xvi. J THE METHOD OF MEANS. 373 



the time of both events with the same telescope, he will be 

 as much too soon in one observation as he is too late in the 

 other, and the mean moment of the two observations will 

 represent with considerable accuracy the time when the 

 satellite is in the middle of the shadow. Error of judg- 

 ment of the observer is thus eliminated, provided that 

 he takes care to act at the emersion as he did at the 

 immersion. 



