ears at a time, it seems to revolve a little faster, and then
again a little slower. The changes are very slight; they can be
detected only by the most laborious and refined methods; yet they must
have a cause, and we should like to know what that cause is.

The moon shows a similar irregularity of motion. For half a century,
perhaps through a whole century, she will go around the earth a little
ahead of her regular rate, and then for another half-century or more
she will fall behind. The changes are very small; they would never have
been seen with the unaided eye, yet they exist. What is their cause?
Mathematicians have vainly spent years of study in trying to answer
this question.

The orbit of Mercury is found by observations to have a slight motion
which mathematicians have vainly tried to explain. For some time it was
supposed to be caused by the attraction of an unknown planet between
Mercury and the sun, and some were so sure of the existence of this
planet that they gave it a name, calling it Vulcan. But of late years
it has become reasonably certain that no planet large enough to produce
the effect observed can be there. So thoroughly has every possible
explanation been sifted out and found wanting, that some astronomers
are now inquiring whether the law of gravitation itself may not be a
little different from what has always been supposed. A very slight
deviation, indeed, would account for the facts, but cautious
astronomers want other proofs before regarding the deviation of
gravitation as an established fact.

Intelligent men have sometimes inquired how, after devoting so much
work to the study of the heavens, anything can remain for astronomers
to find out. It is a curious fact that, although they were never
learning so fast as at the present day, yet there seems to be more to
learn now than there ever was before. Great and numerous as are the
unsolved problems of our science, knowledge is now advancing into
regions which, a few years ago, seemed inaccessible. Where it will stop
none can say.

II

THE NEW PROBLEMS OF THE UNIVERSE

The achievements of the nineteenth century are still a theme of
congratulation on the part of all who compare the present state of the
world with that of one hundred years ago. And yet, if we should fancy
the most sagacious prophet, endowed with a brilliant imagination, to
have set forth in the year 1806 the problems that the century might
solve and the things whic