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 ASTRONOMY Adams that Laplace's investigation of the sub- ject was imperfect ; yet undoubtedly he placed his finger on the true cause of that part of the acceleration which is due to the ordinary forms of perturbation, nor has the cause of the re- maining part of the moon's acceleration been hitherto ascertained. Finally, we may regard the publication of his Mecanique celeste as form- ing a veritable epoch in the history of physical astronomy. Passing over many important con- tributions to the theory of gravitation, we may point to the achievement of Adams and Lever- rier in the discovery of the planet Neptune as perhaps the most conclusive of the evidences yet adduced in support of Newton's theory. A planet hitherto unseen was made known to us, not as in the case of Uranus by a happy chance, but by a study of the deviations of a known planet from the path calculated for it by mathematicians. It may be added that the discovery of Neptune led to the recognition of the mastery which American astronomers and mathematicians had obtained over the more recondite departments of analysis. It has been remarked by Prof. Grant of Glasgow that "the results which have been deduced from Bond's observations of the satellite of Neptune, and the mathematical researches of Walker and Peirce, unquestionably exhibit a degree of consistency with the actual observa- tions of Uranus and Neptune which has not been paralleled by any similar efforts in Europe; while at the same time they tend to throw much interesting light on the theory of both planets." Among the more recent contribu- tions to the mathematics of astronomy must be mentioned Adams's discussion of the moon's secular acceleration and the researches to which that discussion led, Delaunay's exten- sion of the lunar theory, and the inquiries of Prof. Newcomb into the same subject. While mathematical astronomy had been thus advancing, observational astronomy made sim- ilar progress. The discovery of Saturn's ring and largest satellite by Huyghens was soon followed by the discovery of four other satel- lites. Later Sir W. Herschel discovered two other Saturnian satellites, while in compara- tively recent times Bond in America and Las- sell in England discovered an eighth. Uranus was added to the planetary system by Sir W. Herschel in 1781, and at sundry times fourUra- nian satellites have since been discovered, while four others are by some supposed to have been seen by Sir W. Herschel. Neptune and his satellite constitute two other known members of the planetary scheme. But to these must be added 130 small planets (see ASTEROIDS) which travel between the paths of Mars and Jupiter ; while the observations and researches of Bond and Peirce in America and Maxwell in England tend to show that the rings of Saturn are composed of multitudinous small satellites. Apart from these' discoveries, the complexity of the scheme ruled over by the sun has been indicated by the discovery of the fact that multitudes of meteoric sys- tems exist within the confines of the solar domain, and that the component members of these systems must be counted by millions. The recent observations of Profs. Newton and Kirkwood in the United States, Prof. Alex- ander Herschel and Mr. Glaisher in England, Quetelet in Belgium, Schmidt in Athens, Heis in Germany, and Secchi in Rome, have added largely to our knowledge respecting meteors ; while the mathematical researches of Schiapa- relli, Adams, Leverrier, and others, have re- vealed the interesting fact that these bodies are intimately associated with comets. The telescopic study of the starry depths, though it has been prosecuted laboriously by the Her- schels, Struve, Argelander, Madler, and others, must be regarded as still (owing to the vastness of the domain to be explored) in its infancy. The elder Herschel first conceived the daring idea of gauging the celestial depths ; but as a matter of fact the regions surveyed by the two Herschels amount to but a minute portion of the heavens. On the other hand, though Argelander's survey extended over a complete hemisphere, yet the telescopic power employed was but small. Dr. Gould, an American astron- omer, is extending Argelander's system of sur- vey to the southern heavens ; and the result can- not fail to be of the utmost interest and value. We owe to the Herschels nearly all our present knowledge of the strange objects called nebulas or star cloudlets. Of these only 16 were known in Halley's time, and barely 200 when Sir W. Herschel began his telescopic labors. He and his son added between them nearly 5,000 neb- ulae to the list of known objects of this class. At present some 5,700 nebula? are known in all. The theoretical considerations by which the Herschels have endeavored to interpret the scheme of the universe are too important to pass unnoticed in this brief sketch of the his- tory of astronomy. They have presented the galaxy to our contemplation as a scheme of suns, many equalling and many surpassing our own sun in magnitude and splendor, while they have taught that many of the star cloudlets are schemes of suns resembling the galaxy in extent and constitution. If some, as Whewell, Herbert Spencer, and others, do not regard these views as demonstrated or even demon- strable, yet we cannot but contemplate with admiration the activity of mind which enabled the Herschels, after completing unrivalled series of observational researches, to propound theo- ries so magnificent respecting the myriads of orbs which they had examined. The spectro- scopic analysis of the sun and other celestie? bodies, in the hands of Kirchhoff, Huggins, Young, Secchi, Zollner, Lockyer, and Respighi, has revealed many facts of importance. It has been shown that in the sun many of our famil- iar elements exist in the form of vapor. In the planetary atmospheres known vapors, and especially the vapor of water, have been de- tected. The stars have been proved to bo