ASTRONOMY. 183 



reckoning from the first. But they are found to 

 happen later than those intervals, by .quantities 

 which increase progressively, till near the conjunc- 

 tion, when the Earth's distance from Jupiter is 

 the greatest. From this time to the opposition 

 again, a series of anticipations takes place, just 

 equal to the former retardations, and in a reverse 

 order, so as to bring back the eclipses near the 

 opposition, to the time at which they would have 

 happened if none of those inequalities had taken 

 place. 



b. The amount of the retardation, from the opposi- 

 tion to the conjunction, is 16m 26 s nearly. The 

 same inequality is found in the eclipses of the other 

 satellites. For the first satellite, n = 42 hours 

 nearly. 



183. The inequality just described, is com- 

 pletely explained, by supposing light to be pro- 

 pagated in succession, and with a velocity by 

 which it passes from the Sun to the Earth in 



gm ^sec. 



CASSINI and MARALDI discovered this inequality in 

 the first satellite, and the explanation just men- 

 tioned occurred to them, but was rejected, because 

 it implied the existence of the same inequality 

 in the other satellites, which did not appear from 

 their observations to be true. MARALDI also ob- 

 jected to this explanation, that, if just, the ine- 

 quality must be affected by the eccentricity of Ju- 

 piter's orbit. The explanation, however, was re- 

 vived by ROEMER in 1674-, and the inequality was 



shewn 



