ACCORD AS CE OF QUANTITATIVE THEORIES, <<-. 203 



conclusion that the velocity was not really more than 

 185,172 miles per second. No repetition of the experi- 

 ment as thus performed would shake this result, and there 

 was accordingly a discrepancy between the two astrono- 

 mical and the experimental results of about 7000 miles 

 per second demanding explanation. 



Now a very little consideration shows that both the 

 astronomical determinations involve the magnitude of the 

 earth's orbit as one datum, because our estimate of the 

 earth's velocity in its orbit depends upon our estimate of 

 the sun's mean distance. Accordingly as regards this 

 quantity the two astronomical results must count only 

 for one. Though the transit of Venus had been con- 

 sidered to give the best data for the calculation of the 

 sun's parallax and distance, yet astronomers had not neg- 

 lected other less favourable opportunities. Thus Hansen, 

 calculating from certain inequalities in the moon's motion, 

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

 Mars, at 8"' 96 4 ; Leverrier, from the motions of Mars, 

 Venus, and the moon, at 8"' 9 50. Now these independent 

 results agree much better with each other than with that 

 of Bessel (8 //< 578) 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) probably comparable in probability with that of 

 Bessel. It was further found that if Foucault's value for 

 the velocity of light were assumed to be correct, and the 

 sun's distance were inversely calculated from that and the 

 other requisite data, the sun's parallax would appear to 

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

 result. This further correspondence of independent re- 

 sults threw the balance of probability strongly against the 

 results of the transit of Venus, and rendered it desirable 

 to reconsider the observations made on that occasion. 



