We have now to offer an explanation of the curious rings which are
the most characteristic features on the moon. To account for them we
must look for a moment at some objects on the earth. You have all
heard of volcanoes or burning mountains, such as Vesuvius or Etna,
which occasionally break out into violent eruptions, and send forth
great showers of ashes and torrents of molten lava. In the Sandwich
Islands there is a celebrated volcano called Kilauea. It is like a
vast lake of lava, so hot that it is actually molten, and glows with
heat like red-hot iron. The adventurous tourist who visits this crater
can climb to the brink of a lofty range of cliffs which surround it,
and gaze down upon the fervid sea beneath. Suppose that by some great
change the internal heat which keeps this mighty basin glowing were
to decline and go out, the sea of lava would cease to be liquid, and
would ultimately grow hard and cold, and we should then have an immense
flat plain, surrounded by a range of cliffs. Elsewhere in the Sandwich
Islands examples of extinct craters may be found at the present day.
Those who have studied these interesting localities point out how such
terrestrial craters explain the ringed plains in the moon. It seems
certain that in ancient days great volcanoes abounded on our satellite,
and the rings were often much larger than those on the Sandwich
Islands, some of them being one hundred miles or more in diameter.
The volcanoes must long ago have been raging on the moon with a fury
altogether unknown in any active volcanoes which this earth can now
show. We can also surmise how the lofty mountain peak, which so often
rises in the centre of a lunar ring, has been upheaved. When the fires
had almost subsided, and the floor had grown nearly cold, one last
and expiring effort is made by which the congealing surface is burst
through at the centre, and materials are shot forth which remain as the
central mountain to the present day.
I must, however, impress upon you that even our greatest telescopes
never exhibit to us any volcanic eruptions at present going on in the
moon; in fact, it is most doubtful if any change has been noticed in
the features on its surface since the date of the invention of the
telescope. The volcanoes sculptured the crust of the moon into the form
in which we see it, and that form our satellite has preserved for ages,
of which we cannot estimate the duration. All the craters and all the
volcanoes in the moon can only be described as extinct.
It would be interesting for us to compare the present condition of
the volcanoes in the earth with that of the ringed craters in the
moon. The noisy volcanoes on our globe are those most talked about; we
often hear of Vesuvius being in eruption, and in August, 1883, there
was a terrific eruption at Krakatoa, during which a large quantity
of dust was shot up into the air, to such a height that it was borne
right round the earth, and produced beautiful sunsets and unwonted sky
hues in almost every country in the world. The explosion at Krakatoa
made the loudest noise that history has recorded. Fortunately such
convulsions of the earth do not often happen, for, on that occasion,
the sea rushed in on the land, and thousands of lives were lost. There
are said to be one hundred and fifty volcanoes on different parts of
the earth, which are more or less active, but there are many others in
which the fire has gone out, and which seem to be just as cold and
just as extinct as any volcanoes in the moon. Even in our own islands
there are abundant remains of ancient volcanoes. Masses of lava are
found in many places where now there is no trace of an active volcano.
Perhaps there is no more remarkable sight in the British Isles than
that lofty rock which is crowned by Edinburgh Castle; it is the remnant
of a former volcano, while Arthur’s Seat, close by, is another. In
the centre of France is the beautiful district of Auvergne, in which
ancient volcanoes abound; and the lava streams can be traced for miles
across the country. These volcanoes have been extinct for thousands of
years, during which time the lava has become largely covered with soil
and vegetation, and in some places vineyards are cultivated upon it.
We are now able to contrast the earth with the moon, in so far as
volcanoes are concerned. On the earth we have some that are active,
and a much greater number that are extinct. On the moon we find no
active volcanoes, for there all are extinct. I can explain how this
difference has arisen, but first let me show you a simple experiment.
My assistant will kindly bring to me from that furnace two iron balls,
which we placed there before the commencement of this lecture; there
they are, you see, both glowing with a bright red heat, for at present
they are equally hot. We will place them on these stands, and allow
them to grow cold. One of these balls is a small cannon-ball, four
inches in diameter, while the other is only one inch. They are in the
same proportion as the earth is to the moon; but look, even while I am
speaking the balls have ceased to preserve the same temperature, for
the little one has become almost black from loss of its heat, while the
large one still looks nearly as red as it did at the beginning; this
simple experiment will illustrate the principle that two heated bodies
will cool at very different rates, if their sizes be different, while
the other conditions are the same. The small body will always cool
faster than the large one. They need not be globes for this experiment;
if you put a poker and a knitting needle into the fire, and leave both
there until they are red-hot, and then put them out into the fender,
you will speedily find that though they were at the same temperature
when drawn from the fire, they do not long remain so; indeed, the
knitting needle has become cold enough to handle before the poker has
ceased to glow. Our experiments have been made with, no doubt, small
objects only, but the law about which they inform us will remain true,
even for the greatest objects.
Our earth at the present day shows many indications of being much
hotter within than it is on the surface. The volcanoes themselves are
mere outbreaks of incandescent material from inside. Then there are
hot springs of water at Bath, which gush out from the earth. There
are geysers of hot water in Iceland and in the Yellowstone Park in
America, and in other places. And there are other indications also,
with which every miner is familiar. Wherever a deep pit is sunk into
the earth, the rocks below are always found to be warmer than those
on the surface, and the deeper the pit the greater is the heat that
is encountered. Thus, from all over the world we obtain proofs of
the present existence of internal heat. Great as is the earth, we
must still apply the simple common-sense principles that we use in
our everyday life here. Let me give an illustration. Suppose that a
servant came into the room and placed a jug of water on the table, and
that an hour afterwards you went to the jug of water and found it to
be cold, you would not from that fact alone be able to infer anything
with certainty, as to whether the water had been warm or cold when it
was brought in. It might have been perfectly cold, as it is at present,
though on the other hand the water might have been warm at first, and
have since cooled down to the temperature of the room during the hour.
Suppose, however, that when you went to the jug of water, which had
stood on the table for an hour, you found it tepid, no matter how
slightly its temperature might be above that of the room, do you not
see the inference you would be able to draw? You would argue in this
way: that water has still some heat; it must, of course, be gradually
cooling, and therefore it was hotter a minute ago than it is now; it
was hotter still two minutes ago, or ten minutes; and must have been
very hot and perhaps boiling when it was brought in an hour ago.
I want you to apply exactly the same reasoning to our earth. It is,
as I have shown you, still hot and warm inside. Of course, that heat
is gradually becoming lost; so that the earth will from year to year
gradually cool down, though at an extremely slow rate. But we must
look back into what has happened during past ages. Just as we inferred
that the jug must have contained very hot water an hour before from
the mere fact that the water was still warm, so we are entitled to
infer, from the fact that the earth still retains some heat, that it
must in ages gone by have been exceedingly hot. In fact, the further
we look back, the hotter do we see the earth growing, until at last we
are constrained to think of a period, in the excessively remote past,
long ere life began to dawn on this earth, when even the surface of the
earth was hot. Back further still we see the earth no longer covered
with the hard, the dark, and the cold surface we now find; we are to
think of it in these primitive times as a huge glowing mass, in which
all the substances that now form the rocks were then incandescent, and
even molten material.
There is good reason for knowing that in those early times the moon
also was molten with heat; and thus our reasoning has led us to think
of a period when there were two great red-hot globes--one of which had
about four times the diameter of the other--starting on their career
of gradually cooling down. Recall our little experiment with the two
cooling globes of iron; imagine these globes to preserve their relative
proportions, but that one of them was 8000 miles and the other 2000
miles across. Ages will, no doubt, elapse ere they part with their
heat sufficiently to allow the surfaces to cool and to consolidate. We
may, however, be sure that the small globe will cool the faster, that
its outside will become hard sooner than will the surface of the large
one, and long after the small globe has become cold to the centre,
the large one may continue to retain some of its primeval heat. We
can thus readily understand why all the volcanoes on the moon have
ceased--their day is over. It is over because the moon, being so small,
has grown so cold that it no longer sustains the internal fires which
are necessary for volcanic outbreaks. Our earth, in consequence of its
much greater size, has grown cold more slowly. It has no doubt lost the
high temperature on the exterior, and its volcanic energy has probably
abated from what it once was. But there is still sufficient power in
the subterranean fires to awaken us occasionally by a Krakatoa, or
to supply Vesuvius with sufficient materials and vigor for its more
frequent outbursts. The argument shows us that the time will at last
come when this earth shall have parted with so large a proportion of
its heat that it will be no longer able to provide volcanic phenomena,
and then we shall pass into the exhausted stage which the moon attained
ages ago.