STUDIES OF VORTEX-RINGS. 183 



(Fig. 5) will enable us to study, with the help of tobacco-smoke, how 

 little influence the shape of the opening has upon the rings which we 

 drive out through the hole at which we have tilled the box. We may 

 make the hole square or rectangular ; or, if we cut certain combina- 

 tions of long slits or several holes, those composite forms which Thom- 

 son has described as vibrating rings will be produced. 



If two rings are sprung in succession, they will constantly tend to 

 overtake each other and cross each other alternately in a curious play 

 of approaches and withdrawals, but always with some damage to each 

 other if they are large ; and this property is best studied in liquids. 

 Two equal vortices shot from, different boxes will play with each other 

 indefinitely, and the same happens wdien they strike the Avail. But, 

 if they pass each other, only grazing their edges, they will merely 

 change shape without breaking and without getting out of the way, 

 and then immediately bound back to their original form like an India- 

 rubber spring. This power of resistance, or power of annular elas- 

 ticity, is the most striking characteristic of these singular forms, real 

 magazines of energy, in w r hich all the living primitive force collects 

 itself under the influence of external friction. Theoretically, the vor- 

 tical movement, in a perfect fluid, can neither be created nor destroyed ; 

 eternal as matter itself, it has neither beginning nor end. The vortex- 

 ring is indivisible. Attack it as quickly as you will, says Professor 

 Tait, cut at it with the sharpest knife, you can never sever it or mar 

 it ; but, flying away or enveloping the material object, it remains 

 whole, always itself ; and, when we consider its wonderful properties, 

 we need hardly be surprised that philosophers have aspired to build 

 up upon it theories of the constitution of matter, and of the origin of 

 all the physical forces, from gravitation to electricity. 



We conclude with the citation of an extremely simple experiment 

 described by Helmholtz : If we draw the rounded end of a spoon or a 

 knife over the surface of a liquid, we may produce all around a vorti- 

 cal agitation giving the form of a vertical half-ring, the internal con- 

 tour of which corresponds with the semicircular edge of the solid 

 object, while on the right and left, on the surface of the liquid, appear 

 two portions of the same diametrical section, which should reproduce 

 the schemes of Fig. 2, C. Quite plain, in effect, are the scrolls de- 

 signed on the dark surface of a cup of coffee by the white trains of 

 the cream ; and it is very easy to study thus all the mutual reactions 

 of these vortices, similar as they are to those which the waterman sees 

 running along the edge of his oar, and which can be reproduced of 

 satisfactory dimensions on the surface of a bath-tub. Such experi- 

 ments are within the reach of every one. La Nature. 



