1§35-] On the velocky of light. J^93 



it having in the interim gone through all ils varieties of 

 rapid and slow movement. N07/ the effect of a dimi- 

 7iished moticsn of Jupiter is to expedite the occurrence, 

 and diminish the duration of the eclipses of his sfi- 

 tellites^ and an increased motion, to retard the same. 

 The next cause of inequahty in the interval be- 

 tween the consecutive eclipses of the satellites 

 of Jupiter arises from the elliptical form of their or- 

 bits, v/liich, with the secondaries as with the primarief?^ 

 causes their motions to be unequal; and the interval is 

 further rendered unequal by reason of the elliptical 

 figure of Jupiter and consequently of his shadow, and by 

 the orbits of the satellites being inclined to the plane of 

 Jupiter's orbit ; these being the only serious causes of 

 inequality we will now see how far their effects can be 

 avoided in obtaining an approximate value for the ve- 

 locity of light. 



Consulting the Madras observations, we find the ob- 

 servation of two consecutive eclipses of the first satellite 

 as follows : — 



Madras M. Time d, li. m. s. 

 Emers.of Jupiter's 1st Sat. 1821 Nov. 20 14-46-58-47. 



— 22 9-15-38-91. 



Taking the difference we obtain the in- 

 terval between the eclipses of the 



first satellite M8*28'40-44. 



this result from the various causes above noticed, and 

 the uncertainty of a single observation, is probably 

 several seconds in error, it is however of some use as a 

 counter in obtaining a more exact value ; thus, from the 

 above source we obtain the following observations : — 

 d. h. m. s. 



1822 March... 3* 6-48' 1-9. Immer. of Jupiter's 1st Sat. 



1834 January.15-10-28-35 0. 



the interval 433Gd. 3h. 40m. 33'ls. being divided by the 

 above Id. 18h. 2Sm. 40s. 44 shews that between the ob- 

 servatian of March 1822 and that of January 1834 there 



