Excerpt from An Elementary Treatise Physical Astronomy, Vol. 2: Containing Physical Astronomy IT must be in compliance with custom, rather than from any distinct view of good likely to result, when an Author begins his Work by defining the Science he means to treat of. A de finition is not easily laid down. It is not difficult, indeed, to define a branch of' science in general terms but such are seldom intelligible to'the Student. If we enumerate what is too sum marily expressed, and explain a general statement by detailing certain cases comprehended under it, we, probably, ' forestall what belongs to the body of the Work. We attempt to do im maturely what, it is almost certain, will be done imperfectly; and this without an adequate advantage; for, a definition such as we allude to, entailing no consequences, is not required in the beginning of a Work: at the end it is unnecessary. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.