ological prescription alone, but to the vital
sense of humanity. Novel thoughts are rife; fresh impulses stir the
nations; the soughing of the wind of progress strikes every ear. "The
old order changeth" more and more swiftly as mental activity becomes
intensified. Already many of the scientific doctrines implicitly
accepted fifteen years ago begin to wear a superannuated aspect.
Dalton's atoms are in process of disintegration; Kirchhoff's theorem
visibly needs to be modified; Clerk Maxwell's medium no longer figures
as an indispensable factotum; "absolute zero" is known to be situated on
an asymptote to the curve of cold. Ideas, in short, have all at once
become plastic, and none more completely so than those relating to
astronomy. The physics of the heavenly bodies, indeed, finds its best
opportunities in unlooked-for disclosures; for it deals with
transcendental conditions, and what is strange to terrestrial experience
may serve admirably to expound what is normal in the skies. In celestial
science especially, facts that appear subversive are often the most
illuminative, and the prospect of its advance widens and brightens with
each divagation enforced or permitted from the strait paths of rigid
theory.

This readiness for innovation has undoubtedly its dangers and drawbacks.
To the historian, above all, it presents frequent occasions of
embarrassment. The writing of history is a strongly selective operation,
the outcome being valuable just in so far as the choice what to reject
and what to include has been judicious; and the task is no light one of
discriminating between barren speculations and ideas pregnant with
coming truth. To the possession of such prescience of the future as
would be needed to do this effectually I can lay no claim; but diligence
and sobriety of thought are ordinarily within reach, and these I shall
have exercised to good purpose if I have succeeded in rendering the
fourth edition of _A Popular History of Astronomy during the Nineteenth
Century_ not wholly unworthy of a place in the scientific literature of
the twentieth century.

My thanks are due to Sir David Gill for the use of his photograph of the
great comet of 1901, which I have added to my list of illustrations, and
to the Council of the Royal Astronomical Society for the loan of glass
positives needed for the reproduction of those included in the third
edition.

London, _July_, 1902.

PREFACE TO THE FIRST EDITION

The progr