from us, but in opposition* (that is, on the side remote from the sun), all four of his satellites, the least of which is rather less than our own moon, are quite easily seen in the smallest telescopes ever used in astronomical observation. Certainly they can then be all seen with a good telescope one inch in aperture. At such times Jupiter lies at a distance of about 410 millions of miles from us. Now Mars, when he makes his nearest opposition approaches (as for instance in the present autumn), lies at a distance from us of about 35 millions of miles, or less than Jupiter's in the proportion of about seven to eighty-two, or at not much more than one-twelfth of Jupiter's distance. This would cause a self-luminous body to appear about 140 times brighter at Mars's distance than at Jupiter's. But satellites are not self-luminous. Their brightness depends on sunlight, and the nearer they are to the sun the more brightly they necessarily shine. Mars is illuminated, when nearest to the sun, with an amount of sunlight exceeding that which illumines Jupiter when farthest from the sun (these being the cases we are dealing with) in a proportion of more than fifteen to one. So that a satellite near Mars, as large as the least satellite of Jupiter, would shine fifteen times 140 times more brightly, or, in round numbers, fully 2,000 times more brightly, than one of those bodies which the observer can readily see with a telescope only one inch in aperture. But most certainly it is not assuming too much to claim for the most powerful telescopes with which Mars's neighborhood had been searched for satellites an illuminating power exceeding that of so minute a telescope 400 times. This would have made such a moon as we have imagined appear at least 800,000 times brighter than the least of Jupiter's moons actually appears in a telescope one inch in aperture. If, then, instead of being so large as this that is, 2,000 miles or so

*The reader must not understand us here

to mean that it is when in opposition that Jupiter is farthest from us, for the reverse is the case. It is at his successive opposition that he makes his nearest approach to the earth; but he is nearer at some oppositions

than at others, and we are speaking above of those oppositions when his distance is greatest.

in diameter-a moon of Mars had a diameter so much less that the disc were reduced to one-800,000th part of such a moon's disc, it would be as readily visible with one of the very powerful telescopes above mentioned as is Jupiter's least moon with a one-inch telescope. This would be the case if the diameter were reduced to one-895th part (for 895 times 895 is very nearly equal to 800,ooo). So that, were it not for one consideration now to be mentioned, it would have seemed that astronomers might safely have assumed that Mars has not a moon exceeding 24 miles in diameter. The consideration in question is this: a satellite might travel very near to Mars, so that it would always be more or less involved in the luminosity surrounding his disc. The best telescope cannot get rid of this luminosity; for, in fact, it is not an optical but a real light. It is, in fact, our own air, which is lit up by the planet's rays for some distance all round. Now a small satellite amidst this light, even though the planet itself might be kept out of view, would be much less readily viewed than a satellite seen like one of Jupiter's at a great distance from its primary. Yet, as it is known that Jupiter's satellites can be traced right up to the edge of the planet, we do not think so much importance should be attributed to this circumstance as is sometimes done. It should certainly be possible to see a Martian satellite two diameters of the planet, let us say, from the edge, if it shine with twice as much light as would make it visible on a perfectly dark sky. Let us, however, say that the satellite ought to be four times instead of twice as bright. Then the diameter, instead of being 24 miles in order that a satellite close to Mars should just be visible in a very powerful telescope, should be 41⁄2 miles. Certainly we should expect that a satellite five miles in diameter would have been long since revealed under the searching scrutiny to which the neighborhood of Mars has again and again been subjected.

Now it could not but be admitted that a moon five miles or even ten miles in diameter would differ so much from any known moon that the difference must be regarded as rather one of kind than one of degree. No such body had as yet

been heard of at least no such body travelling as an independent moon. A hundred years ago, indeed, men would hardly have been prepared to admit the possibility of a body whose existence, if demonstrated, would have overthrown all their ideas as to the structure of the solar system. They knew of suns, of planets attending on one sun, and of moons attending on several planets, and they knew also of a ring-system accompanying one planet in its course round the sun. Thus they were prepared to recognise new suns, new planets, new moons, and new rings. Sir W. Herschel was nightly engaged in observing hundreds of before unknown suns. He discovered one new planet (Uranus), several new moons attending on Uranus and Saturn, and, as he thought, a pair of new rings attending on Uranus. But that any of the primary planets should be attended by a moon so small as not to admit of being fairly classed with the other known moons of the solar system would have seemed to most of the astronomers of the last century an idea as inadmissible as that an orbital region of the solar system should be occupied by a number of very small planets instead of a single primary planet. In recent times, however, men have become accustomed to recognise how small is our right to assert definitely the characteristics of suns, planets, moons, rings, and other such or ders of bodies in the universe. We have found that, besides such suns as our own, there are some so much larger that they must be regarded as forming a distinct class of giant suns; while others, again, are separated in kind, not merely in degree, from such suns as ours, because of their relative minuteness. We have learned in like manner to distinguish the planets into classes, recognising in the giant planets Jupiter, Saturn, Uranus, and Neptune a family altogether distinct from that of the terrestrial planets, the earth and Venus, Mars and Mercury; while among the minor planets which throng in hundreds, perhaps in thousands, the orbit region between Mars and Jupiter we find another family separated from the terrestrial planets as definitely by their extreme minuteness as are the giant planets by their enormous dimensions. Among ring-systems, again, we had learned to recognise many varieties. In

the rings of Saturn we have a system formed of multitudes of tiny moons trav-. elling so closely together as to appear from our distant station as continuous rings. In the ring of minor planets we have multitudes of tiny planets; but they are so widely strewn that each must be separately sought for with the telescope and no signs of the ring as a whole can be seen in the heavens. Then we have the rings of meteors, oval for the most part in figure and often curiously eccentric as well as extended; sometimes complete rings, or nearly so, like those which produce the August displays of shooting-stars; sometimes incomplete, and at others known only by "the gem of the ring," one rich region in the entire circuit.

But even with our actual knowledge of the diversity existing among the orders of bodies constituting the inaterial universe, we were scarcely prepared to hear of moons like those of Mars. It is not the smallness of these bodies which is so surprising. There would have been nothing very remarkable in the existence of even smaller moons attending on any of the minor planets. Nor is it merely the enormous difference of dimensions between the planet and its moons; for in the case of Jupiter we have a planet whose moons bear a very much smaller proportion to the mass of their primary than our moon bears to the earth; and, though the disproportion is nothing nearly so great as that between Mars and his moons, it would still prepare us for recognizing any degree almost of disproportion between a planet and its satellite. The strange circumstance in the actual case lies in the fact that Mars belongs to a known family of planets, viz., the terrestrial family of which our earth is a leading member; and hitherto it had appeared as if all moons attending on the planets of one and the same class belonged themselves to one and the same class. The range of diversity of magnitude among the moons, for instance, attending on the giant planets, though considerable, is not such as to prevent us from regarding these moons as all of one class. Then, too, it seemed from the fact that our own moon is of the same class as those others, that, speaking generally, diversity of size is not to be

looked for to the same degree among moons even attending on planets of different classes, as among planets or among suns. Certainly there was nothing in the past experience of astronomers to suggest that a planet like Mars, belonging to the same class as our earth, might have a moon or moons belonging to an altogether inferior class.

It was, then, with a sense of astonishment, which would have been mingled with doubt but for the altogether unexceptionable source whence the information came, that astronomers heard of the discovery of two Martian satellites with the great telescope of the Washington Observatory.

The discoverer of the satellites, and the telescope with which they were discovered, both promised well for the truth of what some regarded at first as a mere report.

Professor Asaph Hall, who has long been known as one of that band of skilful and original observers of which American astronomy has just reason to be proud, had during the last few years made many observations showing that, besides scientific skill, he possesses a keen eye. Some of his observations were such as must have taxed even the power of the noble instrument which has lately been erected at Washington. For instance, the faintest of Saturn's satellites, the coy Hyperion, though discovered nearly thirty years ago, had been very little observed, insomuch that the true path of this small moon (a perfect giant, however, compared with the Martian satellites) had not been determined. In 1875, Professor Hall undertook the difficult task of closely observing this body; and now, at last, astronomers at least know where, at any hour, on any night, Hyperion is to be looked for, though the search would be to very little purpose with any save two or three of the most powerful telescopes in existence. Again, amongst other of his observations which required keen vision and patient watchfulness, must be cited the re-determination of the period in which the planet Saturn turns on its axis. This he accomplished in the year 1876. But, undoubtedly, the detection of the Martian satellites must be regarded as a far more noteworthy achievement than either of these.

The telescope which Professor Hall has been privileged to use may fairly be described as the finest refractor yet mounted. Newall, in England, has a telescope 25 inches in aperture, which, until the Washington telescope had been made, was the largest refractor in existence. The Washington instrument has an aperture of 26 inches, making its illuminating power between one-twelfth and one-thirteenth greater. But this telescope is also remarkable for the skill with which it has been made by Messrs. Alvan Clark & Sons, of Cambridgeport, Mass. We know few more interesting histories in scientific biography than that which records the progress of Alvan Clark's labors in the construction of object-glasses-from the first small one which he made (which fell from his hands and was destroyed within a few moments of its completion) to the noble telescope which was mounted at Washington five years ago, after meeting satisfactorily all the tests applied to it by Mr. Clark and his two sons, who inherit his energy and skill. But in this place we must be content with noting that all who have ever used object-glasses constructed by the Clarks have found their optical performance all but perfect; in fact, as nearly perfect as can be obtained from lenses made of a substance which cannot possibly be altogether free from defects, however carefully prepared. Those observers at Washington who have used the great telescope systematically, agree in regarding with peculiar favor the performance of the great compound lens which forms what is technically called its object-glass.

When, then, news came that Professor Hall, using this powerful instrument, had discovered two satellites of Mars, even those who at first supposed the news to be a mere report, felt that the observer and the telescope were alike worthy of being credited with a success of the kind.

But in reality there was no room for doubt from the beginning. The news had been telegraphed to Leverrier by the Smithsonian Institution of Washington, and by Leverrier announced to English and Continental observers. It was known that an arrangement had been made by the oceanic telegraph companies to forward such intelligence, and that the news must of necessity have come

from the source indicated. So that several days or so before details of the discovery reached Europe, the present writer communicated it to the Times (in a letter which appeared on Saturday, August 25), or less than a week after the second moon had been detected, as a discovery not open to doubt or question. Within two days from this, or on August 27, the brothers Henry were able to recognise the outer satellite with the fine telescope of the Paris Observatory; but it was very faint, and could only be seen when the planet was screened from view. In the mean time, however, two other telescopes in America had been used to bring these tiny bodies into view. One of these was the fine 15-inch Merz refractor of the Harvard Observatory, Cambridge, Mass., celebrated in the history of astronomy as that wherewith Sat urn's satellite Hyperion had been detected in 1848. The other was an instrument as large, and doubtless as powerful, as the Washington telescope itself. It will have been noticed, perhaps, that, in speaking of the latter above, we said that it is the finest refractor yet mounted, not the finest yet made. Messrs. Alvan Clark have made a companion instrument for the observatory of Mr. M'Cormick, of Chicago, one of those munificent patrons of science of whom (of late, in particular) America has just reason to be proud. The instrument has not yet left Messrs. Clark's factory, and cannot be said to have been yet (properly speaking) mounted. But the Clarks managed to get it turned upon Mars, and were able to see the Martian satellites. There is another very fine telescope, by the way, also made by Messrs. Clark & Sons, which is now erected at Chicago, where one of the most eminent observers of double stars, Mr. S. W. Burnham, has long pursued his labors. Its object-glass is 18 inches in aperture; and we should have expect ed that, with this aperture and Mr. Burnham's keen vision, the Martian satellites would have been brought into view. We do not hear, however, of their being seen at Chicago. Perhaps unfavorable

* We use the technical term "refractor" as the only convenient way of describing a telescope with an object-glass, as distinguished from a telescope with a mirror or speculum,

which is called a "reflector."

NEW SERIES.-VOL. XXVI., No. 6

weather prevented any observations being made there.

The first news was expressed in telegraph-language, and was imperfect. It ran thus: Two satellites of Mars discovered by Hall at Washington. First elongation west August 18, eleven hours, Washington time. Distance eighty seconds. Period, thirty hours. Distance of second, fifty seconds." This being interpreted (or rather, the latter part being interpreted), means that the outermost, in its circuit around Mars, had reached its greatest apparent westerly range at 11 P.M., Washington time, August 18, or about 4 A.M., August 19, Greenwich time (which Astronomers would call August 18, sixteen hours Greenwich time), and that at this time its seeming distance from the centre of Mars was about one twenty-fourth part of the apparent diameter of the moon. As to the other satellite the news did not convey much information. It implied that the distance was five-eighths that of the outer moon; but whether that was the greatest distance, or the distance at the hour named, there was nothing to show. As it turned out, there was a mistake about this moon, for the greatest range of the moon, east and west of Mars, amounted only to about threefifths of the distance named.

In the circular issued by the Secretary of the United States Navy (the Hon. R. W. Thompson), dated August 21, 1877, a copy of which reached the present writer on September 3, fuller and more correct details are given, in a form, however, which would be quite unsuited to these pages. We will endeavor to present their meaning correctly, but without technical expressions.

The outer satellite travels at a distance from Mars's centre, such that, when the planet is at its nearest, the extreme apparent span of the satellite's path would be about one-eleventh part of the moon's apparent diameter. In actual length this range is about 28,600 miles, half of which represents the distance from the centre of the planet-about 14,300 miles. As Mars has a diameter of about 4,600 miles, the distance of the satellite from his surface is about 12,000 miles, or, roughly, about one-twentieth of the distance which separates the moon from the earth. This other moon trav

43

els round Mars in thirty hours fourteen minutes, the possible error in this determination at present being about two minutes. We have seen that it must be a very small moon. The present writer, in an article in the Spectator which appeared before the circular above mentioned had reached Europe, had indicated ten miles as the greatest diameter which could possibly be assigned to this body. Let us hear what Professor Newcomb, the eminent mathematician who presides over the astronomical department of the Washington Observatory, who has himself seen the satellite, has to say on this point. Writing to the New York Tribune he remarks that "the first question which will naturally arise is, Why have these objects not been seen before? The answer is, that Mars is now nearer to the earth than he has been at any time since 1845, when the great telescopes of the present day had hardly begun to be known. In 1862, when Mars was again pretty near to the earth, we may suppose that they were not looked for with the two or three telescopes which alone would have shown them. In 1875 Mars was too far south of the equator to be advantageously observed in high northern latitudes. The present opportunity of observing the planet is about the best that could possibly occur. At the next opposition, in October, 1879, there is hope that the satellites may again be observed with the great telescope at Washington; but Professor Newcomb thinks that during the following ten years, when, owing to the great eccentricity of the orbit of Mars, he will be much farther from the earth at opposition, the satellites may be invisible with all the telescopes of the world. In the present year it is hardly likely that they will be visible after October. The satellites may be considered as by far the smallest heavenly bodies yet known. It is hardly possible to make anything like a numerical estimate of their diameters, because they are seen in the telescope only as faint points of light. But one might safely agree to ride round one of them in a railway car between two successive meals, or to walk round in easy stages during a very brief vacation. In fact, supposing the surface of the outer one to have the same reflectng power as that of Mars, its diameter

cannot be much more than ten miles, and may be less. Altogether these objects must be regarded as among the most remarkable members of the solar system."

Assigning to this satellite a diameter of ten miles-which we ourselves, for the reasons above indicated, consider too large-it would appear, at a distance of 12,000 miles, with a diameter equal to about the tenth of our moon's, and therefore with a disc equal to about a hundredth of hers in apparent area. But being less brightly illuminated it would shine with less than the hundredth part of her light. Mars receives from the sun (and therefore his moons receive) between one-half and one-third as much light as our earth and moon receive, about half when Mars is at his nearest to the sun, and about one-third when he is at his farthest from the sun. Thus the light given by the farther of his two moons varies from one two-hundredth to one three-hundredth part of our moon's. This part, then, of the Martian moonlight is but small in amount, and certainly cannot go far to compensate the Martians (as compared with us Terrestrials) for their greater distance fom the

sun.

Of course this moon passes through all the phases which we recognise in the case of our own moon. It travels very rapidly among the constellations of the Martian heavens, which are exactly the same in all respects as those we see. In very little over thirty hours it traverses the entire circuit of the heavens; or over what would correspond to one of our zodiacal signs in two and a half hours: whereas our own moon takes more than two and a quarter days traversing one of these signs. Its rate of motion may be best inferred, however, from the statement that, if our moon travelled as fast, she would traverse a distance equal to her own diameter in a little over two and a half minutes, so that her motion among the stars would be quite obvious to ordinary vision. Perhaps the reader may be interested to know which constellations are traversed by this Martian moon in the course of its circuit of the heavens. The zodiac of Mars, or the pathway of the sun and planets, is nearly the same as ours; but her outer moon, instead of travelling, as ours does, within the zodi