Polytechnic Association Proceedings. 87 j 



of seven feet diameter, and driven forty-four feet into the bed of 

 the river, through sand, gravel, rocks and timber, the latter being 

 the debris of the old bridge; perhaps some of the ^vorks left l)y 

 the Romans. 



Dr. Potts' process was at first used, but it was soon found to be 

 insufficient, and the reverse process, the plenum, was then tried, 

 by means of which these columns were driven through the obsta- 

 les already mentioned, in an extraordinarily short time, and at 

 less than one-third of the cost which had been anticipated, by the 

 ordinary method of constructing such works, and to the astonish- 

 ment of the engineers of the day. 



Some years previous, however, Mr. Triger had applied this 

 principle, at Languire, in sinking cast-iron columns of three feet 

 diameter, for a coal shaft, through quicksands, and to a depth of 

 eighty-two feet. 



Soon after the commencement of the Rochester bridge, the same 

 engineer. Sir William Cubitt, built the piers of the Peterboro 

 bridge, with cast-iron caissons six feet square, placed in contact 

 and filled with brick masonry. 



A year or two later. Major Gwynne and L. J. Fleming com- 

 menced the construction of a bridge across the Pedee, in North 

 Carolina, with piers and foundations, composed of columns six feet 

 in diameter, driven about twenty-five feet through sand. 



They, also, commenced by using D\\ Potts' vacuum process, but 

 encountering, with their first column, a large cypress log, Avere 

 compelled to resort to the plenum process, to efiect its removal, 

 and thereafter that method was used exclusively. 



Shortly after, these engineers constructed the Santee bridge, and 

 in 1859, Gen. Smith commenced the Savannah bridge, and the piers 

 of both were built upon the same plan. 



In May, 1860, I commenced the construction of the Harlem 

 bridge, the pivot pier of which was arranged to be composed of 

 ten cast-iron columns of six feet di^njeter, driven to a depth of 

 nearly fifty feet below the water; and subsequently, I arranged a 

 plan of an inverted caisson, in compartments, covering the whole 

 size of the proposed stone piers, the masonry of which I intended 

 to lay by means of the pneumatic process. 



From the year 1850 to this time (1860), and subsequently, a 

 great many bridges were built in Europe and elsewhere, upon this 

 general plan, and various modifications of it; and the better the 

 plan became known, the more favor it met with, until, at last, it 



