All the cranes described above are shown as worked by hand-power, but they may be worked by steam, hydraulic machinery, or electricity. Manual power will be the most economical where the use of a crane is only occasional, but it would be too slow and costly where there is constant heavy work. Water-tanks.--A supply of good water forms an important item in railway working, and ample provision must be made at all principal stations for the requirements of engines and general station purposes. According to the locality, the water may either be procured from the main of some established waterworks company, or be pumped from a well, or forced up from a stream by a ram, or brought down by gravitation in pipes from a spring or stream at a distance. Water thus obtained is conducted into tanks placed at a height of 18 or 20 feet, or more, above the level of the rails, and forms a storage supply from which deliveries can be made at a fair pressure and in large volume. The tanks may be made of cast-iron, wrought-iron, or steel, or even of wood. In the great timber-producing countries abroad, water-tanks, some of them of large capacity, are very frequently made of wood, the circular or half-cask form being preferred; but at home, and on European lines generally, wooden tanks are rarely used except for temporary purposes. Cast-iron being less liable to deterioration from rust than wrought-iron or steel, is much used for water-tanks. [Illustration: Fig. 453, 454, 457, 455, 456, 461, 460, 459, 462, 463, 464, 458]