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.