ir Center of Agitation: about which, there is also a Letter
inserted of {394} M. _Des-Cartes_ to that late Noble and Learned English
Knight, Sir _Charles Cavendish_. The _other_ is, whether Motion can be made
without supposing a _Vacuum_: where 'tis represented, That, if one
comprehend well the Nature, ascribed to the _Materia subtilis_, and how
Motions, called _Circular_, are made, which need not be just _Ovals_ or
_true_ Circles, but are only called Circular, in regard that their Motion
ends, where it had begun, whatever irregularity there be in the Middle; and
also, that all the Inequalities, that may be in the Magnitude or Figure of
the parts, may be compensated by other inequalities, met with in their
Swiftness, and by the facility, with which the parts of the _Subtle
Matter_, or of the first _Cartesian_ Element, which are found every where,
happen to be divided, or to accommodate their Figure to the Space, they are
to fill up: If these things be well understood and considered, that then no
difficulty can remain touching the Motion of the parts of Matter _in
pleno_.

Besides all these particulars, treated of in this _Tome_, there occur many
pretty Questions concerning _Numbers_, the _Cycloid_, the manner of
_Working Glasses for Telescopes_, the way of _Weighing Air_, and many other
Curiosities, Mathematical and Physical.

_II. _ASTRONOMIA REFORMATA_, Auctore _JOHANNE BAPT. RICCIOLI_, Soc. Jesu._

For the Notice of this Book, and the Account of the Chief Heads contained
therein, we are obliged to the _Journal des Scavans_; which informs us,

_First_, That the Design of this Work is, that, because several
_Astronomers_, having had their several _Hypotheses_, there is found so
great a diversity of opinions, that it is difficult thence to conclude any
thing certain; this Author judged it also necessary, to compare together
all the best Observations, and upon examination of what they have most
certain in them, to reform upon that measure the Principles of Astronomy.

_Secondly_, That this _Volume_ is divided into two Parts, whereof the
_First_ is composed of _Ten_ Books; in which the Author {395} considers the
principal Observations, hitherto made of the Motion of the Planets and the
Fixed Stars, of their Magnitude, Figure, and other Accidents; drawing
thence several Conclusions, in which he establishes his _Hypothesis_. The
_second_ contains his _Astronomical Tables_, made according to the
_Hypotheses_ of the Fi