WORKS TO ASSURE SUPPLIES OF WATER did not long remain content with the sup plies river or well provided him with. He must early have realized the advantage of storing water against times of shortage, as rivers and wells run low if they do not even run dry. At what date reservoirs first came into use as an additional aid to a regular supply is unknown, the remains of ancient reservoirs throughout the world, however, testify to the antiquity of the idea ; the earliest form probably consisted of earthen embanked reservoirs, or tanks as they are known in India. The tank suffers from the disadvantage of retaining all the silt contained in the water stored in it. In time this leads to practical obliteration and a new tank has to be constructed elsewhere. Reservoirs with masonry dams
also were built. These too became filled with silt in time, and not until recent years, and until the Periyar dam was built in India, with sufficient sluiceway near its base to pass all the flood waters and only arrest the later clean supply, was the problem met and a reservoir constructed which, in a silty river area, could be depended upon to keep itself clear of detritus. A magnificent example is the Aswan dam on the Nile in Egypt where the whole of the heavy flood passes through the building without dropping any of its silt content and only clear water is stored; thus guaran teeing a practically perpetual life to the reservoir notwithstand ing the vast volume of water it annually contains.