HYDRAULICS, that branch of engineer ing science which deals with liquids (especially water) in motion, the regulation of the Row of liquids, and utilization of the momentum, pres sure, weight, etc., as when confined in pipes and channels, for the operation of machinery, or doing useful work.
Water being the most common liquid, and having considerable weight pounds to the cubic foot), is excellently adapted to do work by its gravity. When allowed to run it acquires momentum, a quality available for the con venient use of the designer of hydraulic mechanisms. Being of very slight elasticity, that is, practically incompressible, it can be used when confined with great advantage, as we note in the hydraulic press and similar machines. Flowing as it does in streams, in almost every locality, it is comparatively easy to direct it for the service of man.
The density of water is apparent from the fact that it supports heavily laden ships, and from the tremendous power with which large waves driven at great velocity by high winds beat upon a shore or breakwater, exerting a pressure in the case of a 30-foot wave of 7,000 pounds per square foot. This density will be better appreciated after studying this table showing feet a minute or 4.4 feet per second. Since 33,000 pounds raised one foot in a minute is the horse power equivalent, it is obvious that this gives a basis for figuring the horse power. If a dam is built, there will be normally 250X264 cubic feet of water weighing pounds per foot coining over the dam every minute, that is, 4,114,000 pounds, a potential of nearly 125 horse power. This water to be utilized may be con veyed by a level mill race to a point where it can fall over an overshot waterwheel, and thus be made to yield from 70 to 90 per cent of its potential horse power, depending upon the me dianical efficiency of the wheel. If the same water were taken from the bottom of the dam to operate a turbine wheel a higher efficiency would be gained. Such a large volume of water as that supposed-1,000 cubic feet per second would suffice to run several ordinary wheels.
The actual horse power developed will be considerably less than the theoretical, owing to friction and imperfect machinery. About 75 per cent of the theoretical is generally consid ered as available for use in practice. Since
no stream flows uniformly, and since the power will be used intermittently, it is obviously neces sary to provide a large storage reservoir for the power plant if the approximate value of i the power is to be obtained. The engineer must therefore calculate how large a dam he needs and can build in that locality, and what it will cost, and how high this dam should be.
If the water in a stream is to be led away in a pipe, it is necessary to figure the carrying capacity of the pipe, for which the following table is useful: Sea water will weigh about one and a half pounds more than fresh water, on which the above table is based. Ice loses weight, or expands on freezing, weighing 57.28 pounds to the cubic foot. A cubic foot of water equals 7.48 United States gallons; a liter of water equals 0.264 United States gallons.
The volume and speed of flowing water in a stream constitutes its theoretical utility. To calculate the horse power available in a stream, it is first necessary to know its cross section. If it can be determined that the average depth is five feet and the average width 50 feet, then the cross section is 250 square feet. If the water flows three miles an hour, that is 264 The above figures are to be taken as approxi mate, because in each actual case there will be differences of friction, depending on the rough ness of the interior of the pipe, Water moving through a pipe at a certain velocity acquires momentum — that is the ve locity multiplied by its weight. When water is suddenly stopped in a pipe it delivers a blow that is called water-hammer, or more properly water-hammering. In a large or long pipe with rapid flow this momentum may be strong enough to rupture the pipe, hence it is extremely dangerous to shut off the flow in a large pipe. A similar effect is produced in pipes carrying great head. In one reported in stance the sudden closing of an automatically balanced valve in a short 42 inch pipe, wider a head of 25 feet, caused a hammer equivalent to a head of 250 feet, and sprung the rivets, causing a leak in a %-inch steel casing. In another instance, with a' 24-inch pipe, a valve was closed by hand in 20 seconds, and the pres sure, measured 2,100 feet away, was found to oscillate a number of times between the ex tremes of 40 and 65 pounds.