VOL. LXXI.] PHILOSOPHICAL TRANSACTIONS. 55 



changed, being well regulated. At 7 h 35 m the spot was in the centre, but did 

 not seem quite to fill the white belt ; nor was it so large and distinct as it was 

 before. 



April 7, 9 h 31 m , 3 dark, spots in the equatorial belt nearly in the centre. 

 April 12, 7 h 50 m , the 3 dark spots in the centre. The southernmost of the 

 3 is nearly quite vanished ; the other 2 are also much fainter. They are, how- 

 ever, distinct enough to be known. 



Observations on Jupiter in 1/79- — April 14, clock 52 s too late. At 8 h 48 m a 

 remarkable bright spot in the equatorial belt towards the north is in the centre. 

 At 8 h 58 ra , the spot a little past the centre. 



April 19, clock true mean time. At 7 h 10 m , a bright spot just in the centre, 

 which, from its shape, seems to be the same that was there April 14th. At 

 7 h 20 m , the spot visibly past the centre. 



April 23, clock shows true time. At 9 h 38 m , the same bright spot in the 

 centre. At 9 h 43 m , it was past the centre. 



Comparing together the observations made in the year 1778, Feb. 24 and 

 March 3, we obtain an interval of 7 days 34 minutes, which being divided by 

 17 revolutions made by Jupiter on his axis, we have the time of 1 synodical 

 revolution equal to 9 h 54 m 56 s .4. 



The dark spot on Feb. 25 was observed some time before, and also just after 

 it was past the centre; therefore supposing it to be in the centre about 8 h 58 m , 

 we have 



1°. From Feb. 25 d 8 h 58 m to March 2 d 8 h 2 m = 4 d 23 h 4 m , which divided by 

 12 rev. gives 1 revol. = 9 h 55 m 20 s . 



2°. From Feb. 25 d 8 h 58 m to March 3 d 13 h 49 m = 6 d 4 h 51 m = 15 revol. 

 which gives 1 rev. = 9 h 55 m 24 s . 



3°. From Feb. 25 d 8 h 58 m O s to March 14 d 7 h 36 m 10 s , allowing l m 10 s for 

 the alt. of the clock, = l6 d 22 h 38 m 10 s = 41 revol. which gives 1 revol. = 

 9 h 55 m 4 S .6. 



4°. From March 2 d 8 h 2 m to March 3 d 1 3 h 49 m = l d 5 h 47 m = 3 revol. which 

 gives 1 revol. = 9 h 55 m 40 s . 



5°. From March 2 d 8 h 2 m s to March 14 d 7 h 36 m 10 s = J l 23 h 34 m 10 s =s 

 29 revol. hence 1 revol. = 9 h 54™ 58 s .2. 



6°. From March 3 d 13 h 49 m s to 14 d 7 h 36 m *T> S = 10 d I7 h 47 m 10 s == 26 

 revol. hence 1 revol. = 9 h 54 m 53 5 .4. 



7°. From April 7 d 9 h 31 m to I2 d 7 h 50 m = 4 d 22 h 29 m = 12 revol. hence 1 

 revol. = 9 h 5l m 35 s . 



Again, comparing together the observations of 1779? which were made with 

 the utmost attention to time, we have, 



