Page:Philosophical Transactions - Volume 001.djvu/182

 before these Observations. There remained to find by Experience, whether Jupiter did turn about his Axis, as many believe, that the Earth turns about her's. And although most Astronomers had conjectur'd, it did so, either by this Analogy, or by other Congruities, yet it was much wish'd, that we might be assured thereof by Observations. And this it is, for which we are obliged to M. Cassini, who, having by the advantage of the same Glasses discover'd several changes, as well in the three obscure Belts, commonly seen in Jupiter, as in the rest of his Diske, and having also observed Spots in the midst of that Planet, and sometimes Brightnesses, such as have bin formerly seen in the Sun, hath at length discovered a Permanent Spot in the Northern part of the most Southern Belt, by the means whereof, he hath concluded, that Jupiter turns about his Axis in 9. dayes, 56. minutes, and makes 29. whole circumvolutions in 12. dayes 4. minutes of ours, and 360. in 149. dayes. For he has found, that this Spot was not caused by the Shadow of any Satellit, as well by reason of its Situation, as because it appeared, when there could be no Shadow. Besides, that its motion differed from that of the Shadows, which is almost equal, as well towards the Edges as towards the Middle of Jupiter: Whereas, on the contrary, this Spot hath all the accidents, that must happen to a thing, which is upon the surface of a round Body moving; for example, to move much more slowly towards the Edges, than towards the Middle, and to pass over that part, which is in the middle of the Diske, equal to the half of the Diameter, in the sixth part of the time, it takes to make the whole revolution; he having seen this half pass'd over, in 99 or 100 minutes just, as it must happen, supposing the whole circumrotation is made in 9. hours 56. minutes.

He hath not yet been able to determine the Situation of the Axis, upon which this motion is made, because the Belts, according to which it is made, have for some years appeared streight, though in the precedent years, other Astronomers have seen them a little crooked: Which sheweth, that the Axis of the diurnal motion of Jupiter is a little inclined to the plain of the Ecliptick. But in time we may discover, what certainty there is in this matter.

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  Rh