RESERVOIRS. In engineering works the word reservoir is under stood generally to apply to the Large collections of water which are made for the purpose, of feeding canals, or of supplying tho head waters of a mill, or of supplying the water required for municipal services. The water, in almost all these cases, is obtained by storing the excess of the rain-fall (which would otherwise pass off from the surface of the land, by the streams or rivers, during the rainy seasons) in artificial ponds, provided with sluices, feeders, or main pipes, to conduct the water to the places where it is required. Under these circumstances the dimensions of reservoirs, and the modes of their construction, must admit of every imaginable variation ; but the general principles con nected with their formation may be briefly stated as follows.
The reservoirs for feeding canals, or inland navigation, may be made without much reference to the quality of the water to be stored in them; but those which are designed for the purpose of a town supply must be made in such places as to obviate any danger from the contamination of the water. It is customary that they should be formed, in either of these cases, if practicable, in the gorges of moun tainous districts, where there might exist a large area of land having a natural fall towards the outlet of the gorge. Writers upon meteorology calculate that upon the average 1 of the total rain-fall of the whole year passes off by means of the superficial natural watercourses ; that another 1 passes off by evaporation; and that the remaining / descends into the gronnd to feed the land springs and the wells. In moun tainous districts, however, these proportions do not hold good ; for, firstly, it happens that, as a general rule, the nucleus of mountain chains consists of dense impermeable rocks, which not only oppose the infiltration of water, but also from the steepness of their escarpments throw off the rain-fall in greater abundance than would be the case in more level districts. Secondly, on mountains the rain-fall is usually more abundant than it is on plains, and it is more evenly distributed over the year; so that the natural surface of the ground does not become so absorbent, nor does the evaporation from that surface take place with the same rapidity, as it would do in the low lands. 'It thus happens that the quantity of water, which may usually be calculated as being likely to find its way into a reservoir placed in a mountainous district, may be reckoned as being about one half of the total rain-fall; but the net quantity disposable will be often very inferior to the quantity:thus originally impounded, because there must always be an active evaporation going on from the water surface, (which, moreover, will take place to the grbatest extent when the water is most wanted) and there must always be a more or less active infiltration through the bed of the reservoir itself. As it will be
necessary to discuss the effects of these conditions of evaporation and infiltration under the article 'WATER SUPPLY, on account of their influence upon gravitation water-works, it may suffice here to say that it is very rarely indeed that the engineer depend upon com manding more than from 1 to / of the total quantity of the rain-fall of his drainage area ; and that it is only exceptionally that he can attain the higher limit above quoted.
Another very important consideration will be found to affect the dimensions of reservoirs, which has only been cursorily noticed above, namely the distribution of the rain-fall over the year. In mammy cases It Is known that rain does not fall for months together; and even in the Bagshot Heath district droughts of four and five months duration have taken place. A reservoir established in such a district must then contain ; hat, the total actual quantity required for the service it is intended to perform ; and it is to bo observed that during droughts the consumption of water must be nearly double that which would be required under normal circumstances, whilst the reservoir must be made large enough to supply the largest consumption, not the arcraye one. 2nd, it must contain, in addition to this quantity, enough water to compensate for the evaporation and infiltration, and to secure the water from Injurious chemical changes. In fact the water of large lakes even, unless constantly kept in motion by streams passing through them, is liable to become vapid end deficient in aeration ; whilst pond waters, in dry, hot weather, become positively nauseous, unless their volume should be very great indeed ; and vegetation, and the forma of animal life, develop themselves with singular activity under the same circumstances. Of course this latter consideration would not materially affect canal reservoirs, but it is an all impor tant one In the case of gravitation water-works, and the singular story of the introduction of the anacharis &Anagram into our canals, proves that it is expedient to watch the growth of aquatic plants even in canal reservoirk.