are, however, of two kinds: the one depending on the principle of refraction, and called the dioptric telescope; the other, on the principle of reflection, and therefore termed the reflecting telescope. Dr. Herschel has made several specula, which are so perfect as to bear a magnifying power of more than six thousand times in diameter on a distant object. 7. Light and colours. The bodies which emit rays of light, as the sun and fixed stars, are called luminous; those that transmit the rays of light, as the air, pellucid; and those that reflect the rays of light, as iron, earth, &c. opaque. These last are specular, or not specular. Specular bodies, are opaque bodies with polished surfaces, as mirrors. The rays reflected from opaque bodies always bring with them to the eye the idea of colour; but this colour is nothing more in the bodies, than a disposition to reflect to the eye one sort of rays more copiously than another. For, particular rays are generally endowed with particular colours; some are red, others blue, yellow, green, &c. Every ray of light, as it comes from the sun, seems a bundle of these several sorts of rays; and as some of them are more refrangible than others; that is, are more turned out of their course, in passing from one medium to another, it follows that after such refraction they will be separated, and their distinct colour observed. Of these the most refrangible are violet, and the least, red; and the intermediate ones, in order, are indigo, blue, green, yellow, and orange. 8. This separation is very entertaining, and may be observed with pleasure in the well known experiment of the prism. A ray being let into a darkened room, throngh a small round aperture in the shutter, and falling on a triangular glass prism, is, by the refraction of the prism, considerably dilated, and will exhibit on the opposite wall an oblong image, called a spectrum, variously coloured;. the extremities of which are bounded by semicircles, and the sides are rectilinear. The colours are commonly divided into seven, which, however, have various shades gradually intermixing at their juncture. Their order, beginning from the side of the refracting angle of the prism, is red, orange, yellow, green, blue, purple, violet. As all these rays differ in refrangibility, so they do in reflexibility, that is, in the property of being more easily reflected from certain bodies, than from others and hence arise all the various colours. White is a compound of all the seven colours. If a circular wheel be painted with the seven colours in due proportion on its rim, that is to say, 45 degrees red, orange 27, yellow 48, green 60, blue 60, indigo 40, and violet 80,--in all 360, and the wheel be turned with considerable velocity, the whole rim or circumference will appear white; consequently, whiteness in bodies, is but a disposition to reflect all colours of light nearly in the proportion in which they are mixed in the original rays. Black, on the contrary, is an entire deprivation of all the colours, and the light is completely absorbed when an object appears black. 9. Rainbow. This beautiful phenomenon is formed by the solar rays entering the drops of falling rain, and being there refracted on the further surface, emerging from the drops after one or more reflexions; and at their emergence, as well as at their entrance, suffering a refraction by which the compound rays are separated, they enter the eye of a spectator, and exhibit the prismatic colours. The back of the spectator must be turned towards the sun. His eye is in the vertex of a cone, in the surface of which all the drops lie which render the rays of any one colour efficacious. Hence it is that the how appears circular. The axis of the cone is directed to the sun; and hence the sun is directly opposite to the centre of the bow. The primary bow is that which is most usually seen in the heavens, and is formed by two refractions and one intervening reflection of the rays. The secondary bow is formed by the rays emerging after two refractions and two intervening reflections. Hence, in the primary bow, the least refrangible rays, and in the secondary bow, the most refrangible rays, are outermost. The colours appear so much the more vivid, as the cloud behind is darker, and the drops of rain fall thick and fast. The drops falling Continually produce a new rainbow every moment; and as each spectator has his particular situation, from which he observes this phenomenon, it so happens, that no two men, properly speaking, can see the same rainbow. This meteor can last no longer than the drops of rain continue to fall. 10. The halo, or corona, is a luminous circle surrounding the sun, the moon, a planet, or a fixed star. It is sometimes quite white, and sometimes coloured like the rainbow. Those which have been observed round the moon or stars are but of a very small diameter: those round the sun are of different magnitudes, and sometimes immensely great. The parhelia or mock-suns generally appear about the size of the true sun, not quite so bright, though they are said sometimes to rival their parent luminary in splendour. When there are a number of them, they are not equal to each other in brightness. Externally, they are tinged with colours like the rainbow. They are not always round, and have sometimes a long fiery tail opposite the sun, but are paler towards the extremity. They are formed by the reflexion of the sun's beams on a cloud. CHAP. VIII.-ASTRONOMY. ASTRONOMY treats of the heavenly bodies, their motions, periods, eclipses, and magnitudes, and the laws by which they are regulated. Geography, chronology, and navigation depend upon an accurate knowledge of astronomy. By the aid of this sublime and interesting science, we traverse the seas, penetrate into foreign courtries, become acquainted with different parts of the earth, and ascertain the dates of many past events with the utmost precision. The solar system consists of the sun in the centre, and the planets and their satellites; to these, comets may be added. The planets which appear to us like stars, are not luminous bodies, but shine by reflexion from the sun. There are two kinds of planets, primary and secondary. The first move round the sun in elliptical orbits which are nearly circular, and respect him only as the centre of their motions: the sun, in fact, being placed in one of the foci of each orbit. The secondary planets, called also satellites or moons, are smaller planets, revolving round the primary, while they, with the primary planets about which they move, are carried round the sun. The planets move round the sun at various distances, some being much nearer to him than our earth, and others being much more distant. 2. There are nine primary planets, which are thus situated with respect to the magnitude of their distances from the sun Mercury, Venus, the Earth, Mars, Jupiter, Saturn, and Herschel, or the Georgium Sidus. Of these our earth is accompanied by one moon, Jupiter by four, Saturn by seven, and the Herschel by six moons. None of these moons, except our own, can be seen without a good telescope. The other five planets do not appear to have any satellites or moons: though it was suspected by Cassini that Venus had one. All the planets move round the sun from west to east, and the moons revolve round their primaries in the same direction, excepting those of the Herschel, which seem to move in a contrary direction. The paths in which they move are called their orbits: they perform their revolutions also in very different periods of time their motions always conforming to the law discovered by Kepler, but first demonstrated by Newton, that the squares of the periodic times are as the cubes of the respective mean distances from the sun, or focal body. 3. The time of performing their revolutions round the sun is called their year, and the time of performing their revolution on their axes, their day. The axis of a planet is an imaginary line conceived to be drawn through its centre, about which it revolves as if on a real axis. The extremities of this line, terminating in opposite points of the planet's surface, are called its poles. A bowl whirled from the hand into the air turns round such a line within itself, while it moves forward; and such are the lines intended, when the axis of the heavenly bodies are named. Venus and Mercury being nearer to the sun than our earth, are called inferior planets, and all the rest, which are without the earth's orbit, are called superior planets: some astronomers distinguish them by the terms interior and exterior, which seem preferable. The following very correct table, taken from the best authorities will give the reader an accurate and comprehensive view of the revolutions, distances, &c. of all the primary planets. A GENERAL VIEW OF THE WHOLE SOLAR SYSTEM. TABLE OF THE REVOLUTIONS, DISTANCES, &C. OF ALL THE PRIMARY PLANETS. Names. Hourly mo, Mean distance from Least distance Rotation on their miles, the Sun in English the Earth in English from the Earth in in English miles. Greatest distance Diameter English miles. miles. Mars 5 48 48 75,222 0 23 56, 15 95,513,794 7,954 2,335 27 7 43 12 95,513,794 1 0.40 Jupiter 11 315 14 39 2 30,358 145,533,667 922,920 254,084 2,172 4,135 |