THE PLANET NEPTUNE. 43 



Neptune, whose action upon Uranus might easily amount 

 to 5" or 1". The uncertainty of the data results from the 

 combination of these three effects, and might amount to 

 10 '' or 12", while the uncertainty of the modern observa- 

 tions does not exceed 2* or 3*. Now the irregularities in 

 the motion of Uranus, which serve as the basis of the dis- 

 cussion, do not exceed at the utmost two minutes of 

 space, and for the most part they are less than one 

 minute ; so that the data were uncertain to one tenth of 

 their whole amount. Le Verrier, therefore, claims that it 

 is unreasonable to demand of him a greater degree of ac- 

 curacy in the positions assigned to his computed planet, 

 than one tenth of their value, and he has attempted to 

 show that these positions are not in error by so large a 

 quantity. Moreover, as he computes the position of his 

 planet only by means of the disturbance which it causes 

 in the motion of Uranus, he can only compute this posi- 

 tion when this disturbance is appreciable. Now this dis- 

 turbance, on account of the distance of the planets from 

 each other, is inappreciable from 1690 to 1812. It was 

 only from 1812 to 1842, that he was furnished with ob- 

 servations in which the disturbing action of Neptune was 

 sensible, and the place of the planet for any other time 

 must be deduced from its motion during these 30 years. 

 He then proceeds to show that the positions he had as- 

 signed to Neptune during this period, and also for a 

 much longer time, were not in error by one tenth of their 

 whole amount. Let us consider, 

 I. The error in the computed longitudes of Neptune. 



