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TO THE SECOND EDITION.

THE rapid sale of the first edition of this work, the author is willing to attribute to the obvious public desideratum of a work of this kind, rather than to any peculiar merit of his production. He is not the first, nor probably will he be the last, to form a more correct judgment of what the public need, than of his own ability to supply that deficiency. The encouragement which he has received, has, however, induced him to correct and somewhat enlarge his work. A great number of facts, omitted in the first edition, are noticed in this, both in the Descriptive and Physical part. To relieve the pupil from a dry narration of facts, or abstract illustration of principles, the author has subjoined to their proper sections and articles, a popular description of several of the most striking natural appearances and phenomena. He has also greatly increased the number of questions. Upon the whole, he feels confident, that the relative value of his work is not diminished by having its size increased.

Several instructers have suggested, that it might be useful to subjoin Tables for calculating eclipses. On this subject the author would only remark, that these Tables and the necessary instructions for applying them, would swell the work to a size, that would in a considerable degree defeat the objects of its publication

Moreover, he cannot very highly appreciate the value of mechanical rules for calculating eclipses, while the grounds and reasons of those rules, and of the tables, to which they refer, are not understood; and nothing but mechanical rules can here be expected. To a vast majority of pupils, an understanding of the reasons and principles of these rules and tables would be much more useful than the ability to apply them.

It is an evil to have frequent alterations in school books of any kind. In some it is unpardonable. But it is a still greater evil to have a book remain imperfect, while it is in the power of the author to improve it, and the book is worth the labour. This is particularly true with regard to books like this. New facts in astronomy are continually coming into notice, which modify and limit the application of established principles. New data for intricate calculations are derived from constant observation. Hence many things, which we now suppose to be true or nearly so, may in a short time be found to be false, or true only under certain circumstances. New and happy illustrations of difficult subjects may also be suggested. All these will cause a difference in the different editions of the same work. The author, therefore, cannot promise that future editions shall not be "improved." He will, however, endeavour to make no alterations, which are not dictated by real utility Boston, Feb. 14, 1823.

INTRODUCTION.

1. THE first change in nature, which the eye, just opened upon the things of this world, notices, is that of light and shade, from day to night and from night to day. The beams of the morning awake the infant from the slumber of the cradle, and call him forth to activity and life; the twilight of the evening insensibly disengages his attention from objects of sight, and the darkness of night finds him again in repose. He soon walks forth under the heavens. He notices, that when darkness gives way to light, the sun becomes visible in the east ; and that when the sun has passed through the heavens, he disappears in the west, and light gradually yields to darkness. At the same time that these changes are noticed by the eye, he feels that warmth or heat increases and decreases, much in the same manner, and in the same degree, that light does; and that at the same time that darkness steals one object after another from his sight, a sensation of cold pervades his frame. He soon comes to this conclusion, that the sun is the grand dispenser of heat and light; that the day is caused by his presence; and that the coldness and darkness of night are nothing but his absence.

2. Besides the changes of heat and cold which a single day exhibits, he will pass but a small part of the

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