Popular Science Monthly/Volume 35/May 1889/The Strange Markings on Mars
|THE STRANGE MARKINGS ON MARS.|
By GARRETT P. SERVISS.
IN the whole planetary empire of the sun there is but one body, if we except the moon, whose actual surface can be satisfactorily examined even with the most powerful telescope. The broad disk of Jupiter presents a most inviting and splendid sight; but it is apparent that we are not looking at the solid shell of a planet, but at a vast expanse of thick clouds, surrounding and concealing the planetary core, and reflecting the sunlight from their shifting surfaces. Saturn presents a somewhat similar appearance, modified by greater distance. Uranus and Neptune are so nearly beyond the present reach of telescopes, so far as the phenomena of their disks are concerned, that we know almost nothing of their surface appearances. Some observations of Uranus, however, indicate that it presents the same equatorial parallelism of exterior markings that characterizes Jupiter and Saturn; and so we may infer that what we faintly discern on its disk are the outlines of cloud-masses, enveloping the planet, and drawn out by the effects of its rotation into belts and streaks. Coming to the nearer planets, we find that Venus, superbly brilliant to the naked eye, and consequently, it might naturally be thought, a promising object for telescopic scrutiny, is nevertheless the most disappointing of all the planets when viewed with a telescope. The splendor of its luminosity in itself forms an obstacle to the study of its surface, where flitting glimpses of shadowy forms and brilliant spots only serve to excite the keenest curiosity. With respect to Mercury, our knowledge is equally unsatisfactory. The surface of the moon, of course, has been well studied, as such maps as those of Beer and Mädler, Neison and Schmidt sufficiently attest. But, after all, the absence of the faintest indication of life robs the wonderful lunar landscapes of a large share of the interest that would otherwise attach to them.
Finally, we look at Mars, and here at last we find a globe whose true surface we can inspect, and which at the same time possesses an atmosphere and other concomitants of vital organization. Since Mars has been selected by more than one astronomer as the probable abode of life (and perhaps the only one besides the Earth in the solar system), and especially since a discussion of the markings seen upon the planet necessarily involves the physical features upon which the theory of Mars's fitness for inhabitation rests, it will be well to recall here the principal facts that have been ascertained respecting that interesting orb.
The diameter of Mars is 4,300 miles, or only some 240 miles more than half of the mean diameter of the Earth. The density of the planet is rather less than three quarters of the density of the Earth, or about four times the density of water. The force of gravity upon its surface is less than two fifths of that upon the Earth; more accurately, 0⋅38. That is to say, if a man from the Earth could visit Mars, he would find that his weight had diminished almost two thirds. Members of terrestrial fat men's clubs could become agile dancers by simply going to Mars. This feebleness of the force of gravity must, it is clear, have an important effect upon the organization of any forms of life that may exist upon Mars, whether animal or vegetable. The mean distance of Mars from the sun is 141,500,000 miles, that of the Earth being 92,900,000. The length of Mars's year is six hundred and eighty-seven days. Its day is only forty-one minutes longer than our day upon the Earth. The inclination of its equator to the plane of its orbit differs but slightly from that of the Earth. But when we come to consider the eccentricity of its orbit, we find a decided difference between the Earth and Mars. The Earth's orbit is so nearly a circle that its greatest and least distances from the sun differ by only 3,000,000 miles, while the orbit of Mars is so eccentric that that planet is 26,000,000 miles nearer to the sun at one extremity of its orbit than at the other. It follows that, while Mars receives, upon the whole, less than half as much light and heat from the sun as the Earth gets, yet that quantity is variable to the extent of about one third of its greatest value—in other words, the sun gives Mars half as much again heat at its perihelion as it does at its aphelion. It is hardly necessary to point out the important climatic effect of such a variation. Another remarkable resemblance between the Earth and Mars comes in here. Just as on the Earth, the summer of the northern hemisphere of Mars occurs when the planet is farthest from the sun and its winter when nearest. The effect, as Mr. Proctor has pointed out, tends to equalize the temperature of the seasons in Mars's northern hemisphere, but to exaggerate their difference in the southern hemisphere.
We may dwell for a moment upon this last-stated peculiarity, for it is exceedingly interesting in its suggestiveness. Having summer occurring in the southern hemisphere of Mars at the planet's perihelion, and winter at its aphelion, we should find there a most remarkable disparity both in temperature and in the brilliancy of daylight between the two seasons. The difference would be the sum of the effects produced by the greater or less distance of the sun and the variation in the inclination of its rays to the surface of the planet. Since the first cause alone would produce an inequality amounting, in the extreme, nearly to the ratio of 3 to 2, it is evident that the addition of the second would increase the difference to such, an extent that the seasonal changes might be fatal to all higher forms of life. We have only to recollect how powerful the effect of the comparatively moderate variations between the seasons of our own planet is upon the human organism in order to understand what must be the condition of things in the southern hemisphere of Mars, where the passage from one season to the other presents the succession of violent winter cold, accompanied by days of gloom and faint sunshine, followed by a blazing summer, with the sun hanging overhead, visibly increased in apparent size by its approach. Telescopic observations show clearly by the great variation in the extent of the polar snows how extensive is the effect of these changes upon the surface of the planet. In the hot summer the snows rapidly retreat toward the pole, and even leave the actual pole itself bare of snow, showing that upon Mars, as upon the Earth, the center, or pole, of greatest cold (at least in the southern hemisphere) does not coincide with the geographical pole of the planet. Then, with the on-coming of winter, the march of the snows begins and they rapidly advance further and further toward the equator, spreading over the antarctic regions until another change of season brings back a flaming sun to melt them away. It should be added that, as Prof. Young has remarked, the climate of Mars, upon the whole, appears to be much milder than we should naturally have expected in view of its distance from the sun.
Bearing in mind these general facts about the size of Mars and its position in the solar system, we shall now proceed to the discussion of its surface phenomena as revealed by the telescope, merely pausing to remark that the atmosphere of Mars is apparently less dense than that of the Earth, and that the spectroscope has demonstrated the presence of watery vapor in it.
The little telescope of Galileo, which had enabled him to discover the phases of Venus, the satellites of Jupiter, the mountains of the moon, the existence of Saturn's ring, and "vast crowds of stars" in the Milky Way, was not powerful enough to show him the markings that diversified the disk of Mars. The earliest drawings of Mars that have come down to us were made by Fontana, in Italy, in 1636 and 1638. They contain very little detail, the best representing the planet simply with a darkish spot in the center of the disk. Twenty odd years later Huygens made much better drawings, and then the work was taken up by Cassini, Maraldi, and others, with the cumbersome telescopes of the time, the most powerful of which consisted of an object-glass suspended high in the air by means of a long pole or other support, while the eye-piece in the hand of the observer on the ground was, with infinite difficulty, brought and kept in line with the optical axis of the instrument. One of these telescopes was no less than three hundred feet in length, the great length being necessary in order to avoid, as far as possible, the chromatic aberration of the single lenses of which object-glasses were then made! Considering the enormous difficulties under which they labored, the results attained by these early observers are astonishing. The delineations of the planet's surface made by Huygens and Hooke were sufficiently exact to be used by modern astronomers in ascertaining the rotation period of the planet within a fraction of a second, while Cassini's observations enabled him to calculate that period with an error of less than three minutes. In fact, Huygens saw enough to suggest to his penetrating mind the existence of an analogy between the surface of Mars and that of the Earth.
In 1666 Cassini made a drawing of Mars, which is reproduced in our cut, showing in rough outline a feature of the planet's surface which has since become well known under the names of Kaiser Sea and the Hour-glass Sea, the last being suggested by its shape. Directly underneath Cassini's drawing I have placed, for the purpose of comparison, a picture of Mars made by Herschel in 1780, and showing the same sea, but with much more detail. Allowing for the difference in the position of the disk (for the two drawings were plainly not made at precisely the same period of the planet's rotation, nor at the same inclination), and also considering the great superiority of Herschel's telescope, the resemblance is sufficiently striking to show that the two observers were looking at the same feature of the planet, and that it was a permanent marking on the disk. The south polar ice-cap is conspicuous in Cassini's drawing.
A word, by the way, in regard to the "seas" and "ice-caps" of Mars. The general color of the planet is ruddy, some observers say rose-color, and this hue is plain in naked-eye observations. But the telescope shows that the disk, instead of being uniformly red, although that tint predominates, is divided into streaks and patches of varying hue. The reddish regions are regarded as being the land-surfaces of the planet, while the dusky or greenish parts are looked upon as probably oceans or seas. At the poles there are seen white caps which, inasmuch as they increase in size when it is winter and decrease when it is summer in their respective hemispheres, are regarded as the arctic and antarctic snow regions of Mars.From the time of Herschel the study of the surface markings of Mars was prosecuted by many observers with more or less success, and Beer and Mädler, those indefatigable portrayers of celestial scenery, made a chart of Mars; but it was not until some twenty years ago that a reasonably full and satisfactory map of the red planet was produced. Then Mr. Proctor, using the drawings of the "eagle-eyed" Dawes as the basis of his work,
A CHART OF MARS
|Old Drawings of Mars.|
had been made since the time of the latter, incomparably greater, in fact, than had been accomplished in the hundred and more years that elapsed between Cassini and Herschel. Yet if we should place any single one of Dawes's drawings side by side with those of the old observers, the difference would not appear by any means so striking, for, the reader must recollect, Mr. Proctor's chart was constructed by inspecting and comparing twenty-seven of Dawes's sketches, representing the planet at different periods of its rotation, so that all sides of it were successively viewed in the best position for observation. If we had an equally numerous series of Cassini's, or preferably of Herschel's sketches, made in a similar manner, we should probably be able to construct from them a chart which, while it certainly would be greatly inferior to Proctor's in its details, would nevertheless make it clear that the earlier observers saw many of the principal markings that are shown in the more modern map.
Still more detailed charts of Mars followed that of Mr. Proctor, notably those of M. Flammarion, and Mr. Green, the latter being a very beautiful work based upon a series of splendid drawings made by Mr. Green in the island of Madeira. But no very considerable advance was made in areography, as the geography of Mars has been called, until Signor Schiaparelli published the results of his surprising observations made during the very favorable opposition of Mars in 1877. Although Schiaparelli has repeated these observations again and again, and they have been confirmed, in part at least, by several able observers, there is a disposition in some quarters to cast doubt upon them, and to ascribe them to the effects of optical illusion or some other hallucination. Considering the wonderful character of these observations, and the immense advance that they constitute in the study of the surface of Mars, there is, perhaps, the shadow of an excuse for some incredulity about them. Yet I think the reader will be convinced, after inspecting Schiaparelli's map, and hearing the story of what he has seen, that to throw discredit upon the substantial accuracy of his observations, marvelous though they may appear, is to do serious injustice to the great Italian astronomer.
And, now, what is it that Schiaparelli has seen on Mars? Many readers will probably at once answer "canals," for the fame of "Schiaparelli's canals" has become wide-spread, and that very word has, perhaps, done as much as anything to foster incredulity in regard to these discoveries. It is true that Schiaparelli himself suggested the name canals to describe the strange lines that he found traversing the continents of Mars, and forming, as it were, a network of intercommunication between its seas; but, at the same time, he indicated that that name was simply to be taken, for lack of a better, as descriptive of their general appearance, and not as implying that they were canals in our sense of the word. Of course, the term was at once restricted, in popular acceptation, to its terrestrial sense, and there have not been wanting speculations about the engineers who constructed those wonderful canals on Mars! Mr. Proctor rather helped on this fanciful interpretation of Schiaparelli's discovery by throwing out the suggestion that, owing to the slight force of gravity on Mars, we should not be too hasty in setting limits to the engineering achievements of the giants who might dwell upon that planet!
But, setting aside the manifestly false analogy which would make of Schiaparelli's "canals" actual artificial water-courses, we shall find that the real facts are not the less wonderful and suggestive of interesting reflections. Schiaparelli's first observations of these singular objects were made, as I have already said, during the opposition of Mars in 1877. It will be remembered that it was at that very same opposition that Prof. Hall, using the great Washington telescope, at that time the most powerful refractor in the world, discovered the moons of Mars. Yet Prof. Hall saw nothing wonderful or very unusual on the disk of the planet; and Schiaparelli, on the other hand, failed to discover the little moons. Hall's discovery was made in August; Schiaparelli's began in September. All this is very singular; but it seems still more strange that, while the moons of Mars, having once been discovered, were afterward seen with comparatively small telescopes, the canals have never been seen with the great glass at Washington, and that only three or four observers besides Schiaparelli have ever seen them. In the last annual report of the Naval Observatory for the year ending June, 1888, it is stated that the great telescope had been in constant use, under the charge of Prof. Hall, and that the surfaces of both Saturn and Mars were constantly and carefully examined, and drawings made from time to time. In the case of the latter planet the canals of Prof. Schiaparelli, though specially looked for, both during and after the opposition, could not be made out. At the very same time the canals were visible not only to Schiaparelli, but to Perrotin and Terby, and, as we shall see further on, some very remarkable phenomena connected with them were observed. At the Lick Observatory, too, they saw the canals, though they did not perceive all the details and peculiarities noted by Schiaparelli and Perrotin. How shall we account for these remarkable discrepancies? I do not for a moment think that they shake the substantial accuracy of the Italian observations. No doubt a clew to the explanation is furnished by what Schiaparelli has recently said of the difficulty of seeing the objects he has described: "On the rare days when these extremely difficult observations are possible, the period of good telescopic images does not last, ordinarily, more than two or three hours during the twilight, or the commencement of night. . . . I have found by experience, at Milan, that one can hardly hope to have an atmosphere sufficiently good during more than eight or ten evenings (during an opposition); sometimes even entire months pass without one's being able to make a satisfactory observation. Much rarer still are evenings of perfect images, those in which one can employ the whole power of an instrument like our Merz equatorial of eighteen inches."
And this is said of the Italian sky, which has long been famous for the steady views that it gives of the heavenly bodies. What could be expected, then, of the mist-haunted atmosphere of the Potomac flats through which the watchers at our Naval Observatory must strain their vision? At Mount Hamilton they have atmospheric conditions that rival those of Italy, and therefore it was to be foreseen that they could hardly fail to confirm the existence of Schiaparelli's strange markings.
It should be said, before proceeding, that while the great majority of the canals have been seen only by Schiaparelli himself and a few other observers, there are two or three which had been recognized, though not under their present designation, and perhaps not in their complete extent, before the Italian astronomer made his discovery. Notable among these is the narrow arm running out of the Kaiser Sea, or Syrtis Magna, as Schiaparelli names it, and which he calls the Nilosyrtis. Herschel, and even earlier observers, seem to have noticed this.But the detection of the dark lines called canals was only the beginning of Schiaparelli's singular discoveries. The next development in this remarkable series of observations was the doubling of the canals. Those that he saw in 1877 were simple lines, or narrow bands, and, strange as their appearance was, the liveliest imagination could hardly have prefigured their aspect at subse-
quent oppositions. In 1879 Scliiaparelli noticed that the canal which he calls Nilus (see his map) was double, or consisted of two streaks running side by side, and perfectly parallel. This observation was made shortly after the time of the vernal equinox on Mars.
"These two regular, equal, and parallel lines," he says, "I confess profoundly surprised me, all the more because a few days before, the 23d and 24th of December, I had carefully examined that same region without discovering anything of the kind. I awaited with curiosity the return of the planet in 1881 in order to see if an analogous phenomenon would present itself in the same place, and I saw the same thing reappear on the 11th of January, 1883, one month after the vernal equinox of the planet, which had occurred on the 8th of December, 1881. The duplication was still plainer at the end of February. At this same date, the 11th of January, another duplication had already been produced, that of the middle section of the canal of the Cyclops at the edge of Elysium (see map). Greater still was my astonishment when, on the 19th of January, I saw the canal Jamuna, which was then in the center of the disk, composed very plainly of two parallel straight lines traversing the space which separates the Niliacus Lacus from the Auroræ Sinus, At first I thought it was an illusion caused by fatigue of the eye and by some new kind of strabismus; but there was no resisting the evidence. From the 19th of January I simply went from surprise to surprise; the Orontes, the Euphrates, the Phison, the Ganges, and most of the other canals showed themselves very clearly, and indubitably divided in two."
It is not a matter for surprise that this announcement of Schiaparelli's, coming upon the heels of his original discovery of the canals as single lines, which in itself was sufficiently remarkable, caused still greater doubt to be entertained of the correctness of his observations. It seemed to many easier to believe that a distinguished astronomer and practiced telescopist had been misled by some deception of the eyes, some optical trick of his instrument or of the atmosphere, than that the globe of Mars was covered, over the larger part of its surface, with a network of lines, apparently connected with the water-system of the planet, and that, at certain times, these lines, canals, or water-courses, or whatever they might be, were doubled up throughout their whole extent. Even the positive assurance of the celebrated astronomer, "I am absolutely certain of what I have observed," could not banish all doubt. The manner in which the doubling of the canals was brought about seemed most mysterious, and added to the apparently dubious character of the whole occurrence. Schiaparelli declared that sometimes he was able to perceive precursory symptoms of the change. A light, hardly visible shade would make its appearance, extended alongside one of the canals. In a few days only a series of whitish spots would appear there. A day or two later the perfect double of the canal would be seen with absolute distinctness lying beside the original, exactly parallel with it, and of equal length, breadth, and depth of color.
"One can," says Schiaparelli, "compare this process of formation to the appearance that would be presented by a multitude of soldiers dispersed without order who, little by little, should arrange themselves in ranks or in columns; so that we are here dealing with formations unknown on the earth, determined by the geographical configuration of the ground, and capable of reproducing themselves periodically in the same places and under the same aspects."
These canals (we must continue to call them by that name for lack of a better) vary in length from a few hundred miles to two or three thousand, while their width is seventy-five or eighty miles. When they become double, the distance between the twin canals is from two hundred and fifty to five hundred miles.
The Italian astronomer's later observations have again and again confirmed the results of his earlier ones. During the oppositions of 1883-'84, 1886, and 1888, under somewhat varying conditions, and with different degrees of visibility, yet always unmistakably, he has seen not only the canals, but the strange phenomenon of their doubling or gemination. The character of the appearances has been always the same, but in details they have differed.
Let the reader compare Schiaparelli's map with the chart of Mr. Proctor, and he can not fail to be impressed by the enormous advance in the matter of minute detail exhibited by the former. Apparently more has been learned about the surface of Mars during the past twelve years than was learned in the previous two hundred years, and the greater part of this gain is the work of a single observer.
While it is more or less idle to speculate on the nature of these singular objects appearing on a globe that never approaches the earth nearer than about forty millions of miles, and that ordinarily is very much farther away, yet it is impossible to avoid indulging a natural curiosity to know what they are. It is known that all the features of Mars's globe are more or less changeable, though upon the whole they preserve the same general aspect, and Schiaparelli declares that in the case of the canals the changes are not only extensive but periodical. It has generally been believed that the variations of appearance in the larger features of the disk were owing principally to atmospheric causes. Large regions of the planet have, at times, been seen apparently hidden under a veil, the gradual withdrawal of which has again revealed their well-known contours, and in such cases the conclusion has seemed irresistible that what had been observed was the formation and subsequent dissipation of vast cloud areas concealing or obscuring the outlines of continents and seas beneath them. So the indistinctness near the edges of the disk and the altered appearance of the planetary features there have been partially ascribed to atmospheric influences as well as to the effects of perspective. Whether the observed changes in the appearance of the canals can be ascribed to similar causes is a question which can not yet be solved. Schiaparelli appears to think that they are principally due to something which occurs on the surface of the planet itself, and which something, in its turn, depends upon the changes of the seasons.
In order to form any opinion whatever upon this question it is necessary to examine a little more closely the varying aspects of the canals. Their discoverer has noted these four points:
1. A canal may remain invisible for a longer or shorter time. This invisibility, he insists, is an actual disappearance of the canal, and is not due simply to unfavorable circumstances of observation. Moreover, he finds here a striking appearance of connection with the seasons. The epoch most favorable for the disappearance of the canals is near the time of the southern solstice of Mars, which, as we have seen, occurs when the planet is nearest to the sun.
2. In many cases, according to Schiaparelli, the presence of a canal begins to manifest itself to the eye in a very vague and uncertain manner, by a slight shading which irregularly extends itself in the direction of its length. This phenomenon is so delicate that, he says, it marks, as it were, the limit between visibility and invisibility.
3. Very often the canals present the appearance of a gray band fading out on each side and having the deepest shade in the middle, which may be dark enough to suggest the appearance of a more or less clearly marked line. Sometimes, but rarely, one side of the band alone is nebulous or indistinct, the other being clearly defined. Various other anomalous appearances have been observed.
4. The most perfect type of the canals, and that which their discoverer says he regards as the expression of their normal condition, "is a dark line, sometimes quite black and well defined, looking as if it had been traced with a pen on the yellow surface of the planet." When the canals appear in this form they are very uniform throughout their length, and Schiaparelli says, on the rare occasions when he has been able to clearly distinguish the two edges, one from the other, he has discerned slight sinuosities or scollopings on the borders. He adds that the width of a canal may change with time from a thread, barely perceptible in the best atmospheric conditions, to a broad black band visible at the first glance.
To all these characteristics must be added, of course, the extraordinary occurrence of the gemination or doubling of the canals. It is natural to suppose that, in such cases, what would be seen would be the apparition of a new canal along side of an old one. That, in fact, is what Schiaparelli described as occurring during his earlier observations; but, during the opposition just passed (1888), he discovered that this was not a general rule, and that it may happen that neither of the new canals, when a doubling takes place, may coincide with the old one. "The identity," he adds, "in the general direction and situation is then merely approximative; every trace of the former canal disappears, giving place to two new lines." Both the width and the distance apart of the twin canals vary in successive seasons.
It would carry us far beyond the limits of space available for this article to enter into a more minute account of Schiaparelli's observation of the many anomalies and changes of appearance which the canals present at different seasons and under varying circumstances. Enough has been said to indicate that it would be impossible to make a map which should show the precise appearance of the surface of Mars at any fixed period, and at the same time contain a representation of all the phenomena which are, from time to time, to be seen there. And it should be said, because this is a matter that has been misunderstood, that Schiaparelli
|Ordinary Appearance.||Appearance in April. 1888.|
|Changes in the Aspect of Libya.|
does not intend his maps to be taken as portraitures of the planet, but simply as sketches showing details of whose existence there is no doubt whatever, but all of which can not be, or have not been, seen simultaneously.
And now there remain yet other remarkable circumstances to be mentioned in order to complete the picture of the surface of Mars, and some of these may have an important bearing upon the question of the nature of the canals. A glance at Schiaparelli's map shows us the disk of the planet divided into areas of land and water, which are about equal in their total extent. Then crossing the land areas in every direction are the canals, which it will be observed always begin and end either at the edge of a sea, or at a point of junction with other canals. Without varying their direction they cross one another, and in some cases several canals radiate from a single center, which then generally appears expanded into a "lake." In addition there are certain regions which Schiaparelli describes as variable in appearance, or intermediate between the seas and the lands, presenting sometimes the character of maritime surfaces and at other times that of continental areas. Among these are the places marked on the map Deucalionis Eegio, Hellas, and the island called Cimmeria. The region named Libya, which ordinarily appears as a continental expanse, seems to belong to this class of variable areas, and within the past year it has obtained great celebrity because it was said to have been submerged by an inundation from the adjoining sea. This region is more than 200,000 square miles in area, and lies just under the equator. In May last M. Perrotin, of the Nice Observatory, made the somewhat startling announcement that the continent of Libya had disappeared. "Clearly visible two years ago," said M. Perrotin, in his report to the Paris Academy of Sciences, "to-day it no longer exists. The neighboring sea, if sea it is, has completely invaded it. In place of the light reddish tint of the continents of Mars the black, or rather dark-blue, color of the seas has appeared there. . . . In sweeping over the continent the sea has abandoned on the south the region that it formerly occupied, and which now appears with a tint intermediate between that of the continents and that of the seas, a light-blue color, analogous to that of a slightly misty sky in winter."
A look at the accompanying cut will show the change which Perrotin detected. This extraordinary aspect of Libya was first seen in April and lasted into May. In June the "continent" seems to have resumed, or nearly so, its ordinary appearance. Perrotin's suggestion that the change observed was probably periodic appears to be borne out by an examination of former observations of this region of the planet. There was a partial "inundation" of Libya in 1883, and a still more extensive one in 1884, both of which were noted by Schiaparelli, who confirms Perrotin's observation of 1888 in a general way, but does not describe the continent as having at any time completely disappeared. Speaking of its appearance in 1884, Schiaparelli says Libya had a flaky look, as if it had been "covered with innumerable little spots all jumbled together." The suggestion of clouds contained in this description is very striking, yet Schiaparelli does not pursue the analogy.
All of the regions which possess this semi-maritime character frequently present a lighter color when viewed obliquely, or near the edge of the planet's disk, than when seen near the meridian. This fact seems strongly to suggest the presence of atmospheric phenomena, which may change or modify the appearance of any district covered by them according to the visual angle under which it is observed. The reader has only to take an ordinary terrestrial globe, and, supposing it to represent Mars, turn it slowly around its axis, in order to perceive how the situation of any region with respect to the center or the edges of the disk may influence its appearance. Near the edge the surface of the planet must necessarily be seen through a far greater depth or thickness of its atmosphere than in the center of the disk, and, if that atmosphere contained clouds or mist, of course its opacity would be greater in proportion to its greater depth, and the reflection of light from the mist would give a whiter tone to the features of the planet seen through it. Nevertheless, the cloud theory fails to account satisfactorily for all the appearances that have been so carefully described by Perrotin and Schiaparelli. Yet if it were possible for us to imagine that masses of clouds of some sort could retain, for considerable periods, a fixed or nearly fixed general form and position in the planet's atmosphere, disappearing and reappearing in the same localities according to the seasons, and occasionally extending their outlines or slightly shifting their positions, then we might be able to account for such phenomena as those presented by Libya, without recourse to such violent, extensive, and rapid geological changes as would seem to be necessary to produce alternate inundations and emergences of large areas of land.
As to the nature of the canals, it is still more difficult to suggest any satisfactory explanation. Several hypotheses have been presented, none of which appears entirely to meet the case. I have already remarked that there has not been lacking the suggestion that these curious streaks represent the lines of actual artificial water-courses on Mars. The straight and undeviating course which they pursue might be regarded as lending some degree of probability to such a view, but the enormous scale on which they exist seems to compel the rejection of the hypothesis. It is true that, if we consider only the influence of the force of gravity on Mars, giants could dwell upon that planet whose mechanical achievements might vastly surpass the greatest performances of our engineers; for a body weighing a ton on the earth would weigh only seven hundred and sixty pounds on Mars, and on the other hand a man on Mars possessing relatively the same activity as one of us might be fifteen feet tall and strong in proportion. But even granting the existence of such, a race of Goliaths on our neighbor world, it is not conceivable that they could have constructed a system of tremendous canals over half the surface of their planet, or that they would have done it if they could. The canals of Mars are enormously disproportioned in magnitude to the most gigantic inhabitants that a due regard for the law of gravitation would suffer us to imagine there.
An ingenious Frenchman has considerately and considerably diminished the difficulty for the inhabitants of Mars by the suggestion that the continents of that planet are so slightly elevated above the level of its seas that frequent and periodical inundations occur over large areas, thus forming temporary channels of communication between the seas which leave only the more elevated points above water to serve as places of refuge for the non-aquatic inhabitants. According to the theory, these inhabitants, possessing a horse sense comparable to that of the descendants of Noah, have, in the course of ages, improved and strengthened their natural places of refuge in times of flood, by excavating the ground from the low lands periodically invaded by the sea, and piling it up on the higher places, thus producing lines of partly artificial hills geometrically placed, and with talus-like flanks.
It will be observed that these attempts at explanation make no reference to the duplication of the canals. Mr. Proctor, always fertile in ingenious theories, undertook to include this strange transformation in an explanation of the canals which he suggested; namely, that they are great rivers, over and along which, in certain seasons, vast fog-banks are formed, or which, perhaps, being frozen in winter, remain covered with snow and ice in spring until the snow is melted along their banks, so that by a phenomenon of diffraction the image of the rivers appears to us as a light line between two dark ones.
M. Fizeau has put forth a theory according to which the canals of Mars are simply glacial productions, enormous crevasses and clefts in the ice covering the planet, like those seen on a smaller scale in our glaciers. But this theory, of course, would imply that Mars is now undergoing the effects of a glacial epoch, involving even the equatorial regions of the planet, while, as a matter of fact, the surface of Mars appears not to suffer from any extreme degree of cold. Attention has also been called to a fancied resemblance between the rectilineal canal system of Mars and the systems of rays seen on the moon, especially that which has. its center at the crater Tycho, and which, under certain illuminations, is one of the most conspicuous features of the lunar surface.
In fact, it may be said, in a double sense, that there is no end of speculations on this curious subject. But nothing has yet been proposed that covers all the appearances presented, and even a combination of atmospheric and geological activities seems insufficient to explain everything. It is possible that some deception of the eye may enter to a minor degree into the observations that have been so carefully described by Schiaparelli and others, but I can not believe that that excellent observer has been mistaken as to the main facts.
Mars is a world having an atmosphere as the Earth has, and possessing a diversified surface, upon which great operations of Nature are taking place under our eyes; and, while it may be idle for us to speculate as to whether those operations involve the weal or woe of a race of intelligent beings dwelling in the midst of them, yet the mind of man will never be satisfied to let such questions as these alone. If he can plant his foot upon one globe only, at least his thoughts can and will range among a million.