Hinton - The Fourth Dimension.pdf

82
THE FOURTH DIMENSION
find the corresponding movement about a plane in
four space.
Now, of all the movements which take place in fluids,
the most important from a physical point of view is
vortex motion.
A vortex is a whirl or eddy—it is shown in the gyrating
wreaths of dust seen on a summer day; it is exhibited on
a larger scale in the destructive march of a cyclone.
A wheel whirling round will throw off the water on it.
But when this circling motion takes place in a liquid
itself it is strangely persistent. There is, of course, a
certain cohesion between the particles of water by which
they mutually impede their motions. But in a liquid
devoid of friction, such that every particle is free from
lateral cohesion on its path of motion, it can be shown
that a vortex or eddy separates from the mass of the
fluid a certain portion, which always remains in that
vortex.
The shape of the vortex may alter, but it always consists
of the same particles of the fluid.
Now, a very remarkable fact about such a vortex is that
the ends of the vortex cannot remain suspended and
isolated in the fluid. They must always run to the
boundary of the fluid. An eddy in water that remains
half way down without coming to the top is impossible.
The ends of a vortex must reach the boundary of a
fluid—the boundary may be external or internal—a vortex
may exist between two objects in the fluid, terminating
one end on each object, the objects being internal
boundaries of the fluid. Again, a vortex may have its
ends linked together, so that it forms a ring. Circular
vortex rings of this description are often seen in puffs of
smoke, and that the smoke travels on in the ring is a
proof that the vortex always consists of the same
particles of air.