Contracted Vein, in Hydraulics, - a term denoting the diminution which takes place in the diameter of a stream of water issuing from a vessel at a short distance from the discharging aperture, owing to the particles nearest the periphery experiencing greater attrition than the rest, and being retarded; in consequence of which the least diameter of the stream is to the diameter of the orifice, on an average as 1 to the square root of 2; and in computing the discharge of fluids, this proportion should be substituted for the real aperture.