beginning of a solar system. The student may
form some idea of how readily local centres may be produced in
materials disseminated in the vaporous state by watching how fog or
the thin, even misty clouds of the sunrise often gather into the
separate shapes which make what we term a "mackerel" sky. It is
difficult to imagine what makes centres of attraction, but we readily
perceive by this instance how they might have occurred.

When the materials of each solar system were thus set apart from the
original mass of star dust or vapour, they began an independent
development which led step by step, in the case of our own solar
system at least, and presumably also in the case of the other suns,
the fixed stars, to the formation of planets and their moons or
satellites, all moving around the central sun. At this stage of the
explanation the nebular hypothesis is more difficult to conceive than
in the parts of it which have already been described, for we have now
to understand how the planets and satellites had their matter
separated from each other and from the solar centre, and why they came
to revolve around that central body. These problems are best
understood by noting some familiar instances connected with the
movement of fluids and gases toward a centre. First let us take the
case of a basin in which the water is allowed to flow out through a
hole in its centre. When we lift the stopper the fluid for a moment
falls straight down through the opening. Very quickly, however, all
the particles of the water start to move toward the centre, and almost
at once the mass begins to whirl round with such speed that, although
it is working toward the middle, it is by its movement pushed away
from the centre and forms a conical depression. As often as we try the
experiment, the effect is always the same. We thus see that there is
some principle which makes particles of fluid that tend toward a
centre fail directly to attain it, but win their way thereto in a
devious, spinning movement.

Although the fact is not so readily made visible to the eye, the same
principle is illustrated in whirling storms, in which, as we shall
hereafter note with more detail, the air next the surface of the earth
is moving in toward a kind of chimney by which it escapes to the upper
regions of the atmosphere. A study of cyclones and tornadoes, or even
of the little air-whirls which in hot weather lift the dust of our
streets, shows that the particles of