562 THE PRINCIPLES OF SCIENCE. [chap. 



certain inequalities in the moon's motion, had estimated 

 it at 8"'9i6; Winneke, from observations of Mars, at 

 S"'g64 ; Leverrier, from the motions of Mars, Venus, and 

 the moon, at 8"-950. These independent results agree 

 much better with each other than with that of Bessel 

 (8"-5 78) previously received, or that of Encke (8"'58) 

 deduced from the transits of Venus in 1761 and 1769, and 

 though each separately might be worthy of less credit, yet 

 their close accordance renders their mean result (8""943) 

 comparable in probability with that of Bessel. It was 

 further found that if Foucault's value for the velocity of 

 lioht were assumed to be correct, and the sun's distance 

 were inversely calculated from that, the sun's parallax 

 would be 8"'96o, which closely agreed with the above 

 mean result. This further correspondence of independent 

 results threw the balance of probability strongly against 

 the results of the transit of Venus, and rendered it desir- 

 able to reconsider the observations made on that occasion. 

 Mr. E. J. Stone, having re-discussed those observations,^ 

 found that grave oversights had been made in the calcula- 

 tions, which being corrected would alter the estimate of 

 parallax to 8"-9i, a quantity in such comparatively close 

 accordance with the other results that astronomers did not 

 hesitate at once to reduce their estimate of the sun's mean 

 distance from 95,274,000 to 91,771,000, miles, although 

 this alteration involved a corresponding correction in the 

 assumed magnitudes and distances of most of the heavenly 

 bodies. The solar parallax is now (1875) believed to be 

 about 8"'878, the number deduced from Cornu's experi- 

 ments on the velocity of liglit. This result agrees very 

 closely witli 8"*879, the estimate obtained from new obser- 

 vations on the transit of Venus, by the French observers, 

 and with 8""873, the result of Galle's observations of the 

 planet Flora. When all the observations of the late transit 

 of Venus are fully discussed the sun's distance will probably 

 be known to less than one part in a thousand, if not one 

 part in ten thousand.^ 



. 1 Monthly Notices of the Royal Astronomical Society, vol. xxviii. 

 p. 264. 



2 It would seem to be absurd to repeat t.lie profuse expenditure of 

 1874 at the approaching transit in 1882. The aggregate sum spent in 

 1874 by various govo'nments and individuals can hardly be less than 



