A POST I,IGHT 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 J'}^ 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 foiuidations 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. 



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