olid nucleus; that is all we can say.

The planet Saturn seems to be very much like that of Jupiter in its
composition. It receives so little heat from the sun that, unless it is
a mass of fiery vapor like Jupiter, the surface must be far below the
freezing-point.

We cannot speak with such certainty of Uranus and Neptune; yet the
probability seems to be that they are in much the same condition as
Saturn. They are known to have very dense atmospheres, which are made
known to us only by their absorbing some of the light of the sun. But
nothing is known of the composition of these atmospheres.

To sum up our argument: the fact that, so far as we have yet been able
to learn, only a very small proportion of the visible worlds scattered
through space are fitted to be the abode of life does not preclude the
probability that among hundreds of millions of such worlds a vast
number are so fitted. Such being the case, all the analogies of nature
lead us to believe that, whatever the process which led to life upon
this earth--whether a special act of creative power or a gradual course
of development--through that same process does life begin in every part
of the universe fitted to sustain it. The course of development
involves a gradual improvement in living forms, which by irregular
steps rise higher and higher in the scale of being. We have every
reason to believe that this is the case wherever life exists. It is,
therefore, perfectly reasonable to suppose that beings, not only
animated, but endowed with reason, inhabit countless worlds in space.
It would, indeed, be very inspiring could we learn by actual
observation what forms of society exist throughout space, and see the
members of such societies enjoying themselves by their warm firesides.
But this, so far as we can now see, is entirely beyond the possible
reach of our race, so long as it is confined to a single world.

VIII

HOW THE PLANETS ARE WEIGHED

You ask me how the planets are weighed? I reply, on the same principle
by which a butcher weighs a ham in a spring-balance. When he picks the
ham up, he feels a pull of the ham towards the earth. When he hangs it
on the hook, this pull is transferred from his hand to the spring of
the balance. The stronger the pull, the farther the spring is pulled
down. What he reads on the scale is the strength of the pull. You know
that this pull is simply the attraction of the earth on the ham. But,
by a universal law of