mind, Lord Kelvin (then Sir William Thomson) was shown by Professor
P. G. Tait, of Edinburgh, an apparatus constructed for the purpose
of creating vortex rings in air. The apparatus, which any one may
duplicate, consisted simply of a box with a hole bored in one side, and
a piece of canvas stretched across the opposite side in lieu of boards.
Fumes of chloride of ammonia are generated within the box, merely to
render the air visible. By tapping with the band on the canvas side
of the box, vortex rings of the clouded air are driven out, precisely
similar in appearance to those smoke-rings which some expert
tobacco-smokers can produce by tapping on their cheeks, or to those
larger ones which we sometimes see blown out from the funnel of a
locomotive.

The advantage of Professor Tait's apparatus is its manageableness and
the certainty with which the desired result can be produced. Before Lord
Kelvin's interested observation it threw out rings of various sizes,
which moved straight across the room at varying rates of speed,
according to the initial impulse, and which behaved very strangely when
coming in contact with one another. If, for example, a rapidly moving
ring overtook another moving in the same path, the one in advance seemed
to pause, and to spread out its periphery like an elastic band, while
the pursuer seemed to contract, till it actually slid through the
orifice of the other, after which each ring resumed its original size,
and continued its course as if nothing had happened. When, on the other
hand, two rings moving in slightly different directions came near each
other, they seemed to have an attraction for each other; yet if they
impinged, they bounded away, quivering like elastic solids. If an effort
were made to grasp or to cut one of these rings, the subtle thing shrank
from the contact, and slipped away as if it were alive.

And all the while the body which thus conducted itself consisted simply
of a whirl in the air, made visible, but not otherwise influenced, by
smoky fumes. Presently the friction of the surrounding air wore the
ring away, and it faded into the general atmosphere--often, however, not
until it had persisted for many seconds, and passed clear across a large
room. Clearly, if there were no friction, the ring's inertia must make
it a permanent structure. Only the frictionless medium was lacking to
fulfil all the conditions of Helmholtz's indestructible vortices. And
at once Lord Kelvin