tion. Placed at the distance of ninety-five millions of miles from the sun, the earth has an orbit of near six hundred millions of miles to travel over. This circuit is performed at the mean rate of sixty-eight thousand miles an hour. Supposing the impulse that appoints this path to be withdrawn, in sixty-four days and a half, the terrestrial would crash with the solar surface, and sink into its mass like a millstone in the sea, owing to its greater density. That diversity of seasons which marks the year of our globe is the joint result of its movements, and of the inclination of its axis to the plane of its orbit. Supposing one of the poles to be always pointed towards the sun, as in fig. 1, then, notwithstanding the daily rotation and the motion in space, the regions from the poles to the equator would be one half in constant light and the other half in constant darkness. Or supposing the equator always pointed directly towards the sun, as in fig. 2, light and darkness would alternate for equal times on all parts of the globe; and there would be different seasons at different places, but no change of seasons in every place as at present. The axis is inclined 231° from a line perpendicular to the plane of the orbit, and remains constantly parallel to itself during the annual revolution. This arrangement causes the same part of the globe to experience days and nights of unequal length, and to receive the solar influence in a more or less oblique direction. The annexed view represents the earth in four different parts of its orbit. It illustrates the varying direction of the sun's rays to the same regions of the globe, and the varying duration of the diurnal exposure to their action, and withdrawment from it. Thus, at the vernal and autumnal equinoxes, it will be seen, that the sun's light reaches equally from pole to pole, so that each part of the surface is carried by the daily rotation into light and darkness of the same duration. At the summer solstice, as the earth rotates, the whole arctic circle enjoys continued day, while the north temperate zone has not only long days and short nights, but receives the sun's rays less obliquely than at the former periods, and consequently experiences an increase of temperature. A precisely opposite effect takes place in the same region at the winter solstice. Hence arises seasonal vicissitude with its resulting phenomena, the repose of vegetation in winter, the renewal of the face of nature in spring, the manhood of its productions in summer, and their autumnal ripeness and garnering. There is a third motion to which the earth is subject, occasioning what is called the precession of the equinoxes. Wherever, in spring and autumn, the sun, in his apparent annual course, crosses the equinoctial, or the circle of the earth's equator extended to the heavens, there are the vernal and autumnal equinoxes. These points are found to have a constant motion westward, at a very small yearly rate, but which effects considerable alterations in a series of ages. To give a familiar illustration of the fact, we may suppose three roads to extend round the earth: one, running due east يلوت Ecliptic Meridian and west, representing the equator or equinoctial; another, proceeding above and below the former, crossing it at two opposite points, representing the ecliptic; a third, crossing the first at right angles, at the points of intersection with the second, representing the prime meridian, or initial point of Equinoctial longitude. We may now suppose a carriage to be started along the ecliptic from the common point of intersection, returning, after a complete circuit of the earth, not to the place from whence it started, but passing the equinoctial about a hundred rods to the west of it. The repetition of this at each circuit, the more westerly intersection of the roads, will illustrate the annual retrograde movement of the equinoctial points, termed on account of its effect in accelerating the time of the equinoxes, their precession, or going forward. This movement, first observed by Hipparchus, causes a progressive increase of the longitude of the stars, and has been clearly shown to be a necessary consequence of the rotation of the earth, combined with its elliptical figure, and the unequal attraction of the sun and moon on its polar and equatorial regions. The mass of matter about the earth's equator being greater than at the poles, the former is more powerfully acted on by the law of attraction than the latter, which produces a slow reeling motion of the axis of the earth from east to west, and the retrocession westward of the equinoctial points. The retrogradation is at the rate of about 501′′ in a year, or 1o in 71 years, so that in a period of 25,000 years the equinoxes will accomplish a complete revolution along the circle of the ecliptic. While the plane of the earth's equator, or the equinoctial, thus experiences a constant displacement from the action of the sun and moon, it is a remarkable and well ascertained fact that the plane of the earth's orbit, or the ecliptic, is subject to a slow annual displacement, which diminishes its obliquity, an effect due to the baiting which our globe endures from the other planets, chiefly from the attacks of Jupiter and Venus. In consequence of this, the tropics are slowly and steadily approaching the equinoctial, so that the sun does not now come so far north of the equator in summer, nor decline so far south in winter, by a degree, as he must have done six thousand years ago. The obliquity of the ecliptic, or the angle which the plane of the earth's orbit makes with that of the equator, has been observed with care in different ages, and the following results obtained. Thus, in the interval of two thousand years, the obliquity of the ecliptic has decreased by only 23' 43"-5. There is little ground, therefore, for the apprehension of the seasons being annihilated owing to the ecliptic coinciding with the equator, and equalising the length of our days and nights. The event is far away in the womb of the future, even supposing the diminution to go on without check. But the theory of gravitation tells us that it will at length cease, and an increase of the obliquity set in, to decrease again and increase, oscillating about a mean position within very restricted limits. One of the most important attributes of our globe as a planet is the gaseous envelope that encloses it on all sides the atmosphere-essential to the vitality of its organised occupants, man, brute, bird, and plant, but chiefly interesting to the astronomer from its influence in displacing celestial phenomena by its refractive power, and diffusing the rays of light so as to surround us on every hand with visible glories. The atmosphere rapidly diminishes in density as we recede from the surface. The diagram exhibits various strata of air resting upon the earth, the upper pressing upon the lower, and causing the interior to be more dense than the exterior strata. It is not known how high this elastic medium extends, but "to breathe The difficult air on the ic'd-mountain's top," is an experiment which shows the atmosphere to be exceedingly rare at no greater height than what is reached by many of the superficial elevations of the earth. Mr Green the aeronaut ascended in a balloon upwards of five miles-the greatest altitude ever reached by man. Visible clouds, however, are supposed to be sometimes twice that height, and atmospheric phenomena have been noticed, conceived to have had an elevation of forty miles. But the chief peculiarity of our planet is the presence of a secondary body as a satellite, reflecting to its surface the light of the sun in the absence of his direct rays. It has indeed been supposed that Venus is similarly dignified, and we are not in circumstances to say positively that this is not the case, either in relation to her or Mercury, as such an attendant, if small, might exist, and have hitherto escaped notice, owing to the position occupied by those bodies. Cassini and others have imagined they perceived a satellite attending Venus, but the observation has not been verified. The terrestrial world occupies a favoured place in the system, a position from which nearly all the sister planets are visible to the naked eye. It will not be seen itself by the inhabitants of Uranus, and be scarcely perceptible to those of Saturn. As an interior planet to Jupiter and Mars, it will appear occasionally as a spot upon the sun's disk, performing similar transits to those of Venus and Mercury. Its great divisions of land and water, the outlines of its continents and seas, with its masses of ice and snow at the poles, will be seen from Mars; and at the time of the inferior conjunction of Venus, when she is not more than twenty-six millions of miles removed from us, our globe will exhibit a full orb, shining with great splendour through the whole of her night. CHAPTER III. THE MOON AND LUNAR PHENOMENA. races. EXT to the greater light that rules the day, the most useful and interesting to us of all the bodies in the universe, is the lesser light that rules the night. The proximity of the moon, the relation in which she is linked to the earth, the power she exerts upon the ocean in drawing up its billows, and the great importance of the lunar theory to safe navigation, have intently fixed the eye of science upon her orb; while the mild radiance with which she shines in the heavens, the advantage of her light to the terrestrial traveller, and the beauty and regularity of her changing phases, have elicited the admiration of barbarian and polished The unfailing performance within a definite period of a synodical revolution, or the cycle included between each conjunction with the sun, when she is invisible, called synodical, from the Greek word signifying a coming together, has rendered the moon a convenient time-keeper to men in rude states of society, and won for her the love and respect of savage tribes. Among the wandering hordes of the western continent, such a number of moons measures the duration of a journey, and the lapse of events; and successive lunar appearances are discriminated by coincident terrestrial occurrences, as the wild-strawberry moon, the wild-rice gathering moon, the ice moon, the deer-rutting moon, and the leaf-falling moon. Some of the sacred ceremonies of the Jews, in the early periods of their history, were regulated by the sign of the lunar crescent in the heavens, and the rabbins relate, that persons were stationed on the tops of the mountains to watch for the first appearance of the moon, which event was proclaimed by signal fires throughout the land. In all ages, the eye of man has gazed with delight upon her face, whether in courtly or in rustic life, from old baronial halls or cottages obscure. The meek splendour, the quietude, the fidelity, of which the luminary is a visible image, bewitch the senses, excite the imagination, and have originated some of the most captivating strains of poetic description, among which the Trojan bivouac scene in the Iliad still stands peerless. "The troops exulting sat in order round, O'er heaven's clear azure spreads her sacred light; An imaginary soliloquy, put into the mouth of Milton by a living writer, strikingly expresses the emotions of such a mind, upon first perceiving the curtain about to fall |