223 ON LIGHT. 



distances of the two planets at different points of their 

 respective revolutions. At present, in our almanacs a 

 due allowance of time for the transmission of light at 

 this rate, assuming a uniform velocity, is made in the 

 calculation of these eclipses; and the discrepancy in 

 question between the observed and predicted times has 

 ceased to exist. 



(10.) Taking the diameter of the earth's orbit, as con- 

 cluded from the sun's observed parallax,* at 24,000 

 diameters of the earth itself, and the latter diameter 

 at 7925! miles, t this gives a velocity of 192,700 miles 

 per second. 



(n.) So vast a speed seemed at first incredible; to 

 some indeed even more so than an instantaneous com- 

 munication. The one might be conceived as the result 

 of some sort of spiritual communication : the other 

 seemed, in those days, to transcend all imaginable limits 

 of mere physical agency. But it soon received a very 

 unexpected confirmation from Dr Bradley's discovery of 

 the ABERRATION of light : to conceive which, let any 

 one imagine a long tube held perpendicularly, at perfect 

 rest, while a falling body (a drop, suppose of a shower of 

 rain), descending also perpendicularly, should pass down 

 its axis. If it entered at the centre of its upper orifice, 

 it would issue at that of the lower ; and, judging from 

 this indication alone, and knowing the tube to be exactly 

 vertical, a spectator would truly conclude from it that 

 the descent of the drop was so also. Supposing him and 



* See p. 196. note. 



f This is tne equatorial diameter. 



