Читать книгу Star-land: Being Talks With Young People About the Wonders of the Heavens - Robert S. Ball - Страница 14

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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.

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.

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.