(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 combinations of long slits or several holes, those composite forms which Thomson 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 when 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 elasticity, is the most striking characteristic of these singular forms, real magazines of energy, in which all the living primitive force collects itself under the influence of external friction. Theoretically, the vortical 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 vortical agitation giving the form of a vertical half-ring, the internal contour 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 designed 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 experiments are within the reach of every one.—La Nature.