================+=======+=======+=======+=========+=========+========= | | | | |PREVIOUS | | TOTAL |ORGANIC|ORGANIC|CHLORINE.| SEWAGE |HARDNESS. |IMPURI-|CARBON.| NITRO-| |OR ANIMAL| | TIES. | | GEN. | |CONTAMIN-| | | | | | ATION. | ----------------+-------+-------+-------+---------+---------+--------- Worst Condition.| 94. | 2.160 | .274 | 13.10 | 10.090 | 35.58 Best “ | 24.60 | .108 | .055 | 4.05 | 17.920 | 9.20 Average “ | 64.02 | .982 | .191 | 6.36 | 10.443 | 33.09 ================+=======+=======+=======+=========+=========+========= Much depends upon the knowledge of the climatic influences and rain-fall, extended, as it should be, through years of observation in determining the available quantity of water. Engineers, however, are liable to be too sanguine of the resources from water-sheds, by assuming, as a general rule, the average, rather than the minimum, rain-fall. In 1868 nearly all the cities and towns of England, supplied by gravitation, suffered a water-famine, because of the overestimate of the available rain-fall, and in an insufficient provision of storage for an unusually long drought. Although the rain-fall for the year was above the average, yet it was unequally distributed. The authorities of Manchester were obliged to publish official notices cautioning the inhabitants against waste, and, on the 3d of August, limited the supply to the city to twelve hours of the day, stopped the street watering, and diminished the trade supplies by one-half. In the middle of September the general supply of the town was further limited to eight hours per day. Many persons were prosecuted for waste or undue use of water. 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.