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.