[Illustration PATH OF BIELA'S COMET.] LETTERS ON ASTRONOMY, IN WHICH THE ELEMENTS OF THE SCIENCE ARE FAMILIARLY EXPLAINED IN CONNECTION WITH BIOGRAPHICAL SKETCHES OF THE MOST EMINENT ASTRONOMERS. WITH NUMEROUS ENGRAVINGS. BY DENISON OLMSTED, LL.D., PROFESSOR OF NATURAL PHILOSOPHY AND ASTRONOMY IN YALE COLLEGE Revised Edition. INCLUDING THE LATEST DISCOVERIES. NEW YORK: HARPER & BROTHERS, PUBLISHERS, 329 & 331 PEARL STREET, FRANKLIN SQUARE. 1855. Entered according to Act of Congress, in the year 1840, by MARSH, CAPEN, LYON, AND WEBB, in the Clerk's Office of the District Court of Massachusetts. ADVERTISEMENT TO THE REVISED EDITION. SINCE the first publication of these Letters, in 1840, the work has passed through numerous editions, and received many tokens of public favor, both as a class-book for schools and as a reading-book for the family circle. The valuable discoveries made in the science within a few years have suggested an additional Letter, which is accordingly annexed to the series in the present revised form, giving a brief but comprehensive notice of all the leading contributions with which Astronomy has of late been enriched. The form of _Letters_ was chosen on account of the greater freedom it admits, both of matter and of style, than a dress more purely scientific. Thus the technical portion of the work, it was hoped, might be relieved, and the whole rendered attractive to the youthful reader of either sex by interspersing sketches of the master-builders who, in successive ages, have reared the great temple of Astronomy, composing, as they do, some of the most remarkable and interesting specimens of the human race. The work was addressed to a female friend (now no more), who was a distinguished ornament of her sex, and whose superior intellect and refined taste required that the work should be free from every thing superficial in matter or negligent in style; and it was deemed by the writer no ordinary privilege that, in the composition of the work, an image at once so exalted and so pure was continually present to his mental vision. YALE COLLEGE, _January_, 1853. CONTENTS. PREFACE, 3 LETTER I. Introductory Observations, 9 LETTER II. Doctrine of the Sphere, 16 LETTER III. Astronomical Instruments.--Telescope, 29 LETTER IV. Telescope continued, 36 LETTER V. Observatories, 42 LETTER VI. Time and the Calendar, 59 LETTER VII.