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POPULAR SCIENCE REVIEW. 
something to the history of this object ; — he looked for it in 1660 and 1861, 
but it was “ nowhere.” The last observation of D' Arrest is Jan. 12, 1856, 
although the writer of this notice is of opinion that it was seen by him in the 
latter end of that year. 
Companion of Sirius. — The existence of a companion of this, the brightest 
star of the Northern heavens, is now beyond doubt. As previously noticed, 
it was discovered by Alvan Clark, with his new equatorial of 18-1 inches aper- 
ture, at the beginning of the year, and his observation was speedily con- 
firmed by Professor Bond, of Cambridge (U.S.), on February 10 ; at Paris on 
March 20, and, finally, by M. Lassell at Malta, on April 11. But although 
thus distinctly verified, the micrometrical measurements, as M. Lassell 
notices, greatly differ. For instance, on February 10, the distance was mea- 
sured at 10 seconds and 37 hundredths ; on March 20,* at 7 seconds and 
4 tenths ; on April 11, at 4 seconds and 92 hundredths. M. Lassell how- 
ever adds, “ I cannot accept the conclusion that there can have been a real 
motion in the star sufficient to reconcile these measures.” The difficulty of 
micrometrical measurements with a flashing star like Sirius will be readily 
comprehended by all astronomers. It remains to be seen whether the com- 
panion is physically or only optically connected with the large star — in the 
last case similar in type to Beta Orionis and Antares. 
Mountains on the Moon. — Air. Birt announces the rediscovery of the missing 
lunar crater Alhazen, situated on the western border of that conspicuous 
object — the Mare Crisium. It was first noticed by Schroeter, who employed 
it for measuring the libration, after which some confusion appears to have 
arisen, and it was even supposed to have disappeared altogether. The Rev. 
Mr. Webb has given the full history of its changes as observed by Schrceter, 
who considered that its appearance was too variable to be accounted for by 
the varying angle of illumination. At first Schrceter observed it as a dark 
grey spot ; at other times he perceived it as a very deep crater, with a small 
shadow. Mr. Birt, upon attentively considering this object, has reason to 
think “ that it consists of two nearly parallel ranges of mountains — the 
Eastern range forming a part of the actual border.” He appears to think 
that the appearance of a crater is produced by the varying shadows on the 
surface of the moon by those two mountain-ranges. It is satisfactory to 
know that the observations of different observers between 1823 and 1829 
Slave been confirmed. 
An important work on the topography of the moon has just been announced, 
which calls into requisition the valuable aid of photography to give a “ local 
Siabitation and a name ” to the various features of the lunar surface. It is 
entitled “ Our Satellite, a Selenography according to the present State of 
Science, by Dr. A. Le Vengeur D’Orsan.” It is inscribed by permission to 
Lord Brougham, and receives the high commendation of the Astronomer 
IBoyal The first photographers of the day are said to have borne testimony 
to the value of the illustrations, which include numerous woodcuts 
* There appears to be some error here. On referring to M. Chacornac’s 
measures, we find that the distance on March 20 was 10*4", and on March 25, 
10*43”, agreeing very closely with Mr. Bond’s measures on February 10. M. 
Lassell d.erives his information from Galignani. 
