AJUTAGE

, or Adjutage, in Hydraulics, part of the apparatus of a jet d'eau, or artisicial fountain; being a kind of tube fitted to the aperture or mouth of the cistern, or the pipe; through which the water is to be played in any direction, and in any shape or figure.

It is chiefly the diversity in the ajutage, that makes the different kinds of fountains. So that, by having several ajutages, to be applied occasionally, one fountain is made to have the effect of many.

Mariotte, Gravesande, and Desaguliers have written pretty fully on the nature of ajutages, or spouts for jets d'eau, and especially the former. He affirms, from experiment, that an even polished round hole, made in the thin end of a pipe, gives a higher jet than either a cylindrical or a conical ajutage; but that, of these two latter however, the conical is better than the cylindrical figure. See his Traite du Mouvement des Eaux, part 4.

The quantity of water discharged by ajutages of equal area, but of different figures, is the same. And for like figures, but of different sizes, the quantity discharged, is directly proportional to the area of the ajutage, or to the square of its diameter, or of any side or other linear dimension: so, an ajutage of a double diameter, or side, will discharge 4 times the quantity of water; of a triple diameter, 9 times the quantity; and so on; supposing them at an equal depth below the surface or head of water. But if the ajutage be at different depths below the head, then the celerity with which the water issues, and consequently the quantity of it run out in any given-time, is directly proportional to the square-root of the altitude of the head, or depth of the hole: so at 4 times the depth, the celerity and quantity is double; at 9 times the depth, triple; and so on.

It has been found that jets do not rise quite so high as the head of water; owing chiefly to the resistance of the air against it, and the pressure of the upper parts of the jet upon the lower: and for this reason it is, that if the direction of the ajutage be turned a very little from the perpendicular, it is found to spout rather higher than when the jet is exactly upright.

It is sound by experiment too, that the jet is higher or lower, according to the size of the ajutage: that a circular hole of about an inch and a quarter in diameter, jets highest; and that the farther from that size, the worse. Experience also shews that the pipe leading to the ajutage, should be much larger than it; and if the pipe be along one, that it should be wider the farther it is from the ajutage.

For the other circumstances relating to jets and the issuing of water under various circumstances, see EXHAUSTION, Flux, Fountain, Jet d'Eau, &c, to which they more properly belong.