by Anthology
"It's like this, M'Allister," I said. "You measure things with a two-foot rule, which is something you can actually handle, and you know it is made according to a standard measure and must contain exactly 24 inches. If, however, your rule was 24-1/4 inches long, yet still divided into twenty-four equal parts, you could measure work with it just the same, but would know that every measurement was just a little bit out. If you had no possible means of obtaining another rule, you would have to put up with a little inexactitude.
"That is just the position in which astronomers are placed; they have to put up with a measure which they know is not perfectly accurate, yet it is the best which can be secured.
"Their two-foot rule, so to speak, may be the distance from the earth to the sun, or the length of the whole diameter of the earth's orbit, and these cannot be handled like your rule; and although we know the measurements of these are nearly correct, they are not quite so. Yet the distances of the moon, planets, stars, &c., have to be measured by these rules, so it is clear we can only know those distances with a near approximation to accuracy.
"For this reason astronomers are always trying different means of ascertaining the sun's exact distance from the earth in order to obtain a perfectly correct measure; but there are so many difficulties and complications which affect the result, that it will be a long time yet before they succeed in their work.
"You will therefore understand that all these figures as to distances and dimensions of planets and stars are only as near approaches to correctness as is possible to attain in our present circumstances. They must not be regarded as literally exact, although they are usually sufficiently accurate for all general purposes. Astronomers know this and allow for it; but general readers of books, when they find figures which do not agree with others they have seen, are apt to regard them as all being mere guesses, and in this they are doing an injustice to the painstaking labours of generations of astronomers and mathematicians.
"I shall presently be mentioning the heights of mountains, the size of ring-plains, craters, &c., but the same reasoning applies to them; the dimensions given are averages of measurements made by different observers, and, though not quite accurate, are as near the truth as the difficult conditions under which they have to be measured will allow."
"Thank you, Professor," said M'Allister as I concluded. "I'm glad I don't have to work with such rules as those you mention, for measurements a little bit out of correctness would ruin any machine in the world."
"Still, M'Allister," I said, "you would have the advantage over astronomers with your two-foot rule, because you would know that it was a quarter of an inch too long. Their difficulty is that they do not know exactly how much their rule is out of correctness, so cannot obtain absolute accuracy however they may try."
We now set the machines going very slowly and moved toward the northern part of the moon, where I pointed out the position of the lunar north pole, and explained that, owing to the very slight inclination of the lunar axis, there can be but very little variation of seasons in any one particular part of the moon. Thus, if at one place it were spring, it would practically always be spring there, but with very cold nights all through the lunar year. Where it was summer it would practically always be summer, also with very cold nights, and so on.
I further explained that, as the moon revolves on its axis in the same time that it takes to make one revolution round the earth, those on the earth always see the same side of the moon, except when occasionally, owing to inequalities in the lunar motions, they are afforded a peep just round portions of the edges at different periods. The remainder of the other side of the moon has never yet been seen from the earth by human eyes, and in all probability never will be seen for millions of years to come.
John, who as usual was smoking like a factory chimney, here removed his pipe from his mouth and said, "Professor, you stated just now that the nights on the moon would always be intensely cold, and I should like to know whether there is any really reliable information respecting the temperature of the lunar days and nights. I have seen so many contradictory theories on the subject that I scarcely know what to believe."
"In that respect," I answered, "I fear that my position is much the same as yours, for I have absolutely no certain knowledge on the point, but will just state shortly how the matter stands at present.
"During the past century many investigations have been made by scientific men respecting the temperatures on the moon, and their results have differed to an amazing extent. It would take too long, and be too wearisome, to quote all the authorities, so a few must suffice. Lord Rosse, who used a thermopile in his experiments, found that in order to produce the results he obtained, the sunlit surface of the moon must be heated to a temperature of 500 degrees on Fahrenheit's scale. Sir John Herschel had previously concluded that the temperature must be much greater than that of boiling water. On the other hand Ericsson and, more recently, Professor Langley—who used a bolometer of his own invention for measuring the heat of the sun's invisible rays—came to the conclusion that even under continued sunshine the temperature would rarely, if ever, rise above the freezing point of water.
"Professor Very, however, who has continued the delicate experiments with the bolometer, and also made other experiments and calculations of quite recent date, has obtained results more nearly in accordance with those first quoted, for he found that near the end of the second week's sunshine on the moon the temperature of the rocks, soil, &c., must rise to at least 80 degrees Centigrade above the heat of boiling water."
"My word!" said M'Allister, "that's hotter than a ship's engine-room, and I shouldn't care for such a very high temperature."
"As this is so recent," I proceeded, "and the work of one of our highest authorities, I think we must accept it as being more correct, especially as Professor Very has taken into consideration some factors which had not previously been allowed due weight.
"In connection with this matter of temperature it is necessary to remember that the days and nights upon the moon are both very long, for the full lunar 'day' is equal to a month, so the actual lunar day is equal to fourteen of our days, and the lunar night is of the same duration. Our 'day' of twenty-four hours is divided into day and night in unequal proportions, according to the changes in the seasons; but, as I before remarked, the seasonal changes on the moon are very slight, so the variations in the lengths of the days and nights are very small.
"But, whatever may be the difference of opinion as to the heat of the lunar day, there seems to be a pretty general agreement that, owing to the absence of an atmosphere, the nights must be so intensely cold as to be almost beyond our conception—probably approaching nearly to the absolute zero of outer space. Even with an atmosphere the long nights in our polar regions are so cold that only very strong people can endure them, notwithstanding every device for obtaining warmth.
"You will gather from this that although the moon appears so beautiful from a distance, it must be anything but a desirable place of residence even from a climatic point of view, for we should practically be fried at midday, while at midnight—or even in the daytime when out of the direct rays of the sun—we should soon be frozen stiff."
As I said this John chimed in with: "Professor, all things considered, I think I could smoke my pipe more comfortably upon the earth than upon the moon. I really don't like such extremes of temperature."
"I am of the same mind," I replied, "and it is because I prefer a more equable temperature that I have carefully kept our martalium blinds drawn over those windows of our vessel upon which the sun is shining."
Chapter V
WE VIEW THE LUNAR SCENERY IN THE NORTHERN HEMISPHERE
"Now, Professor," exclaimed M'Allister, jumping up with a shrug of the shoulders, "you've given our friend John a considerable amount of information on a wee bit dry subject, so, mayhap, you will now give us something more interesting, and go on with the description of the natural features of the moon down yonder."
"Yes do, please, Professor," said John; "M'Allister's own temperature is evidently rising rapidly. Strange, isn't it, that a douche of cold facts should make our friend so warm!"
"Well, not altogether," I replied laughingly; "there should always be a healthy reaction after a cold douche. Much depends on the intensity of the cold applied, and you know that if you touch extremely cold metal it burns you like hot iron!"
"Professor," chimed in M'Allister, "maybe I was a bit warm, but really your facts were not so cold as to make me hot."
"I'm glad to hear you say so," I answered.
"At all events, Professor," continued John, "whatever may be M'Allister's actual temperature, I'm simply burning to know something about that very striking formation with the steel-grey coloured flooring which is situated not very far down from the North Pole, and a little to the east of the central meridian."
"That," I said, "is a large walled plain called Plato, and, being on a receding curve of the moon, it is seen from the earth foreshortened, so that it appears to be elliptical in shape. It is about sixty miles in diameter, and encloses an area of 2700 square miles, which is just about the area of Lincolnshire. The general height of the mountain walls is over 3600 feet; one mountain on the east is nearly 7500 feet high, and others on the north and west are but little lower.
"You will notice that there are several breaks in the walls, and a large one on the south-west; whilst on the inner slope of the mountains you can see where a great landslide has occurred.
"It is rather singular, John, that in your first selection you have chosen a formation which is one of the lunar mysteries!"
"Ah! Professor," said John, smiling, "I always was lucky! What is this dreadful mystery?" he asked, with an assumed expression of awe.
"Oh, it's not a ghost story, John, nor anything to make your flesh creep," I said rather grimly. "Usually the floor of a walled plain becomes brighter as the sun rises higher and higher in the sky, but Plato actually becomes darker under a high sun. By some it has been thought that this is merely the effect of contrast with the very bright surroundings of this formation, and that there is no actual darkening of the tint. This is certainly not the case, for I have examined it carefully myself with the telescope—shutting out all the bright surroundings from the field of view, but the floor still appeared equally dark.
"Others have suggested that the hot sun causes the growth of some kind of vegetation all over the plain, the ripening of which makes the floor darker in tint. As regards this suggestion, it is the fact that upon Mars the old sea-beds are the places where vegetation is most luxuriant at the present time; so, if Plato were at one time an enclosed sea, it might not be impossible that vegetation in some low form might grow and be nourished by the crude gaseous remains of a former atmosphere. A greenish tint has occasionally been noticed by some observers, also several light streaks across the floor, as well as several small craterlets, which have been duly noted on the maps.
"But before we go any further we will have a better means of seeing, for it is rather uncomfortable looking directly down upon the moon. So, John, just lend a hand and we'll fetch one of those large mirrors."
This was done, and the mirror suspended with the upper part projecting forward, so that when adjusted at the proper angle we could sit and look straight into the mirror before us and see the reflection of all that was below. We could still look down at the objects, if we wished to do so, without shifting our position.
"There, John," I remarked, as we completed this arrangement, "I have already arranged mirrors in the proper positions at the windows in the forepart of the vessel, so that in future M'Allister will be able to see what is nearly straight ahead of him. Now you will understand that I had a scientific use for the mirrors I provided, and did not require them merely to admire my beautiful face in as you suggested."
John laughed as I recalled his suggestion, saying, "All right, Professor, I know you generally have a good reason for what you do."
Now, being more comfortably seated, I drew their attention to some small isolated mountains on the area to the south of Plato, pointing out Pico, an isolated mountain over 8000 feet in height, and another with three peaks not very far from it. To the north-east of these, some distance away, are the Teneriffe and Straight Ranges; also isolated groups.
"You will remember," I remarked, "that I said there were several formations which seemed to me to owe their present appearance to the action of water. Now look well at all this district before us—does it not seem to bear out my contention? Those numerous small mountains and isolated groups were not, I think, originally isolated, but connected with the adjoining ranges. If we assume that Plato was once an enclosed sea, or lake, which burst through the mountain walls—possibly owing to their being weakened or broken by volcanic action—there would have been a tremendous outrush of water, which must have carried away a good deal of the softer material of these hills and mountains; whilst, in after years, the continual wash of the waters, combined with aerial denudation, would gradually have worn away all but the hardest parts of these formations.
"Most probably the whole of the surrounding area was also at some time a sea, though volcanic action has since altered its surface conformation, and in places it bears evidence of having been covered with lava. It is not unusual on our world for volcanoes to burst up from under the sea, so even the evidence of volcanic action does not, as some seem to think, negative the possibility of water ever existing here; and it may not be inappropriate to point out that our hydrographers have proved that our ocean-beds are not always smooth, but are often diversified by high hills and deep valleys."
M'Allister here interjected: "Professor, would you kindly tell us something about that fine range of mountains over yonder, just to the right hand?"
"Oh yes," I replied; "I was just about to mention that mountain range, which is called the Alps after those in Switzerland; and that peak on the front portion, just south of the great valley you see, is named Mont Blanc, and is about 12,000 feet in height.
"You will notice a very large number of peaks in this and the other neighbouring ranges—in fact, several thousands have been marked on our large maps.
"Cutting diagonally in a north-westerly direction, completely through the Alps, you will notice a long and deep valley. This is known as the 'Great Alpine Valley,' and is over eighty miles long, and varies from about three miles to six and a half miles in width. At the eastern end it is some 11,000 feet deep, debouching on to the plain in several comparatively narrow passes, whilst at its north-western extremity it is very shallow, and emerges on to what is known as the Sea of Cold, which covers an area of about 100,000 square miles. This valley seems to afford another example of formation by the action of water.
"Amongst the three thousand peaks comprised in the Apennine range just below the Alps, are several mountains of considerable altitude," I remarked, pointing out Mount Huygens, nearly 20,000 feet high, Mount Hadley, 15,000 feet, and Mount Woolf, 12,000 feet in height. "This range curves round towards the east, and finishes with a fine ring-plain called Eratosthenes—some thirty-seven miles in diameter, with a floor depressed 8000 feet below the lunar surface. It encloses a central mountain, and on the east wall there is one peak which rises 16,000 feet above the floor.
"The ranges in this part of the moon are, perhaps, more like those on our earth than others to be found on its surface, but much more wild and rugged.
"Eastward and northward of these ranges is the Sea of Showers, on which there are several fine ring-mountains and walled plains—notably Autolychus and Aristillus, two very perfect ring-mountains some 9000 feet high.
"One of the most striking, on account of its size and situation, is that large one which is called Archimedes, and is about fifty miles in diameter; and you will notice that a rugged mass of mountains and high hills extends from it to a distance of over a hundred miles on the south. The floor of this walled plain is only about 600 feet below the general level, and the
mountain walls average about 4000 feet in height; but there is at least one peak some 7000 feet high.
"You will see a little below and westward of Archimedes the commencement of a system of large cracks or crevasses in the lunar surface which are known as 'rills.' Many such systems are found in various parts of the moon; some of the cracks are comparatively shallow, but, according to Professor Langley, others are known to be at least eight miles deep, and may be infinitely deeper, though I cannot say I understand how these great depths have been arrived at. The length of the cracks varies from a few miles to over three hundred miles, and from a few hundred yards to some miles in width. They are attributed partly to volcanic action, but mainly to the contraction of the crust of the lunar globe as it became cold. Being so much smaller, the moon would cool much more rapidly than the earth, and the disruptive effects would necessarily be greater."
John here touched my arm, and pointing to some mountains on the borders of a large elongated oval area, close to the north-western terminator where the sun was setting, asked me what they were. I explained that the dark area was known as the Mare Crisium, or Sea of Conflicts, and is possibly the deepest of the large lunar depressions.
"It is about 280 miles long from north to south, and 355 miles wide from east to west, but, owing to its position, the width is seen from the earth very much foreshortened, so that it really looks nearly twice as long as it is wide. It contains an area of about 75,000 square miles, thus being as large as the combined area of Scotland and Ireland, and the five largest northern counties of England. It is surrounded by mountains, some being over 11,000 feet high, reckoning from the dark floor."
I drew their attention to Proclus—a ring-mountain on the eastern side of this sea—which is about eighteen miles in diameter, and the second brightest of the lunar formations. "From its neighbourhood several bright streaks diverge in different directions, two extending a long way across the dark area, and there is a longer one striking towards the north and another towards the south at an angle of about 120 degrees with each other.
