Belas — On the Structure of Water-Jets. 205 



supporting the nozzle, a long portion of the stream may be 

 detached. (Plate XIX., fig. 2.) 



If now, instead of allowing the column to break up arbitrarily, 

 we impress upon it vibrations of a regular character, a marked 

 change takes place in its appearance. Placing a sounding tuning- 

 fork on the stand, the first thing to be noticed is that the jet begins 

 to separate into drops a good deal nearer the orifice, while lower 

 down (if the conditions be favourable) it assumes a beautiful wavy 

 outline, consisting of perfectly regular swellings and contractions. 

 On removing the fork from the stand, the jet regains its former 

 appearance, though the fork may be vibrating strongly, showing 

 that the vibrations transmitted through the air have little or no 

 effect. 



Let us now consider the cause of these swellings. 



If swellings are to be found in a jet, the regularity in the 

 separation of the drops must not only take place in such a manner 

 that (1) all the drops are equally great, but (2) the time that 

 elapses between the formation of any two of them must remain 

 constant. For it is only under these conditions that all drops on 

 arriving at the same part of the stream are in the same phase. 

 At the middle of a swelling they are drawn out to the greatest 

 extent in a horizontal direction ; and in the space between two 

 swellings, they are most elongated vertically. It is therefore 

 on account of their assuming these different diameters that they 

 produce the appearance referred to. 1 



On examining the photographs, we can see clearly what has 

 taken place. A large drop is cast off elongated horizontally ; next 

 follows a small one, formed by the narrow neck of liquid ; then a 

 large one ; but, by the time the second large drop is cast off, the 

 first has swung through half a vibration, and is now elongated in 

 a vertical direction. (Plate XX.) 



This regular alternation of drops is confined chiefly to the 

 upper portion of the stream. Lower down, collisions occur between 

 the large and small drops, while at the lowest part the small drops 

 have disappeared, and the large ones have all become more or less 

 spherical, the vibrations having died out. (Plate XXI.) 



In a jet possessing no swellings, the detached masses are not 



1 Magnus, "Hydraulic Researches," Phil. Mag., 1859, vol. xviii., p. 175. 



