SCIENTIFIC SUMMARY. 
371 
considerably more accurate than the previous Greenwich twelve-year one, 
partly due to the greater correctness of the places of the standard stars as 
determined by the latter, and partly to the greater stability of the new 
transit-circle, which was employed in the observations of stars included in 
, the former work. The last catalogue issued from the Greenwich Observatory, 
known as the Seven-year Catalogue, he considers even more accurate than the 
Six-year one, and claims for it a high position among standard catalogues for 
the great accuracy of the fundamental right ascensions. The stars of refer- 
ence chosen by him in conducting the investigation, were Eta Pegasi, Pollux, 
Spica, and Alpha Aquilae. 
Transit Instruments and Transit Circles. — Professor Airy institutes a com- 
parison between the transit instrument as formerly employed and the present 
non-reversible transit circle, which is alluded to in the last paragraph. Even 
in the small points of difference of the flexure in the cones of support or of 
the telescope tube, he finds that when the error of the horisontality of the 
axis is obtained by a spirit-level, that in the old construction, although the 
optical axis may describe a great circle and a meridian, it may not be the 
meridian of the place, whilst by using a surface of mercury, as must be done 
in the Greenwich instrument ; this is obviated. But the more important 
advantage of the non-reversible instrument is that the same pivot is always 
in the same bearing ; and thus its action is more accurate, whilst at the same 
time the determination of the forms of the pivots is much less complicated 
than with the reversible form. In the determination of the level-error by 
means of a spirit-level, the difference of the diameters and forms of the pivots 
and of the angles of the forks of the level must be taken into consideration 
in the latter, but are not required when using the reflected image of the wires 
in mercury. 
Semidiameter of Venus. — Mr. Stone has made a new determuiation of this 
element, making use of the vertical diameters observed with the old mural 
circle at Greenwich between 1839 and 1850, and those with the transit 
circle between 1851 and 1862 ; the former being 219 in number, the latter 
370. He divides them into two groups, viz., when the observed diameter is 
greater than twenty seconds, and when it is less than that quantity. The 
former gives as the semidiameter of Venus, at the mean distance of the Sun 
from the Earth, 8,560 seconds of arc, the latter 8,427, with a small probable 
error. The mean would therefore be 8,472, which is larger than the value 
adopted in the Nautical Almanac (8,250), or that found by Encke from the 
transits of V enus. 
Mr. LasselVs Observations at Malta. — In a letter, dated December 31, 
1864, Mr. Lasseil gives a resume of the observations made during his resi- 
dence at Malta. Numerous measures have been made of the diameter and 
satellite of Neptune. In regard to the latter, Mr. Lasseil is firmly convinced, 
from the ease with which it is seen even ui bright moonlight (especially when 
it is more than eight or nine seconds central distance from the planet), that 
the planet is attended by no other satellite — at least by any whose brightness 
could be compared to it, or equal to that which Dione or Bhea bears to Titan 
among Saturn’s satellites. It may be possible, however, that a very distant 
one exists. In respect to the satellites of Uranus, Mr. Lasseil has the fullest 
conviction that the Uvo satellites discovered by Sir W. Herschel in 17-87, 
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