296 On tJie velocity of light, [July 



former to the tenor of the latter or vice versa, to do which 

 we require to know the interval occupied by the satellite to 

 pass over the shadow of Jupiter, for this purpose we must 

 again consult observation ; from the best authorities we 

 find that the first satellite arrives at its greatest elonga- 

 tion from the planet at a distance of 3,034 equatoreal 

 diameters of the planet, or the circumference of the or- 

 bit= 19,000 35 diameters, which is performed as we have 

 found above in \d. 18/?. 27w. 335. 32; hence 1 diame- 

 ter is performed in 2Ji. \^m. 5s. which from the proxi- 

 mity of the satellite to the planet may be assumed as the 

 duration of the eclipses of this satellite — if we now apply 

 this number to the observation of 1834, October^ 16 we 

 obtain the time of Emersion, I8h. 35m. 46s — Subtract- 

 ing the first from the third observation we determine that 

 a certain number of complete synodic revolutions had tak- 

 en place in ]80d. I2h. 52m. 1 5s. 0 which being divid™ 

 ed by the time of one revolution shews the number to be 

 102 ; pursuing the same course with the second and 

 third observations it appears that 9 complete synodic 

 revolutions were performed in I5d. 22h. 20m. 37s. 

 Now the times of performing 102 and 9 sidereal 

 revolutions are 180^/. lOh. 50m. 38^. 64, and 

 I5d. 22h. 7/?2. 59c>\ 88 respectively, in which periods (see 

 the Nautical Almanac) Jupiter has advanced in his or- 

 bit 14° 27' and 1° 23' respectively, and the times neces- 

 sary for the satellite to perform these angles are 

 Ih. ^2?n. \5s. 3, and 9?n. 47s. 3, which added to the 

 above times of performing the sidereal revolutions gives 

 I80d. \2h. 32m. 53s. 9 and I5d. 22h. 17m. 47s. 18 

 for the true interval betv/een the eclipses which were ob- 

 served as above, whereas on account of the progressive 

 motion of light the observations in each case were ob- 

 served later by 19m. 2\s. I and 27n. 49s. 82. Now iu 

 the interval between the first and third observation Ju- 

 piter had increased his distance from the earth 1 58;3CO,OO0 



