AIDS OF THE NEWTONIAN PERIOD. 483 



stars appears to gain ground among astronomers. The parallax of 61 

 Cygni, as determined by Bessel, is 0" - 34 ; about one-third of a second, 

 or 1-10000 of a degree. That of a Centauri, as determined by Ma- 

 clear, is 0""9, or 1-4000 of a degree.] 



But besides the fixed stars and their corrections, the astronomer has 

 the motions of the planets for his field of action. The established 

 theories have given us tables of these, from which their daily places 

 are calculated and given in our Ephemerides, as the Berliner Jahrbuch 

 of Encke, or the Nautical Almanac, published by the government of 

 this country, the Connaissance des Terns which appears at Paris, or 

 the Effemeridi di Milano. The comparison of the observed with the 

 tabular place, gives us the means of correcting the coefficients of the 

 tables ; and thus of obtaining greater exactness in the constants of the 

 solar system. But these constants depend upon the mass and form of 

 the bodies of which the system is composed ; and in this province, as 

 well as in sidereal- astronomy, different determinations, obtained by dif- 

 ferent paths, may be compared ; and doubts may be raised and may 

 be solved. In this way, the perturbations produced by Jupiter on dif- 

 ferent planets gave rise to a doubt whether his attraction be really pro- 

 portional 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 outer- 

 most Satellite, is found to agree, though different from the mass pre- 

 viously 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 observations 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 prob- 

 able 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 ex- 

 istence of a new inequality. Such an inequality was at length found to 

 result theoretically from the attraction of Venus. Encke, in his exam- 

 ination of his comet, found a diminution of the periodic time in the 

 successive revolutions ; from which he inferred the existence of a resist- 

 ing medium. Uranus still deviates from his tabular place, and the 

 cause remains yet to be discovered. (But see the Additions to this 

 volume.) 



