DISTANCE COMPUTED BY PARALLAX. 



the side of a pier of solid masonry, erected upon a foundation of 

 rock. This instrument was screwed into such a position that 

 particular stars, as they crossed the meridian, would necessarily 

 pass within its field of view. Micrometric wires were, in the usual 

 manner, placed in its eye-piece, so that the exact point at which 

 the stars passed the meridian each night, could be observed and 

 recorded with the greatest precision. The instrument being thus 

 fixed and immovable, the transits of the stars were noted each 

 night, and the exact places where they passed the meridian 

 recorded. This kind of observation was carried on through the 

 year ; and if the earth's change of position, by reason of its annual 

 motion, should produce any effect upon the apparent position of 

 the stars, it was anticipated that such effect would be discovered 

 by these means. After, however, making all allowance for the usual 

 causes which affect the apparent position of the stars, no change of 

 position was discovered which could be assigned to the earth's motion. 



18. Notwithstanding the numerous difficulties which beset the 

 solution of this problem, by means of observations made with the 

 ordinary instruments, Professor Henderson, during his residence, 

 as astronomer at the Royal Observatory, at the Cape of Good 

 Hope, succeeded in making a series of observations upon the star 

 designated a in the constellation of the Centaur, which, being 

 afterwards submitted by him to the proper reductions, gave a 

 parallax of I". Subsequent observations made by his successor, 

 Mr. Maclear, at the same observatory, partly with the same in- 

 strument, and partly with an improved and more efficient one of 

 the same class, have fully confirmed this result, giving 0-9128, or 

 j^ths of a second as the parallax. 



It is worthy of remark, that this conclusion of Messrs. Hender- 

 son and Maclear is confirmed in a remarkable manner by the fact, 

 that like observations and computations, applied to other stars in 

 the vicinity of a Centauri, and therefore subject to like annual 

 causes of apparent displacement, such as the mean annual variation of 

 temperature, gave no similar result, showing thus that the displace- 

 ment found in the case of a Centauri could only be ascribed to parallax. 



Since the limits of error of this species of observation affecting 

 the final result cannot exceed the tenth of a second, it may then 

 be assumed as proved, that the parallax of a Centauri is 1", and, 

 consequently, that its distance from the solar system is such that 

 light must take 3 '234 years to move over it. 



19. Notwithstanding the great multitude of stars to which in- 

 struments of observation of unlooked-for perfection, in the hands 

 of the most able and zealous observers, have been directed, the re- 

 sults of all such labours have hitherto been rather negative than 

 positive. The means of observation have been so perfect, and their 



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