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CLASS-BOOK

OF

ASTRONOMY

BY

FRANCIS BULLOCK, LL.D.

AUTHOR OF "CLASS-BOOK OF MODERN SCIENCE," "CLASS-BOOK SCIENCE
READINGS; SELECT LESSONS IN PHYSICAL SCIENCE," "SMALLER
CLASS-BOOK OF SCIENCE," "CLASS-BOOK OF ROMAN HISTORY,"
"ILLUSTRATED SCHOOL HISTORY OF ENGLAND

ETC.

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PREFACE.

THE chief object of this book is to supply a convenient and trustworthy "Class-Book of Astronomy," especially fitted for use in Schools. At the same time the Author has sought to make the work equally serviceable to General Readers who, with limited opportunities, wish to gain such a knowledge of Astronomy as will help them to an intelligent conception of the Phenomena and Mechanism of the Heavens.

Most people know, or they are supposed to know, that the apparent revolutions of the Celestial Sphere are occasioned by the real motions of the Earth; yet, according to the late Professor De Morgan (in his Budget of Paradoxes), "there is no subject on which there is so little accurate conception as that of the motions of the heavenly bodies."

Not content with tracing the movements, or with noting the periods and aspects of the nearer and more conspicuous of the heavenly bodies, Astronomers venture to determine the shape, the size, and even the weight of worlds so distant as to be barely visible to the naked eye. Pushing still further their researches, they investigate the relations and the physical constitution of Suns and Systems of Suns, which the Creator has lit up in the immeasurable depths of space. The boldness of many of the conclusions announced is sufficiently startling when it is remembered that the Astronomer possesses no exclusive means of information, but has to work out his results with merely human appliances. Even a Student, who is not yet acquainted with the resources of Modern Science, may well be staggered by the assurance that "the distance of the Sun and Stars is ascertained by a yard measure;" that "the weight of the Sun and the Planets is measured by the

pound weight avoirdupois;" that the nature and intensity of the force that controls the motions of worlds can be determined by comparison with the force that causes a pendulum to swing, or a stone to fall to the Earth; and that the constituent elements of the Sun and Stars can be ascertained from the light of those distant bodies. But the difficulty of accepting the established results of this Science often arises from ignorance of the kind of evidence on which the Astronomer bases his conclusions. This Class-Book furnishes such indications of the nature of that evidence as will, it is believed, help the Student over the difficulty and add much to his interest in the study of Astronomy.

The interesting results of the most recent discoveries in Astronomical Science are given in the following pages; and the distances, magnitudes, and masses of the heavenly bodies are based on the revised value of the Solar Parallax. Prefixed to the several Chapters are brief Historical Notices of the progress of Astronomy, from which the Student may see, without the necessity of reference to other books, when many of the most eminent promoters of the Science flourished, and what are the principal discoveries with which their names are associated. Throughout, the one great aim has been to make the "Student's Class-Book of Astronomy" a book which may be pleasantly and successfully used in Schools, and profitably studied at Home as a reliable Introduction to one of the most interesting and instructive of the Sciences.

The Author ought to state that, in revising the proof. sheets, he has been favoured with the assistance of Dr. T. A. Bullock, joint author with himself of the Series of Science "Class-Books" which are now extensively used in Great Britain and her Dependencies.

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