566 PROCEEDINGS OF SECTION J. 



below water, it is lined with iron. It was sought in this case to 

 avoid the expense and delay of passing the excavated material 

 through the airlocks, by constructing separate dredging tubes open 

 to the air, which passed through the caisson and below the level 

 of its cutting edge and of the compressed level of water. 



The workmen excavated around the foot of these shafts in the 

 caisson, so that the material sunk through the water and under 

 the edge of the dredging tube, inside of which ordinary dredging 

 grabs, working in the usual method, removed it ; but it was not a 

 success, as, notwithstanding the care that was taken to keep 

 the foot of the tubes below water, tliere was a great loss of 

 compressed air. 



These caissons are among the largest ever constructed, being 

 one hundred and two feet by one hundred and sixty-eight feet, 

 each with nine feet six inches deptli of chamber, the Brooklyn 

 pier beiiig founded on trap boulders embedded in clay and sand, 

 forty-five feet below high water, and the New York one is 

 sunk through quicksands to rock at a depth of seventy-eight feet 

 below the same level. 



Standing on this enormous structure which lises one hundred 

 and thirty -five feet above the crowded river, with a span of nearly 

 1,600 feet, and turning from the magnificent view around, one 

 can hardly idealize, how the lower parts of the solid masonry of 

 the huge towers could have been actually, so to say, floated and 

 sunk with such steadiness and accuracy as to be effectual for their 

 purpose. 



In the Forth Bridge the caissons, which are of iron, are circular, 

 seventy feet in diameter at the base ; four of these forming one of 

 the great piers. There is a seven feet working chamber at bottom, 

 but the walls are carried up above the roof, and pockets inside the 

 circumfei'ence of these upper walls are constructed so as to enable 

 any point to be loaded with concrete, and the sinking to be regu- 

 lated. Notwithstanding this, one of the caissons tilted over in 

 sinking, and much trouble, delay, and expense were incurred befoi'e 

 it was righted. The weight on these foundations, including the 

 tilting action of a wind pressure of fifty-six pounds per square 

 foot, is about six tons per square foot. 



The foundations of the four legs or pedestals, of the great 

 Eifiel Tower in Paris are similar in tyi)e, four caissons being used 

 for each foot. The pressure, including the enormous leverage 

 produced by the eflfect of wind on a structure nearly 1000 feet 

 high, is estimated at under four tons per square foot. 



The centre pier of the Harlem River Bridge, New York, 

 supporting the thrust of a five hundred and ten feet arch on each 

 side of it, stands on a timber caisson one hundred and four feet 



