THE question of the habitability of “other worlds than ours” has always been of great interest to us as inhabitants of this earth, and speculations have at all times been entertained as to the probable nature and mode of existence of extra-terrestrial beings. Fontenelle, an ingenious French writer of some note in the early eighteenth century, composed a -series of dialogues. between himself and a fashionable lady, “the Marchioness,” on the different planets of the solar system, attributing to them inhabitants whose qualities were in accordance- with the infiuences respectively attributed to these bodies in astrological systems. The inhabitants of Mercury were of hot, “mercurial” temperent, those of Venus soft and amorous by nature, those of Mars warlike and belligerent, and so forth. • In later times, the famous Dr. Whewell indulged in some speculations as to the possible physical nature of the inhabitants of the various planets of our system. However, it must be confessed that little scientific evidence, one way or the other, exists as to such matters, and; indeed, there are many who consider—not altogether without ' reason on their side—that our earth alone in the universe is the abode of organized beings. This,' no doubt, may seem an extreme view, though it may at once be conceded that it is not easy to imagine how many of the forms of life with which we are conversant on this planet could exist even on the nearest of our neighbors in space. The very variety of conditions under which we know life to be possible here, however, should of itself preclude us from asserting that life can only - exist, in our own solar system at least, on this planet. Nothing seems to prevent the existence of totally different beings on every one of the planets (with the possible exception of the moon), organized and just as fitted for the conditions of their existence as we are for our own. As for the “fixed” stars, though their intense heat seems to preclude the present existence of life, yet, as they are in all probability surrounded by revolving planets not unlike those in our own system, there is no doubt that on some of these latter conditions prevail more or less akin to those existing here. Where we know nothing we may speculate without fear of contradiction. Our own solar system presents a considerable variety of bodies at varying distances from their central orb, and of very various sizes. The great distances of the planets Jupiter and Saturn from the sun, and the probability that their surfaces are still more or less incandescent, seem to be against the possibility of their being inhabited, though. there is no reason whatever, so far as we know, why some of their satellites, at least, should not be the abode of living beings. The planet Mercury, from its proximity to the sun, and from its turning one side only toward the latter, seems at present unfitted for habitation. The case is different, however, with Venus and Mars, our two nearest neighbors in the planetary system. Though much less is known as to the physical conditions of Venus than might be expected from its comparative proximity, approaching as it does at times within 26,000,000 miles, a distance less than that of any other heavenly body (the moon only excepted), yet sufficient is known to show a 80nsiderable analogy of conditions with those prevailing on our own planet. Its size is slightly less than that of the earth (diameter 7,700 miles, surface and volume correspondingly less than in our planet), and from markings detected on its surface, it has been concluded that it turns round once on its own axis in a period of 23 h. 21 m., so that its “day” is nearly of the same length as our own, though some have contended for a much longer period. Observers in Italy and other more favored regions have distinctly recognized markings on the planet's surface indicating the existence of continents and oceans “dimly visible,” while intensely bright spots; like the polar caps so well seen on Mars, have also been seen at times. Schroter and other observers long ago concluded the existence of high mountains upon the surface. When Venus is near the sun, distinct evidence of the existence of an extensive atmosphere twice as dense as our own is obtained; and the spectroscope shows the presence of water-vapor in some ' abundance. The dark portion of' the planet's disk (that turned away from the sun) is occasionally seen faintly illuminated, in a manner, says Prof. Young, recalling the aurora and other electrical manifestations on the earth. For a long time more or less conflicting evidence as to the existence of a possible satellite or attendant on Venus was given; but it seems fairly certain that if any such body exists it must - be a very small object., On the other hand, the absence of a “moon” is to a great extent made up for by the earth. It may, perhaps, seem strange to some to think of our own apparently dark earth as a luminary; but there is no doubt that, as seen from Venus, if there are any inhabitants, it must be at times a brilliant object. When Venus is nearest to the earth (nearly between earth and sun), its dark side is turned toward us, and, being also in the region of the sky close to the sun, it is invisible to us (except on the rare occasion of a “transit”); but, on the other hand, the earth, having its illuminated side turned wholly toward Venus, must be conspicuously visible as a '''full” disk of its maximum size, shining during the night, in the sun's absence, with a brilliancy greatly exceeding the maximum brightness of Venus as seen by us. Thus the want of a moon to the planet may be to a great extent made up for in this way. The distance of Venus from the sun is only about three-quarters that of the earth, or about 67,000,000 miles, so that any area of its surface must receive about twice the amount pf light and heat that an equal area on the earth receives; but, as we have already said, the presence of- a more extensive atmosphere may to a considerable extent mitigate this, to our idea, excessive amount. “In size, density, and general constitution,” says Prof. Young, “this planet is our earth's twin sister.” Its diameter is slightly smaller, its period of rotation slightly less (day), while its year is only a little over seven months. Air, water, lands, continents, mountains, polar snows, etc., all seem to be present. Thus, so far as our limited knowledge extends, the. evidence for the existence of- living beings, of a character not so very dissimilar from those with which we are familiar, seems as complete as we can reasonably expect. Turning now to Mars, we meet with a planet very considerably smaller. Its diameter is only about 4,200 miles, its mass less than one-ninth that of our own earth, and its volume only one-seventh. Its distance from the sun varies- from about 135 to 148 millions of miles, and it completes one revolution in 687 days, turning on its axis once in 24 hours 37 minutes. Its equator makes an angle with the plane of its orbit of about 25 deg., which is not much greater than the angle between the earth's equator and its orbit; so that, so far ' as the seasons depend upon this condition, there should not be much difference in this respect for the two planets. Like our own earth, Mars is not quite spherical, but slightly flattened at the pole, and this flattening is nearly the same in both cases. There are two small satellites, or moons; but they were only discovered in 1877 by the Washington observers with the great 26-inch refractor. Neither of these bodies exceeds ten miles in diameter, so that they cannot give much light to their primary, or produce any considerable tides. Phobos, the inner of these two satellites, revolves round Mars in less than eight hours—a period much shorter than that in which the planet makes one rotation on its axis. Thus it will appear to rise in the west and set in the east. The outer satellite, Deimos. will rise in the east, like the other heavenly bodies; but its orbital motion in the reverse direction is nearly as great, so that it will take about 130 hours between rising and setting, undergoing all its changes of phase—”new,” “full,” etc. —four times in that interval, its period of revolution being only thirty hours. Frequent eclipses of these satellites and transits across the sun's disk take place. In a small telescope some markings may usually be detected upon the planet's surface. In a more powerful instrument much detail can be perceived. The planet's disk will be seen to be not so distinctly reddish as it appears to the naked eye, but still of that color, though there will also be, found green and purple patches of more or less distinct outlines. Near either pole are to be seen brilliant white spots, which periodically increase and diminish according to their respective presentation away from or toward the sun, the spot at the pole turned away from the sun increasing, the other diminishing, just like the polar snows on our planet, whence the idea naturally arose that these spots on Mars are due to a similar cause. The details of the surface vary considerably from time to time. Spots and markings seen distinctly at one time are scarcely visible at another, while other markings are seen instead (the color of the surface varies at times), in addition, of course, to the change produced by the rotation of the planet. These variations are probably due to the atmosphere of the planet, laden with clouds and mists, obscuring more or less of the underlying features of the planet. By comparison of drawings made from time to time, maps have been made giving the permanent features of the planet's surface. There seems, so far as we can judge, a much greater proportion of land to sea than is the case with the earth, few great continents and oceans, but numerous narrow “seas” penetrating and dividing the “land.” But the most remarkable features of this planet's surface are a series of long, straight markings connecting the larger “seas,” first seen by Schi- aparelli in 1877, to which he gave the name of “canals.” In 1881 he saw them again, and this time many of these canals were double. Some of his observations have been confirmed by other astronomers; and others, again, have denied the existence of these canals, or, at least, their duplicity. M'. Terby considers the “double canals” to be purely an optical phenomenon, and remarks that they present exactly the appearance that single markings would present if viewed through a double-image prism. Some experiments made hy Mr. Maunder at Greenwich point to a similar conclusion. On the other hand, Prof. Lowell, firmly convinced of the reality of these markings, regards them as artificial works due to Martian engineers. He considers that the planet is now all but devoid of water, none remaining but a little in the polar regions. When the “snow-caps” melt, the canals convey the precious fluid toward the equator, and vegetation bursting round their banks produces the periodical changes of tint that we see. However, there is considerable uncertainty as to the true nature of these polar caps. At first it was thought, from terrestrial analogies, that these were composed of snow, melting on the approach of the planet's summer; yet there is much reason to suppose that they may ' consist of solid carbon dioxide, which much resembles snow in appearance, and evaporates at a very much lower temperature. The atmosphere of Mars is probably much rarer than our own, and, being considerably farther from the sun, both winter and summer temperatures must be lower; but yet the polar caps diminish to a much greater extent than could be expected. Dr. Johnstone Stoney supposes, accordingly, that it is solid carbon dioxide (carbonic acid gas frozen), and not snow, of which they are composed. Thus all the evidence we possess seems to point to this planet being a much older one than our own. If we suppose both earth and Mars originally incandescent bodies, the much smaller planet would 000I more quickly, and it would grow old and decrepit much sooner. The spectroscope shows the presence of water-vapor in the Martian atmosphere, and some are of opinion that there is a very considerable amount of it; but most observers consider that the amount both of air and water is considerably less, not merely absolutely, but in proportion to its bulk, than on our planet. Though some enthusiastic observers are convinced of the existence of rational beings, in an advanced state of civilization, inhabiting Mars, we may well pause before we arrive at this conclusion. All the evidence we certainly possess so far merely' indicates the possibility of a narrow equatorial zone being the abode of life, at least of a kind akin to that with which we are familiar. If the polar caps be composed of solid carbon dioxide, the temperature prevailing during the planet's winter seasons must be excessively low. Nevertheless, this by no means precludes the possibility of the existence of races of beings of a totally different character to anything we can imagine.—English Mechanic and World of Science.