to us observations made in those times, which, so far as we can trust
them, we may add to the observations made in our own times. The
materials in short which Science has to handle are obtained by
experience.

But on the other hand Science can deal with these materials only on the
condition that they are reducible to invariable laws. If any observation
made by the senses is not capable of being brought under the laws which
are found to govern all other observations, it is not yet brought under
the dominion of Science. It is not yet explained, nor understood. As far
as Science is concerned, it may be called as yet non-existent. It is for
this very reason possible that the examination of it may be of the very
greatest importance. To explain what has hitherto received no
explanation constitutes the very essence of scientific progress. The
observation may be imperfect, and may at once become explicable as soon
as it is made complete; or, what is of far more value, it may be an
instance of the operation of a new law not previously known, modifying
and perhaps absorbing the law up to that time accepted. When it was
first noticed in Galileo's time that water would not ascend in the
suction pipe of a pump to a greater height than 32 feet, the old law
that nature abhors a vacuum was modified, and the reasons why and the
conditions under which Nature abhors a vacuum were discovered. The
suction of fluids was brought under the general law of mechanical
pressure. The doctrine that Nature abhorred a vacuum had been a fair
generalization and expression of the facts of this kind that up to that
time had been observed. A new fact was observed which would not fall
under the rule. The examination of this fact led to the old rule being
superseded; and Science advanced a great step at once. So in our own day
was the planet Neptune discovered by the observation of certain facts
which could not be squared with the facts previously observed unless the