CHAPTER V 



I Hi; IMS. OYKBY OF URANUS 



Tm; third paper sent by Herschcl to the Royal Society 

 was in the form of a letter to Dr. Watson from Mr. 

 William Herschel of Bath, dated October 18, 1780. 

 It was a record of observations made in the tlm 

 years from 1777 to 1779, with the view of determining 

 whether our day is of the same length year after year. 

 A point so difficult could be settled, he thought, only 

 by observing the length of the day in otlirr ]>l.m<ts. 

 This had been done, or attempted, for Venus and 

 Jupiter, by watching the time it took for a spot on 

 the face of the planet to return to the same position. 

 But in Venus, on account of her exceeding brilliance, 

 it had been done so imperfectly that her day was put 

 down roughly as of 23 hours' length. For Jupiter 

 the time of rotation on his axis was set down more 

 precisely at 9 hours 56 minutes, a result arrived at 

 by keeping careful watch on spots that may not be 

 fixed points on his disc, but movable on what we may 

 call trade-wind belts of clouds in his equator. These 

 spots "change so often that it is not easy, if at all 

 possible, to ascertain the identity of the same appear- 

 ance for any considerable length of time." Sometimes 

 a bright, at other times a dark spot, or belt, was 

 observed, but the time of its revolution round the 



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