BY DENISON OLMSTED, A. M.

PROFESSOR OF NATURAL PHILOSOPHY AND ASTRONOMY.

CALIFORNIA

NEW YORK:

COLLINS, KEESE, & Co.

1839.

Entered, according to Act of Congress, in the year 1839,

By DENISON OLMSTED,

In the Clerk's office of the District Court of Connecticut.

Printed by B. L. Hamlen, New Haven, Conn.

PREFACE.

NEARLY all who have written Treatises on Astronomy, designed for young learners, appear to have erred in one of two ways; they have either disregarded demonstrative evidence, and relied on mere popular illustration, or they have exhibited the elements of the science in naked mathematical formulae. The former are usually diffuse and superficial; the latter, technical and abstruse.

In the following Treatise, we have endeavored to unite the advantages of both methods. We have sought, first, to establish the great principles of astronomy on a mathematical basis; and, secondly, to render the study interesting and intelligible to the learner, by easy and familiar illustrations. We would not encourage any one to believe that he can enjoy a full view of the grand edifice of astronomy, while its noble foundations are hidden from his sight; nor would we assure him that he can contemplate the structure in its true magnificence, while its basement alone is within his field of vision. We would, therefore, that the student of astronomy should confine his attention neither to the exterior of the building, nor to the mere analytic investigation of its structure. We would desire that he should not only study it in models and diagrams, and mathematical formulæ, but should at the same time acquire a love of nature herself, and cultivate the habit of raising his views to the grand originals. Nor is the effort to form a clear conception of the motions and dimensions of the heavenly bodies, less favorable to the improvement of the intellectual powers, than the study of pure geometry.

But it is evidently possible to follow out all the intricacies of an analytical process, and to arrive at a full conviction of the great truths of astronomy, and yet know very little of nature. According to our experience, however, but few students in the course of a liberal education will feel satisfied with this. They do not need so much to be convinced that the assertions of astronomers are true, as they desire to know what the truths are, and how they were ascertained; and they will derive from the study of astronomy little of that moral and intellectual elevation which they had anticipated, unless they learn to look upon the heavens with new views, and a clear comprehension of their wonderful mechanism.

Much of the difficulty that usually attends the early progress of the astronomical student, arises from his being too soon introduced to the most perplexing part of the whole subject, the planetary motions. work, the consideration of these is for the most part postponed until the learner has become familiar with the artificial circles of the sphere, and conversant with the celestial bodies. We then first take the most simple view possible of the planetary motions by contemplating them as they really are in nature, and afterwards proceed to the more difficult inquiry, why they appear as they do.

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Although we have found it convenient to defer the consideration of the fixed stars to a late period, yet we would earnestly recommend to the student to begin to learn the constellations, and the stars of the first magnitude at least, as soon as he enters upon the study of astronomy. A few evenings spent in this way, assisted, where it is practicable, by a friend already conversant with the stars, will inspire a higher degree of enthusiasm for the science, and render its explanations more easily understood. It is recommended to the learner to make a free use of the Analysis, especially in reviewing the ground already traversed. If, by repeated recurrence to these heads, he associates with each a train of ideas, carrying along with him, as he advances, all the particulars indicated in these hints, he will secure to them an indelible place in his memory.

With such aids at hand, as Newton, La Place, and Delambre, to expound the laws of astronomy, and such popular writers as Ferguson, Biot, and Francoeur, to supply familiar illustrations of those laws, it might seem an easy task to prepare a work like the present; but a text book made up of extracts from these authors, would be ill suited to the wants of our students. We have deemed it better therefore, first, to acquire the clearest views we were able of the truths to be unfolded, both from an extensive perusal of standard authors, and from diligent reflection, and then to endeavor to transfuse our own impressions into the mind of the learner. Writers of profound attainments in astronomy, and of the highest reputation, have often failed in the preparation of elementary works, because they lacked one qualification-the experience of the teacher. Familiar as they were with the truths of the science, but unaccustomed to hold communion with young pupils, they were incapable of apprehending the difficulty and the slowness with which these truths make their entrance into the mind for the first time. Even when they attempt to feel their way into young minds, by assuming the garb of the instructor, and employing popular illustrations, they often betray their want of the experience and art of the professional teacher. In this number perhaps we may place Sir John Herschel, whose work on the Elements of Astronomy, no sooner made its appearance in this country, than it was seized upon with avidity by many of the professors of our colleges, and among others by the author of this treatise. But with great deference to his exalted rank in the scientific world, and with the highest respect for his profound and varied attainments, we feel compelled to say, after several years trial, that his work is wholly unsuited to the case of the college student, and, indeed, but ill adapted to the wants of the general reader.

Although the mathematical complexion of some parts of this work, may seem repulsive to the general reader, yet the propositions demonstrated are generally also illustrated in a popular way, so that they may be understood by those who are unable or unwilling to toil through the demonstrations. By occasionally omitting these, such may find this treatise well suited to give them a concise and comprehensive view of the latest discoveries of astronomy; and we hope it may prove no less worthy of the library of the scholar, than of the lecture room of the student.

Yale College, May, 1839.