226 ON LIGHT. 



And in the way of experiment, the contrivances of 

 clock-work enable us to register the subdivisions of what 

 we call " an instant" into hundreds, nay, thousands, of 

 equal and exactly measurable portions applying, so to 

 speak, a microscope to time, and estimating, by unde- 

 niable calculation, portions of it utterly eluding all our 

 powers of perception. The question has been asked in 

 both these modes, by astronomical observation and 

 by direct physical experiment, and the answer, from 

 each, has been affirmative ; and from both agreeing, 

 in a manner which may well be considered wonder- 

 ful. 



(9.) The planet Jupiter is attended by four satellites 

 which revolve round it in orbits veiy nearly circular, 

 and whose dimensions, forms, and situations with respect 

 to that of the planet itself are now perfectly well known. 

 The periodical times of their respective revolutions are 

 also ascertained with extreme precision, and all the par- 

 ticulars of their motions have been investigated with 

 extraordinary care and perseverance. The three interior 

 of them are so near the planet and the planes of their 

 orbits so little inclined to that in which it revolves 

 round the sun, that they pass through its shadow, and 

 therefore undergo eclipse, at every revolution. These 

 eclipses have been assiduously observed ever since the 

 discovery of the satellites, and their times of occurrence 

 registered. As they afford a means of determining the 

 longitudes of places, the prediction beforehand of the 

 exact times of their occurrence becomes an object of 

 great importance : and it is evident enough that, all the 



