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volume scarcely needs a preface, for the title-page tells its aim, and implies the motives which have inspired it. It may, however, be confessed that the great public interest which seems to have sprung up in connection with the expected return this year or next of "Halley's Comet" had something to do with the non-publication of the book last year and its non-postponement till next year.

I have modelled it on the Comet chapters of my well-known Handbook of Astronomy, but every line of those chapters has been rewritten, altered, corrected, or expanded, and new chapters added, to embody the different conditions of our knowledge in 1909 contrasted with the circumstances of nearly a quarter of a century ago.

I have to thank various friends for advice and assistance in making the book more useful than it would have been without such advice and assistance; in particular, Mr. A. C. D. Crommelin, B.A., F.R.A.S., of the Royal Observatory, Greenwich, and Mr. W. E. Rolston, F.R.A.S., of the Solar Physics Observatory, South Kensington; and Mr. D. Smart, F.R.A.S.; whilst as regards the illustrations I owe thanks to the Council of the Royal Astronomical Society; to