Prof. Magnus's Hydraidic Researches. 195 



Jets which contain air in their interior. 



113. Besides the cases already mentioned, in whicli air pene- 

 trates with the jet into the water, there is one case essentially- 

 different from these ; for if the water in the vessel from which 

 the jet issues begins to rotate, which easily happens when no 

 tranquilizer is employed, there is formed after some time a fun- 

 nel-shaped cavity. This often draws itself downwards, not 

 only to the efflux orifice, but even, when this is not too small, 

 beyond the orifice into the jet, which hence assumes a peculiar 

 appearance. If it issues from a circular orifice, and is without 

 dilatations, the air often draws itself down in it, changing 

 the jet into a tube, which becomes narrower with increasing di- 

 stance from the aperture ; but if the jet possesses dilatations, or 

 spiral-shaped windings, § 80, these appear filled with air, and it 

 has then the appearance of a spiral-formed hollow tube. When 

 the jet issues from a polygonal aperture, air is only drawn in, 

 if it has already assumed a spiral-shaped form in consequence of 

 rotation. This is seldom regular, and becomes more irregular 

 by drawing air in. Hence the form which it assumes is difficult 

 to determine, and it can only be said in general terms to be 

 similar to a spiral hollow jet from a circular orifice. 



113. If the hollow jet meets the surface of perfectly tranquil 

 water, the air contained in the jet does not penetrate far below 

 the surface as long 'as the motion is quite regular; but after 

 a short time the motion becomes irregulai-, rotation ensues, and 

 now the air goes down in little bubbles to a great depth with the 

 water. 



114. After this discussion of the different conditions under 

 which air penetrates into water, the so-called water-bellows, 

 which I have discussed in the appendix to a previous treatise 

 " On the Motion of Liquids," needs no further explanation. For 

 it is manifest, either that the air, by a funnel-shaped motion of 

 the water, gets into the jet, and is carried down with this into 

 the water of the pipes, or that the jet, without containing air, is 

 so set in motion where it reaches the water, that air is enclosed 

 and carried down with the water rushing in. 



115. Lookinp; at the preceding investigations, it will be con- 

 ceded that they give a clearer insight into the phpenomena of 

 efflux. The remarkable forms of jets are explained on the simple 

 laws of motion, of resistance, and of cohesion. The influence 

 which a motion in the vessel from which the jet issues exercises 

 upon its form, and the manner in which this influence is exerted 

 over the whole length of the jet, are so far exjjlained that it is 

 possible to produce at pleasure all the different forms of jet. 

 And not only this, but the reverse also : to determine from the 



