wing. Now
that the Earth hath such a motion about its own _Axis_ (whereby it might be
fitted to carry about the Moon) is evident by its _Diurnal_ motion. And it
seems as evident that the Moon hath not; because of the same side of the
Moon alwaies turned towards us; which could not be, if the Moon carried the
Earth about: Unlesse we should say, that it carries about the Earth in just
the same Period, in which it turnes upon its own Axis: Which is contrary to
that of the Sun carrying about the Planets: the shortest of whose Periods,
is yet longer than that of the Suns moving about its own Axis. And the like
of _Jupiter_, shorter than the Period of any of his _Satellites_; if at
least the Period of his conversion about his Axis, lately said to be
observed, prove true. (Of _Saturn_ we have not yet any Period assigned; but
it's likely to be shorter, than that of his _Satelles_.) And therefore we
have reason to believe, not that by the Moons motion about its Axis the
Earth should be carried by a contemporary Period (whereby the same face of
the Moon should be ever towards us;) but that by the Earths revolution
about its Axis in 24. hours, the Moon should be carried about it in about
29. dayes, without any motion on its own Axis: And accordingly, that the
_Secondary_ Planets about _Jupiter_ and _Saturn_, are not (like their
_Principals_) turned about their own Axis. And therefore I am not at all
inclined to believe, that the _Menstrual_ Period of the Tides with us, is
to be salved by such an Hypothesis.

In stead of this, that _Surmise_ of mine, (for I dare not yet, with
confidence give it any better name,) of what I have spoken to you
heretofore, (and which hath occasioned this present account which I am now
giving you,) is to this purpose.

The Earth and Moon being known to be Bodies of so great connexion (whether
by any Magnetick, or what other Tye, I will not determine; nor need I, as
to this purpose;) as that {272} the motion of the one follows that of the
other; (The Moon observing the Earth as the Center of its _periodick_
motion:) may well enough be looked upon as _one Body_, or rather _one
Aggregate of Bodies_, which have _one common center of Gravity_; which
Center (according to the known Laws of _Staticks_) is in a streight Line
connecting their respective Centers, so divided as that its parts be in
reciprocal proportion to the Gravities of the two Bodies. As for Example;
Suppose the Magnitude (and therefore probabl