TANK. The sense usually attached to this word, in England, is that of a rather large vessel, for holding water or other liquids, either placed above or below the ground ; in India, the word is applied to the receptacles formed for the purpose of storing rain-water, some of which have an area of 364 (Uilwara) or 437 acres (Kalingur), with enclosure dykes of 38 feet In height.
Tanks intended to hold rain-water collected from houses, or the drainage water of farm yards in agricultural districts, are rarely made of a greater capacity than 50 or CO 'cubic yards; and they are con structed of the most impermeable hydraulic masonry that can be obtained ; but prtssautions must always be taken that the materials so employed should not be of a nature to Affect the chemical properties of the waters. Brickwork set in Roman cement is, for instance, pre ferable for such muss to masonry of limestone, set in ordinary lime mortar; and a backing of good clay puddle ie preferable to one made of concrete. Provision must be made for fixing the suction pipes of the pumps, for connecting the inlet and overflow pipes, and for menus of access for examination and repair ; ventilation pipes must also be provided. As it almost always happens that dead leaves, or other organic impurities, are carried into tanks of the description under con sideration, it is advisable, if the water be required for domestic use, to provide some moans of filtration for the water, and this object is effected in many very Ingenious manners, two of which may here be mentioned. ]or instance, in the great chalk plateau of Upper Normandy the rain water tanks arc sunk into the boulder clay, or the chalk, as the case may be, and they are made of sufficient dimensions to allow of the formation of a subsidiary lateral chamber, shut off from the body of the tank by a cress wall of it porous materinl, through which the water from the larger chamber filters into the sub sidiary one ; the suction pipe being placed in the latter, and the inlet pipes in the larger chamber. The other description of rain water task is used in Venice, and it is formed by firstly making a large water tight enclosure, usually circular in plan, and then an inner, concentric wall is built, leaving a large annular space between them : this annular space is filled in to a certain height with filtering materials of sand, gravel, ttc., through which the water passes until it reaches a series
of holes made in the lower portion of the inner wall ; it then accumulates in this species of well, from whence it is drawn by means of buckets. It must, however, always be observed that rain waters so stored are not of a proper quality for human consumption, and that they should be exclusively applied to washing, or to analogous household purposes.
The large vessels used for storing oils, or for the various operations of manufacturing chemistry, are made of iron, wood lined with lead, of zinc, or of other materials according to the nature of the liquids to be contained. The principles of their construction are derived from the ordinary laws of HYDROSTATICS.
As to the Indian cateh-water tanks it may be observed that they are in fact Resenvolus, and the remarks made under that head will apply to them. Generally speaking the tanks in question are principally used for irrigation, and their dams are formed of earth-work faced towards the up stream with stone, either laid dry, or with mortar. They are provided with overflow dams, escape weirs, sluices, and all the ordinary appliances of reservoirs; and they would appear to have been constructed upon the same principles at the present century as they were in the remotest antiquity. Much information on this sub ject is to be found in General Baird Smith's works on and in the Selections from the Records of the Bombay Government,' or in the 'Selections from Public Correspondence,' published by authority of the late East India Company.
Agricultural Tanks are sometimes large open receptacles, or ponds, formed by excavating the ground and disposing the removed earth in the form of banks to retain the water ; but the tanks which will here be especially treated of are the smaller covered reservoirs used to collect and retain liquid manure.