230 HISTORY OF PHYSICAL ASTRONOMY. 



required sensible alterations, and that the mass of 

 Venus ought to be reduced about one-ninth, and 

 that of the moon to be sensibly diminished. In 

 1827, Professor Airy 28 , of Cambridge, compared 

 Delambre's tables with 2000 Greenwich observa- 

 tions, made with the new transit-instrument, and 

 deduced from this comparison the correction of the 

 elements. These in general agreed closely with 

 Burckhardt's, excepting that a diminution of Mars 

 appeared necessary. Some discordances, however, 

 led Professor Airy to suspect the existence of an 

 inequality which had escaped the sagacity of La- 

 place and Burckhardt. And, a few weeks after this 

 suspicion had been expressed, the same mathema- 

 tician announced to the Royal Society that he had 

 detected, in the planetary theory, such an inequality, 

 hitherto unnoticed, arising from the mutual attrac- 

 tion of Venus and the Earth. Its whole effect on the 

 earth's longitude, would be to increase or diminish 

 it by nearly three seconds of space, and its period is 

 about 240 years. " This term," he adds, " accounts 

 completely for the difference of the secular motions 

 given by the comparisons of the epochs of 1783 

 and 1821, and by that of the epochs of 1801 and 

 1821." 



Many excellent tables of the motions of the sun, 

 moon, and planets, were published in the latter part 

 of last century; but the Bureau des Longitudes, 

 which was established in France in 1795, endea- 



28 Phil. Trans. 1828. 



