Determination of the Sun's Distance. 129 



European governments to secure good observations, and 

 expensive expeditions were fitted out for all parts of the world, 

 the celebrated one under command of Captain Cook being 

 dispatched to the South Seas. The transit was fully observed 

 in both hemispheres, the resulting parallax being calculated 

 as 8 //- 58, corresponding to a distance of 95,023,000 miles. 

 This distance has been accepted up to within the last two or 

 three years. Mr. Airy, however, in a paper read before the 

 Royal Astronomical Society, in May, 1857, stated that great 

 doubts were attachable by many astronomers to this result, 

 inasmuch as " it happened that it depended almost entirely 

 upon the observations made by Father Hell, of Wardhoe, 

 and to these great suspicion has been attached, many having 

 without hesitation designated them as forgeries." 



Encke, the well-known astronomer and mathematician, has 

 discussed these observations on the transits of Venus with 

 every possible care, and the numbers given above are the 

 results of his computations ; and all reliance may be placed on 

 their correctness as far as the calculation is concerned, but 

 circumstances have since arisen, apart from the suspicions 

 which some attach to the genuineness of some of the northern 

 observations of these transits, which indicate very strongly 

 that the sun's distance derived from them is too large. 



The well-known French philosopher, Foucault, from some 

 recent experiments on the velocity of light, with apparatus 

 from which all uncertainty appears to be excluded, has come 

 to the conclusion that we have hitherto attributed too great 

 a velocity to light, and that instead of 192,000 miles it moves 

 through only 185,170 a second. Now, from eclipses and 

 other phenomena, we know exactly that light takes 8m. ] 8s. to 

 move from the sun to the earth's surface, from which, by a 

 simple calculation, the sun's distance becomes a little over 

 92,000,000 of miles, giving a parallax of 8 //- 86 ; being more than 

 3,000,000 of miles less than the accepteddistance. Again, Mons. 

 Le Yerrier, the Director of the Imperial Observatory at Paris, 

 some time since arrived at the conclusion that, in order to 

 satisfy the theory of some of the planetary perturbations, the 

 sun's distance must be diminished, and theoretically assigned, 

 on purely physical grounds, irrespective of instrumental 

 measures, a parallax of 8 //- 95, equal to a distance of 91,066,350 

 miles, less, by over 4,000,000 miles, than that previously 

 adopted. I must here anticipate, to draw attention to the 

 singular coincidence, that the purely theoretical determination 

 of the. sun's parallax agrees closely with the practical 



