to my fellow-workers, Professors Hann, Mohn, Supan, Köppen, Angot, and W. M. Davis, and also to Dr. Fridtjof Nansen, for permission to reproduce some of their maps and diagrams in the present volume. Mr. Henry S. Mackintosh, of Keene, N. H., has very kindly helped me in the proof-reading.

ROBERT DE C. WARD.

Harvard University, Cambridge, Mass., December, 1907.

NOTE TO SECOND EDITION

I have taken the opportunity offered by the publication of a second edition of this volume to make some revision of the chapters on The Characteristics of the Polar Zones and on Changes of Climate, in order that these subjects may be more accurately presented in the light of recent investigations. A few other changes have also been made, and all errors which occurred in the first edition have been corrected.

ROBERT DE C. WARD.

August, 1917.