In plan the tower was an irregular rectangle, its outside surfaces being broken up by heavy buttresses. It stood lengthwise against the shore, 140 feet long, 59 feet wide. So at a height of one hundred feet, it was still broader than it was high, still only a little more than a third as high as it would eventually go, and only nineteen feet short of the height of the roadway. But already it was considerably higher than anything else around it.
Moreover, the tower kept gaining all the time, as though it were coming up out of the river, growing organically, instead of being slowly, methodically added to stone by stone. The change was never enough to notice from one day to another. Like the movement of an hour hand its progress was best seen at intervals. A Brooklyn dock worker on Furman Street might one morning notice that the stonework had gotten up above every ship mast since the last time he looked that way or a homebound commuter at the rail of a ferry pulling away from New York might realize one evening for the first time that this great blunt shaft with its feet in the water now topped the rise of the Brooklyn skyline.
The intended purpose of the structure would have been rather hard to figure at this stage if one did not already know. In the very early morning, when the ferries still had their running lights on and before anyone was at work on the tower, it might have been taken for an ancient harbor defense, a gray solitary battlement standing guard over the swarm of ships to either side of it. And when the sun began coming up and lit the top of the tower, the derricks bristling there looked for all the world like medieval war machines, the trajectory of which, from such a height, would surely be enough to hit New York. But in the full light of day, with the sun glaring on its clean buff-colored granite, the tower looked very new indeed, and more like the beginnings of a gigantic astronomical observatory perhaps, or the pedestal for some breath-taking triumphal monument.
But everyone did know its real purpose, of course, and could do little but marvel at its growth and at the way it seemed to diminish the size of everything else nearby. The ferryhouse, the most imposing Fulton Street stores, the newest business blocks, did not look so grand any longer. At the end of day, when the sun was a red ball hanging low over New Jersey and the west face of the tower seemed to be glowing from within, the granite pink nearly, everything in back of the tower stood in shadow for a block and more.
When the Eagle claimed there was nothing on earth, save the Pyramids, to rival “this Brooklyn tower of ours,” nobody thought that especially high-blown. And now Roebling had introduced a new vision to stir the public imagination. Now, one need only look at the Brooklyn tower and picture the same thing concealed below water directly across the river. As far as this tower reached above the river, the other one reached below, like a gigantic rock taproot. (Roebling undoubtedly wrote what he did a number of weeks before it was published, at the time when the Brooklyn tower was indeed at about seventy-eight feet.) For every stone the crowds on the ferry had seen hauled up the face of the Brooklyn tower, another had been added to the burden of the New York caisson. And those seemingly fearless figures working along the uppermost rim of the tower were no farther above the surface of the East River than the men in the caisson were below it.
Work inside the caisson was to be finished in another month. So it was the end of the first great stage in the building of the bridge, a clear dividing place. From here on the problems to be overcome, the work to be done, would be of an entirely different nature. Roebling made quite a point of this in his report, expressing congratulations to the Board of Directors “on the success which has attended the last of the two great tower foundations.” At the start of the work, he said, the foundations had been the principal engineering problem. The work to come—the building of the towers, the cable spinning, building the superstructure—was all work that had been done before on other bridges, on a smaller scale, “but upon the tower foundations rests the stability of the entire work.” Then he remarked almost as an afterthought, “Considerable risk and some degree of uncertainty was necessarily involved in their construction.”
All the extensive preparations for receiving the caisson had been completed by the end of the first week in September 1871. The tower was to fill a space formerly occupied by two ferry slips, between Piers 29 and 30. The riverbed had been dredged out to a depth of thirty-seven feet, or a little more than twice as deep as at the Brooklyn site. A hundred feet of Pier 29 had been torn away, and a huge pile dock had been built, itself a bridge more than a hundred yards long between the new foundation and the shore. At the end of the dock a square enclosure for the caisson had been built of six-inch pine planks—this to break the force of the tidal current, which was decidedly stronger on the New York side.
Borings made from the end of Pier 29 indicated bedrock anywhere from seventy-seven to ninety-two feet down. How far the caisson would have to go or whether even it was essential that it go clear to bedrock were questions that had still to be decided. But in any case the strata appeared to be chiefly gravel and sand, with layers of quicksand from fifteen to twenty feet thick. It was very different terrain from that at Brooklyn.
The machinery needed was all standing by on the dock: three huge boom derricks similar to those used in Brooklyn, the same clamshell dredging equipment as before, hoists, steam engines, and pile-driving gear. Workshops and offices had been built, a blacksmiths’ shed, sheds for cement, tools, general stores, a compressor house (the largest building) with its air-pumping machinery set up inside—thirteen Burleigh compressors ranged in a single row, each with its own steam boiler, as compared to the six compressors used for the Brooklyn caisson.
On September 11 the colossal wooden box was towed up from the Atlantic Basin, where in the four months since it was launched seven additional courses of timber, all laid with cement between, had been built on top. Once the pile enclosure was completed on the river side and the caisson confined to its permanent position, a final ten courses of timber were added, bringing the total height of the structure to just over thirty-one feet.
Particular care had been taken this time to guard against sea worms. The protection was needed only during the time the caisson was afloat and before it was entirely submerged below the riverbed, where the sea worm, the teredo, never penetrates. But this microscopic animal, less than a sixteenth of an inch in diameter, can bore into any crevice water can get through, so the precautions had to be quite substantial. Every outside seam was caulked. The entire outside surface was heavily coated with a composition of coal tar, rosin, and a hydraulic cement, which, all by itself, was supposed to have enough body and grit to dull the teredo’s boring apparatus. Then this had been finished off with a sheet of tin covering all four sides and the top of the sixth timber course. All seams in the tin had been soldered airtight and layers of tar paper had been put in both above and below the entire sheet. Finally, the whole caisson had been sheathed in four-inch yellow pine saturated with creosote.
“The great timber foundation was now complete!” Roebling wrote. “It contains 22 feet of solid timber above the roof of the air chamber, seven courses more than the Brooklyn caisson, and since the strength of such structures varies as the square of the depth, we may consider it to be nearly twice as strong as its Brooklyn brother.”
In their general features the two caissons were almost identical. The sides of the New York caisson were again of yellow pine and tapered from nine feet thick on top to an iron cutting edge eight inches wide. The timbers used in the roof were again a foot square. Headroom inside the work chamber was nine and a half feet as before. The base dimensions were 102 by 172 feet, making the new caisson just four feet longer than the one in Brooklyn. The heavier roof had been built to carry what Roebling figured would be a significantly greater load, since this caisson would have to go much deeper and therefore carry far more stone. But there were several other differences as well.
The light skin of iron boiler plate that lined the interior would not only provide fire protection, but make the caisson more airtight; and to i
mprove visibility the whole inside had been given a heavy coat of whitewash. The water shafts this time, instead of being square, were round (they would be stronger this way Roebling had decided). In addition, some fifty iron pipes, four inches in diameter, had been installed throughout the work chamber as a way of removing sand.
In the Brooklyn caisson there had been no means of communication between the men below and those working up on the surface. But here Colonel Paine had devised a simple, ingenious mechanical signaling system. One of the sand pipes was capped below and an inch tube was passed through the cap with index pointers attached above and below. Underneath each pointer was placed a small plan of the caisson, showing the position of every pipe and shaft. By rotating this tube immediate attention could be called to any of the points. In addition, a small rod was passed down through the pipe and its weight offset by a weight above that was attached to a cord that passed over a pulley. Small indexes were then fixed above and below. These moved up or down on vertical boards on which were printed such messages as “stop,” “start,” “bucket is caught,” and so forth.
The arrangement of air locks was also quite different. This time there were two double locks, each large enough to accommodate thirty men, which meant that a full shift of 120 could enter or leave at one locking. And instead of being mounted on top of the caisson, as had been done in Brooklyn, the locks were built into the roof of the work chamber, so they actually projected down into the chamber about four feet. Each set of locks was connected to the top of the caisson by a spiral stairway enclosed in an iron shaft.
The arrangement was essentially the same as Eads had used in St. Louis and it was over this particular feature that Eads and Roebling were to have their bitter falling out. The advantages to be gained were these, supposedly: the men could now step directly from the air lock into the work chamber, and at the end of the day they would not have to make the climb to the top while still under pressure.
On October 31 the last timber course was finished, the first stone of the new tower laid. By December 12 enough stone was in place to hold the caisson on the river bottom at high tide. The compressed air was turned on and Roebling, Paine, Collingwood, and a complement of some thirty men went down inside. (Since the water was thirty-seven feet deep and the caisson with all its timber courses stood thirty-one feet high, this left the top of the caisson just six feet below water at high tide and about two feet below at low tide.)
Tearing out the temporary floor took another two weeks. When the digging began, the work proved nowhere near so difficult as it had been in Brooklyn, but much more disagreeable, for the caisson was standing in the middle of what for years had been New York’s principal dumping ground. Moreover, a street sewer was still emptying into the river close by.
The ground itself was a clay silt turned black by sewage and thick, with the putrid remains of animals, garbage, and what Farrington called “sewage abominations.” All this had been odorless so long as it was embedded below salt water, but once the black muck was turned over inside the caisson, the smell, even in the compressed air, came forth in all its original strength, as Roebling wrote. The stench was such that several men were actually overcome and had to be helped back up to the surface. Only by keeping a skim of water over the entire caisson floor could the men keep on working.
But beneath this foul dock mud, which was only a few feet deep, they hit clean river sand and gravel, and things took an immediate turn for the better. The skim of water was expelled by compressed air, leaving a perfectly dry footing, and by now, too, the lights inside were fully operating and in combination with the white roof and walls they lit up the entire chamber as bright as day. From then on the great timber box descended into the earth extremely rapidly.
Above ground the scene was one of great energy and purpose. A reporter described it this way:
At the foot of Roosevelt Street, where the New York tower is being erected, one of the busiest scenes in the city is met with and has been for months—dozens of workmen hurrying hither and thither with wheelbarrows and hods and spikes and shovels; engines puffing away, lifting huge blocks of stone with huge derricks from the barge at the side of the dock, drawing lumber from the foot of the pier, driving the piles of the cofferdam, and condensing and compressing the air to be used by the submarine workmen; men chopping and planing and sawing the immense timbers used in constructing the enormous derricks; others shoveling gravel and sifting sand for the cement; little knots of threes driving immense piles through the heavy timbers of the caisson with their sledges and kept steadily at work by an overseer who evidently enjoys his employment; some wheeling cement for others to lay between the large granite blocks, boring and hammering and cutting stone and carrying iron rails, everything indicating that the work is being pushed rapidly forward.
There were almost no boulders to contend with this time, indeed little else but sand. The average rate of descent would work out to about two feet a week, but at one stage, for several weeks, the caisson was sinking six to eleven inches a day. In the Brooklyn caisson, during the first discouraging month of excavation, the rate of descent had not averaged six inches a week. Now everything was working to perfection. The dredges had no trouble digging the pools below the water shafts and the sand pipes worked like a charm.
How the pipes were to be used exactly had been left undecided until it came time to give them a try. Either the sand could be forced out with water, using a new kind of sand pump devised by Eads, or it could be blown out by compressed air. The latter would be the simpler, less costly way, of course, if it worked. It would also greatly aid ventilation. An air chamber with an iron skin such as this one had inside would be practically airtight, but a certain quantity of new air had to be fed into the work area at all times to keep the atmosphere fresh enough to live in. In the Brooklyn caisson this had been no problem since air kept escaping under the shoe or into the timber roof. But compressed air lost that way did little work. Roebling’s thought, therefore, was that with all the compressors he had at hand, why not allow air to escape through the pipes and take sand out with it?
So the air system had been tried and after that there was no more talk of sand pumps. For everyone who remembered how it had been in Brooklyn this was the smoothest sailing imaginable. Morale could not have been better.
The sand pipes extended down through the roof and into the chamber to within a foot or so of the work surface. Sand, loose earth, and fine gravel were shoveled around the pipe in a cone-shaped pile two to three feet high. When the pipe was opened, the pile vanished up the pipe. It was as neat and uncomplicated as could be, and the deeper the caisson went—the greater the pressure in the chamber became—the better the system worked. When the caisson was down about sixty feet, for example, the air was blasting out of the sand pipes with such force that fourteen men could stand in a circle around one pipe and shovel sand under it with all their strength and the sand would disappear as fast as they could shovel. At least three sand pipes were kept going all the time and some sixty men did nothing but feed these pipes, which was about the most tiring work imaginable. Time and time again the pipes had to be shut off to give the men a rest.
Up on top the sand blasted out with such velocity that it became a serious problem. At first, when there were only vertical discharge pipes, the sand was shooting four hundred feet in the air. To deflect this great geyser off at right angles, so it would feed into big scows tied up beside the caisson, iron elbows were fixed to the tops of the pipes. But the furious blast of sand would cut right through these, sometimes in a matter of minutes. Iron an inch thick would not last a regular workday. When elbows fixed with thick caps or patches of a special chilled iron were tried, they lasted only two days. Toward the end of the work the elbows would be dispensed with altogether and heavy granite blocks would be placed on supports directly above the mouths of the pipes. The sand would strike the blocks, then fall back. But the sharp, abrasive force of the sand was such that in three or four days’ time i
t would cut a hole through the granite block.
Once, a man passing by in a rowboat, with one hand resting on the gunwale, had the end of a finger shot off by a pebble fired from a sand pipe. Another time a workman was drilled through the arm in the same way. And down inside the caisson Farrington at one point thoughtlessly placed his hand over the open end of a sand pipe and found he was unable to remove it. Only with several others pulling on his arm was he able to get his hand free and then found that his whole palm had been drawn up into the pipe like a stopper.
Boulders were encountered only on occasion and slowed things down but a little, except when one appeared directly below a water shaft. The shaft had to be capped then, the water removed, the boulder excavated out of the dry hole, the same as had been done so many times in Brooklyn. But relatively little time was lost that way. Paine’s “mechanical telegraph” between the men on top and those below worked amazingly well. “The downward movement of the caisson has been under perfect control,” Roebling wrote. Indeed, the work went smoother, faster than anyone could have hoped for. Everything functioned as it was supposed to in theory and as it seldom had in practice over in Brooklyn. Nothing very unexpected happened. The heavy, tiresome digging by the men inside and the noisy work of dredges and stone derricks up above continued day after day, six days a week, and on into winter. The caisson, itself as high as a four-story building, kept descending steadily, evenly, uneventfully, the lights inside burning twenty-four hours a day, and all the while an enormous load of limestone blocks piling up on its back just as steadily, evenly, and uneventfully.
As far as Roebling and his staff were concerned, there were only two problems to be considered—the effects of the compressed air on the men and the depth to which they might have to go before stopping. In Brooklyn every foot of ground gained had to be fought for and the physical discomfort of working under pressure had been but part of the problem. But here the mercury gauges on the big Burleigh compressors kept inching up just as steadily as the caisson was descending.
David McCullough Library E-book Box Set Page 109