Vulcan and his boat seemed a pretty way of accounting for the sun’s
apparent motion. The chief drawback was that it was all work and
no play for poor Vulcan. There were also a few other difficulties.
Captains of ships told us that they had sailed out on the great sea,
and that so far from finding that the ocean extended on and on in one
flat plain forever, the water seemed to bend round, so that, in fact,
after sailing far enough in the same direction, they found that they
would be brought back again to the place from which they started. They
also knew a little about the north. They told us that there could be no
such ocean as that which Vulcan in this fable was supposed to navigate.
It also appeared that ships had been voyaging all over the globe
night and day in every direction, and that no captain had ever seen
the sun coming down to the sea, and still less had he ever met with
Vulcan in the course of his incessant voyages. Thus it was discovered
that the earth could not be a never-ending flat, but that it must be
a globe, poised freely in space without any attachment to hold it up.
It was thought that the change from day to night might be accounted
for by supposing that the sun actually went round the earth through
the space underneath our feet. This is, indeed, what it seems to do.
But there was a great difficulty about this explanation, which began
to be perceived when the size and distance of the sun were considered.
It required the sun to possess an alarming activity. He would actually
have to rush round a circle one hundred and eighty million miles in
diameter and complete this astonishing voyage once every day.
[Illustration: FIG. 21.--How we illustrate the Changes between Day and
Night.]
A little reflection will show that a very much simpler explanation was
available. It was shown that the sun need not revolve round the earth
once every day, but that everything would be explained if the earth
itself turned round in such a way as to produce the changes from day
to night. We may illustrate the case by this figure (Fig. 21). The
small globe is the earth, which I can turn by the handle. The lamp will
represent the sun, and, as at present shown, the side of the earth,
on which England lies, is towards the lamp and in full day. On the
opposite side of the globe are other countries such as New Zealand, and
there it is dark. You see that by simply turning the handle I can move
England gradually round so that it passes into the dark side, and then
night falls over the country. At the same time New Zealand is turned
round to enjoy the smiles of day. This is a very simple method of
accounting for the succession of day and night, and it is also the true
method. We have already seen that the sun turns round, and now we find
that the earth also turns, but the little body, the earth, goes much
the faster, for it makes twenty-five turns while the sun goes round
once.
Our earth is at this moment spinning round at a speed so great that
London moves many hundreds of miles every hour. A town near the
equator would gallop round at a pace of more than a thousand miles an
hour--quicker, in fact, than a rifle-bullet. Don’t you think that we
ought to perceive that we are being whirled about in this terrific
fashion? We know that when we are flying along in a railway train, we
feel the jolting and we hear the noise, and we feel the blast of air
if we put our heads out of window, and we see the trees as they appear
to rush past. All these things tell us that we are in rapid motion.
But suppose these sensations were absent. Imagine a line so perfectly
laid that no jolts are perceptible, and that no racket is heard; draw
down the blinds so that nothing can be seen, how then are we to know
that we are moving? Indeed, your grandfathers used to be able to enjoy
such a tranquil locomotion. I remember seeing in my childhood the
fly-boats, as they were called, on the Royal Canal, wherein passengers
were conveyed from Dublin to the West of Ireland, before the railway
was made. The fly-boat was a sort of Noah’s ark in appearance, drawn
by a horse cantering along the towing-path. In the cabin of such
a vessel, where there was not the slightest motion of rolling or
pitching--nothing but noiseless gliding along the canal--no one would
be conscious of motion, so long as he did not look through the cabin
windows. No one was ever seasick in a fly-boat; it was the perfection
of travelling for those who loved ease and quiet.
The motion of the earth round its axis is, so far, like that of the
fly-boat. It is so absolutely smooth that we do not feel anything, and
we only become conscious of it by looking at outside objects. These
are the sun, or the moon, or the stars. We see these bodies apparently
going through their unvarying rising and setting, just as, in looking
out from the fly-boat, the passengers in that quaint old conveyance
could see the houses and trees as they passed.
Seeing is believing; and I should like here, in this very theatre, to
show you that we are actually turning round; and this I am enabled to
do by the kindness of my distinguished friend, Professor Dewar.
[Illustration: FIG. 22.--A Pendulum.]
I am tempted to wish that I had Aladdin’s lamp for the moment, for I
would rub it, and when the great genie appeared, I would bid him take
the Royal Institution, and all of us here, to a place which everybody
has heard of, and nobody has seen--I mean the North Pole. It would be
so easy to describe the experiment I am about to show you, there. It is
not so easy here. But it will be sufficiently accurate for our purpose
to suppose that we actually have made the voyage, and that this is the
Pole at the centre of the lecture-table. The direction of the axis
round which the earth is turning is a line pointing up straight to the
ceiling. This lecture-table and all the rest of the theatre is going
round. In about six hours it will have moved a quarter of the way, and
in twenty-four hours it will have gone completely round. That is, at
least, what would happen if we were actually at the Pole. As we are
not there, for the Pole is many miles away from the Royal Institution,
I must slightly modify this statement, and say that the table here
takes more than twenty-four hours to go round. And now I want some
way of proving that such is actually the case. There is no use in our
merely looking at it, because we ourselves, and this whole building,
and the whole of London, are all turning together. What we want is
something which does not partake of the motion. Here is a heavy leaden
ball (Fig. 22). It is fastened to the roof by a fine steel wire, and
you see it swings to and fro with a deliberate and graceful motion. I
want it to oscillate very steadily, so I draw it to one side and tie
it by a piece of thread to a support, and then I burn the thread, and
the great ball begins to swing to and fro. It would continue to do so
for an hour, or indeed for several hours, and it is a peculiarity of
this motion that the vibration always remains in the same direction
in space. Even the rotation of the earth will not affect the plane of
this great pendulum, so far at least as our experiment is concerned.
Here, then, we have a method of testing my assertion about the turning
round of this theatre. I mark a line on the table, directly underneath
the motion of the ball to and fro. If we could wait for an hour or so,
we should see that the motion of the ball seemed to have altered to
a direction inclined to its original position, but it is really the
table that has moved, for the direction of the motion of the ball is
unaltered. We cannot, however, wait so long, therefore I show you the
ingenious method which Professor Dewar has devised. By a beam from
the electric light, he has succeeded in so magnifying the effect that
even in a single minute it is quite obvious that the whole of this
room is distinctly turning round, with respect to the oscillations of
the pendulum. This celebrated experiment proves by actual inspection
that the earth must be rotating. By measuring the motion we might even
calculate the length of the day, though I do not say it would be an
accurate method of doing so.
The proper way of finding how long the earth takes to turn round is
by observing the stars. Fix on any star you please, and note it in a
certain position to-night; if you then observe the moment when the star
is in the same place to-morrow, the interval of time that has elapsed
is the true duration of one complete rotation. When accurately measured
its length is found to be 23 hours 56 minutes 4 seconds, or about four
minutes shorter than the ordinary day, measured from one noon to the
next.