712 ANNUAL BEPOET SMITHSONIAN INSTITUTION, 1913. 



is 4,650 feet, and the maximum depth of the water is 190 feet. The 

 area of the water surface is 12.8 square miles, and in preparing the 

 bottom it was necessary to remove seven villages, with a total popu- 

 lation of 2,000. Forty miles of highway and 10 bridges had to be 

 built. In the construction of the dam and dikes it was necessary to 

 excavate nearly 3,000,000 cubic yards of material, and 8,000,000 

 cubic yards of embankment and nearly 1,000,000 cubic yards of ma- 

 sonry had to be put in place. The maximum number of men em- 

 ployed on the job was 3,000. 



THE 9 2-MILE AQUEDUCT. 



The water is conducted from Ashokan Reservoir as a huge under- 

 ground artificial river. The aqueduct is 92 miles in length from 

 Ashokan to the northern city line, and it should be explained that 

 it is built on a gentle grade, and that the water flows through this 

 at a slow and fairly constant speed. The aqueduct contains four dis- 

 tinct types : The cut-and-cover, the grade tunnel, the pressure tunnel, 

 and the steel-pipe syphon. The cut-and-cover type, which is used on 

 55 miles of the aqueduct, is of a horseshoe shape and measures 17 feet 

 high by 17 feet 6 inches wide, inside measurements. It is built of 

 concrete, and on completion it is covered in with an earth embank- 

 ment. This type is used wherever the nature of the ground and the 

 elevation allow. Where the aqueduct intersects hills or mountains 

 it is driven through them in tunnel at the standard grade. There are 

 24 of these tunnels, aggregating 14 miles in length. They are horse- 

 shoe in shape, 17 feet high by 13 feet 4 inches wide, and they are 

 lined with concrete. When the line of the aqueduct encountered 

 deep and broad valleys, they were crossed by two methods : If suit- 

 able rock were present, circular tunnels were driven deep within this 

 rock and lined Avitli concrete. There are 7 of these pressure tunnels 

 of a total length of 17 miles. Their internal diameter is 14 feet, and 

 at each end of each tunnel a vertical shaft connects the tunnel with 

 the grade tunnel above. If the bottom of the valley did not offer 

 suitable rock for a rock tunnel, or if there were other prohibitive 

 reasons, steel siphons were used. These are 9 feet and 11 feet in 

 diameter. They are lined with 2 inches of cement mortar and are 

 imbedded in concrete and covered Avith an earth embankment. There 

 are 14 of these pipe siphons in a total length of 6 miles. At present 

 one pipe suffices to carry the water. Ultimately three will be re- 

 quired for each siphon. 



Of the many siphons constructed, by far the most interesting and 

 difficult is that which has been completed beneath the Hudson River. 

 The preliminary borings made from scows in the river showed that 

 great depths would have to be reached before rock sufficiently solid 

 and free from seams was encountered to withstand the enormous 

 hydraulic pressure of the water in the tunnel. After failing to 



