=66. Hydraulic Ram.=--The hydraulic ram (see Fig. 47) is an automatic device that is much used for raising water from springs to houses located on higher ground. Water flows through the pipe _A_ through the opening at _B_. The pressure closes the valve at _B_. The increased pressure in the pipe due to the closing of _B_ opens the valve _C_ and some of the water flows into the air chamber _D_. This reduces the pressure against the valve _B_ so that it drops and allows a little water to escape. Just as this happens, valve _C_ closes. The pressure in the pipe then closes _B_ and forces water past _C_. This action being continually repeated, the air in _D_ becomes so compressed that it has elastic force enough to raise the water in a steady stream to a height of many feet. [Illustration: FIG. 47.--Cross-section of a hydraulic ram.] =67. The Balloon.=--Since air is a fluid, Archimedes' principle applies to it as well as to liquids. Therefore any object in the air is lifted up by a force equal to the weight of the air it displaces. The object will rise, if it weighs less than this displaced air and will continue to rise until both weights are equal. _The Balloon_ (Fig. 48) rises because it weighs less than the air it displaces, and therefore it is pushed up by the heavier air, the "lifting power" being the difference between its weight and that of the air displaced. The neck at the bottom is left open to allow for expansion of the gas. When the aeronaut wishes to descend, he opens a valve at the top allowing some of the gas to escape. [Illustration: FIG. 48.--Winner of international championship race, Paris, 1913.] _Hydrogen_ is the lightest gas, weighing 0.09 kg. per cubic meter, and so gives the greatest lifting power, but as it is expensive to make, coal gas, density 0.75 kg. per cubic meter, is ordinarily employed. Helium has recently been used to fill military balloons because it cannot be set on fire. _The Parachute_ (Fig. 49) is an umbrella-shaped device for use in descending from a balloon. After falling a few seconds it opens, the large surface exposed to the air causing it to descend slowly. The hole in the top keeps the parachute upright by allowing the air to escape through it, thus relieving the pressure. [Illustration: FIG. 49.--A parachute.] [Illustration: FIG. 50.--Cross-section of a Westinghouse air brake.]