192 Prof. Magnus's Hydraulic Researches. 



moderate force, and only deviates as far as the section of the 

 falling jet requires. The water can then draw itself up the jet, 

 as Prof. Tyndall* has observed, just as in a glass rod. But if a 

 sufficiently strong pressure be employed, and the cup-shaped 

 cavity is produced, it is manifest that it can only remain so long 

 unchanged as the water in which it is produced does not disturb 

 its form by other motion. As soon as this is the case, the air 

 which it encloses is carried down with the water into the liquid. 



100. If a water-jet from a perfectly circular aperture 3 millims. 

 in diameter, and under a pressure of a column of water 2 or 3 

 metres high, be allowed to fall vertically on the smooth surface 

 of water contained in a vessel O'^'G in height and width, and 

 if care be taken that the jet meets the surface (which must 

 be only a few centimetres from the efflux orifice) when the former 

 is quite clear and transparent, the above-mentioned cavity can 

 be distinctly, and for some time seen. But after some time 

 the water in the vessel into which the jet falls always begins to 

 move. This motion is very perceptible in particles of dust or 

 very fine bubbles which float on the surface. They are seen to 

 move slowly in a circle about the place where the jet meets the 

 water. This rotation becomes more rapid, and at a certain velo- 

 city a cavity is formed which is drawn downwards in the liquid in 

 screw fashion, and carries with it innumerable small air-bubbles. 



101. The formation of this screw-shaped cavity is promoted 

 by setting in rotation, in any other manner, the liquid into which 

 the jet falls. If this be done about the place where the jet meets 

 the surface, the cavity winds like a screw either to the right or 

 left, according as the rotation has the one or the other direction. 



102. As this screw-shaped cavity can be formed by setting the 

 water in rotation, so it can be stopped by preventing the rotation. 

 If a solid plane, as a thin board or a metal sheet, be held vertically 

 in the water into which the jet falls, so that one of its edges is 

 parallel to, but at a small distance from, the axis of the jet, the 

 screw-shaped cavity is not formed, or if already formed, it dis- 

 appears. 



103. In order to be able to produce or stop this rotation more 

 certainly, I made use of the tranquilizer described in § 38. If 

 this be so placed in the water that the jet falls inside the space 

 abcdfg, fig. 13, the rotation is prevented, and no screw-shaped 

 cavity is formed. But if the tranquilizer be rapidly turned about 

 the prolongation of the jet, and the water be thus set in rotation, 

 the cavity is formed immediately, and lasts as long as the rota- 

 tion, but disappears as soon as the apparatus is held firm. 



104. If, instead of the tranquilizer, the water be moved in any 

 other way, it is only seldom that it rotates exactly about the pro- 



* Phil. Mag. S. 4. vol. i. p. 105. 



