Fig. 593.
A tank is an artificial receptacle for liquids, thus: a tank engine is one which carries the water and fuel it requires, thereby dispensing with a tender; tank-iron or steel is common plate used in building tanks. Steel is cheaper than sheet-iron. A cistern is primarily a natural reservoir—a hollow place containing water; more commonly an underground reservoir or tank. Closed pressure tanks are usually cylindrical shells similar to a horizontal steam boiler, having bumped or rounded heads to save bracing. Closed pressure tanks are used extensively in connection with hydraulic elevators; the requisite pressure for these was formerly derived from an open tank installed upon the roof of the building, but the closed pressure tank, located in the engine room, now very generally takes the place of the open tank.
A closed pressure tank is shown in Fig. [596] in use with a hydraulic elevator.
A reservoir is a place where water is collected and kept for use when wanted, so as to supply a fountain, a canal or a city by means of aqueducts or to drive a mill-wheel or the like.
Fig. 594.
A receiving reservoir is a principal reservoir into which an aqueduct or rising main, delivers water and from which a distributing reservoir draws its supply.
A graduated tank is one fitted with water gauges and indicating marks, at different heights, between which, the capacity of the tank is shown.
A ship’s ballast tank is the compartment for water to be pumped in and out for the purpose of insuring the proper stability of the vessel, to avoid capsizing and to secure the greatest effectiveness of the propelling power.