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card. Ordinary experience teaches that in such a case the flow does not follow the walls round the corner, but shoots across as a jet, which for-a time preserves its individuality and something like its original section. Since the velocity is not lost, the pressure which would replace it is not developed. It is instructive to compare this case with another, experimented on by Savart* and W. Froude f, in which a free jet is projected through a short cone, or a mere hole in a thin wall, into a vessel under a higher pressure. The apparatus consists of two precisely similar vessels with, apertures, in which the fluid (water) may be at different levels (fig. 7, copied from Froude). Savart found that not a single drop of liquid was spilt so long as the pressure in the recipient vessel did not exceed one-sixth of that under which the jet issues. And Froude reports that so long as the head, in the discharge cistern is maintained at a moderate height above that in the
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Fig. 7.
recipient cistern, the whole of the stream enters the recipient orifice, and there is " no waste, except the small sprinkling which is occasioned by inexactness of aim, and by want of exact circularity in the orifices." I am disposed to attach more importance to the small spill, at any rate when the conoids are absent or very short. For if there is no spill, the jet (it would seem) might as well be completely enclosed; and then it would propagate itself into the recipient cistern without sudden expansion and consequent recovery of pressure. In fact, the pressure at the narrows would never fall below that of the recipient cistern, and the discharge would be correspondingly lessened. When a decided spill occurs, Froude explains it as due to the retardation by friction of the outer layers which are thus unable to force themselves against the pressure in front.
Evidently it is the behaviour of these outer layers, especially at narrow places, which determines the character of the flow in a large variety of cases.
* Ann. de Chimie, Vol. LV. p. 257, 1833. f Nature, Vol. xin.  p. 93, 1875.
R. VI.
16servation of energy;ory can give a satisfactory account.length.