well-known and widely spread geological phenomena, I show how the great
features of Mars--the 'canals' and 'oases'--may have been caused. This
chapter will perhaps be the most interesting to the general reader, as
furnishing a quite natural explanation of features of the planet which
have been termed 'non-natural' by Mr. Lowell.

Incidentally, also, I have been led to an explanation of the highly
volcanic nature of the moon's surface. This seems to me absolutely to
require some such origin as Sir George Darwin has given it, and thus
furnishes corroborative proof of the accuracy of the hypothesis that our
moon has had an unique origin among the known satellites, in having been
thrown off from the earth itself.

I am indebted to Professor J. H. Poynting, of the University of
Birmingham, for valuable suggestions on some of the more difficult
points of mathematical physics here discussed, and also for the critical
note (at the end of Chapter V.) on Professor Lowell's estimate of the
temperature of Mars.

BROADSTONE, DORSET, _October_ 1907.



TABLE OF CONTENTS.


CHAPTER I.

EARLY OBSERVERS OF MARS,
--Mars the only planet the surface of which is