If you look out from your window at the midnight sky, or take a walk
on a fine clear night, you will occasionally see a streak of light
dash over the heavens, thus forming what is called a falling, or a
shooting, star (Fig. 77). It is not really one of the regular stars
that has darted from its place. The objects we are now talking of are
quite different from stars proper. To begin with, the shooting stars
are comparatively close to us when we see them, and they are very
small, whereas the stars themselves are enormous globes, far bigger
than our earth, or often even bigger than the sun. Sometimes a great
shooting star is seen which makes a tremendous blaze of light as bright
as the moon, or even brighter still. These objects we call meteors,
and you will be very fortunate if you can ever see a really fine one.
Astronomers cannot predict these things as they predict the appearance
of the planets. Bright meteors consequently appear quite unexpectedly,
and it is a matter of chance as to who shall enjoy the privilege of
beholding them. But it is not about the great meteors that we are now
going to speak particularly; they are often not so interesting as the
small ones.
[Illustration: FIG. 77.--A Brilliant Meteor.]
These little meteoroids, as we shall call them, have a curious history.
They become visible to us only at the very last moment of their
existence--in fact, the streak of light which forms a shooting star is
merely the destruction of a meteoroid. You must always remember that
we are here living at the bottom of a great ocean of air, and above
the air extends the empty space. Air is a great impediment to motion;
a large part of the power of a locomotive engine has to be expended
solely in pushing the air out of the way so as to allow the train to
get through. The faster the speed, the greater is the tax which the
air imposes on the moving body. A cannon-ball, for instance, loses
an immensity of its speed, and consequently of its power, by having
to bore its way through the air. In outer space beyond the limits of
this atmosphere, a freedom of movement can be enjoyed of which we
know nothing down here. I spoke of this when discussing the movements
of Encke’s comet. Even this very unsubstantial body could dash along
without appreciable resistance until it traversed the atmosphere
surrounding the sun. But now we have to speak of the motion of a little
object both small and dense, resembling perhaps a pebble or a fragment
of iron, or some substance of that description. It is a little body
such as this which produces a shooting star.
For ages and ages the meteoroid has been moving freely through space.
The speed with which it dashes along greatly exceeds that of any of the
motions with which we are familiar. It is about 100 times as swift as
the pace of a rifle-bullet. About twenty miles would be covered in a
second. You can hardly imagine what that speed is capable of. Suppose
that you put one of these flying meteoroids beside an express train to
race from London to Edinburgh, the meteoroid would have won the race
before the train had got out of the station. Or suppose that a shooting
star determined to make the circuit of the earth, it might, so far as
pace is concerned, go entirely around the globe and back to the point
from which it started in a little more than twenty minutes. But the
fact is, you could not make any object down here move as fast as a
shooting star. No gunpowder that could be made would be strong enough,
in the first place, and even if the body could once receive the speed,
it would never be able to force its way through the air uninjured.
So long as a little shooting star is tearing away through open space
we are not able to see it. The largest telescope in the world would
not reveal a glimpse of anything so small. The meteoroid has no light
of its own, and it is not big enough to exhibit the light reflected
from the sun in the same manner as a little planet would do. It is
only at the moment when it begins to be destroyed that its visibility
commences. If the little object can succeed in dashing past our earth
without becoming entangled in the atmosphere, then it will pursue its
track with perhaps only a slight alteration in its path, due to the
pull exercised by the earth. The air which surrounds our globe may be
likened to a vast net, in which if any little meteor becomes caught
its career is over. For when the little body, after rejoicing in the
freedom of open space, dashes into air, immediately it experiences a
terrific resistance; it has to force the particles of air out of the
way so as to make room for itself, and in doing so it rubs against them
with such vehemence that heat is produced.
I am sure every boy knows that if he rubs a button upon a board it
becomes very hot, in consequence of the friction. There are many other
ways in which we can illustrate the production of heat in the same
manner. One is a contrivance by which we spin round rapidly a piece of
stick pressed against a board. Quantities of heat are thus produced
by the friction, and volumes of smoke rise up. We have read how some
savages are able to produce fire by means of friction in a somewhat
similar manner, but to do so requires a rare amount of skill and
patience. There is another illustration by which to show how heat can
be produced by friction. A brass tube full of water is so arranged that
it can be turned around very rapidly by the whirling table. We apply
pressure to the tube, and after a minute or two the water begins to get
hot, and then at last to boil, until ultimately the cork is driven out
and a diminutive and, fortunately, harmless explosion of the friction
boiler takes place. Engineers are aware how frequently heat is produced
by friction, when it is very inconvenient or dangerous. Indeed, unless
the wheels of railway carriages are kept well greased, the rubbing
of the axle may generate so much heat that conflagrations in the
carriage will ensue. Nature, in the little shooting star, gives us a
striking illustration of the same fact. Perhaps you may be surprised to
hear that the whole brilliancy of the shooting star is simply due to
friction. As the little body dashes through the air it becomes first
red-hot, then white-hot, until at last it is melted and turned into
vapor. Thus is formed that glowing streak which we, standing very many
miles below, see as a shooting star.
A bullet when fired from a rifle will fly into pieces after it has
struck against the target, and if you quickly pick up one of these
pieces you will generally find it quite hot. Whence comes this heat?
The bullet, of course, was cold before the rifleman pulled the
trigger. No doubt there was a considerable amount of heat developed
by the burning of the gunpowder, but the bullet was so short a time in
contact with the wad, through which so little heat would pass, that we
must look to some other source for the warmth that has been acquired.
Friction against the barrel as the bullet passed to the mouth must have
warmed the missile a good deal, and when rubbing against the air the
same influence must have added still further to its temperature, while
the blow against the target would finally warm it yet more.
In comparing the shooting star with the rifle-bullet we must remember
that the celestial object is travelling with a pace 100 times as swift
as the utmost velocity that the rifle can produce, and the heat which
is generated by friction is increased in still greater proportion.
If we double the speed, we shall increase the quantity of heat by
friction fourfold; if we increase the speed three times, then friction
will be capable of producing nine times as much heat. In fact, we must
multiply the number expressing the relative speed by itself--that is,
we must form its square--if we want to find an accurate measure for
the quantity of heat which friction is able to produce when a rapidly
moving body is being stopped by its aid. The shooting star may have
a pace 100 times that of the rifle-bullet, and if we multiply 100 by
100 we get 10,000; consequently we see that the heat produced by the
shooting star before its motion was arrested in dashing through the air
would be 10,000 times that gained by the rifle-bullet in its flight.
If the temperature of the rifle-bullet only rose a single degree by
friction, it would thus be possible for the shooting star to gain
10,000 degrees, and this would be enough to melt and boil away any
object which ever existed. Thus we need not be surprised that friction
through the air, and friction alone, has proved an adequate cause
for the production of all the heat necessary to account for the most
brilliant of meteors.
It is rather fortunate for us that the meteors do dash in with this
frightful speed; had the little bodies only moved as quickly as a
rifle-bullet, or even only four or five times as fast, they would have
pelted down on the earth in solid form. Indeed, on rare occasions it
does happen that bodies from the heavens strike down on the ground.
The great majority of those that fall on the ground, however, become
entirely transformed into harmless vapor. The earth would, indeed,
be almost uninhabitable from this cause alone were it not for the
protection that the air affords us. All day and all night innumerable
missiles would be whizzing about us, and though many of them are
undoubtedly very small, yet as their speed is 100 times that of a
rifle-bullet, the fusillade would be very unpleasant. It is, indeed,
the intense hurry of these celestial bullets to get at us which is the
very source of our safety. It dissipates the meteors into streaks of
harmless vapor.