500 Trans. Acad. Sci. of St. Louis. 
and north on the other. The thread intervals of the reticule 
do not need to be known, and if the error of collimation 
remains constant its effect is wholly eliminated from the 
result. 
By this method it is customary to reduce the transits of 
a star across each thread separately and when the thread 
intervals are entirely unknown it is necessary to do so. The 
mean of the results for any thread circle south and circle 
north, corrected by a small quantity 4 ¢ gives a value of the 
latitude. But when the thread intervals of the reticule are 
approximately known, the labor of the computations may be 
very much reduced without sacrifice of rigor by computing 
the correction for reducing the mean of the transits across 
the threads to the time of transit across the mean of the 
threads. This correction may be applied directly to the mean 
of the transits across the threads, or it may be used to com- 
pute a correction to the latitude deduced on the supposition 
that the mean of the transits across the threads is the time of 
transit across the mean of the threads. The effect of any 
small error in the assumed thread intervals is eliminated by 
the reversal of the instrument. The formulae necessary for 
the computation of this correction are to be found on pages 
246 and 248 of Chauvenet’s Spherical and Practical Astron- 
omy, Vol. 11. 
This abbreviated process was employed in the reduction. 
The equatorial thread intervals from the mean of the threads 
which were used in computing the corrections 4 ¢, and 4 @s 
were determined in September, 1890. They are— 
B C 
— 6§.25+-0°.01 — 88.20-+.0°.01 —0%.08-.0*.01 
D E 
+3°.09+-0°.01 +65.44-+-0°.01 
The intervals had no doubt changed somewhat but, as 
above stated, they do not need to be known accurately for this 
purpose. 
_ The observations of four stars at both east and west trans- 
