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LEFT ASTRONOMY 320 ASTRONOMY the host of lines of the solar spectrum, of which he counted 600 and mapped 324, and which are to-day known as the "Fraunhofer Lines." Though he did not have the time to carry this dis- covery to its legitimate conclusion, this being afterward done by Kirchhoff and Bunsen, Fraunhofer's labors may be considered as the beginning of the new astronomy. Frledrich Georg Wilhelm Struve (1793-1864) rendered his name immor- tal by the accurate determination, with the 9^/^ inch Dorpat refractor, of the position-angles, distances, colors and relative brightness of 3,112 double and multiple stars, about 2,200 of which were new discoveries. Friedrich Wil- helm August Argelander (1799-1875) ranks next to Bessel among the great astronomers of the 19th century. A pupil of the latter, he thoroughly im- bibed the ideas of exactitude in astro- nomical observations for which his great master was pre-eminent, and he carried them out in all his subsequent work. His first work was the observations, made while his observatory at Bonn was being completed, for the formation of the "Uranometria Nova," the accepted standard of stellar magnitudes. John F. W. Herschel (1792-1871), fol- lowing in the footsteps of his father, in 1834 began at the Cape of Good Hope a survey of the southern heavens, using an 18-inch reflector of his own con- struction. With this, in the course of four years, he accumulated a vast store of material, in the way of new double and multiple stars, nebulae and star- clusters, photometric measures of stellar brightness, " soundings " or " star- gauges" in the Milky Way, to show the laws of the distribution of the stars in space, all of which form the starting point of our knowledge of the southern heavens. The work of Sir George Bid- dell Airy (1801-1892) next deserves attention. Appointed in 1835 to the directorship of the Royal Observatory of Greenwich, he first carried to completion the great work begun at his suggestion two years before — the complete reduc- tion, on a uniform system, of all the Greenwich planetary observations from 1750 onward. The greatest event of the century was the discovery of the planet Neptune. The work of Lagrange and Laplace in the domain of gravitational astronomy was continued and vastly extended in the 19th century by several eminent mathematicians, astronomers, notably by Leverrier. His life was devoted to the perfection of the theory of the planetary motions. Adams, the equal sharer with him in the glory of the discovery of Neptune, has also made very important additions to our knowledge in the same field, and in the United States we have, in the persons of Simon Newcombe and George W. Hill, their worthy successors and collaborators. The amount of work which Newcomb published in the line of fundamental star places, the discussion of old eclipses and occultations, with their bearing on the theory of the moon's motion, the motion of Mercury, etc., was prodigious. The theory of the moon's motion, or the lunar theory, as it is generally called, has from the beginning attracted the attention of the ablest mathematical investigators. The two who stand out prominently before all others are Han- sen and Delaunay. In various other branches of gravitational astronomy, several names deserve special mention. Olbers, besides being the discoverer of several comets and of the second and fourth planetoid, is best known for his development of the best method of com- puting cometary orbits. Encke, a pupil of Gauss, developed the best methods of applying the method of least squares to computation, determined a value of the solar parallax, which stood a long time as giving the accepted value of 95,000,- 000 miles as the distance of the sun, but is best known for the discovery of the remarkably short period of the comet which bears his name. Hall's detection of the two minute and remarkable satellites of Mars ranks next to that of Neptune as the most brilliant of the century. It was not an accidental picking up of easily visible object in sweeps, but the result of a well planned and careful search at the most favorable time, the opposition of 1877, after the erection of the 26-inch refrac- tor of the United States Naval Obser- vatory. Hall also kept up systemati- cally the observation of the difficult sat- ellite systems of Saturn, Uranus, and Neptune after taking charge of the 26- inch Washington refractor. Next came the discovery of the fifth satellite of Jupiter, by Prof. Edward E. Barnard, of the Lick Observatory, Sept. 9, 1892. This was followed by the discovery, March 18, 1899, of the ninth satellite of Saturn, by Prof. William H. Picker- ing, of the Harvard Observatory. The work done at Cordoba, in the Ar- gentine Republic, by Dr. Benjamin Ap- thorp Gould and his assistants in 1870, must next be mentioned. Dr. Gould began the observation for a uranometry of the southern heavens, to include all stars down to the seventh magnitude. This great work contains the names,