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Updated: May 5, 2025
To render the lunar observations more continuous, Airy employed Troughton's successor, James Simms, in conjunction with the engineers, Ransome and May, to construct an altazimuth with three-foot circles, and a five-foot telescope, in 1847.
In case any one should be inclined to doubt the accuracy of the latitudes taken by the back observation with Troughton's pocket sextant; I think it proper to mention that I have observed at Sansanding alternately with the horizon of the river, and the back observation in water and the artificial horizon; and never found them to vary more than four minutes, but generally much nearer.
The transit, the zenith sector, and the mural quadrant, with which Bradley had done his incomparable work, retained their places long after they had become deteriorated by time and obsolete by the progress of invention; and it was not until the very close of his career that Maskelyne, compelled by Pond's detection of serious errors, ordered a Troughton's circle, which he did not live to employ.
Endeavoured to take the meridional altitude of the sun, by the back observation with Troughton's pocket sextant; and after carefully examining his rise and fall, with the intervals betwixt each observation, I was convinced that it can be done with great accuracy, requiring only a steady hand and proper attention.
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