194 Prof. Magnus's Hydraulic Researches. 



depends on the strength of the spu'al winding of the jet, as well 

 as on the pressure with which it acts on the liquid. 



109. If an aperture of 3 millims. diameter in a thin wall be 

 so used that the pressure under which the water issues can be 

 increased at pleasure to the pressure of a column of water 2™'5 

 high, then if the jet under a small pressure meets the surface at 

 the distance of a few centimetres from the orifice, no bubbles 

 penetrate into the water, even if it be set in motion or rotation 

 in any other manner; but if the pressure is increased, the jet 

 changes, the spiral-shaped cavity is formed, and air-bubbles 

 penetrate into the liquid. If the rotation of the water be then 

 prevented by means of the tranquilizer, no air-bubbles penetrate, 

 even under the application of a pressure of 2°'"5. But if the 

 efflux aperture be changed for a wider one, for instance of 9 mil- 

 lims. diameter, the jet sets the water in such a whirlpool-shaped 

 motion that the tranquilizer has no effect, and it is impossible to 

 prevent the penetration of air-bubbles. 



110. If the water be met by the jet when it is no longer con- 

 tinuous, air always penetrates ; but since the place where the jet 

 ceases to be continuous depends on the vibrations communicated 

 to the vessel from which it flows (§ 86), and since such vibra- 

 tions may be produced by the most insignificant circumstances, 

 it may easily occur, that, while everything seems unchanged, 

 air-bubbles suddenly penetrate. I will mention an experiment 

 very easy to perform, but which is very surprising. 



111. If from a vessel of moderate size (that used was about 

 O'^'S in diameter and height, and stood upon a not very firm 

 frame) a jet be allowed to flow through an orifice 3 millims. in 

 diameter, and if it be caught in a glass held in the hand, the 

 bubbles are first seen, if the glass is gradually lowered, where 

 the jet ceases to be continuous. If the glass be held at a some- 

 what higher place where no air-bubbles penetrate, and if, when 

 the first glass is full, the excess of water be caught in a second, 

 likewise held in the hand, no air-bubbles are seen to penetrate 

 into the water of the first glass. If the water fi'om the second 

 be let fall into a tin vessel standing on the ground, a noise is 

 produced, and air-bubbles penetrate into the first glass. Hence 

 by an easy movement of the second glass, which is quite sepa- 

 rated from every other glass, air can be made to penetrate or not 

 at pleasure into the first, which remains fixed in its place. It is 

 very surprising to see bubbles making their appearance in the 

 first glass as soon as a small quantity of water falls into the 

 lowest tin vessel, and disappearing again as soon as the falling 

 of the water ceases. 



