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ASTRONOMY. PAET I. of the article on Astronomy in the ninth edition of this Encyclopaedia treats of the history of the science, and needs no supplement. In the first six chapters of Part II. are developed those general principles and methods which form the basis of the science and are not subject to material revision. Our present purpose is to supplement the chapters from VII. onwards. We shall first treat the recent developments relating to the solar system, and afterwards briefly describe the progress of knowledge in regard to the fixed stars. In arranging the first subject we shall set forth the general results of telescopic and spectroscopic observations of the planets, in advance of results derived from the mathematical investigation of their motions. The subject thus divides itself into three sections :— I. The physical and other general features of the solar system. II. Gravitational and theoretical astronomy. III. The recent development of our knowledge of the fixed stars. I. Physical Features of the Solar System. The development of this subject since 1880 is principally due to the increased power of our instruments and the more extended use of the spectroscope. Sixty years ago the spectroscope was unknown, and the meaTia' ^elescoPes most used were from 4 to 9 inches telescopes. aperture. About 1840, the successors of Fraunhofer made two telescopes of 14 inches’ aperture for the observatories of Pulkowa and Harvard University. The extreme limit of size was then thought to be attained, but by 1880 the aperture was carried up to 26 inches, and now it is 40 inches. The great Yerkes telescope of the university of Chicago gives about seven times the light of the Harvard and Pulkowa instruments. It might be thought that this enormous increase of power would have resulted in the final settlement of all questions, formerly the subject of debate, as to the physical phenomena presented by the planets. But such is far from being the case, and the vast mass of observations that have been accumulated has thrown less light than might have been expected on the question of the rotation or physical constitution of these bodies, though, at the same time, critical examination may show that the results to be derived are not so discordant or doubtful as might appear from the seeming divergence of the observations themselves. The observers who, since 1880, have made careful studies upon the appearance of the planets in the telescope, are so numerous that only a few of the best known can be mentioned. The systematic work of the British Astronomical Association, which has organized sections for the special study of particular objects, offers a good example of the energy with which a large and popular body may pursue scientific investigation. This body does not, however, command the instruments or enjoy the atmospheric conditions necessary to settle the most difficult questions. Among the individual observers Schiaparelli may be assigned the first place, in view of his long-continued studies of the planets under a fine Italian sky, the conscientious minuteness of his examinations, and his eminence as an investigator. In what concerns the means and opportunities of observation, Barnard at the Lick Observatory enjoyed advantages offered to no other observer. The observatory at Flagstaff, Arizona, was founded by Mr Percival Lowell, of Boston, for the special

purpose of studying the physical phenomena presented by the planets, particularly Mars, and its situation is believed to be one of the best as regards atmospheric conditions. Taking up the planets in order, we begin with Mercury. Only on rare occasions have observers succeeded in discerning any well-defined features on the disc of this planet. Up to 1890 the period of its rotation Mercurywas generally said to be nearly twenty-four hours, a statement which rested mainly on observations made by Schroeter, the indefatigable observer of Lilienthal, about the beginning of the century. Cautious and conservative astronomers have been more and more sceptical as to the correctness of this period, on account of the failure of observers with better instruments than Schroeter’s to see permanent markings that could be followed from day to day. In 1882 Schiaparelli began a careful study of the face of the planet, with a refractor of 8 inches’ aperture, subsequently replaced by one of 18 inches’. His unexpected conclusion was that the rotation of Mercury resembles that of the moon, in having its period equal to that of the orbital revolution. As the moon always presents the same face to the earth, so Mercury always presents very nearly the same face to the sun. Schiaparelli also announced that the axis of rotation of the planet is very nearly perpendicular to the plane of its orbit. The rotation being uniform, while the orbital motion, owing to the great eccentricity of the orbit, is affected by a very large inequality, it follows that there is a libration in longitude of nearly 24° on each side of the mean position. Mr Lowell, in 1897, took up the question anew by combining a long series of measured diameters of the planet with drawings of its apparent surface made from time to time. The latter showed long narrow markings, similar in their general character to the supposed channels seen by Schiaparelli upon Mars, and the constancy of these markings was considered to confirm the view of Schiaparelli as to the slow rotation of the planet. The diameter was found to be 0‘8" greater than the value which had before been generally accepted. The most curious result of Mr Lowell’s work was that the body of the planet is possibly somewrhat ellipsoidal, the longer axis being directed toward the sun. This, however, cannot be regarded as established. It is worthy of remark that the larger diameter and the ellipsoidal form may be connected with a curious discrepancy found in discussing the meridian observations of the planet. This consists in the fact that in the general average, when the planet is east of the sun the right ascension derived from observations seems to be too small, and when west of the sun too great. In either case, it is impossible to observe the real centre of the planet, because the latter always shows a phase like that of the moon, being sometimes a crescent, sometimes a semicircle, and sometimes gibbous, according to the position in the orbit. The observers sometimes directed their view upon the bright limb which, of course, was always towards the sun, and sometimes upon the apparent centre of the illuminated surface; hence reduction from the point observed to the centre of the planet was necessary to derive the position of the centre supposed to be given by the observations. This reduction ^Yas necessarily a fraction of the assumed diameter of the planet, which had to be determined from other sources. The discrepancy shows that the reduction actually applied was, in the general average, too small by a considerable fraction of a second, but it is impossible to say with certainty whether this arose from a personal equation of