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With sextant. – Many observations made for practice and by the officer of the day for regulating the post time are not reported. The following table includes those made in connection with sextant latitude and longitude observations.
Each officer is required to submit at least one complete set of observations for determining the latitude by the method adopted by the Corps of Engineers for reconnaissances. Ten altitudes of an east and ten of a west star (both near the prime vertical and at similar altitudes) are taken for time, and corresponding sets on Polaris and on a south star, at its culmination, for latitude—all made on the same night. By this method, errors due to eccentricity and to ill-determined index error, if the sextant be carefully handled, are both eliminated. No limit is fixed as to the number of trials, the object being to encourage that careful practice with the instrument which is so needful to attain skill in its use.
A set of similar observations for determining the longitude by the method of lunar distances is also required, chiefly for practice with the instrument, as the inherent inaccuracy of the method renders it of little use for land purposes.
OBSERVATIONS FOR LATITUDE. Observations for latitude have been made during the past year with the Würdemann zenith telescope, with the Lingke and Russian combined instruments, and with the sextant. The results by each instrument will be reported.
With zenith telescopes. The following observations were made to find the constants of the three instruments:
Constants of zenith telescopes.
* A Ursæ Minoris.
+ Terrestrial object.
The Safford catalogue was used in selecting the pairs of stars for determining the latitude. Each officer prepared his own list, and observed on three nights as nearly consecutive as the weather permitted. The results are given in the following table, the latitude deduced by each officer appearing at the foot of his column-computed by the rule that no observation should be rejected; first, unless marked doubtful in the record book; second, unless giving a latitude at least one minute in error (which may fairly be attributed to mistaking one of the stars or to wrongly reading the micrometer); or, third, unless rejected by Peirce's Criterion, applied to his whole set of observations. Rejections for the first two reasons do not appear in the table, but their number (trifling) is reported at the end. Those for the last are entered, marked : *