138 LIGHT SCIENCE FOR LEISURE HOURS. 



nearly three thousand miles too great ; while his third 

 made it ten thousand miles too great. His contem- 

 poraries were even less successful. Maskelyne, after a 

 long and careful series of observations, assigned to the 

 planet a diameter eight thousand miles too small ; the 

 astronomers of Milan gave the planet a diameter more 

 than twenty thousand miles too great ; and Mayer, of 

 Mannheim, was even more unfortunate, for he assigned 

 to the planet a diameter exceeding its actual diameter 

 of thirty-four thousand miles, by rather more than 

 fifty thousand miles. It will be understood, therefore, 

 that Herschel might well leave unattempted the task 

 of comparing the different diameters of the planet. 

 This task required that he should estimate a quantity 

 (the difference between the greatest and the least 

 diameters) which was small even by comparison with 

 the errors of his former measurements. 



But besides this, a peculiarity in the axial pose of 

 Uranus has to be taken into account. I have spoken 

 of the uprightness of Jupiter's axis with reference to 

 his path ; and by this I have intended to indicate the 

 fact that if we regard Jupiter's path as a great level 

 surface, and compare Jupiter to a gigantic top spinning 

 upon that surface, this mighty top spins with a nearly 

 upright axis. In the case of Uranus the state of 

 things is altogether different. The axis of Uranus is 

 so bowed down from uprightness as to be nearly in the 

 level of the planet's path. The result of this is that 

 when Uranus is in one part of his path his northern 

 pole is turned almost directly towards us. At such a 



