EOYAL IEISH ACADEMY. 
225 
triangulation, and so render much more easy the detection of those 
regular variations of place which enable us to pronounce the moving 
body to he a planet. 
Induced by these considerations, and stimulated by zeal for the ad¬ 
vancement of his favourite science, our fellow-Academician, Mr. Cooper, 
undertook the laborious and formidable task of determining the position 
of all the stars in the neighbourhood of the ecliptic, to the twelfth mag¬ 
nitude inclusive. His Catalogue contains no fewer than 60,066 stars, 
of which by far the greater part have been determined for the first time. 
This gigantic undertaking was commenced in 1848, and has occupied 
nearly eight years. In that time upwards of 72,000 observations were 
made, including the two co-ordinates of right ascension and declination 
of the observed body; in other words, 9000 observations yearly. The 
Catalogue has been published by the aid of the Parliamentary grant of 
the Royal Society, and extends to four volumes; but Mr. Cooper has 
announced his intention of publishing a fifth, or supplementary volume, 
to complete the work. 
I must, however, call your attention to a singularly interesting and 
remarkable circumstance, which was discovered during the progress of 
this undertaking. A large number of stars (as many, I believe, as 
seventy-seven) which had been previously observed, and their positions 
noted, were found to have disappeared. Of these, fifty had been cata¬ 
logued by Mr. Cooper in the earlier years of the progress of his work, 
but, when sought for of late, are found wanting. The remainder are stars 
that were noted in the catalogues of foreign astronomers. 
This remarkable fact of the disappearance of stars recently observed, 
has been fully confirmed by the labours of M. Chacornac, who has been 
engaged, simultaneously with Mr. Cooper, in forming a catalogue of 
ecliptic stars, and who has already published eighteen charts of their 
positions. 
It is, of course, quite possible that some cases of supposed disappear¬ 
ance may be apparent only, arising from the errors of former observers, 
and some, perhaps, also by the discovery of small planets, belonging to 
the group between Mars and Jupiter, whose nature was not recognised at 
the time of observation, and which were mistaken for stars. But it is 
quite certain that by far the greater number are real disappearances, and 
can only be accounted for by an actual variability in the stellar systems, 
whether periodical or otherwise. The number of known va/riable stars— 
stars whose brightness varies periodically,—has been greatly augmented 
since the attention of astronomers has been directed to stars of inferior 
magnitude; and it is not improbable that the stars which have disap¬ 
peared belong to this class, and that they will consequently be found 
to reappear at some future time. But we cannot without great pre¬ 
sumption conclude that all are of this class, and that all the stars now 
seemingly extinguished will reappear at distant intervals;—in other 
words, we cannot presume to assert that there are no permanent changes 
in the stellar system not compensated by opposite fluctuations; and if 
this be so, the observations published by Mr. Cooper, and others of a 
