AT WASHINGTON

THE "BROKEN-BACKED COMET-SEEKER"

NEBULA IN ORION

DIP OF THE MAGNETIC NEEDLE IN VARIOUS LATITUDES

STAR SPECTRA

PROFESSOR LANGLEY'S AIR-SHIP

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 Debt to Astronomy" is from The Chautauquan; and
"An Astronomical Friendship" from the Atlantic Monthly.

SIMON NEWCOMB. WASHINGTON, JUNE, 1906.

I

THE UNSOLVED PROBLEMS OF ASTRONOMY

The reader already knows what the solar system is: an immense central
body, the sun, with a number of planets revolving round it at various
distances. On one of these planets we dwell. Vast, indeed, are the
distances of the planets when measured by our terrestrial standards. A
cannon-ball fired from the earth to celebrate the signing of the
Declaration of Independence, and continuing its course ever since with
a velocity of eighteen hundred feet per second, would not yet be
half-way to the orbit of Neptune, the outer planet. And yet the
thousands of stars which stud