Deep wells are those which, passing through one or m< re impervious beds, tap a sheet of water in some lower stratum, which has percolated along it from its outcrop on hills or higher ground at a considerable distance. They may be dug and steined, or bored, or dug to a certain depth, below which they are continued by boring. It is always advisable to leave the tube in situ, and the steining should be constructed with special wedge-shaped vitrified bricks set in cement-mortar. If common rectangular bricks have been used, they should be faced with cement, down at least to the level of an impervioos stratum not leas than twenty feet from the surface, so as effectually to exclude any land-springs or surface-water, by which the water in the well might be contaminated. Even when sunk in hard rock, the lining should not be omitted, for these wells are especially liable to the entrance of polluted surface-waters; above all. wells sunk in the chalk are exposed to such risks, from the passage of polluted waters from

Mas-pits, etc, through the fissures by which this formation is everywhere traversed.

Deep-well waters are usually hard; those in chalk or limestone from carbonate of lime, in the dolomite from lime and magnesia, and in the lias and some oolitic beds from chlorides, sulphates, etc., which may render them actually unfit for drinking, or give them the character of medicinal and aperient waters. Those, however, from the chalk, mountain-limestone, upper oolite, and coal measures, ire among the very best, though all more or less hard.

Fig. 726.   Diagram showing ground water, deep water, spring, stream, and different kinds of well:

Fig. 726. - Diagram showing ground-water, deep water, spring, stream, and different kinds of well: -.

A. A superficial bed of pervious sand or gravel.

B. Ground-water emerging at L as a spring, which finds its way to the stream M.

CC.An imperviouis bed or stratum of clay.

DD.A deep pervious bed of limestone or sandstone, forming in its lower part a large sub-terranean sheet or reservoir of water. E E, the level of which is O,O,O,O.

F F. An impervious formation of solid rock sup-porting the mass of water, E, in the pervious bed, D.

G G. A seam of gravel traversing the bed of clay, C, receiving the surface drainage at G, and giving rise in wet weather to a so-called land-spring at G.

H. A house with privy and cess-pit, which may pollute the ground-water, B.

I. A surface-well sunk into the ground-water, B.

K. A deep well, steined as far as the clay so as to exclude the ground-water, and tapping the subterranean water, E.

N. An artesian well, in which, the mouth being below the line O, O, O, O, the water rises under pressure in the form of a natural fountain, requiring no pump as all the others do.

P. A deep well steined to exclude the land-spring in G G.

Q. A well which, though drawing from the same source, and at the same depth as P, is technically a surface well, since it does not pass through any impermeable stratum.

Through a pedantic ignorance the term "artesian well" is almost constantly applied to machine-bored wells, whereas it properly describee a peculiar form of deep well from which the water rises up in a natural fountain; the name is derived from the French province of Artois, where the phenomenon was first observed. The requisite conditions are rarely met with, viz. a hollowed, saucer-like bed of clay overlying pervious, water-bearing rocks, the lowest point of the depression being below the level of the ground-water in the surrounding strata. Given these conditions, it follows that as soon as the dense bed in question is pierced, the water spouts upwards under its own pressure.

Far too much reliance is placed by the public on the results of a chemical analysis of water. Because in his examination into the proportions in which certain constituents of nearly all waters are present, the analyst finds no evidence of sewage pollution in that particular sample, the character of the water is supposed to be conclusively vindicated. This is, however, a dangerous error. From what has been already stated, it will be easily understood that all ammonia, or material convertible into ammonia by boiling with an alkaline permanganate), and known technically as "albuminoid ammonia", is evidence of the presence of organic matter in the process of change, while nitrites indicate a further, and nitrates the ultimate stages in the conversion. Nitrates themselves are quite harmless, hut when they are associated with nitrites, and saline and albuminoid ammonia, the continuity of pollution - as distinguished from one long-past, or a single and accidental contamination - is demonstrated Chlorides, if exceeding 15 parts of chlorine per million parts of water, are very generally accepted as evidence of sewage pollution recent or remote, but it should not be forgotten that in some springs, as those of the natural medicinal waters, chlorides, however abundant, may be wholly of mineral origin. It is quite true that from a consideration of the chlorides, nitrates, nitrites, ammonia, and albuminoid ammonia, and the oxygen required for the complete oxidation of the cruder organic matters, the chemist may form a notion as to the amount, nature, and age of the pollution, and whether it be continuous or a thing of the remote past; but unless he has a fair knowledge of the geology and hydrology of the district, the general character of its waters, the density of the population, and the nature of its manufactures, and is acquainted with the surroundings of the well or spring, he had far better confine his report to a mere statement of percentages and of the physical character of the water, leaving all expression of an opinion as to its wholesomeness to those who have the wider knowledge and are familiar with the local conditions.