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Rh Bradley (edited by Rigaud), from which the portrait has been taken. My account of Halley's work is based to a considerable extent on his own writings; there is a good deal of biographical information about him in the books already quoted in connection with Newton and Flamsteed, and there is a useful article on him in the Dictionary of National Biography. I have made a good deal of use in this chapter of Wolf and Delambre, especially in dealing with Continental astronomers; and for special parts of the subject I have used Grant's History of Physical Astronomy, Todhunter's History of the Mathematical Theories of Attraction and the Figure of the Earth, and Poynting's Density of the Earth.

Chapter XI.—Most of the biographical material has been taken from Wolf from articles in various encyclopaedias and biographical dictionaries, chiefly French, and from Delambre's Eloge of Lagrange. The two portraits are taken respectively from Serret's edition of the Oeuvres de Lagrange and from the Academy's edition of the Oeuvres Complètes de Laplace. Gautier's Essai Historique sur le Problème des Trois Corps and Grant's History of Physical Astronomy have been the books most used for my account of the scientific contributions of the various astronomers dealt with; I have also consulted various modern treatises on gravitational astronomy, especially Tisserand's Mécanique Céleste, Brown's Lunar Theory, and to a less extent Cheyne's Planetary Theory and Airy's Gravitation. For special points I have used Todhunter's History, already referred to. Of the original writings I have made a good deal of use of Laplace's Mécanique Céleste as well as of his Système du Monde; I have also consulted a certain number of his other writings and of those of Lagrange and Clairaut; but have made no systematic study of them.

Students who wish to know more about gravitational astronomy but have little knowledge of mathematics should try to read Airy's Gravitation; Herschel's Outlines of Astronomy and Grant's History (quoted above) also deal with the subject without employing mathematics, and are tolerably intelligible.

Chapter XII.—The account of Herschel's career is taken chiefly from Mrs. John Herschel's Memoir of Caroline Herschel, from Miss A. M. Clerke's The Herschels and Modern Astronomy, from the Popular History of Astronomy in the Nineteenth Century by the same author, and from Holden's Sir William Herschel, his Life and Works. The last three books and the Synopsis and Subject Index to the Writings of Sir William Herschel by Holden & Hastings have been my chief guides to Herschel's long series of papers; but nearly everything that I have said about his chief pieces of work is based on his own