JET D'EAU. A fountain which throws up water to some height in the air. Ac cording to the theory of hydrostatics, the velocity with which water issues from an orifice is equal to that which would be acquired by a heavy body in falling through a height equal to the difference between the levels of the orifice and the fountain head ; whence, if the resistance of the air and other impediments were removed, the height of the jet would be equal to that of the surface of the reser voir. Among the causes which prevent the jet from obtaining the height which theory assigns to it, the following are the principal : 1. The resistance of the air, which is proportional nearly to the square of the velocity. 2. The friction against the ' sides of the pipe and the orifi..e through which the water issues. 3. The velocity of the particles diminishing at every instant as they ascend, the lower particles of the ascending column press against those next above them ; and the pressure being by the nature of fluids communicated in all directions, the consequence is, that the column is enlarged and proportion ally shortened. 4. The water at the top
of the jet does not fall off instantaneously when its velocity is destroyed ; it rests for a moment at the top of the column, where its weight opposes an obstacle to the particles next succeeding, which re tards their velocity, and this retardation is communicated to the whole column. This last obstacle may be avoided by slightly inclining the jet from the verti cal; and it is found by experience that a jet so inclined plays higher than one quite upright, though the effect is thereby rendered less pleasing. It is necessary that the diameter of the adjutage or ori fice be considerably less than that of the pipe. (See Desagulier's Erperimental Phi losophy; Mariotte, ifouvement des Eaux.)