After this excellent Discovery, he hath calculated many _Tables_, whereof he gives the Explication and Use in the Letters by him addressed to the Abbot _Falconieri_. By the means of them, one may know, _when_ this _Spot_ may be seen by us. For, having first {173} considered it in relation to the _Sun_, in respect whereof, its motion is regular, he considers the same in relation to the _Earth_, where _We_ observe it; and shews by the means of his _Tables_, what is to be added or subtracted, to know, at what time the said _Spot_ is to come into the middle of _Jupiter_'s Diske, according as he is Oriental or Occidental. He hath also considered it in relation to an unmovable point, which he has supposed to be the first point of _Aries_, because we thither refer here upon Earth the beginning of all the Celestial motions, and _there_ is the _Primum mobile_, that one would imagine, if we were in _Jupiter_, as we do here imagine Ours of 24. hours. The Discovery is one of the best, that have been yet made in the Heavens; and those, that hold the Motion of the earth, find in it a full Analogy. For, _Jupiter_ turning about the Sun, does nevertheless turn about his _Axis_; and although he be much bigger than the Earth, he does nevertheless turn much more swiftly than it, since he makes more than two Turns, and a third part, for its one; and carries with him 4. Moons, as the Earth does one. This Observation ought to excite all Curious persons to endeavour the perfecting of _Optick Glasses_, to the end that it may be discovered, whether the other _Planets_, as _Mars_, _Venus_ and _Mercury_, about whom no Moon hath as yet been discovered, do yet turn about their _Axes_, and in how much time they do so; especially _Mars_, in whom some _Spot_ is discover'd, and _Venus_, wherein M._Burattini_ hath signified from _Poland_, he has observ'd Inequalities, as in the Moon. It will be worth while, to watch for the seeing of _Jupiter_ again this Spring, that this happy Observation may be confirmed in divers places, and endeavours used to make new ones. * * * * * _An Account of some Books, lately published._