National Geographic : 1913 Jan
A POST LIGHT ON THE MISSISSIPPI RIVER Post lights are maintained on about 5,500 miles of rivers in the United States A caisson of boiler iron 36 feet wide, 46 feet long, and 61 feet deep was built in port. This caisson was towed to the site and sunk in position. Eight feet above the lower or cutting edge of the caisson was a diaphragm, forming a working chamber, from the center of which rose a cylindrical shaft with an airlock. The caisson was sunk by the pneumatic process to a depth of 73 feet below low water, the sand being removed from the working chamber by a sand blast; the caisson was filled with con crete and masonry and the light-tower erected on this foundation. Two years later, in 1887, the first light house in the United States built on a submarine foundation and sunk in a sand bottom by the pneumatic process was completed on Fourteen-foot Bank, Delaware Bay, in 20 feet of water. A timber working chamber 40 feet square was built, with cutting edge 7 feet deep. On this was placed an iron cylinder 35 feet in diameter and 18 feet high, built of cast-iron plates bolted together by their flanges. This was towed to the site and placed in position. It was sunk, by digging and blowing out the sand, to a depth of 33 feet below the surface of the shoal, the cylinder being built up until it was 73 feet high and filled in with concrete (see page 20). Cast-iron cylinders have been used also on other shallow submarine sites affording stable foundations or on rocks nearly awash. Wooden cribs floated to the site have been similarly employed, an example of which is Detroit River lighthouse. Recently reinforced concrete caissons have been used, sunk in place on the bottom, for minor light stations.