by the explosion of a shell. This idea is enforced by the look of the
outer portion farthest from the bright half of the nebula, where
sharply edged clouds with dark spaces behind seem to be billowing away
as if driven by a wind blowing from the center.

Next let us consider what scientific speculation has done in the
effort to explain these mysteries. Laplace's hypothesis can certainly
find no standing ground either in the Orion Nebula or in those of a
spiral configuration, whatever may be its situation with respect to
the grand Nebula of Andromeda, or the ``ring'' and ``planetary''
nebulæ. Some other hypothesis more consonant with the appearances must
be found. Among the many that have been proposed the most elaborate is
the ``Planetesimal Hypothesis'' of Professors Chamberlin and Moulton.
It is to be remarked that it applies to the spiral nebulæ
distinctively, and not to an apparently chaotic mass of gas like the
vast luminous cloud in Orion. The gist of the theory is that these
curious objects are probably the result of close approaches to each
other of two independent suns, reminding us of what was said on this
subject when we were dealing with temporary stars. Of the previous
history of these appulsing suns the theory gives us no account; they
are simply supposed to arrive within what may be called an effective
tide-producing distance, and then the drama begins. Some of the
probable consequences of such an approach have been noticed in Chapter
5; let us now consider them a little more in detail.

Tides always go in couples; if there is a tide on one side of a globe
there will be a corresponding tide on the other side. The cause is to
be found in the law that the force of gravitation varies inversely as
the square of the distance; the attraction on the nearest surface of
the body exercised by another body is greater than on its center, and