For as _Kepler_ did before conjecture, from the motion of the Primitive Planets about the Sun as their Center, that the Sun moved about its own Axis, but could not prove it, till by _Galileo_ and _Shiner_ the Spots in the Sun were discovered; so it hath been thought reasonable, from the Secundary Planets moving about _Jupiter_, that _Jupiter_ is also moved about his Axis; yet, till now, it hath not been evinced by Observation, That it doth so move; much less, in what Period of Time. And the like reason there is to judge so of _Saturn_, because of the Secundary Planet discovered by Monsieur _Hugens de Zulichem_ to move about it; (though such motion be not yet evinced from Observation:) as well as that of the _Earth_, from its Attendant the _Moon_. Whether the same may be also concluded of the other Planets, _Mars_, _Venus_, and _Mercury_, (about whom have not yet been observed any Secondary Planets to move,) is not so evident. Yet there may be somewhat of like probability in those. Not onely, because it is possible they may have Secundary Planets about them, though not yet discovered; (For, we know, it was long after those of _Jupiter_, before that about _Saturn_ was discovered; and who knows, what after times may discover about the rest?) But because the Primary Planets being all in like manner inlightned by the Sun, and (in all likely hood) moved by it; it is likely that they be moved by the same Laws and Methods; and therefore, turn'd about their own Axis, as it is manifest that some of them are. But, as for the Secundary Planets, as well those about _Jupiter_, as that about _Saturn_; it is most likely that they have no such Rotation upon their Axis. Not so much because, by reason of their smalness, no such thing hath been yet observed, (or, indeed, could be, though it were true;) But because they being Analogical to our _Moon_, it is most likely that they are moved in like manner. Now, though it be {145} true, that there is some kind of _Libration_ of the Moon's body, so that we have not precisely just the same part of it looking towards us; (as is evident by _Hevelius_ observations, and others;) yet is there no Revolution upon its Axis; the same part of it, with very little alteration, always respecting us, as is to be seen in _Hevelius_ his Tratise _de Motu Lunae Libratorio_, and indeed, by all those who have written particularly of the spots on the Moon; and is universally known to all that have with any curiosity viewed it with Telescopes. * * * * * _Of some Philosophical and curious Books, that are shortly to come abroad._ 1. Of the _Origine_ of _Forms_ and _Qualities_, deduced from _Mechanical_ Principles; by the Honorable _Robert Boyle_ Esq. 2. _Hydrostatical Paradoxes_, by the same. Both in _English_.