C. Abbe on the Repsold Portable Circle. 315 
As fundamental points of reference for the longitudes the fol- 
U1. : 
lowing list was selected of points determined in previous years. 
Station. Latitude. Longitude. 
I. Poulkova, 59° 46/ oh Om 
Louga, 58 43 —() 
III. Noygored, 58 30 +0 3 47°86 
IV. Nova La ; 60 7 + 7 68°8 
V. Borovaytsche, 58 24 +0 14 19°98 
VLT s 59 39 +0 1 
Of these, however, it was preferred to include Iv and v1 
among those to be independently determined on the present oe- 
casion, and they have received the numbers 16 and 11 in the 
previous list. 
Among several similar expeditions that haye come to my 
notice, [ may mention those before referred to, of Messrs. Dél- 
Jen and Hiibner in 1855-56 in the government of Perm, where 
many points have been unusually carefully determined with 
reference to Perm and Ekaterineberg; and that of Captain Bol- 
scheff in Finnland in 1863-66, each of whose 150 points has 
been twice independently determined, } 
An extensive series of observations made at Poulkova with 
a vertical circle, made in 1857 by Messrs. Repsold, still farther 
attests, if any farther proof were wanted of the, reliability of 
the results afforded by this instrament—to observe with which 
is in fact a luxury of convenience. The observations made on 
distant terrestrial signals—the reflection from ed church 
domes—gave for perfect condition of images the probable acci- 
dental error of a zenith distance resulting from two pointin 
in opposite positions of the circle +:0’"52, a large portion of 
Which may fall upon the level, which was afterwards improved 
by the addition of a narrow mirror reflecting the divisions hori- 
zontally to the observer's eye. 
A suggestion of Mr. Déllen as to the interest that would 
attempt the same in the determination of latitude. 
37 East 20th st., New York, January, 1867. 
€ 
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