The methods of storing and delivering water will vary with its source. In rural districts, deep wells and springs are the best sources of supply; but in large towns they are found to be insufficient for the wants of a rapidly-increasing population; and they can only be multiplied in a given district within certain limits, as every well drains a large surrounding area. The supply from surface wells in gravel or sand beds or in chalk districts is liable to fail in seasons of drought; but deep wells in oolite or chalk formations, and in the new red sandstone, generally yield a constant and abundant supply. When the water is supplied from upland surfaces, springs, or small streams, =a collecting reservoir= is required. This is generally a natural valley below the level of the source of supply, but of sufficient elevation above the place supplied to allow the water to be distributed by gravity, without any pumping apparatus. The reservoir should be large enough to hold five or six months’ supply, and its embankment should be perfectly water-tight, and of great strength. When water is collected from upland surfaces, it is important to know the amount of rainfall to be reckoned on. If we know the area of the surface which drains towards the reservoir, and the average rainfall, the _total_ rainfall is easily calculated. This will, however, differ greatly from the _available_ rainfall, owing to the losses from penetration into the ground, evaporation, and other causes. The amount lost will vary, according to the season, from one-half to seven-eighths of the total rainfall; and according to the soil (page 68). The proportion of percolation in the chief water-bearing strata surrounding London varies from 48 to 60 per cent. (Prestwich). It is less when the ground is steep and the rainfall rapid, and usually less in winter than in summer. Water collected near its actual place of fall, and from uncultivated districts, is always purer than that collected further from its source, and from cultivated land. From the collecting or impounding reservoir, water is carried by the aqueduct or conduit either directly into the service-pipes, or when the pressure is too great, into a second =service-reservoir=, resembling the impounding reservoir in general structure, and capable of holding a few days’ supply. This must be high ground, above the level of the highest houses to which water has to be supplied, as water cannot rise above its own level. When this cannot be arranged, the water is pumped into tanks at a higher level, and distributed from them.