Chap. VIII.
42 5
or stem fixed in the middle, 18 feet in length, divided into feet and inches, were used toexamine the level of the bottom, and to ascertain if the bed was ready to receive thecaissoon.
The caissoon being finished, with a sluice towards the bottom, all the seams wellcaulked, and the bottom and outsides pitched over, on the 15th January, 1739, it waslaunched and fixed in its position, by means of a lighter that had been previously moored200 feet distance from the shore; from the head and stern of which two cables passed tothe two ends of the caissoon, guiding its motion, after it was launched, without any personon board to direct it.
The masons then began hoisting the stone into the caissoon, and as soon as the firstcourse was laid, the gate of the sluice was raised near the time of low water, and thecaissoon was sunk to ascertain how it sat and grounded. Some loose stones having fallenin, the sluice was again shut, and after two hours’ pumping, the caissoon floated; theground was again levelled, and two days afterwards it sunk to its proper bed when it wasa second time raised, and after the masons had cramped the stone of the first course, andset and cramped the second course, the gate of the sluice was raised, and the- caissoon sunk,when it was found to bed itself, and set perfectly level on the hard gravel. The water wasthen pumped out of it, and it floated as before.
The third course of stones being cramped, the caissoon was sunk for the last time; thestone-work of this pier was brought up within 2 feet- of the common low water mark.About two hours before low water, the sluice was let down, and by the help of four pumpsof 8 inches square, three men to each, and a small spare pump of 3 inches in diameter,worked by one man, the water was pumped out without waiting for the lowest ebb of thetide, for the masons to set and cramp stones of the succeeding courses; before the tide hadagain flowed or risen sufficiently high to endanger the caissoon and stone-work, the masonsdesisted for that tide, and the sluice was again opened. By the masons working at everylow water, night and day, the first course of the solid shaft was finished by March 24th.
The sides of the caissoon were floated off over the sides of the pier on the 30th March,and applied to the caissoon of the other large pier.
When this was done, the masons placed their crab or engine, with which they hoisted thestone, on a temporary floor, fastened to some fender piles, which guarded the north point ofthe pier.
On the 20th April the last stone of the torus or cordon was cramped, when the sheersand the crab, as well as the other necessary apparatus were taken away, and the first pierwas so far completed.
This new method of building piers having succeeded so well, the commissioners con-tracted for the two abutments, including the two small arches and the two abutment piersclose in-shore, and it was decided that the wooden structure might be dispensed with.
In April, 1740, the masons’ work for the three middle arches was contracted for byAndrew Jelfe and Samuel Tuffnell, who had already built the pier.
Mr. James King also contracted to supply the three centres, and subsequently fiveothers, his design for them being preferred by Labelye to his own.
The Portland stone was brought by sea upwards of 250 miles, and the Purbeck fromSandwich in Dorsetshire , a distance of 220 miles. The moorstone was from Devonshire orCornwall, and the Kentish rag from Maidstone .
The soffite of every arch is turned, and built quite through, the same as in the fronts,with large Portland blocks, over which, bonded in with the Portland, is another arch ofPurbeck stone, 4 or 5 times thicker on the reins than over the key.
The breadth of the Thames where the bridge is built is 1220 feet. The length of the twoabutments is 226 feet, the section of the twelve piers 174 feet, and the span or opening ofthe thirteen arches 800 feet, the voids or water-way being double that of the solids.
The breadth of the bridge is 44 feet, which was also the clear width of way at the top;the parapet walls being fixed over the sally or projection of a cordon or rustic cornice,which serves also as a weathering to the stone work.
The road-way is 30 feet in breadth, and each foot-way 6 feet. Over each pier arerecesses for shelter. The piers are terminated by a right angle at each end. The spandrillsof the arches were filled in with regular rubble stones, with proper bond, and the joints ofthe work preserve a tendency to the centre.
The entire construction of this bridge occupied II years and 9 months; it was completedthe 10th day of November, 1750, and the total sum expended upon it is said to have beensomething under 400,000 pounds. Some delay occurred from the unequal settlement ofone of the piers, occasioned by the removal of some gravel from the bed of the river intendedfor the roadway of the bridge; this took place near the third pier on the western side ofthe centre arch; the gravel slipping from under the platform, in consequence of the holemade below the foundations, the pier sunk so much that it was necessary to take down thetwo arches which vested upon it.
On the pier being examined, it was found to have settled 18 inches at one end, and that