabout the year 1799, was the model for WaterlooBridge ; the estimate for its construction was 1 2,876/.,which was not exceeded.
Die old bridge at Kelso was constructed of sixsemicircular arches, with a water-way of 318 feet,where the river was 561 feet in width.
The roadway of this bridge is perfectly level; overeach pier are two columns, which sustain an en-tablature that runs the entire length ; these columnsproject about three quarters of their diameter, andare of graceful proportions. The width of thebridge measured on the soffite of the arches is 26feet. The scenery around this structure is admiredfor its beauty, and the conflux of the Tweed and theTeviot rivers is enriched by the engineer; at theperiod this bridge was constructed, similar workswere carried on in France , but none excel thisexample in simplicity or style of execution. Theobjections made to the introduction of columnsat the bridge of Blackfriars do not hold good inthe Kelso Bridge, for they are of one proportionthroughout, and are not diminished in heighttowards the abutments to adapt them to the cur-vature of a cornice, which rakes with an inclined
f