Deep wells are derived from springs. A spring is a flow of water which essentially depends upon the following conditions. A permeable stratum crops up on the surface of the ground, resting upon an impermeable stratum; rain falling upon the permeable stratum percolates through until it is stopped by the impermeable stratum. If the stratum again crops up at a lower level the spring there bursts forth, if this does not happen, the water is stored up as in a reservoir, and can be reached by sinking a well.

Wells are therefore sunk through impervious strata into pervious strata, the water in the latter being confined by impervious strata below. Imagine a sheet of india-rubber, and on the sheet a thick layer of sand, and above a layer of putty or some other doughy material, if water is now made to pass into the sand it is confined between the two layers, the under one preventing the water running away, the upper preventing evaporation; this is an illustration of most of the water-bearing stratum into which wells are sunk.

(86) Artesian Wells. 1

These are tube wells often sunk to a very great depth. The most common and successful artesian wells have been sunk into the chalk.

From many of the artesian wells the water rises above the mouth in a stream, and pumping machinery is unnecessary. Thus the new La Chapelle well sunk in the Paris basin has a depth of 720 metres and the water rises 35 metres above the mouth. This well furnishes 6,000 cubic metres in 24 hours. It is even proposed to utilize this force to drive a dynamo for the production of electricity. At Ponce de Leon, Florida, the water from an artesian well drives a turbine wheel and dynamo, and by means of this motive force the hotel is lit by electricity. In other cases the water has to be pumped to the surface.

(87) III. Lakes.

When lakes are used as reservoirs for the supply of large towns, the enormous quantity of water drawn off might lower the water in the lake and so injure various vested rights; to obviate this, the water-level in the lake has to be raised by heightening the barrier

1 The word "Artesian" is derived from "Artois" a place where the first Artesian well is said to have been sunk,

at its outlet, the height to which this barrier has to be raised depends wholly upon the area of the lake and the quantity likely to be drained from it. Loch Katrine, 3,000 acres in area, only required the barrier to be raised 4 feet, in order that with a maximum lowering of 7 feet, it might provide a storage of 5,687 million gallons for a supply of 50 million of gallons per day. Thirlmere, which is to supply Manchester, has an area of 350 acres, and the barrier has to be raised 50 feet in order to furnish a storage of 8,100 million gallons for a similar daily supply.

(88) Quantity of Water per Head.

The actual quantity per head supplied to towns varies within very wide limits. The following interesting table is given by the late Major Bolton, based upon Sir J. W. Bazalgette's figures :

The quantity required for a town supply is stated usually as follows:

It is impossible to give a large daily supply of water to any place which has not suitable channels, such as good sewers for its removal, but provided a town is properly supplied with drains, there should be no limit to the water supply; in the middle and upper classes of houses, the general employment of baths1 much increases the demand for water. It is the writer's opinion that no town in this climate can be considered adequately supplied unless the quantity per head amounts to at least 30 gallons, and if there are manufactories which draw upon this supply, then it must be proportionately increased.

(89) Catchment Areas, Contour Lines, Ridge Lines.

66

In taking a survey of a catchment area for the purpose of supplying a town with water, a plan is necessary showing by means of "contour lines" the undulations and general slopes of the ground, and here will be a convenient place to insert a brief account of what is meant by the terms "contour lines," " datum," "ridge lines." In the ordnance map there are seen a number of dots usually following the course of the main roads, with figures upon them; these figures express in feet and decimals of a foot, the heights of the ground at the various places mentioned, thus B.M. 355-2 means that the particular spot is 355.2 feet above the ordnance datum, and the ordnance datum level for Great Britain, is the level of mean tide at Liverpool as ascertained by a series of observa1. An ordinary full length bath will take from forty to sixty gallons of water.

tions taken by the ordnance survey in 1844; it is 8 inches below the general mean level of the ocean around our coasts. If the place indicated by any of these figures be visited there will be found a broad arrow marked on some permanent object such as a mile-stone, or a church, or a rock.

Contour lines are "lines of equal altitude," and are "what would represent the water's edge, supposing water to stand at the various levels" marked out by the figures. The contour lines marked out in the 6-inch ordnance map are drawn at every 25 feet (vertical) of elevation apart and these lines may be taken as the basis. But for the purpose of water-supply or drainage, contour lines require to be drawn closer than this; they are usually drawn for local purposes every 8 or 10 feet of elevation apart. Ridge lines are also called water-shed lines. As the name indicates, ridge lines along the whole of their course are higher than the ground immediately adjacent on each side, hence the ground slopes downwards from them at both sides.

Since all water supply comes either directly or indirectly from the rain, the amount of rain which can be utilized for the supply of a town depends upon the depth of rainfall and the catchment area.

A catchment area is also called "catchment basin," "drainage area," or gathering ground. It is in almost every case a district enclosed by a ridge line, which line is continuous, save where the water finds exit. The ridge line may, and commonly does, give off branches, and thus there are produced subsidiary or secondary catchment areas. All the details of such an area are obtained from a series of levels, and the drawing of contour lines as above described.

(90) Rainfall in relation to Water Supply.

The most important data respecting the depth of rainfall, are as follows:

(1) The least annual rainfall.

(2) The mean annual rainfall.

(3) The greatest annual rainfall.

(4) The distribution of the rainfall at different seasons and especially the longest continuous drought.

(5) The greatest flood rainfall, or continuous fall of rain in a short period.

L

For the purposes of water supply, the most important of these are the least annual rainfall and the longest period of drought, but for drainage works, the greatest annual rainfall and the greatest flood rainfall.

The amount of water from a gathering ground may be approximated from the formula Q = 62·15 A (RE) where Q is the supply in gallons daily, A the catchment area in acres, R the average annual rainfall, and E the loss from evaporation both in inches.

In constructing reservoirs, the storage capacity is calculated from the number of rainless consecutive days, likely to be experienced in any given locality. In our climate this is seldom more than a month.

(91) General Principles of a Town Water Supply.

A supply of water to a town should properly be at such a height that by gravitation alone the water may be cast in a jet 20 feet above the highest houses. Such a system in its most perfect character, does away with the necessity for fire engines of the ordinary type; for by the multiplication of hydrants, facilities are given to deal promptly with fires. The growing height of modern buildings, enables this system only to be adopted partially. In towns of great extent and irregular levels, care has to be taken that certain portions of the mains and branches are not exposed to excessive pressure, this is obviated by causing "loss of head," either by passing the water through small inlets or by the use of loaded valves.

The points to be considered in water mains for streets are that they should be made of a material that will not contaminate the water, that they be of sufficient size and strength, and that the joints be absolutely water-tight. The usual material is cast-iron, and the empirical rule as regards strength is "that the thickness of a cast iron pipe is never to be less than a mean proportional between its internal diameter and one forty-eighth of an inch." A more accurate formula is as follows, the limit of safety being six times the working pressure

thickness greatest working pressure on fect of water

diameter

=

12,000