PREFACE

In preparing and issuing this collection of essays and addresses,
the author has yielded to what he could not but regard as the too
flattering judgment of the publishers. Having done this, it became
incumbent to do what he could to justify their good opinion by
revising the material and bringing it up to date. Interest rather
than unity of thought has determined the selection.

A prominent theme in the collection is that of the structure,
extent, and duration of the universe. Here some repetition of
ideas was found unavoidable, in a case where what is substantially
a single theme has been treated in the various forms which it
assumed in the light of constantly growing knowledge. If the
critical reader finds this a defect, the author can plead in
extenuation only the difficulty of avoiding it under the
circumstances. Although mainly astronomical, a number of
discussions relating to general scientific subjects have been
included.

Acknowledgment is due to the proprietors of the various
periodicals from the pages of which most of the essays have been
taken. Besides Harper's Magazine and the North American Review,
these include McClure's Magazine, from which were taken the
articles "The Unsolved Problems of Astronomy" and "How the Planets
are Weighed." "The Structure of the Universe" appeared in the
International Monthly, now the International Quarterly; "The
Outlook for the Flying-Machine" is mainly from The New York
Independent, but in part from McClure's Magazine; "The World's