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SUPPLY OF WATER TO TOWNS.
BALDWIN LATHAM, C.E., A.I.C.E.,
MEMBER OF THE COUNCIL OF THE SOCIETY OF ENGINEERS,
E. & F. N. SPON, 16, BUCKLERSBURY.
UPON THE SUPPLY OF WATER TO TOWNS.
If we look to man as the lonely inhabitant of the plain, or find him congregated with his fellow-men in our cities or towns; or to the beasts, birds, or reptiles that roam the wilds of nature unfettered and unrestrained; or to the majestic tree that rears its lofty head in the primeval forest; or the tiny floweret that we tread beneath our feet; or direct our attention still closer to a world of beings so minute, that, aided by the most powerful microscopes, many of them are only distinguishable by that quivering movement which indicates the presence of life, we find that they are all mutually dependent for life and existence upon water; not only are the animal and vegetable kingdoms so dependent on it for existence, but it also enters so largely into combination with the mineral matters composing the crust of the earth, that in its absence the fabric of this world would collapse and all nature become an arid waste. A subject of so much vital importance as water supply, especially to the human race, is deserving of careful consideration; as its influences,
from a sanitary point of view, are attended with immense results for good or evil. The subject of water supply is one which has more or less occupied the attention of men of all ages, and in an historical point of view is well worthy of consideration, especially as with all the boasted advancement of the present age it is a question if our modern works for supplying water surpass those of ancient Judea, Greece, Rome, Mexico, and other places ; indeed, all history goes to prove that wherever man had arrived at any considerable degree of civilisation, the subject
of water supply had a share of his solicitude. The volume of sacred writ contains records of many works of this character, and the works of Vitruvius not only urge upon us the necessity that exists for the supply of pure water, but point out the modes in which it may be procured and purified. So impressed were our progenitors with the importance of a good and abundant supply of water, that they were always careful to settle in those localities where it could be the most easily procured. In the course of events, it may have been that the original settlement increased in numbers, and extended itself to such a degree, as to have affected the quality or rendered the quantity of water insufficient for the demand, when, from necessity, more distant sources of supply have been laid under tribute to furnish what has been required. This has been the case with many of the towns of this and other countries. It was the case with ancient Rome and many of the cities of the East.
In these papers it is purposed to inquire into the various methods that have been, and that are still, adopted, to furnish supplies of water to large cities or towns, and as all supplies are furnished by the rainfall, this part of the subject will be first treated of, and following it, the various sources of supply will be dealt with in order as they are more immediately connected with it.
All supplies of water being derived from the rainfall, this subject should receive careful attention, as the sufficiency and constancy of all works for the supply of water are dependent upon it. There are, however, circumstances connected with each mode of procuring a supply that in some measure may modify or alter the quantity, or affect the quality, of the rainfall of every district; yet there is no source, however remote it may appear from the rainfall, that is not directly or indirectly affected by the amount of that rainfall.
Rain is the result of the condensation of aqueous vapour, which is, at all temperatures, more or less suspended in the atmosphere. The quantity of aqueous vapour capable of being suspended in the atmosphere increases in a greater ratio than the temperature, and the phenomenon of rain occurs when the air, saturated with moisture, loses its temperature, and precipitates the excess it is no longer capable of containing in an aëriform state, either in the form of dew, rain, hail, or snow. It is found that the first causes of rain are identical with those that produce the winds and currents in the atmosphere—viz. the changes of temperature, to some extent the electrical state of the
atmosphere, and the magnetic state of the earth ; consequently it very naturally follows that the winds have a very close connexion with the rainfall. Thus, winds blowing from a warm climate over a great expanse of sea would be completely saturated with vapour, which, upon coming into a cooler climate, would be precipitated; on the other hand, a wind blowing from the frozen regions of the Arctic ocean, and deriving its moisture from the ice and snow of those severe regions, would, when it arrived in a warmer climate, by lowering the temperature of the atmosphere of that climate, diminish its power to retain aqueous vapours; and if, at the time, it was surcharged with moisture, a fall of rain must ensue. The physical conditions of every locality have some effect upon the rainfall. Thus, from observation, it has been ascertained that the rainfall is greater in mountainous districts than in level countries, which is probably owing to currents of air saturated with vapour striking against the mountain-sides, and losing temperature by contact or by reason of being compelled to ascend into higher and colder regions; there is, however, a limit to the effects produced by elevation, for there are regions too high to experience any heavy rainfall, for the rains of Switzerland and the Alpine regions are not greater than those in the north of this country. The amount of rainfall is considerably influenced by the position of the locality with respect to the currents in the atmosphere : for example, it is found that the prevailing winds in this country are westerly, and come to us from a warmer climate; after sweeping over the face of the great Atlantic ocean they are naturally saturated with moisture, and, striking the ridges and high lands on our westerly coast, discharge the greater portion of their burden there. Thus we find it is nothing uncommon in the counties of Cumberland, Westmoreland, and Lancashire, to have an annual depth of rainfall equal to 6 ft., while in Cambridgeshire, on the eastern side of the country, the annual rainfall seldom exceeds 22 in. The rainfall also varies with the seasons of the year, the rainfalls of this country being greater in winter than in summer, because the temperature of the atmosphere is decreasing in winter, and with it its capacity for retaining vapour, while in summer the opposite is the result. But it will also be found, as a rule, that heavier rains fall in summer than in winter, although there may be fewer showers; because in summer the atmosphere has greater powers for retaining moisture, so that when the causes that induce a fall of rain are brought into action, there is a larger amount of moisture to be precipitated. In Germany the rainfalls of winter and summer are about equal; at St. Petersburg the rainfall of winter is but little more than one-third the rainfall of summer ;