426 HISTORY OF SCIENCE. 



of the attractive forces between the planets ; and it will readily be 

 observed that, until the year 1822, the action of Neptune on Uranus 

 accelerated the movement of that planet, and that after 1822 the action 



changed into a retarding one. Neptune 

 has been found to have a diameter of 

 about 36,000 miles, and to revolve round 

 the sun in 164*6 years at a distance of 

 about 2,746 millions of miles. This 

 distance is notably less than that re- 

 quired by the so-called law of Bode in 

 the proportion of 300 to 388, and it is 

 by far the most marked deviation from 

 it. On the loth of October, 1846, Las- 

 sell discovered a satellite of Neptune. 



The orbit in which Neptune was 

 FIG. 190. actually found to revolve is, therefore, 



very much smaller than that which had 



been assigned by Le Verrier and Adams; and the other elements which 

 had been hypothetically determined were consequently very different 

 from the truth. There were in the problem four unknown quantities to 

 be determined, but these might be modified in a variety of ways, so- as 

 to compensate and correct each other. Thus it happened that, the 

 distance having been assumed too great, as it was afterwards found, the 

 error was compensated by adopting a mass greater than the reality. 

 This was possible, because the observations of Uranus only embraced 

 a few years before and after the conjunctions of the planets. Had ob- 

 servations of Uranus over a very extended space of time been accessible, 

 the inadequacy of the false assumptions to explain the whole of the phe- 

 nomena would have been noticed, and a nearer approach to the truth 

 obtained. After Neptune had been seen, and the elements deduced 

 from observation, some astronomers affirmed that this was not the 

 theoretical planet of Le Verrier and Adams ; and that the agreement of 

 Neptune's position, when discovered, with that of the hypothetical 

 planet, was only a happy coincidence. The final conclusion is that 

 Adams and Le Verrier did at about the same time investigate the 

 irregularities of Uranus, on the supposition that they were due to an 

 exterior planet, and that independently of each other they arrived at 

 a very approximate determination of the position of the perturbing 

 planet. Each is, therefore, entitled to be considered the independent 

 discoverer of Neptune. 



Lassell discovered also a satellite of Uranus revolving at a distance 

 nearer to the planet than the innermost of the six observed by Sir W. 

 Herschel. The number of Uranian satellites was again added to in 

 1847, when O. Struve discovered one which is the eighth in the order 

 of discovery, and takes the second place in order of distance from its 

 primary. On September i6th, 1847, an additional satellite to Saturn, 



