408 ON THE DISCHARGE OF GASES UNDER HIGH PRESSURES afterwards with greater rapidity, until it vanishes when the pressure come equal. The work of Saint-Yenant and Wantzel was fully discussed by Sto" his Report on Hydrodynamics*. He remarks "These experiments sho-\ when the difference of pressure in the first and second spaces is conside we can by no means suppose that the mean pressure at the orifice is to the pressure at a distance in the second space, nor even that there a contracted vein, at which we may suppose the pressure to be the sa; at a distance." But notwithstanding this the work of the French \\ seems to have remained very little known. It must have been unkno O. Reynolds when in 1885 he traversed much the same ground f, a< however, the important observation that the maximum reduced ve occurs when the actual velocity coincides with that of sound unde conditions then prevailing. When the actual velocity at the orifice re this value, a further reduction of pressure in the recipient vessel doe influence the rate of discharge, as its effect cannot be propagated back against the stream. If ry = T408, this argument suggests that the disc reaches a maximum when the pressure in the recipient vessel falls to -c and then remains constant. In the somewhat later work of Hugonic the same subject there is indeed a complimentary reference to Saint-V and Wantzel, but the reader would hardly gather that they had in upon the difference between the pressure in the jet at the orifice a the recipient vessel as the explanation of the impossible conclusion ded from the contrary supposition. In the writings thus far alluded to there seems to be an omissi consider what becomes of the jet after full penetration into the rec .The idea appears to have been that the jet gradually widens in section leaves the orifice and that in the absence of friction it would ultin attain the velocity corresponding to the entire fall of pressure. The fi deal with this question seem to have been Mach and Salcher§, but the elaborate examination is that of R. Emden|, who reproduces inter* pictures of the effluent jet obtained by the simple shadow method of Dvc Light from the sun or from an electric spark, diverging from a small api as source, falls perpendicularly upon the jet and in virtue of differen refraction depicts various features upon a screen held at some di< behind. A permanent record can 'be obtained by photography. E thus describes some of his results. When a jet of air, or better of ca: * S.A. Report for 1846; Math, and Phys. Papers, Vol. T. p. 176. t Phil. Mag. Vol. xxi. p. 185 (1886). t Ann. de Chim. t. ix. p. 383 (1886). § Wied. Ann. Bd. XLI. p. 144 (1890). |j Wied. Ann. Bd. LXIX. pp. 264, 426 (1899). IF Wied. Ann. Bd. ix. p. 502 (1879).e magnitude of the cohesion between liquids and solids, as well as of the particles of fluid with each other, is more directly shewn by an experiment on the continuance of a column of mercury, in the tube of a barometer, at a height considerably greater than that at which it usually stands, on account oi the pressure of the atmosphere. If the mercury has been well boiled in the tube, it may be made to remain in contact with the closed end, at the height of 70 inches or more " (Young's Lectures, p. 626,1807). If the mercury be wet, boiling may be dispensed with and negative pressures of two atmospheres are easily demonstrated.