Liverpool, Sheffield, Bristol, and several other large cities were obliged to resort to like severe methods enforced at Manchester. New York has been using every gallon that the aqueduct is capable of supplying; and, during the drought of last summer, when the head of water at Croton Lake was diminished, the capacity of the aqueduct was so reduced that the flow of water to the city was reduced, and a water-famine averted only by a Providential rain-fall.
The rule observed among engineers, in Great Britain, in determining the calculated rain-fall, is the deduction of one-sixth from the average rain-fall of twenty years for an average annual rain-fall of the three driest consecutive years in that period. But, as Mr. Homersham, C. E., observes, the axiom in mechanics, that the strength of a beam is the strength only of its weakest parts, applies also to gravitation water-works, their real strength or power of supply being only the minimum quantity they may be reduced to.
Allowance for absorption depends upon the geological formation and stratification, and for evaporation, upon local influences.
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:
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.