Part 7

You may desire to know how we are able to measure the heights of mountains on the moon. That is what I am now going to show you; and for this purpose we shall look at our imitation lunar crater. Here is the great ring, or circular enclosure, surrounded by cliffs, and here is a sharp mountain peak rising in the centre. I shall ask to have the beam from the electric lamp turned on our model. You see how prettily it is lighted up. I have placed the lamp so that the beams are sloping; and I have done this with the express object of making the shadows long. In fact, as we look at a lunar crater, which lies on the border between light and shade, the sun illuminates the object under the same conditions as those shown in the figure. I dare say you have often noticed what long shadows are cast at sunset. Similar shadows are made to teach the astronomer the altitudes of the lunar mountains; for he measures the length of the shadow, and then by a little calculation he can find the height of the object by which that shadow has been cast. I shall suppose that we want to measure the height of a flagstaff (Fig. 43). It is quite possible to do this by merely measuring the length of the shadow which that flagstaff casts at noon. It would not be correct to say that the height of the flagstaff is the length of its shadow. This will, indeed, be the case if you are fortunate enough to make your measurement at or near London on either the 6th of April or the 5th of September. On all other days in the year a little calculation must be made, which I need not now mention, but which the astronomer, with the aid of his Nautical Almanac, can do in a very few minutes. In a similar manner, by measuring the lengths of the shadows on the moon, and by finding the number of miles in the shadow, we are able to calculate the altitudes of the lunar mountains and of the ranges of cliffs by which the walled plains are surrounded.

ON THE ORIGIN OF THE LUNAR CRATERS.

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.

THE MOVEMENTS OF THE MOON.

Though the moon is going round and round the earth incessantly, yet it always manages to avoid affording us a view of what is on the other side. Our satellite always directs the same face towards the earth, and we may reasonably conjecture that the other side is covered, like the side we know, with rings and other traces of former volcanoes. In this respect the moon is quite a peculiar object. The other great celestial bodies, such as the sun or Jupiter, turn round on their axes, and show us now one side and then the other, with complete impartiality. The way in which the moon revolves may be illustrated by taking your watch and chain, and as you hold the chain at the centre making the watch revolve in a circular path. At every point of its path the ring of the watch is, of course, pointed to the centre where the chain is held. If you imagine your eye placed at the centre, to represent the earth, the movements of the watch would exemplify the way the moon turns round it.

One more point I must explain about the moon before we close this lecture. There is nothing more familiar than the fact that a heavy body will fall to the ground. Indeed, it hardly matters what the material of the body may be, for you see I have a small iron ball in one hand and I hold a cork in the other. I drop them at the same moment, and they reach the ground together. Perhaps you would have expected that the cork would have lagged behind the iron. I try the experiment again and again, and you can see no difference in the times of their falling, though I do not say this would be true if they were dropped from the top of the Monument. In general we may say that bodies let drop will fall sixteen feet in the first second. Even a bit of paper and a penny piece will fall through the same height in the same time if you can get over the difficulty of the resistance of the air. This is easily managed. Cut a small piece of tissue paper which will lie flat on the top of the penny, and hold the penny horizontal with the paper uppermost. Though there is nothing to fasten the paper to the penny, you will find that they fall together. If we could conduct the experiment of dropping the penny and the bit of paper in a vacuum, then, whether the paper was laid on the penny or placed in any other way, the two objects would reach the table at the same moment if released at the same moment at equal heights.

Wherever we go we find that bodies will always tend to fall in towards the centre of the earth; thus in New Zealand, at the opposite side of our globe from where we are now standing, bodies will fall up towards us, and this law of falling is obeyed at the top of a mountain as it is down here. No matter how high may be the ascent made in a balloon, a body released will fall towards the earth’s centre. Of course, we can only ascend some five or six miles high, even in the most buoyant of balloons; but we know that the attraction by which bodies are pulled downwards towards the earth extends far beyond this limit. If we could go ten, twenty, or fifty miles up, we should still find that the earth tried to pull us down. Nor, even if you could imagine an ascent made to the height of 1000 miles, would gravitation have ceased. A cork or an iron ball, or any other object dropped from the height of 1000 miles, would assuredly tumble down on the ground below.

Suppose that by some device we were able to soar aloft to a height of 4000 miles. I name that elevation because we should then be as high above the earth as the centre of the earth is below our feet. We should have doubled our distance from the centre of the earth, and the intensity of the gravitation would have decreased to one-quarter of what it is at the surface. A body which at the earth’s surface falls sixteen feet in a second would there fall only four feet in a second, and the apparent weight of any body would be so much reduced that it would seem to weigh only a quarter of what it weighs down here. Thus, the higher and higher we go, the less and less does gravity become; but it does not cease, even at a distance of millions of miles. Therefore you might say that as gravity tries to pull everything down, wherever it may be, why does it not pull down the moon? This is a difficulty which we must carefully consider. Supposing that the earth and the moon were simply held apart, both being at rest, and that then the moon were to be let go, it would no doubt drop down directly on the earth. The movement of the moon would, however, be very different if, instead of being merely let fall, it was thrown sideways. The effect of the earth’s pull upon the moon would then be shown in keeping the moon revolving around us instead of allowing it to fly away altogether, as it would have done had the earth not been there to attract it.

We can explain this by an illustration. On the top of a mountain I have placed a big cannon (Fig. 44). We fire off the cannon, and the bullet flies away in a curved path, with a gradual descent until it falls to the ground. I have made the mountain look hundreds of times larger than any mountain could possibly be; and now I want you to imagine a cannon far stronger and gunpowder more potent than any powder or cannon that has ever yet been manufactured. Fire off a bullet with a still greater charge than the last time, and now the path is a much longer one, but still the bullet curves down so as ultimately to fall on the earth. But make now one final shot with a charge sufficiently powerful, and away flies the bullet, following this time the curvature of the earth, for the earth’s attraction has the effect of bending the path of the bullet from a straight line into this circular form. By the time the bullet has travelled a quarter of the way round, it is no nearer to the earth than it was at first, nor has it parted with any of its original speed. Thus, notwithstanding its long journey, the bullet has practically just as much energy as when it first left the muzzle of the cannon. Away it will fly round another quarter of the earth, and still in the same condition it will accomplish the third and the fourth quarters, thus returning to the point from which it started. If we have cleared the cannon out of the way, the bullet will fly again over the mountain top without having lost any of its speed by its voyage round the earth. Therefore it will be in a condition to start again, and thus to revolve around the earth permanently. If, then, from the top of a mountain 240,000 miles high a great bullet 2000 miles in diameter had once been projected with the proper velocity, that bullet would continue forever to circle round and round the earth, and even though the mountain and the cannon disappeared, the motion would be preserved indefinitely. This illustration will, at all events, show how a continuous revolution of the moon round the earth can exist, notwithstanding that the earth is constantly pulling our satellite down towards its surface.

ON THE POSSIBILITY OF LIFE IN THE MOON.

Astronomers are often asked whether any animals can be living on the moon. No observations we can make with the telescope can answer that question directly. There are great plains to be seen on the moon, of course, but even if there were elephants tramping over those plains, our telescopes could not show them. Nor will our instruments pronounce at once whether plants or trees flourish on the moon. The mammoth trees of California are so big that a tunnel has been cut through the trunk of one large enough to give passage for a carriage and pair. Even if there were trees as big as this on the moon, they would not be visible from the most famous observatories.

Let us think what we should ourselves experience if we could in some marvellous manner be transferred from the earth to its satellite, and tried to explore that new and wonderful country. Alas, we should find it utterly impossible to live there for an hour, or even for a minute! Troops of difficulties would immediately beset us. The very first would be the want of air. Ponder for a moment on the invariable presence of air around our own globe. Even if you climb to the top of a high mountain, or if you take a lofty voyage in a balloon, you are all the time bathed in air. It is air which supports the balloon, just as a cork is buoyed up by water. In all circumstances, we must have air to breathe. In that air is oxygen gas, and we must have oxygen incessantly supplied to our lungs to reinvigorate our blood. We require, too, that this oxygen shall be diluted with a much larger amount of nitrogen gas, for our lungs and system of circulation are adapted for abode in that particular mixture of gases which we find here. The atmosphere becomes more and more rarefied the higher we ascend, and apparently terminates altogether some two or three hundred miles over our heads. Beyond the limits of the atmosphere it seems as if empty space would be met with all the way from the earth to the moon. We could not procure a single breath of air, and life would be, of course, impossible. Even at a height of three or four miles, respiration becomes difficult, and doubtless life could not possibly be sustained at a height of ten miles.

It is therefore plain that for a voyage to the moon we should require an ample supply of air, or, at least, of life-giving oxygen, which in some way or other was to be inhaled during the progress of the journey. When at length 240,000 miles had been traversed, and we were about to land on the moon, we would first of all ascertain whether it was surrounded with a coating of air. Most of the globes through space are, so far as we can learn, covered and warmed with an enveloping atmosphere of some kind; but, unhappily, the poor moon has been left entirely, or almost entirely, without any such clothing. She is quite bare of atmosphere at all comparable in density or in volume to that which surrounds us, though possibly we do now and then perceive some traces of air, or of some kind of gas, in small quantities in the lunar valleys.