528 



BRIDGE. 



Practice. wa s bestowed in adjusting this first frame, because 

 * "V ' upon this depended the having the coffer-dam, and, 

 of course, the entrance and wings of the sea-lock, in 

 a proper direction. On the upper side of this lead- 

 ing beam, previous to sinking it, there were mortices 

 cut, three on each side, and three in the front, which 

 were to receive tenons made on the lower end of the 

 standard, which were placed upright therein, and 

 cut off at the level of one foot below high water 

 neap tides, in order to receive the middle leading 

 frame. This last- mentioned frame, after having been 

 made on the shore, in all respects similar to the first, 

 was floated into its place ; but, after two or three 

 fruitless attempts to rest it on the top of the upright 

 standards as the tide left it, this mode was abandon- 

 ed, it being found impracticable, in so strong a tide, 

 to steady such a large frame on such low unconnect- 

 ed points. This led to placing a pile engine upon 

 the deck of a sloop, and driving three piles along the 

 front, and two on each side, all close to the outer 

 side of the lower leading frame. Large brackets 

 were nailed to the inside of these piles, level with 

 the tops of the upright standards. The frame was 

 then floated again into its place, when it rested upon 

 the brackets, and upon the top of the standards, and 

 was screw-bolted to the piles. Upon the second 

 leading frame were also upright standards fixed in 

 mortices, and cut off at the level of high water of a 

 high spring tide. The upper leading frame was 

 floated off and fixed upon the standards. For doing 

 this, advantage was taken of one of the highest 

 spring tides in the beginning of October. This 

 frame was likewise bolted to the piles. A tempora- 

 ry scaffold was made above the top frame, by laying 

 large beams across, and driving piles in the inside of 

 the space to support them. The whole was now 

 loaded witli stones. A large pile engine was placed 

 on the scaffold, and a number of piles driven around 

 the outside of these frames, at fifteen and twenty feet 

 apart from each other. They were bpltcd to the up- 

 permost frame, to keep the whole steady during the 

 winter storms. 



In March 1808, the work was recommenced by 

 putting down the main or framing piles, and fixing 

 them in the rock by means of iron- dowels. For this 

 operation there was first constructed a cylinder of 

 three inch fir plank, 22 inches diameter in the inside, 

 and eight feet long. The joints were made perfect- 

 ly correct, and dowelled together ; and it was hoop- 

 ed with flat iron bars. The lower end was shod 

 with a circular iron shoe, fitted on the edge of the 

 timber, to prevent it from being damaged by stones 

 while driven down to the rock. Near the upper end, 

 and on the opposite sides of the cylinder, two strong 

 eyes were fixed by means of strong iron clamps, 

 which embraced the cylinder, and were rivetted to 

 its sides. This was done in order to receive a strong 

 chain to lift up the cylinder, and also for drawing it 

 out of the sand and mud after the pile was fixed. 

 At low water, this cylinder was placed in the situa- 

 tion where the centre of the main pile was to lie. 

 At this spot there were three feet of low water, and 

 eight and a half of silt and gravel upon the top of 

 the rock. The cylinder being lifted by ropes fixed 

 on the top of the pile engine frame, it was set per- 



pendicularly on the surface of the sand, and close to 

 the inside of the lower leading frame. On the top 

 of it was placed a block of ash timber, two feet high ; 

 on the lower end of this six inches were turned, to 

 fill exactly the inside of the cylinder, to prevent the 

 block from shifting, and also to prevent the cylin- 

 der from crushing during the driving. This block 

 was strongly hooped with iron, as was also a pile, 

 twelve inches square, that stood on the top of it, and 

 reached as much above the top of the upper scaffold 

 as the cylinder had to sink into the mud. The 

 whole were now lashed, in such a manner as to keep 

 them perpendicular, and, at the same time, allow 

 them to sink. The driving was then commenced 

 with a pile engine 30 feet high, and a ram 1008 Ib. 

 At first the cylinder went freely ; but, by the time 

 it had sunk three feet and a half, it went so stifly, 

 that it was found advisable to begin emptying the 

 matter out of the inside. For this purpose an in- 

 strument, named a sand auger, was made, (Sec 

 Plate XCVII.) having a quadrant of a circle of the 

 same diameter as the inside of the cylinder. The 

 circular side, and one of the straight sides of this 

 quadrant, for six inches high, were made close, of 

 thin rolled iron rivetted to the ribs, which were fas- 

 tened to the corners of the quadrant, from which 

 proceeded an upright shaft, the other straight side 

 being open. To the bottom of it four flat teeth, 

 two inches and a half long, were fixed, with an in- 

 clination downwards ; so that, when the auger turn- 

 ed round, these teeth loosened the sand, and prepa- 

 red it to enter easily into the body of the auger. 

 To keep it steady while turning, there was fastened 

 to the lower side of that corner of the quadrant which 

 is the centre of the circle, a pivot, six inches long, 

 which passed into the sand, and served as a centre 

 for the auger to turn upon. Immediately above this 

 pivot stood the upright shaft, which for ten feet 

 was of iron, one inch and a half square, and for 

 twelve feet more a piece of ash timber four inches 

 square. Upon this two cross handles were placed, to 

 turn it with ; and they were to slide up and down as 

 the auger rose and fell. At five feet below the up- 

 per scaffold, where the pile engine stood, a tempora- 

 ry scaffold was erected, on the top of the second 

 leading beam, where four men might stand and turn 

 the auger. In this lower scaffold was a round hole, 

 through which the wooden shaft of the auger pass- 

 ed, and kept it steady. When at work, two com- 

 plete turns filled it. It was then lifted up above the 

 water, by a purchase from the top of the pile engine, 

 and the sand was cleaned out with a small shovel, by 

 a man who stood in a boat for that purpose. The 

 operations of boring and cleaning out were repeated, 

 until the sand was taken out to the bottom of the 

 cylinder ; which was then driven farther down, and 

 a similar operation of sand boring gone through ; 

 driving and boring alternately, till the lower part of 

 the cylinder rested upon the rock, and all the sand 

 was taken out, as low as the pivot would allow for 

 the rock. There was then a frame, whic^i fitted the 

 inside of the cylinder, introduced into its upper end, 

 and sunk to the bottom by means of two half hun- 

 dred weights. Down a square hole in the middle of 

 this frame, was introduced a pipe, four inches sqnare 



Praci 



PLAT* 



XCVI 

 Fig. 3. 



