The illustration above, from the May 1883 issue of Harper’s Magazine, courtesy of the New York Public Library, gives a better sense of the Brooklyn Bridge caissons than yesterday’s working drawing (repeated below) even if it’s got less technical information. Note that May 1883 was when the bridge officially opened, and the caissons had at that point been literally buried for a decade.
We see here the basic construction of the caisson – an open-bottom wood box with a heavy grillage as its top – supporting the stone of the bridge tower. The caisson, during the construction process, was full of pressurized air to keep the river water out. You can see the partitions that divide the working chamber (the inside of the box) into separate rooms. There are six vertical elements through the roof, arranged in mirrored pairs. The outermost and largest are the water shafts. This is literally a tube full of water that opens on the top to the sky and opens on the bottom into a pit dug into the floor full of water. This is a barometer: the air pressure on the surface of the water in the pit balances against the weight of the water in the tube and the air pressure on the outside. (Mercury, used in most barometers, is 13.7 times the density of water. A mercury barometer has a vertical column about 30 inches high, depending on the weather; a water barometer will therefore have a vertical column 13.7 times 30 inches or about 34 feet high. And that is what we see in this illustration.) Each of the water shafts had a clamshell bucket on a set of pulleys within it. If you shoveled loose soil into the pit, the scoop could then pick it up and carry it up to the top working surface for disposal. The scoop in the left water shaft is closed, so it’s probably going up with some spoil; the scoop in the right water shaft is open, and so is headed down for a load.
I’m not 100 percent sure what the next inward pair of vertical elements are. Given their location, I’d guess they are the means to supply pressurized air.
The innermost pair of vertical elements are the airlocks and their access. The airlock itself is just above the wood roof of the caisson, at the base of the stone work, with the domed top. The narrow shaft below that is access within the pressurized air, consisting of a ladder in a tube. You can see men in the open space above the airlocks on the ladders from there up to the top of the working surface of the stone.
In short, the men in the caissons dug out the soil while the men on top built up stone. The weight of the stone and the reduced support from the digging drove the caissons down into the riverbed. More about that tomrrow.
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