I HEKSCHEL 699 large, and a second examination showed it to have changed its place. Finally he pronounced it a comet, and it was so published in the "Philosophical Transactions" (1781). This announcement drew the attention of astrono- mers to the supposed comet, and they began to endeavor to compute its course. The presi- dent Saron first pronounced it a planet, and then Lexell and Laplace, almost simultaneous- ly, computed its elements, and found it to have an elliptical orbit, whose great axis was about 19 times greater than that of the earth, and the period of its revolution to be 84 years. Herschel had taken no part in the mathemati- cal calculations, but on its being pronounced a planet, he proposed to name it the Georgium Sidus. It has often been called Herschel, but the name Uranus, applied to it by Bode, has been generally adopted. Herschel now turned his attention most carefully to this planet, de- termined the apparent diameter (about 4") for its mean distance from the earth, and discov- ered two of its satellites, revolving in a plane nearly perpendicular (at an angle of 78 58') to its orbit, and contrary to the order of signs (that is, from east to west). He thought he had also detected four other satellites ; but it is now generally believed that he mistook faint stars for satellites, and that Uranus has only four, two of which were discovered by Lassell, of England, in 1851. The discovery of Uranus attracted the attention of all Europe, and Herschel was made private astronomer to the king, with a salary of 400 and a house near Windsor, first at Datchet, and finally at Slough. With funds advanced by the king, Herschel constructed his celebrated 40-foot reflecting tel- escope, the metal speculum of which was 4 ft. in diameter, 3^ in. thick, and over 2,-000 Ibs. in weight. The plane mirror of the instrument was dispensed with, and the observer sat in a swinging chair with his back to the object ob- served, and facing the object end of the tube, in which the image, by an inclination of the speculum, was thrown to one side and observed through a single lens. He conjectured that with this instrument 18,000,000 stars might be seen in the milky way. Though Herschel dis- covered an almost unprecedented number of new bodies in the planetary system, yet his glory is greatest in sidereal astronomy, of which he laid almost the foundations. His leading discoveries in this branch of the science were the following : I. The binary system of stars, and the orbits of several revolving stars. Double stars had been noticed even before the introduction of the telescope ; but while Her- schel was observing them to learn their annual parallax, he noticed a steadily progressive change in their position and distance ; and in 1802, 23 years after he began his observations, he announced in the "Philosophical Transac- tions " his discovery that both stars were re- volving round their common centre of gravity, and all his instances have been confirmed. II. He classified the nebulae, and advocated the nebular hypothesis, since supposed to be dis- proved by the discoveries made with the great telescope of Lord Rosse, but now accepted as demonstrated by the results of spectroscopic analysis. He discovered that these nebulous spots cover at least -fa of the visible firma- ment, and in 1802 he indicated the positions of 2,500 nebulae or clusters of stars. He classified them as : 1, clusters of stars ; 2, nebulae proper ; 3, nebulous stars. III. The law of grouping the entire visible firmament. He "gauged" the heavens, by counting the whole number of stars visible in the field of his 20-foot reflector, and taking the average for each region. The result showed a remarkable and steady law of decrease, from the central zone of the milky way in opposite directions to the northern and southern poles. IV. The determination of the fact of the motion of our system, and the di- rection of that motion. It was already known that the stars were not fixed, but had a proper motion. Herschel, from the proper motions of about 20 stars, with great penetration, di- vined that our system was moving in the direc- tion of A Herculis, a point whose right ascen- sion is 270, and north declination 25. Be- sides discovering the satellites of his own planet, Herschel discovered two new satellites of Saturn, now called, from their being next the ring, the first and second, and determined the rotation of the rings of the planet to be in 10 h. 32 m. He found also that the time of the rotation of the satellites of Jupiter was just equal to the period of their revolution about the planet. When his age made it ad- visable for him to discontinue his observations in the heavens, he turned his attention to the properties of heat and light. He also gave some valuable opinions concerning the spots on the sun, attributing them to occasional open- ings in the luminous coating, which seems to be always in motion. Herschel contributed papers, sometimes several in a year, to the "Philosophical Transactions" from 1780 to 1815. He married in 1788 Mrs. Mary Pitt, a widow of considerable fortune, and had by her one son, John. II. Caroline Lucretia, sister of the preceding, born in Hanover, March 16, 1750, died there, Jan. 9, 1848. She lived in Hanover till her 22d year, when she went to England to join her brother at Bath. Here she turned her attention to astronomy, and gave him great assistance, not only taking the part of an amanuensis, but frequently perform- ing alone the long and complicated calculations involved in the observations. For this she received a pension from George III. Mean- while she took her own separate observations of the heavens, with a small Newtonian tele- scope which her brother had made for her. She devoted herself particularly to a search for comets, and between 1786 and 1805 discovered alone eight of these bodies, of five of which she was the first observer. Her contributions to science, most of them in her brother's works and under his name, are very valuable. She
