on the bright limb of the moon for a second or more, and then suddenly fades from the sight.

This phenomenon, as also another of most startling character attending sometimes the total eclipse of the sun, when blood-red streaks in radiations are found to shoot suddenly from behind the moon's limb, are supposed by some to demonstrate the existence of a lunar atmosphere. Much attention has been bestowed on the total eclipses of the sun during the past twenty years, for the express purpose of solving, if possible, these mysterious radiations of red light. Some entertain the opinion that they are due to the coloured glasses used to soften the intense solar light, as seen through the telescope. We can only say that these phenomena remain without satisfactory explanation, and that the physical condition of the moon is yet a problem of the deepest interest. We can assert the irregularities of her surface, her deep cavities and lofty elevations, her extended plains and abrupt mountain-peaks, but beyond this, our positive knowledge does not extend.

We shall resume the consideration of our satellite when we come to discuss the great theory of universal gravitation.

CHAPTER V.

MARS, THE FOURTH PLANET IN THE ORDER OF DISTANCE FROM THE SUN.

Phenomena of Mars difficult to explain with the Earth as the Centre of Motion.-Copernican System applied.-Epicycle of Mars.-Better instruments and more accurate observations.-Tycho and Kepler.-Kepler's method of investigation.-Circles and Epicycles exhausted.-The Ellipse, -Its Properties.-The Orbit of Mars an Ellipse.-Kepler's Laws.Elliptical Orbits of the Planets.-The Elements of the Planetary Orbits explained.-How these Elements are obtained.-Kepler's third Law.Value of this Law.-The Physical Aspect of Mars.-Snow-zones.— Rotation of the Planet.-Diameter and Volume.-Speculation as to its Climate and Colour.

THIS planet is distinguished to the naked eye by its brilliant red light, and is one of the planets discovered by the ancients. To the old astronomers Mars presented an object of special difficulty. Revolving as it does in an orbit of great eccentricity, sometimes receding from the earth to a vast distance, then approaching so near as to rival in brilliancy the large planets Jupiter and Venus, on the old hypothesis of the central position of the earth, and the uniform circular motion of the planets, Mars presented anomalies in his revolution most difficult of explanation.

These complications were partially removed by the great discovery of Copernicus, which released the earth from its false position, and gave to Mars its true centre, the sun; but even with this extraordinary advance in the direction towards a full solution of the mysterious movements of this planet, there remained many anomalies of motion of a most curious and incomprehensible character. It will be remembered that Copernicus, in adopting the sun as the centre of the planetary orbits, was compelled to retain the epicycle of the old Greek theorists, to account for the facts which still distinguished the planetary revolutions. As in

the revolution of the earth about the sun there was an approach to, and recess from, this central orb, so in the revolution of Mars it was manifest that there was a vast difference between the aphelion and perihelion distances of

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the planet. The epicycle was then retained to account for this anomaly in the motion of Mars; and it will be readily seen from the figure above how this hypothesis rendered a general explanation of the facts presented for examination.

The large circle, having the sun for its centre, represents the orbit of Mars; that is, a circle whose radius is equal to the average or mean distance of the planet. The small circles represent the epicycle, in the circumference of which the planet revolves with an equable motion, while its centre moves uniformly round on the circumference of the large circle. When the planet is at A, it is in perihelion, or nearest the sun. While the centre of the epicycle performs a quarter-revolution, the planet also performs in its epicycle a quarter of a revolution, and reaches the position B. A half-revolution brings it to aphelion in C, and three quarters of a revolution in the epicycle locates the planet at D, and an entire revolution brings it again to A, the point of departure. Thus it will be seen that the planet

must describe an oval curve, traced in the figure A B C D, and for general purposes this exposition of the phenomena seemed entirely satisfactory. It is true that it only accounted for the movement from east to west, or in longitude, while the motion north and south of the earth's orbit, or in latitude, was accounted for by supposing the plane of the epicycle to vibrate or rock up and down, or right and left of the plane of the ecliptic, while its centre moved uniformly round in the great circle constituting the orbit of the planet.

So long as observation was so defective as to yield but rough places of the heavenly bodies, the deviations from the path marked out by the theory of epicycles escaped detection. The erection of the great observatory of Uraniberg, by the celebrated astronomer Tycho Brahe, and the furnishing it with instruments of superior delicacy, introduced a new era in the history of astronomical observation. The instruments employed by Copernicus were incapable of giving the place of a star or planet with a precision such as to avoid errors amounting to even the half of one degree, or an amount of space equal to the sun's apparent diameter. The instruments employed by Tycho reduced the errors of observation from fractions of degrees to fractions of minutes of arc; and when thus critically examined, the planets, as well as the sun and moon, presented anomalies of motion, requiring to account for them a large accumulation of complexity in the celestial machinery. Such was the condition of theoretic and practical astronomy at the era inaugurated by the appearance of the celebrated Kepler. This distinguished astronomer early became a devoted advocate of the Copernican system of the universe, adopting not only the central position of the sun, but also the ancient doctrine of uniform circular motion, and the theory of epicycles. The investigations of Kepler on the motions of the planet Mars commenced after joining Tycho at Uraniberg, in 1603, and, based upon the accurate observations of this later astro

nomer, finally led to the overthrow of the old theory of epicycles and circular motion, by introducing the true figure of the planetary orbits; and with the elliptical theory of planetary motion, commenced the dawn of that brighter day of modern science, which in our age sheds its light upon the world.

The history of the great discoveries of Kepler presents one of the most extraordinary chapters in the science of astronomy. It must be remembered that the doctrine of circular motion, at once so beautiful and simple, had held its sway over the human mind for more than two thousand years. Such, indeed, was its power of fascination, that even the bold and independent mind of Copernicus could not break away from its sway. When Kepler commenced his examination of the movements of Mars, it was under the full and firm conviction that the theory of circles and epicycles was unquestionably true. His task, then, was simply to frame a combination such as would account for the new anomalies in the motions of Mars discovered by the refined observations of Tycho. The amount of industry, perse.. verance, sagacity, and inventive genius displayed by Kepler in this great effort, is unparalleled in the history of astronomical discovery. His plan of operation was admirably laid, and if fully and faithfully carried out, could not fail, in the end, to exhaust the subject, and to prove at least the great negative truth, that no combination of circles and epicycles could by any possibility truly represent the exact movements of this flying world. It is useless to enumerate the different hypotheses employed by Kepler. They were no less than nineteen in number, each of which was examined with the most laborious care, and each of which, in succession, he was compelled to reject. Having adopted an hypothesis, he computed what ought to be the visible positions of the planet Mars, as seen from the earth, throughout its entire revolution. He compared these computed places or positions with the observed places, or those actually occupied by the planet,