In making investigations to ascertain what quantity of water may be relied upon to supply a reservoir, whether natural or artificial, for the use of a town or city, a survey is first taken of the district of territory which naturally is drained into the reservoir, and thus the number of square miles of surface is ascertained. Then the rain-tables are consulted, and the fall of rain upon the surveyed district is computed. The ascertained proportion of rain-fall, which usually goes off by evaporation, is then deducted, which leaves with sufficient accuracy, the amount of water which flows both upon the surface, and through the soil, to the reservoir. With proper deductions for waste by freshets, when the water will overflow the reservoir, and for other known losses, a reliable estimate is readily made, in advance, of the quantity of water supplied to the reservoir. Now, these reservoirs Nature has placed in all our valleys, in the form of lakes and ponds, and the drainage into them is by natural springs and streams; and the annual amount of the water thus naturally flowing into them may be readily computed, if the area within their head-waters be known. If the earth's surface were, like iron, impervious to water, the rain-water would come in torrents down the hill-sides, and along the gentle declivities, into the streams, creating freshets and inundations in a few hours. But instead of that, the soft showers fall, often on the open, thirsty soil, and so are gradually absorbed. A part of the rain-water is there held, until it returns by evaporation, to the clouds, while a part slowly percolates downward, finding its way into swamps and springy plains, and finally, after days or weeks of wandering, slowly, but surely, finds its outlet in the stream or pond.