VOL. I.] PHILOSOPHICAL TRANSACTIONS. 6l 



of Jupiter ; and, observing it from time to time, he found that within two 

 hours after, the said spot had moved from east to west, about half the length 

 of the diameter of Jupiter. 



According to M. Cassini there are two sorts of spots to be seen in the disk 

 of Jupiter ; the one being only the shadows of his satellites, the other sort re- 

 sembling those that are seen in the moon ; and they are perhaps of the same 

 nature, with those called belts. They move from the eastern to the western 

 limb ; their apparent motion is unequal, and swifter near the centre than the 

 circumference ; and they are never seen so well as when they approach the cen- 

 tre ; for in approaching the circumference they become very narrow, and 

 almost imperceptible ; which seems to argue that they are flat and super- 

 ficial. 



Among these spots, there is none so observable as that situated in the 

 northern part of the southern belt. Its diameter is y^- of Jupiter's ; its centre 

 when nearest is distant from that of Jupiter about i of the semidiameter of 

 that planet. 



M. Cassini, after many observations during the summer l665, found that 

 the period of its apparent revolution is 9 hours 56m. He continued to observe 

 this spot till the beginning of 1666, when Jupiter approached to the beams of 

 the sun ; but after he got out of them it was difficult to be discerned : This 

 giving grounds to think that it might be of the nature of the solar spots, 

 which appearing for a while, disappear for ever, M. Cassini intermitted his 

 observations. 



But, Jan. 19, 1672, N. S. observing Jupiter at 4|- hours in the morning, he 

 perceived in the same place of his disk the figure of the same spot, adhering to 

 the same southern belt. It had already gone over the half of this belt, and he 

 saw it advance gradually towards the w^estem limb, to which it seemed very near 

 at 6j- hours. 



By the celerity of its motion near the centre, and by the place where he had 

 begun to see it, he judged it might have been in the middle of the belt at 4 

 hours 35 m. in the morning. And as he set about making ephemerides of its 

 motion for 1672, he perceived that in those he made for 1666, this spot had 

 been in the middle of Jupiter the same day, viz. the 19th of Jan. at the same 

 hour, so that in six years, of which one is a bissextile, it is found to have made, 

 in respect of the earth, at least 5294 revolutions, each of 9 hours, 55 m. 58 

 sec, one revolution with another ; and at most, 5295 revolutions of 9 hours, 

 55 m. 51 sec. forasmuch as he was assured of the preciseness of one mean revo- 

 lution to 4- of a minute. 



Till that time he never observed an immediate return of this spot after 9 



