12. Besides, he declares, that those _Astronomers_ were too hasty, in determining, after 5 or 6 Observations only, in how much time _Mars_ finish's his Revolution; and denies it to be perform'd in 13 hours: adding, that, though Himself had observ'd for a much longer time, than they; yet he durst not for a great while define, Whether _Mars_ made but _one_ Turn in 24 hours 40 minuts or _two_; and that all, that he could, for a long time affirm, was onely this, that after 24 h. 40 m. this Planet appear'd in the same manner he did before. 13. But since those first Observations, He affirms to have found cause to determine, that the Period of this Conversion is made in the said space of 24 h. 40 m; and not oftner than once within that time; Alledging for proof; 1. That, whereas _Febr._ 6. (st.n.) he saw the Spots of the first Face of _Mars_, moving from eleven of the Clock in the night, until break of day, they appear'd not afterwards in the Evening after the rising of that Planet (witness several intelligent persons, which he names, that were present at the Observations) Whence he infers, that after 12 hours and 20 minuts, the same Spots did not come about; since that the same, which in the morning were seen in the middle, upon the rising of _Mars_; after 13 or 14 hours, might have appear'd neer the Occidental Limb. But, because he might be imposed upon by Vapors, whilst _Mars_ was yet so neer the _Horizon_, he gives this other determination, _vid._ 2. Whereas he saw the first Face of _Mars_ the 6 of _February_ at 11 of the clock of the night following; he did not see the same after 18 daies at the same hour; as he ought to have done, if the Period were absolved in the space of 12 h. 20 m. [Illustration] 3. Again, whereas he saw _Febr._ 24. in the Evening, the other Face of _Mars_, he could not see the same, the 13. and 15. day of _March_, to wit after 17 and 19 days; as he should have done, if the Revolution were made in the newly mention'd time. 4. Again, whereas the 27. of _March_ in the Evening he saw {245} the second Face of _Mars_, he could not see it the 14. and 16. of _April_. From all which Observations he Judges it to be evident, that the Period of this Planets Revolution is not perform'd in the space of 12. hours 20, minutes, but in about 24 hours 40 minutes; more exactly to be determin'd by comparing distant Observations: And that those who affirm the former, must have been deceived by not well distinguishing the two Faces, but that having seen the second, taken it for the first. All which he concludes with this Advertisement, that, when he defines the time of the Revolution of _Mars_, he does not speak of its _Mean_ Revolution, but onely of that, which he observ'd, whilst _Mars_ was opposite to the Sun; which is the shortest of all. _The Figures of the _Principal_ Observations, represented in the Book here discoursed of, may be seen in the annexed _Scheme_; _videl.__