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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, Mathematic and Physical.
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
