I T 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 definition 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 summarily 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 immaturely 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.

EVERY one strives to adapt means to ends, and when the author prepared his large work on Astronomy, he had no other end in view than to teach Astronomy to such as may be competent to the task and fully prepared to learn it. His first aim was to produce a book of the right tone and character, without any regard to the number of persons who might be prepared to use it. That effort was entirely successful, but the book is not adapted to the great mass of pupils, because it requires of the learner considerable mathematical knowledge, and a corresponding discipline of mind, therefore but few persons, omparatively speaking, feel qualified to study that book. At the same time a book of like tone, character, and spirit, is demanded by teachers for the use of their more humble pupils, except that it must be on a lower mathematical plane, and this book is designed to supply that demand.

THE present volume is the first of a projected series having the double purpose of developing the elements of Practical and Theoretical Astronomy for the pecial student of the subject, and of serving as a handbook of convenient reference for the use of the working astronomer in applying methods and formulae. The plan of the series has been suggested by the author's experience as a teacher at the Johns Hopkins University, and as an investigator. The first has led him to the view that the wants of the student are best subserved by a quite elementary and condensed treatment of the subject, without any attempt to go far into details not admitting of immediate practical application. As an investigator he has frequently been impressed with the amount of time consumed in searching for the formulae and data, even of an elementary kind, which should be, in each case, best adapted to the work in hand.

THE present volume originated in a desire to present, in small compass, a record of the marvellous progress in astronomy during the past hundred years. Indebtedness should be acknowledged to the valuable works of Professor Newcomb, Professor Schiaparelli, Professor Lowell, Professor Young, Sir Robert Ball, Mr Gore, M. Flammarion, and Miss Clerke, who, as the historian of modern astronomy, occupies a place at once authoritative and unique.

THE following work is translated and abridged from M. Flammarion's LJ 'Atmosphere, Paris, 1872. That some curtailment of the text of the original work was requisite will be apparent when it is stated that the French Edition contains 824 large pages of closely printed matter, and is of more than twice the extent of the present volume. Not only was some compression necessary in order to bring the work within a reasonable compass, but, independently of this, one or two chapters, such as that on the Eespiration and Alimentation of Plants, appeared to have so remote a connection with the subject of the work the Atmosphere that their omission would in any case have been desirable.