VORTEX MOTION, the motion of a fluid in a closed curve; it is unequal, that is, the fluid does not rotate as a mass. If two vortices of unequal size approach, the smaller floats through the larger one. One of the most striking .ex amples of vortex motion is seen in smoke rings emitted occasionally by locomotives and other high pressure steam-engines when exhausting slowly. Similar and more definite rings are easily produced also by devices now common in physical laboratories. Helmholtz, who was the first to investigate the theory of vortex motion, showed that vortex rings and filaments, or combinations thereof, in a perfect liquid (which is an ideal frictionless fluid of constant density) are indestructible. Extending this idea to per fect fluids, Lord Kelvin suggested that the atoms and molecules may be vortex rings or filaments, or combinations thereof,. in the ether. The mo tion of a mass of fluid is Icnown if the motion of every infinitesimal portion, or particle, of the mass is known. The motion of a particle
is investigated by. referring the successive posi tions it occupies m its journey to a set of three straight lines, or axes, which pass through the same point and are mutually rectangular. With respect to these axes the motion of the particle may assume either one of two characteristic types. It may move without rotation about either of the axes, or it may move with rota tion about one or more of the axes. The for mer type is called irrotational motion, and the latter type is called rotational motion, or vor tex motion. The vortex type of motion is by far the more common in nature and manifests itself in an infinite variety of ways. For the theory of vortex motion and references to the literature of the subject, consult Lamb, 'Hydro dynamics.)