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CONTENTS.

Speculations respecting their Origin.—Errors of Radius Vector.—Researches ofGeometers.—Bessel.— Adams.—Inverse Problem of Perturbation.—Account ofAdams’ Researches relative to the existence of a Planet exterior to Uranus. —Re-sults obtained by him.—Researches of the French Astronomers on the Theory ofUranus.—Eugene Bouvard.—Le Verrier.—Account of his Researches.—NearAgreement of his Results with those of Adams.—Steps taken by Airy and Challisfor the purpose of discovering the Planet.—New Results obtained by Adams.—Explanation of Errors of Radius Vector.—Account of the second part of Le Ver-rier’s Researches on the Trans-Uranian Planet. —Address of Sir John Herschel at Southampton.—The Planet discovered at Berlin by Galle.—Admirationexcited by the Discovery.—Account of Challis’ Labours.—Public Announce-ment of Adams’ Researches_Impression produced by it.—Historical Statement

of the Astronomer Royal. — Publication of the Researches of Le Verrier andAdams.—Remarks suggested by the Discovery of the Planet .... 164

CHAPTER XIII.

The Elements of the Planet Neptune deduced from Observation.—They are foundto be discordant with the Results of Theory. —The cause of Discordance as-signed.—The Planet observed by Lalande.—Theory of its Perturbations.—Re-searches on the value of its Mass.—Uncertainty respecting this Element.—Re-searches of M. Hansen on the Lunar Theory.—Conclusion of the History ofPhysical Astronomy.201

CHAPTER XIY.

Researches on the Solar Parallax.—Modern Determinations of this Element.—Discovery of the Solar Spots.—Consequences deduced from this Discovery.—Period of the Sun ’s Rotation.—Theories of the Solar Spots.—Wilson.—Herschel.

—Researches on the Lunar Parallax.—Ellipticity of Mercury.—Researches onthe Rotation of Venus.—Discovery of the Ultra-Zodiacal Planets.—Microme-trical measures of Jupiter ’s Satellites.—Micrometrical measures of Saturn , and ofhis Ring.—Discovery of the eighth Satellite of Saturn.—Researches on theSatellites of Uranus.—Lassell’s Discovery of the Satellite of Neptune.—Re-searches on Comets.—Halley’s Comet.—Comet of 1843 . 211

CHAPTER XV.

General Aspect of Comets.—Translucency of Cometic Matter.—Structure and Di-mensions of the Envelope. —Description of the Tail.—Its Direction and Curva-ture.—Peculiarities of Structure.—Dimensions.—Phenomena observed duringthe Passage of Comets through their Perihelia.—Comet of Halley.—Comet of1799.—Variation of the Volume of Comets.—Hevelius.—Newton.— Struve.—Herschel.—Dissolution of Comets.—Historical Statement of Ephorus.—Cometof Biela.—Developement of the Tail.—Comet of 1680.—Comet of 1769.—Anomalous Appearances in the Tail.—Instances of Remarkable Comets.—Hypo-theses respecting their Physical Constitution.—Theories of the Variation of aComet ’s Volume.—Newton.—Valz.—Herschel.—Theories of the Tails of Comets.—Kepler.—Newton.—Electrical Theory.—Light of Comets. —Appearance ofPhases.— Cassini. — Cacciatore. — Polarization of the Light of Comets. —Re-searches of Arago.—Question respecting the Solidity of Comets.—Newton.—La-place.—Smallness of a Comet ’s Mass.—Ultimate condition of Cometary Bodies.—Opinions of Newton, Laplace, and Herschel on this point .... 292