of the measured arcs of terrestrial meridians. General Mudge accounts for the remarkable variations in the lengths of the degrees measured in England by supposing that his plumb-line was made to incline towards the north, at all the stations, by attractive forces, which augmented in intensity in going from south to north; and it is easy to conceive that such attractions may be, partly, the cause that the experiments made on the vibrations of pendulums in rent places, for determining the ratio of the equatorial, to the polar diameter of the earth, give results disagreeing with each other and with those obtained from geodetical admeasurements. Fortunately these differences are small in value, and it has been with justice observed, that our present knowledge of the form and dimensions of the earth is quite sufficient for any of the most delicate operations of astronomy, geography, or navigation.

CHAPTER XXIII.

THE LATEST DISCOVERIES IN THE HEAVENS.

Herschell's remarks on the solar spots.- Opinions concerning the state of the moon's surface.-Discovery of the Georgium Sidus.—And of the four new planets. Notices of some ancient comets.-The orbits of comets suspected by Cassini to be curves returning into themselves.-Dr. Halley computes the elements of a comet's orbit.-Uncertainty of the computations founded on the observed places of comets.-The knowledge of the fixed stars is yet imperfect. Changes observed in the appearances of the stars.-Opinion of Sir W. Herschell concerning their proper motions.-Observed movements in the double and multiple stars.- -Opinion of Herschell concerning the nebula.-An idea of Kepler concerning the distributions of the fixed stars. Catalogues of stars.

THE highly improved state of the instruments employed in making celestial observations, and the diligence exercised by the illustrious men who, during the eighteenth, and in the beginning of the nineteenth centuries, devoted themselves to the cultivation of astronomy, while they brought to light many facts relating to the constitution of the bodies which compose our system, were the means of greatly extending, if we may so speak, the regions subject to the dominion of the sun, and of augmenting the number of planets and comets which, like vassal princes, own the influence of his power. The nature of the spots which occasionally appear on the sun himself were attentively examined by Sir W. Herschell, during several years, with the finest telescopes in existence, and his observations have led him to the conclusion that they are caused by an elastic gas, which, issuing from the solid matter composing the body of the sun, piercing the luminous strata surrounding it, and dispersing itself every way about the orifice, permits the dark nucleus at the bottom of the perforation to become visible to a spectator on the earth, when the latter is in the direction in which the perforation has been made; at times, also, Sir William was able to look obliquely down the opening and distinguish by a difference of colour, on the side opposite to the eye, the direction of the thick

ness of the luminous mass. The elasticity of the gas appears, frequently, insufficient to allow the particles to make their way through the matter which surrounds the sun, and then it swells it into ridges or nodules, appearing more brilliant than the rest of the surface; or, escaping in small quantities, it disturbs the upper surface only, causing gentle swellings or shallow indentations; the last of which are rather more obscure than the other parts of the sun, and contain many small dark points, like pores, through which some of the ascending gas has passed.

The objects which diversify the visible disc of the moon have not been so minutely distinguished as might be expected from the magnifying powers of our telescopes: though the nearest to us of all the celestial bodies, she is still too remote to afford any indication of the productions of nature on her surface which, to judge from its appearance, should be as well fitted for the support of vegetable and animal life as the earth itself. Her mountains, occasionally, are disposed in chains or groups, like the Alps and Pyrennees, and rise to the height of five miles, nearly; but, more generally, they assume the appearance of annular ridges enclosing spaces, from 1800 to 16000 feet deep, like the beds of lakes or seas, or the craters of volcanoes long since extinct frequently an isolated mountain rises from a plain, with portions of rock projecting from its sides, and round its base are scattered fragments which seem to have been separated by time from the principal mass. The absence of those variations of light and shade which would be produced by clouds floating above her surface, and the irregularities of the ground, visible at the bottom and on the sides of her cavities, have given reason to believe that no atmosphere surrounds her and that she is destitute of rivers and seas. Such are the opinions generally entertained concerning the moon; but M. Schroeter, a German astronomer, ventures to assert that our satellite is the abode of living and intellectual beings: he has perceived some indications of an atmosphere which, however, he admits, cannot exceed two miles in height, and certain elevations which appear to him to be works of art rather than of nature. He considers that a uniformity of temperature must be produced on her surface by her slow rotation on her axis, by the insensible change from day to night, and the attenuated state of her atmosphere, which is

never disturbed by storms; and that light vapours, rising from her valleys, fall in the manner of a gentle and refreshing dew to fertilize her fields 2.

To this astronomer we are indebted for some observations on the figure and appearances of Mars, and for a knowledge of the times in which Venus and Mercury perform their revolutions on their axes; previously to his time, the rotation of Venus had been uncertain, and that of Mercury quite unknown; but, in 1793, Schroeter discovered both, from the recurrence of like appearances at the cusps of the planets when nearly in conjunction with the sun.

In 1781, the attention of Sir William Herschell was first attracted to a star, in the constellation Gemini, which appeared to be larger and less bright than the rest; and, continuing to observe it during a few nights, he found that its place in the heavens gradually changed: as soon as this circumstance was communicated to the learned, the principal astronomers of Europe made the star the subject of their nightly observations, and being satisfied that it was really in movement, they applied themselves to the consideration of its nature. At first, it was generally thought to be a comet, and attempts were made to compute the figure of its path on this supposition; but M. Lexell, of St. Petersburgh, discovered that it revolves about the sun in an orbit which is circular or nearly so, and that its distance from the sun is about twice as great as that of Saturn; and La Lande computed that the period of its revolution is about eighty-two years. These determinations being confirmed by all subsequent observations, the star newly observed was acknowledged to be a planet and received the name of Uranus, of Herschell, or, in honour of the reigning monarch, of the Georgium Sidus; the last of which, only, has been adopted in this country. Within a year from its discovery, La Place, by means of several excellent observations, ascertained the ellipticity of the orbit, its principal elements, and the perturbations to which it is subject by the attractions of Jupiter and Saturn; and it is worthy of remark that, among these, he found two, pro

Voiron, Hist. de l'Astronomie.

ducing equations of its mean motion, not exceeding two minutes and a half of a degree, of which one is accomplished in 90 years, and the other in 569 years. This, as Voiron observes, is an astonishing proof of the power of analysis; by observation alone the variations in the movements of the planet could not be ascertained till after long periods of time, and in proportion as they became sensible to the observer; but the eye of the mathematician penetrates into the depths of ages to come, and, besides foreseeing several other inequalities, it is enabled to discover a correction of its mean motion amounting to the small quantity above mentioned, and compensated, periodically, in above five hundred years. The power of Herschell's telescope made known, between 1787 and 1794, that the planet was accompanied by six satellites, two of which are remarkable for having the planes of their orbits nearly perpendicular to that of the planet. On examining the registers of M. Le Monnier for the year 1765, several positions of a newly observed star are there found marked, which, from the subsequent determinations of the places of the above planet, shewed the star and planet to be the same; and it is evident, therefore, that if the French astronomer had taken the trouble to compare together his own observations, he would have anticipated Dr. Herschell in the discovery which has rendered the latter so justly celebrated.

From the days of Kepler it had been suspected, on account of the disproportion which the interval between Mars and Jupiter bears to those between the other planets, that a planet might exist in that interval; and this notion was thought to be verified when, in 1801, M. Piazzi, at Palermo, observed a small star, in Taurus, which was moveable; the star was, afterward, lost in the sun's rays, and was not again discovered till January in 1802, when the fact that it was a planet was fully ascertained. To this the name of Ceres was given, and the elements of its orbit were presently determined; it presented, however, this peculiarity, that the inclination of its orbit to the ecliptic was far greater than that of any other planet, being found to vary from 11 to 18 degrees. But the supposed order of the distances of the

a Ut suprà.