VOL. VII.] PHILOSOPHICAL TRANSACTIONS. 707 



are so well seen as when they approach to the centre, as they are very narrow, 

 and almost imperceptible when they approach to the circumference : which 

 makes us believe that they are flat and superficial to Jupiter. 



Among these spots of the second sort, there is none so sensible as one tliat is 

 situated between the two belts, which in the disk of Jupiter are usually seen 

 extended from east to west ; the largest of which is between the centre and the 

 northern limb, and the narrowest is beyond the centre towards the southern 

 limb. This spot is always adhering to the southern belt, its diameter is about 

 the tenth part of that of Jupiter; and at the time that its centre is nearest to 

 that of Jupiter, it is distant from it about the third part of the semidiameter 

 of that planet. 



Signer Cassini, after he had made many observations of this spot during the 

 summer of the year ] 665, found that the period of its apparent revolution is nine 

 hours and 56 minutes. Jan. 19, of this present year 1672, (N. S.) when he 

 obser\^ed Jupiter at 44 o'clock 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 

 was already gone beyond the moiety of this belt, and he saw it advance little by 

 little towards the western limb, to which it seemed to be very near at 6-j- o'clock: 

 but then it appeared so small and little, and so little sensible, that he was obli- 

 ged to cease from observing it. 



By the calculation he made 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 

 nine hours 55 minutes, 58 seconds, compensating one revolution by another, 

 and at most 5295 revolutions of nine hours, 55 minutes, 51 seconds; forasmuch 

 as he was assured of the preciseness of one mean revolution to one 8th of a 

 minute, which will be verified by future observations. 



Until then he had never yet seen an immediate return of this spot after g 

 hours and 56 minutes, because it had not happened that Jupiter, after the ap- 

 pearance of the spot, had stood, in one and the same night, long enough above 

 the horizon, at least at a sufficient height to observe him with due distinctness. 

 He had only concluded the time of this revolution by returns observed after 

 about 20, 30, and 50 hours ; and he had more precisely limited it by observa- 

 tions more distant. But the night after the 1st of March, at 7-I- o'clock in the 

 evening, he saw this spot in the midst of the belt ; and the same night, at 5 

 o'clock, and 26 minutes in the morning, he saw it again returned precisely to 

 the same place. Next day he made a report of these observations to the Koyal 

 Academy of the Sciences, and predicted, that the spot would arrive again at the 

 midst of the belt on the 3d of March, at 8 minues after 9 o'clock at night, 

 whereupon that assembly deputed M. Buot and M. Mariotte to be present at 



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