Hydraulics
The science of hydraulics treats of liquids in motion, whether issuing from orifices, or running in pipes or the beds of streams. If an opening be made in the side or bottom of a vessel containing a liquid, as molten metal, etc., the latter will at once be forced through it, as the particles at that point are acted upon by the pressure of those above. The rapidity of a stream flowing out of an orifice depends upon the depth of the latter below the surface of the liquid. A liquid issues from a given orifice with equal velocity as long as the liquid is kept at the same height in the vessel, but if the pressure is diminished by a lack of supply above, the liquid gets lower, with a proportionate diminution in the velocity of the stream. The weight of water or molten metal is as the quantity, but the pressure exerted is as the vertical height. Fluids exert an equal pressure in every direction; hence any vessel containing a fluid sustains a pressure equal to as many times the weight of the column of greatest height of that fluid as the area of the vessel is to the sectional area of the column. See PRESSURE OF MOLTEN METAL; WEIGHTING COPES; HYDROSTATIC BELLOWS.