When the source of a water supply is at a higher elevation than the point at which the water is to be delivered, and there are no intervening obstructions between the two points, the supply may be delivered by gravity.
When there is a hill or rise of land between the source and the point of delivery, however, the only method that may be employed is to convey the water by means of siphonage.
If the intervening elevation rises above the source to a height to which atmospheric pressure cannot force the water, the siphon cannot be made to work. Theoretically, the siphon will raise water to a height somewhat above 33 ft., but in actual practice, owing to friction and the lack of an absolutely perfect vacuum, this height cannot be reached by several feet. The great obstacle to obtaining a supply of water by siphonage is the accumulation of air at the high point or points on the supply line. This trouble may be remedied by the use of cocks located accessibly at the high points, through which to vent the collections of air. They must be opened frequently in order that the siphon may operate properly, and such constant attention is always a matter of inconvenience. If not given frequent attention, however, the siphon will soon cease entirely to deliver water.
There is comparatively little trouble experienced from air-lock in siphons that lift water through distances of 10 ft. or under, and empty it at a point low enough to develop a strong flow. Under such circumstances, the air mixed with the water is carried along with it. In the case of lifts much greater than 10 ft., however, air begins to give trouble, the trouble increasing rapidly as the lift is increased, especially when the crown of the siphon is sharp and unable to contain much air. Under the latter conditions, the siphon will cease working in a very few hours. Another method sometimes employed to relieve the siphon of air, is the placing of an air pump on the crown of the siphon, for use in pumping out the air that may have collected. The interval between successive operations of such a' pump cannot be definitely stated, as the nature of the water sometimes affects this matter, as well as the height through which the water is raised.
There is still another method, more effective than either of those already described, which may be applied as follows. A connection should be made at the top of the siphon into a galvanized sheet-iron tank of 2 or 3 gallons capacity. Between this tank and the siphon a shut-off is located, and also one above it, a funnel being soldered into the upper end of it. Close the lower cock and open the upper one to allow water to be poured in, which should fill the tank and the funnel.
If the upper cock is then closed and the lower one opened, the water will drive out the air in the siphon and maintain the siphon in this condition until the tank becomes empty. When the tank has drained out, close the lower cock, open the upper one, and refill the tank. Now again open the lower cock and close the upper one, and the tank is prepared to perform its work as a receiver for the air that accumulates at the crown of the siphon.
By the use of such a device as this, the siphon may be kept free of air for a considerable length of time. The larger the tank used, the longer the interval between the successive fillings. Galvanized wrought-iron pipe and galvanized cast-iron fittings are better suited for siphons than other materials.
The use of cast iron with caulked lead joints for large siphons, is very poor policy, as experience shows that much difficulty is experienced in keeping it air-tight, a very essential feature in the proper operation of a siphon.
The siphon may be made to cover a very wide range of work, as siphons of large size may be used as successfully as those of smaller size. In the use of large-pipe siphons, however, it is necessary to use special starting apparatus and to provide for constant attention to the removal of air at all high points. These siphons have been made to carry water through distances of many miles. The same principle is successfully applied to the disposal of sewage under similar circumstances, large amounts of sewage being thus handled.