in the text the values of dimensions and distances in the solar system as based upon Encke's determination of the sun's parallax. The coming transits of Venus in 1874 and 1882 will furnish " a full and precise settlement of this important question." Both new and old values are inserted in the tables. I am indebted to the courtesy of the Director of the U. S. Naval Observatory, and to the Assistant in charge of the Coast Survey Office, at Washington, for valuable information; to Prof. Henry Morton, Ph. D., of the Franklin Institute, Philadelphia, through whose unsolicited kindness I am able to present the superior engraving of the moon, reduced from a 24-inch photograph in his possession, taken by Mr. Louis M. Rutherford, of New York; and to Mr. H. H. Vail, whose careful scrutiny of the proof-sheets has been invaluable. HIGH SCHOOL, CHICAGO, June 15, 1869. S. H. P. : ELEMENTS OF ASTRONOMY. CHAPTER I. ASTRONOMICAL IDEAS REFERRED TO THE POSITION OF THE OBSERVER. 1. The Horizon.-Any person in the open air upon level ground, or on the water, finds himself at the center of a large circle bounded by the sky. The sky seems to be the half of a hollow sphere, or a dome, which rests upon the outer edge of the plane, on which he stands. The line at which the earth and sky appear to meet is called the visible, or sensible, horizon. The plane which contains this line is the plane of the horizon; any plane which is parallel to the plane of the horizon is a horizontal plane. 2. The horizon is often hidden by houses, forests, or hills, while high some high place, Fig. 1. DIP HORIZON (9) |