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 probably, the Gravity) of the Moon to be about an _One and fourtieth part_ of that of the Earth; (and thereabouts _Hevelius_ in his _Selenography_ page 203. doth out of _Tycho_, estimate the proportion; and an exact certainty is not necessary to our present businesse.) And the distance of the Moons Center from the Center of the Earth, to be about _fifty six Semidiameters_ of the Earth, (as thereabouts he doth there estimate it, in its middle distance; and we need not be now very accurate in determining the numbers; wherein Astronomers are not yet very well agreed.) The distance of the Common Center of Gravity of the two Bodies, will be from that of the Earth, about a two and fourtieth part of fifty six Semidiameters; that is, about 56/42 or 4/3 of a Semidiameter; that is about 1/3 of a Semidiameter of the Earth, above its surface, in the Air, directly between the Earth and Moon. Now supposing the Earth and Moon, joyntly as one Body, carried about by the Sun in the great Orb of the _Annual_ motion; this motion is to be estimated, (according to the Laws of _Staticks_, in other cases,) by the motion of the common Center of Gravity of both Bodies. For we use in _Staticks_, to estimate a Body, or Aggregate of Bodies, to be moved upwards, downwards, or otherwise, so much as its Common Center of Gravity is so moved, howsoever the parts may change places amongst themselves.