298 Professor Piazzi Smyth on a Centauri, 
determined the parallax of a number of stars, and can 
state the average parallax of stars of the first, second, 
and other magnitudes ; while M. Faye, of the Paris Obser- 
vatory, supposed that he had found the star, No. 1830 Groom- 
bridge, to have a larger parallax (7. e., a closer distance) 
than a Centauri. But this result has been completely over- 
thrown by subsequent, more extended, and accurate observa- 
tions by M. O. Struve at Pulkowa Observatory, and Mr 
Main at the Greenwich Observatory; the former making it 
only 0”-03 at most, and the latter finding it something less 
than nothing, according to the rigid interpretation of his ob- 
servations, while M. Faye’s quantity was 1”08. Then M. 
Peter’s parallaxes having been arrived at by means of a 
meridian instrument, and being also very small, so as to be 
barely within the power of such means, never commanded 
much confidence; and more, they have had a further doubt 
thrown on them by Mr O. Struve’s admirable observations 
with the great equatorial of the same Observatory, and by 
the Greenwich Observatory; for the star which they have 
very satisfactorily determined to have an insensible parallax, 
or certainly not more than “03, M. Peters had attributed so 
large a quantity as’"25. a Centauri and 61 Cygni thus re- 
main the only hold that we have on the whole of the sidereal 
system; and, as they are both double stars, they will give 
the means of estimating the mass or weight of the com- 
ponents of the system as compared with our sun, when the 
orbits of the stars round their mutual centre of gravity shall 
have been well determined. 
To this end, the former object presents by far the more 
favourable opportunity ; first, from its very much closer dis- 
tance to us, and the consequent greater angle that the orbit 
is seen under; second, from the period of the orbit being 
very much shorter, so that a determination may be obtained 
much sooner; and, third, from the stars being not only 
really larger, thence exhibiting a greater amount of measu- 
rable attraction; but apparently longer also, and therefore 
visible and observable in the day as well as by night, and 
so continuedly throughout the year; as wellas to our haying 
much earlier micrometrical observations of it than of the other. 
And further, it claims the more pressing attention just 
SEDI. erie S cnet RS 
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