Kepler with the existence of a power pertaining to matter, varying
inversely as the squares of the distances, to which power he showed the
weight of terrestrial bodies was owing, and also the revolution of
the moon about the earth. Since Newton's day, those deviations from the
strict wording of Kepler's laws, have been referred to the same law,
and the avowed object of the author of the "Mechanique Celeste," was to
bring all the great phenomena of nature within the grasp of analysis, by
referring them to one single principle, and one simple law. And in his
Introduction to the Theory of the Moon, he remarks: "Hence it
incontestibly follows, that the law of gravitation is the sole cause of
the lunar inequalities."


BESSEL'S OPINION.

However beautiful the conception, it must be admitted that in its _à
priori_ aspect, it was not in accordance with human experience and
analogy to anticipate a successful issue. In nature law re-acts upon
law, and change induces change, through an almost endless chain of
consequences; and it might be asked, why a simple law of matter should
thus be exempt from the common lot? Why, in a word, there should be no
intrinsic difference in matter, by which the gravitation of similar or
dissimilar substances should be affected? But experiment has detected no
such differences; a globe of lead and a globe of wood, of equal weight,
attract contiguous bodies with equal force. It is evident, therefore,
that if there be such differences, human means are not yet refined
enough to detect them. Was the issue successful then? Generally
speaking, we may say yes. But where there is a discrepancy between
theory and observation, however small that may be, it shows there is
still something wanting; and a high authority (Professor Bessel) says in