PoLTTECHXio Association Proceedings. n. g^j^g; ^Ar 



outside, where it is worked by hand on signal. When the rolnmn- — ^ 

 has been entirely cleared down to the bottom, the workmen ascend 

 into the air-lock, and, closing the lower valve, the compressed air 

 in the air-lock is allowed to escape, the upper valve falls open, the 

 men pass out, and the bags of material are removed. Men are 

 then stationed at the guy ropes, and the cock in the second curved 

 pipe is opened, and the compressed air in the column allowed to 

 escape quickly. The pressure of the'air in the column on the upper 

 surface neutralizes its weight to an extent which is governed by 

 the depth of the bottom of the column. By allowing the air to 

 escape quickly in the manner mentioned, this M'eight is suddenly 

 restored with an effect similar to a blow, while at the same time 

 the rapid inrush of water at the bottom, causes a complete scour- 

 ing of the material at and under the sharp rim of the column, so 

 that the resistance to driving it is simultaneously removed. 



The friction of the outside of the column against the material 

 through which it penetrates, is greatly diminished by the current 

 of water passing along its surface from the river on its way down- 

 ward to the inside. If no rocks, logs or similar obstructions are 

 encountered, the column will continue to settle quite rapidly during 

 the time the air is escaping, and afterwards until the material has 

 stopped scouring under the edges, and has compacted itself under 

 the pressure of the water sufficiently hard to sustain its weight. 

 The amount of settling in one operation will frequently amount to 

 ten or twelve feet, or even more. 



When boulders or logs are met with, the column stops, and it is 

 then recharged with air. The workmen descend and remove the 

 obstruction, and the process already described is repeated. In 

 this manner, columns of almost any dimension may be sunk to any 

 required depth, even to one hundred feet or more. 



In the construction of Harlem bridge, my brother and myself 

 were in the habit of entering the columns daily, and frequently, in 

 conducting some of the more important and delicate operations 

 required, have spent hours under air pressure. The greatest depth 

 at which we have been under water was over fifty feet. The 

 pressure due to this depth was about twenty-two pounds to the 

 square inch over the atmospheric pressure, or, with the latter added, 

 thirty-seven pounds; but this again was frequently increased by 

 the extra pressure required to drive out the water through the 

 compacted material around the outside of the column, so that 

 the pressure was often increased by as much as an additional atmos- 



