The illustrations contained in the present work are worthy of as careful study as is the text, and many of them are intended as an aid to experimental work and accurate measurement, e. g., the star maps, the diagrams of the planetary orbits, pictures of the moon, sun, etc. If the school possesses a projection lantern, a set of astronomical slides to be used in connection with it may be made of great advantage, if the pictures are studied as an auxiliary to Nature. Mere display and scenic effect are of little value. A brief bibliography of popular literature upon astronomy may be found at the end of this book, and it will be well if at least a part of these works can be placed in the school library and systematically used for supplementary reading. An added interest may be given to the study if one or more of the popular periodicals which deal with astronomy are taken regularly by the school and kept within easy reach of the students. From time to time the teacher may well assign topics treated in these periodicals to be read by individual students and presented to the class in the form of an essay. The author is under obligations to many of his professional friends who have contributed illustrative matter for his text, and his thanks are in an especial manner due to the editors of the Astrophysical Journal, Astronomy and Astrophysics, and Popular Astronomy for permission to reproduce here plates which have appeared in those periodicals, and to Dr. Charles Boynton, who has kindly read and criticised the proofs. GEORGE C. COMSTOCK. UNIVERSITY OF WISCONSIN, _February, 1901_. CONTENTS CHAPTER PAGE I.--DIFFERENT KINDS OF MEASUREMENT 1 The measurement of angles and time. II.--THE STARS AND THEIR DIURNAL MOTION 10 Finding the stars--Their apparent motion-- Latitude--Direction of the meridian--Sidereal time--Definitions. III.--FIXED AND WANDERING STARS 29 Apparent motion of the sun, moon, and planets--Orbits of the planets--How to find the planets. IV.--CELESTIAL MECHANICS 46 Kepler's laws--Newton's laws of motion--The law of gravitation--Orbital motion--Perturbations-- Masses of the planets--Discovery of Neptune-- The tides. V.--THE EARTH AS A PLANET 70 Size--Mass--Precession--The warming of the earth--The atmosphere--Twilight. VI.--THE MEASUREMENT OF TIME 86 Solar and sidereal time--Longitude--The calendar--Chronology.