< 39 4 ) 
M. Cartes to that late Noble and Learned Englifh ■Knight,'’ 
Sir Charles Cavendijh , The other is , whether Motion can be 
made without fuppofing a Vacuum*, where ’tis reprefented , That 
if one comprehend well the Nature , afcribed to the Materia fnb~ 
tills y and how Motions, called Circular ,, are made, which need 
not be juft 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 5 .and alfo , that all the Ine- 
qualities , that may be in the Magnitude or Figure of the parts* 
may be compenfated by other inequalities , met with in their 
Swiftnefs , and by the facility * with which the parts of the 
Subtle Matter , or of the firft Cartefian Element , which are found 
every where happen to be divided , or to accommodate their 
Figure to the Space, they are to fill up : If thefe things be well 
imderftood and confidered,that then no'difficulty can remain touch- 
ing the Motion of the parts of Matter in flew. 
. Befides all thefe particulars , treatedof in this Tome, there oc- 
cur many pretty Queftions concerning Numbers , the Cycloid 
the manner of Working Clajfes for Tele [copes , the way of 
Weighing Air , and many other Curiofi ties , Mathematical and 
Bhyfical , 
Hi ASTRONOM I A REF OR M AT A , Auctore 
JOHANNE BA.PT. RICCIOLI, . Soe. 
fefu,, 
For the Notice of this Book , and the Account of the Chief 
Heads contained therein , we are obliged to the Journal des Sea - 
vans • which informs us, 
Firft , That the Defignof this Work is , that 3 becaufe feve- 
ral Aftronomers , having had their feveral Hypothefes , there is found 
fo great a diversity of opinions , that it is difficult thence to con- 
clude any thing certain $ this Author judged it alfo neceffary, to. 
compare together all the beft Qbfervations , and upon examinati- 
on of what they have moft certain in them , to reform upon that 
meafure the Principles of Aflronomy, 
Secondly , That this Volume is divided into two Parts 5 where- 
of, the. Firft is compofed of Ten. Books 5 in which the Author 
confi- 
