The opinion which it has lately been attempted to convert Into a law, " that the nearer the source. the purer the /supply," seems to be one of those half-truths which require to be accepted with a reserva tion. Thus, the waters rising frotu deep-seated springs in the chalk formation contain at their source an abnormal dose of the bicarbonate of lime, which they part with if allowed to flow in the open air in a clear channel. Indeed, so much do waters generally improve by the flow in open channels, that some of the ablast physiologists hold that river waters alone should be used for town supplies; but in such cases, It is essential to adopt precautions with a view to the prevention of the contamination of the latter, by the organic impurities washed down from the lands they traverse. A prolonged course over a rough channel also removes earthy impurities, and the hydrous oxides, which are often held in suspension in spring waters; but it does not seem to have any influence upon the sulphates of lime, or of magnesia, or upon the chief-idea of calcium or of sodium. In all cases wherein such a course is able to do good, it must, however, be observed. that the water must flow with considerable velocity ; and it would appear that the aeration of the water thus produced constitutes one of the superiorities of river waters over those obtained from ordinary wells ; because the latter are exposed, occasionally, to become stagnant. When a water supply has to be obtained from a great distance, the considerations of the deposition of the earthy salts become of great importance, on account of the effect they are likely to produce upon the delivery of the conducting channels ; and it was precisely because the engineer of the Marseille waterworks, M. de Montricher, feared that the waters of the Durance would choke a reversed syphon that he was led to erect the gigantic aqueduct of Roquefavour. The channel through which any stream is to be led must, it may also be added, be protected from accidental impurities derived from the atmosphere, or from any other source ; and even at the present day, the hydraulic engineer might derive many useful lessons from the practice of the Romans in these details of their aqueducts. They took care, in fact, to cover their channels as far as possible; to provide ventilating shafts from distance to distance ; and to place drips in the line of the invert, in order to produce small cataracts, with a view to increasing the aeration of the water.
The objections to the use of ordinary well-waters, on account of their tendency to become stagnant, apply with variable force (according to the quantities considered) to the waters of ponds, of reservoirs, or of lakes ; and therefore they have an important bearing upon the system of the supply to towns, known in England by the name of the " catch scaler y aritation system." In works of this description, the water, falling upon elevated districts around the town to be supplied, is stored in large artificial reservoirs, in such a manlier as to allow the excess of the winters rains to be distributed in the dry season; and evidently these reservoirs must be established so as to ensure the disposal of the maximum quantity for distribution precisely at the season when there would be the least rainfall to renew the water. During the dry season, in fact, the water in the reservoirs cannot be renewed, and it must. be
exposed to all the deleterious actions which are known to take place in stagnant water from the development of animal and of vegetable life in them, under the influence of light and heat. The purity of the waters thus stored must, however, depend greatly upon the nature of the surface from whence they flow, upon the nature of the soil of the reservoirs themselves, and upon the exposure, the outline in plan, and the transverse sections of the reservoirs ; for the primary qualities of the water depend on the first of these considerations, and its preserva tion in a wholesome state depends greatly upon the influence of the latter in maintaining a surface agitation. The lands which exercise the most deleterious action upon the waters to be impounded are those which are covered by pent, or by agricultural land; or those which are likely to give off mineral or earthy salts in large quantities ; and, therefore, the primary or secondary crystalline rocks and the pure silicious sands are the best adapted for the purpose of forming gathering-grounds. In England. and in most countries where gravi tation waterworks can be established from reservoirs, such as are above alluded to, it fortunately happens that the gathering-grounds occur in the hilly districts of the primary formations; but in India, and in tropical alluvial plains, it is often necessary to store the excessive rainfall of the wet season in tanks, for consumption during the dry season, because the superficial water-courses cease to yield water, and there are no perennial springs in such districts. The waters so stored must, under these circumstances, be filtered, or treated in some way, in order to counteract their otherwise deleterious effects upon the human frame ;•the great difficulty, however, arising from the tempe rature to which they are raised by the sun's action upon their surface.
It would appear, after all, that the waters which unite the greater number of favourable qualities for a town supply are those which flow from deep-seated springs, or from the insoluble clay slates, crystalline limestones, or are obtained by continuous pumping from wells sunk in the pure silicious strata, etc. It is not often that these wells yield water in sufficient quantities for a town supply ; and there is always a danger of their being interfered with by the operations of neighbours, or by injurious infiltrations from cesspools or dead wells. The history of the Artesian wells round London, of those at Tours and Paris, and of the deep wells in the new red-sandstone at Liverpool, proves that there are serious objections to allowing the supply of a large population to depend on sources so uncertain, and so easily diverted, as these deep seated springs evidently are. In no country in Europe is there any legislation creating a right of property in underground waters; for law-makers have not hitherto been geologists, and they have openly avowed their ignorance of the principles which regulate the flow of deep-seated springs. It thus happens that the inhabitants of large towns are compelled, practically, to resort for their water supply either to rivers or superficial water-courses, or to create reservoirs upon either the gravitation or the tank system, to impound the rainfall of the wet seasons.