This embankment is supported and protected on its lower side by a heavy protection wall 20 feet wide at base. The portion of the dam over which the water flows, together with the abutment and bulkhead adjacent, is of hydraulic stone masonry their united length is ;

105 feet. The width of the sheet of water flowing over the dam is 90 feet, and it is supposed in times of a freshet will be from four to six feet deep it has already in the last ;

autumn been equal to three feet. The area of the Croton Lake is about 400 acres, the available capacity of the lake taken at seven feet deep, (the depth to which the water can

180 MEMOIR OF THE be drawn down,) and of an average area of more than 300 acres, may be stated at 600 millions of gallons. The total capacity of delivery of the aqueduct is about 50 millions of gallons for 24 hours, but in case the lake is being reduced seven feet, by a discharge through the aqueduct, not more than 35 millions can be calculated on as the average delivery during that time. The flow of the Croton in ordinary low water exceeds 50 millions for 24 hours, and the minimum flow after long continued drought has been ascertained to exceed 25 millions, consequently, assuming the Croton at its lowest rate of flow, there would elapse 60 days before the lake would be reduced seven feet, and 60 days before the delivery of the aqueduct would fall below 35 millions of gallons in 24 hours.