AIDS OF THE NEWTONIAN PERIOD. 



form of the bodies of which the system is com- 

 posed ; and in this province, as well as in sidereal 

 astronomy, different determinations, obtained by 

 different paths, may be compared ; and doubts may 

 be raised and may be solved. In this way, the 

 perturbations produced by Jupiter on different 

 planets gave rise to a doubt whether his attraction 

 be really proportional to his mass, as the law of 

 universal gravitation asserts. The doubt has been 

 solved by Nicolai and Encke in Germany, and by 

 Airy in England. The mass of Jupiter, as shown 

 by the perturbations of Juno, of Vesta, and of 

 Encke's comet, and by the motion of his outermost 

 satellite, is found to agree, though different from 

 the mass previously received on the authority of 

 Laplace. Thus also Burckhardt, Littrow, and Airy, 

 have corrected the elements of the solar tables. In 

 other cases, the astronomer finds that no change of 

 the coefficients will bring the tables and the obser- 

 vations to a coincidence ; that a new term in the 

 formula is wanting. He obtains, as far as he can, 

 the law of this unknown term ; if possible, he traces 

 it to some known or probable cause. Thus Mr. 

 Airy, in his examination of the Solar Tables, not 

 only found that a diminution of the received mass 

 of Mars was necessary, but perceived discordances 

 which led him to suspect the existence of a new 

 inequality. Such an inequality was at length found 

 to result theoretically from the attraction of Venus. 

 Encke, in his examination of his comet, found a 



