The corresponding percentages for the cities of New York, Philadelphia, and Chicago for the same periods are as shown in the following tabular statement: ------------+-----------------------------+------------------------ | Population. |Percentages of Increase. ------------+---------+---------+---------+---------+-------------- | 1870. | 1880. | 1890. |1870-80. | 1880-90. ------------+---------+---------+---------+---------+-------------- New York |1,626,119|2,131,051|2,821,802| 31 | 32 Philadelphia| 726,247| 921,458|1,162,577| 27 | 26 Chicago | 310,996| 550,618|1,075,158| 77 | 95[6] ------------+---------+---------+---------+---------+-------------- [6] Includes added territory. Obviously every estimate of this kind must be made upon the merits of the case under consideration. The probable increase of population for any particular city is sometimes estimated by considering the circumstances of growth of some other city of practically the same size, and if possible with the same commercial industries or residential environment, or making suitable allowances for variations in these respects. Since it is imperative to secure as accurate estimates as practicable, both methods or other suitable methods should be employed, in order that the results may be confirmed or modified by comparison. In every case the supply system should be designed to meet reasonable estimated requirements for the longest practicable future period, preferably not less than twenty years. =161. Sources of Public Water-supplies.=—One of the most important features of a proposed water-supply is its source, since not only the potable qualities are largely affected by it, but frequently the amount also. The two general classes into which potable waters are divided in respect to their sources are surface-waters and ground-waters. Surface-waters include rain-water collected as it falls, water from rivers or smaller streams, and water from natural lakes; they are collected in reservoirs and lakes or impounding reservoirs. Ground-waters are those collected from springs, from ordinary or shallow wells, from deep or artesian wells, and from horizontal galleries, like those sometimes constructed near and parallel to subsurface streams or in subsurface bodies of water, affording opportunity for filtration from the latter through sand or other open materials to them.