48 HISTOEY OF ASTRONOMY. 



to the mean distance, eccentricity, and mass of the planet? 

 The answer is plain : they were misled by placing too 

 great confidence in Bode's law of the planetary distances. 

 Since it was necessary, in the first instance, to make some 

 hypothesis with regard to the distance of the disturbing 

 body from the sun, both computers started with that sup- 

 position which was generally thought most probable. 

 The distance of Saturn from the sun is nearly double 

 that of Jupiter ; the distance of Uranus is almost exactly 

 double that of Saturn ; hence it seemed probable that the 

 planet they were in search of, would be found at a dis- 

 tance about double that of Uranus. Accordingly this as- 

 sumption was made the basis of their first computations ; 

 but neither of the computers accepted this as his final re- 

 sult, without attempting to verify it. They both varied 

 the assumed distance, and found that by bringing the 

 planet a little nearer the sun, the observed irregularities 

 of Uranus were still better explained. The distance of 

 36*154 (or about 3435 millions of miles), finally adopted 

 by Le Yerrier, was that which appeared to reconcile all 

 the observations most satisfactorily. This distance corre- 

 sponds to a period of two hundred and seventeen years. 

 Le Yerrier found (or thought he found) that whether he 

 increased or diminished this distance, the observations of 

 Uranus were not so well represented. He hence inferred 

 that the mean distance from the sun could not be less 

 than 35*04, nor greater than 37*90. (Le demi-grand axe 

 de 1'orbite ne pent varier qu'entre les limites 35*04 et. 

 37*90.) The periods of revolution corresponding to these 



