54 On the Transit Instrument in determining Latitude. 
nection. 
The graduation of the circle is very accurate. The difference 
between the readings of the opposite verniers seldom amounts to 
to 15”, and from its law of variation, it is manifestly due, in the 
main at least, to a very slight excentricity in centering the circle 
on the graduating engine, not to imperfection in the engine itself. 
e micrometer-screw has 75 threads to the inch, or one rev- 
olution of the head equal to 78''74. 
The optical performance of the instrument is very satisfactory. 
Transits of Polaris are readily taken at midday, and i 
ble states of the atmosphere, at that hour, even by reflection in 
mercury. Transits of Aldebaran have been taken when the star 
was within 6° of the sun’s center. The eye-piece commonly used 
is a diagonal one of four lenses, with prismatic reflector,—the 
magnifying power about 190. . 
the whole instrument, the star bisected with the micrometer 
wire, and the readings of the level and micrometer noted. The 
axis is then reversed, the bubble brought to zero as before, and 
the other star observed in the same manner. ad 
e pairs for the observations now to be considered, were ta: 
ken from the Catalogue of the British Association. The stars 
are to the sixth magnitude, and mostly within 25° of the zenith, 
the difference of zenith distances of the stars of a pair being 
usually less than 25’—averaging about 12’. 
The individual results of 92 pairs are exhibited in the follow- 
ing table, which includes, with two exceptions, all the observa- 
tions made, whether in favorable or unfavorable states of the 
atmosphere. In a few cases where the B. A.C. differs widely 
from other catalogues, the B. A.C. results are enclosed in brack- 
ets [ ] and are omitted in computing the mean. That the fault is. 
in the B. A. C., is obvious from the accordance of the correspond- 
ing results from other catalogues with the general average. 
e contents of the table are as follows: 
Col. 1 and 2. The numbers from the British Association Cata- 
logue designating the stars of a pair. 
