V]

PREFACE.

lifetime is far too short. Information as to any errors ormisprints in this volume will therefore he most gratefullyreceived.

Although the Lectures have been printed from the originalmanuscript used in their delivery, alterations and additionshave been freely introduced. At Gresham College they werecopiously illustrated by means of a lime-light lantern. Theillustrations now supplied are necessarily far less numerous.

In accordance with the author’s usual practice in his Lec-tures, references for further information are in general madeto such works upon Astronomy as are most easily accessible,rather than to the original authorities, which he has himselfin almost every instance consulted.

In footnotes various numerical calculations are introduced.When it is possible to obtain a result by means of arithmeticit seems advisable that the process should be shown. School-masters, pupil-teachers, and others have attended the Lectures,who have found such problems useful and interesting.

Some readers may perhaps think that too much space isgiven to the celestial phenomena which would be seen by anobserver upon the various planets. Their investigation, how-ever, affords a useful mental exercise, and enables a studentto realize more easily other celestial movements, a thoroughacquaintance with which is essential.

So rapid is the progress of Astronomy , that, even while thepresent volume has been passing through the press, advanceshave occurred which have rendered some of its pages almostantiquated. The only reference to Dr. Siemens’ theory of theConservation of Solar Energy is in a footnote printed veryshortly after his Memoir was read to the Royal Society , andbefore it had been possible properly to appreciate it. In likemanner it has been impossible to discuss Professor Langley’sresearches with regard to the atmospheric absorption of theSun’s radiations and the relation of the possible temperatures