198 CELESTIAL MEASURINGS AND WEIGHINGS. 



illusory; and it was not till the year 1839 that Mr Hen- 

 derson, having returned from filling the situation of As- 

 tronomer Royal at the Cape of Good Hope, and discuss- 

 ing a series of observations made there with a large 

 ' : mural circle " of the bright star a Gentauri, was enabled 

 to announce as a positive fact the existence of a measur- 

 able parallax for that star : a result since fully confirmed 

 with a very trifling correction by the observations of his 

 successor, Sir T. Maclean 



(22.) The parallax thus assigned to a Centauri is so 

 very nearly a whole second in amount (o'^S) that we 

 may speak of it as such. It corresponds to a distance 

 from the sun of 206,265 times that of the sun from the 

 earth, which, as we have already seen, is itself 23,222 

 polar semi-axes of the latter, thus making a total of 

 4,789,880,000 such semi-axes (or 10,000,000 times 

 that number of geometrical cubits), equivalent to 

 18,918,000,000,000 (nearly nineteen billions) of British 

 statute miles. Its near neighbour (3 of the same con- 

 stellation and other stars adjacent exhibit no such 

 annual displacement, and are therefore beyond the 

 reach of our measurement. Such, then, is the length 

 of the sounding-line with which we have first touched 

 bottom in the attempt to fathom the great abyss of the 

 sidereal heavens. At such a distance, the vast globe 

 filling the earth's orbit, above spoken of, would be 

 covered from sight by a human hair held at twenty-five 

 feet from the eye.* 



(23.) The other mode in which this great question 

 * Supposing the pupil reduced to a point. 



