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Mars that, notwithstanding its smallness, it will seem, when seen overhead from the surface of the planet, to be about two-thirds the size of our moon. The more distant and smaller satellite will appear but about a quarter of the size of its rival.

As the satellites revolve in paths vertically above the equator of their primary, the one less than 4,000 miles and the other only some 14,500 miles above the surface, it follows that they can never be visible from the poles of Mars; indeed, to see Phobos, the observer's planetary latitude must not be above 683°, or the satellite will be hidden by the body of Mars, just as we, in the British Islands, would be unable to see an object revolving round the earth a few hundred miles above the equator. For the same reason Phobos will only be rather more than one-third of each revolution above the horizon of a spectator on the planet.

Before passing from the attractive subject of the satellites, we may just mention two points of a literary character. Mr. Hall consulted his classical friends as to the names which should be conferred on the two satellites. Homer was referred to, and a passage in the "Iliad " suggested the names Deimos and Phobos. These personages were the attendants of Mars, and the lines in which they occur have been thus construed by my friend Professor Tyrrell :

"Mars spake, and called Dismay and Rout

To yoke his steeds, and he did on his harness sheen."

A curious circumstance with respect to the satellites of Mars will be familiar to those who are acquainted with "Gulliver's Travels." The astronomers on board the flying Island of Laputa had, according to Gulliver, keen vision and good telescopes. Gulliver announced that they had found two satellites to Mars, one of which revolves round Mars in ten hours. The author has thus not only made a correct guess about the number of the satellites, but he actually stated the periodic time with considerable accuracy! We do not know what can have suggested the latter guess. A few years ago any astronomer reading the voyage to Laputa would have said this was absurd. There might be two satellites to Mars no

doubt; but to say that one of them goes round in ten hours would be to assert what was "impossible!" impossible!" Yet the truth has been even stranger than fiction.

And now we must bring to a close our account of this beautiful and interesting planet. There are many additional features of it. over which we are tempted to linger, but there are so many other bodies which claim our attention in the solar system, so many other bodies which exceed Mars in size and intrinsic importance, that we are obliged to desist. Our next step will not, however, at once conduct us to the giant planets. We find outside Mars a host of planets, small indeed, but of the greatest interest; and with these we shall find abundant occupation for the following chapter.

CHAPTER XI.

THE MINOR PLANETS.

The Lesser Members of our System-Bode's Law-The Vacant Region in the Planetary System-The Research-The Discovery of Piazzi-Was the small Body a Planet?-The Planet becomes Invisible-Gauss undertakes the Search by Mathematics-The Planet Recovered-Further Discoveries-Number of Minor Planets now known-The Region to be Searched-The Construction of the Chart for the Search for Small Planets-How a Minor Planet is Discovered -Physical Nature of the Minor Planets Small Gravitation on the Minor Planets-The Berlin Computations-How the Minor Planets tell us the Distance of the Sun-Accuracy of the Observations-How they may be Multiplied-Victoria and Sappho-The most perfect Method.

In our chapters on the Sun and Moon, on the Earth and Venus, and on Mercury and Mars, we have been usually discussing the features and the movements of globes of vast dimensions. The least of all these bodies is the moon, but even our moon is a ball 2,000 miles from one side to the other. In approaching the subject of the minor planets, we must be prepared to find planets of dimensions quite inconsiderable in comparison with the great globes of our system. No doubt these minor planets are all of them some few miles, and some of them a great many miles, in diameter. Were they close to the earth they would be conspicuous, and even splendid objects; but as they are so distant they do not, even in our greatest telescopes, become very remarkable, while to the unaided eye they are totally invisible.

In a diagram of the orbits of the various planets, it is shown that a wide space exists between the orbit of Mars and the orbit of Jupiter. It was often surmised that this wide region must be tenanted by some other planet. The presumption became much stronger when a remarkable law was discovered which exhibited, with considerable accuracy, the relative distances of the great planets of our system. Take the series of numbers 0, 3, 6, 12, 24,

48, 96, whereof each number (except the second) is double of the number which precedes it. If we now add four to each, we have the series 4, 7, 10, 16, 28, 52, 100. With the exception of the fifth of these numbers (28) they are all sensibly proportional to the distances of the various planets from the sun. In fact, the distances are as follows:-Mercury, 3.9; Venus, 72; Earth, 10; Mars, 15-2; Jupiter, 52.9; Saturn, 95-4. Although we have no physical reason to offer why this law-known as Bode's law-should be true, yet the fact that it is so nearly true in the case of all the known planets, tempts us to ask whether there may not also be a planet revolving around the sun at the distance represented by 28.

So strongly was this felt that a large number of astronomers decided, at the end of the last century, to make a united effort to search for the unknown planet. It seemed certain that the planet could not be large, otherwise it would have been found long ago. If it should exist, then means must be found for discriminating between the planet and the hosts of stars strewn along its path.

The search for the small planet was soon rewarded by a success which has rendered the evening of the first day in this century memorable in astronomy. It was in the pure skies of Palermo that the observatory was situated, where the memorable discovery of the first minor planet was accomplished by Piazzi. This laborious and accomplished astronomer had organised a most ingenious system of exploring the heavens, which was eminently calculated to discriminate a planet from among the stars. On a certain night he would select a series of stars to the number of fifty, more or less according to circumstances. With his meridian circle he determined the places of all the fifty stars. The following night, or at all events as soon as convenient, he re-observed the whole fifty stars with the same instrument and in the same manner, and the whole operation was repeated on two, or perhaps more, nights subsequently. When the observations were compared together he was in possession of some four or more places of each one of the stars on four nights, and the whole series was complete. He was persevering enough to carry on these observations for very

many groups of stars, and at length he was rewarded by a success which amply compensated him for all his toil. It was on the 1st of January, 1801, that he commenced for the one hundredth and fifty-ninth time to observe a group of stars. Fifty stars this night were viewed in his telescope, and their places were carefully recorded. Of these objects the first twelve were undoubtedly stars, and so to all appearance was the thirteenth, a star of the eighth magnitude in the constellation of Taurus. There was nothing to distinguish the telescopic appearance of this object from all the stars which preceded or followed it. The following night Piazzi, according to his custom, re-observed the whole fifty stars, and he did the same again on the 3rd of January, and once again on the 4th. He then, as usual, brought together the four places he had found for each of the stars on his list. When this was done it was at once seen that the thirteenth object on the list was quite a different body from the remainder and from all the other stars which he had ever observed before. The four places of this mysterious object were all different; in other words, the object was in movement, and was therefore a planet.

A few days' observation sufficed to show how this little object, afterwards called Ceres, revolved around the sun, and how it circulated in that vacant path intermediate between the path of Mars and the path of Jupiter. Great, indeed, was the interest aroused by this discovery, and great, indeed, is the influence which it has exercised on the progress of astronomy. The majestic planets of our system were now to admit a much more humble object to a share of the benefits dispensed by the sun.

After Piazzi had obtained a few observations of the object, and after he had fully identified its character as a planet, the season for observing this part of the heavens had passed away, and the new planet of course ceased to be visible. In a few months, no doubt, the same part of the sky would be above the horizon after dark, and the stars would of course be seen as before. The planet, however, was moving, and would continue to move, and by the time the next season had arrived it would have passed off into some distant region, and would be again confounded with the stars

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