The following is taken from Hughes’ Water-Works: “A flat, low-lying country is seldom well adapted for the impounding of water by embanking across the valleys. In such a district, long and shallow embankments would be required, and these would cause the water to spread out over a great area with a very shallow depth. Under these circumstances, the water is apt to vegetate and become highly impure. Again, in low-lying districts of flat countries the rain-fall is seldom nearly so great as in upland districts, so that much larger drainage areas must be sought.” In addition to the general configuration of the valleys, which ought to be deep and with precipitous sides, flanked by lofty hills, there are several other points which require attentive examination in projects for collecting water from drainage areas: 1. The area of water-shed. 2. The geological character of the soil as affecting its capacity to absorb rain, and to allow the infiltration of water through it. 3. The character of the surface soil as affording soluble ingredients which may be taken up by the water and serve to contaminate its quality. In this point of view, districts of decomposing peat, districts of arable agricultural land richly manured, and places thickly covered with population, are often highly objectionable. 4. The rain-fall of the district, and especially the minimum fall in any one year. 5. The nature of the surface-soil as affording facilities for procuring puddle and constructing retentive reservoirs. 6. The consideration of compensation to mill-owners and possibly to land-owners where the water is used for irrigation. The geological structure is extremely important in estimating the capacity of a drainage area. It is not alone the rain which falls on the sloping surface of the hills, and finds its way by gravitation to the lower levels; but the effect of springs is also very great in augmenting the quantity of water. Many drainage areas are also valleys of elevation, in which the strata dip in opposite or anticlinal directions on opposite sides of the valley. In this case it is evident that much of the rain falling on a porous surface will insinuate itself between the partings of the strata, and flow off in a direction contrary to that of the surface drainage. From Mr. Beardmore’s work we take the following, as the proportion or percentage of rain-fall which flows off the surface: “From twenty examples we have 89 as the largest per centage, the lowest 29 per cent., and the average 64 per cent. “The Eaton Brook water-shed, in Madison County, New York, of 6,800 acres, with steep slope and compact soil, underlaid by hard greywacke rock, elevated 1,350 feet above the sea, availed 66 per cent. of the rain-fall as surface flow.