70 HISTORY OF THE [1830-40 be repeated, led at once to a Committee being appointed to consider the practicability of doing so. A Government grant was obtained, and Baily undertook the work, which, however, was not finished and published till 1843. If from the bodies of the solar system, including the earth, we pass to the investigation of the elements required for the reduction of observations, we may begin by mentioning that among the papers published in the Memoirs during the previous decade had been H. Atkinson's study of the decrease of temperature in the atmosphere and its effect on refraction (vol. 2). He followed this up by a second investigation on the fluctuations of temperature near the earth's surface, and their effect on the refractions at very low altitudes.* Unfortunately the author's death prevented the completion of his work. Another paper (of a different kind) on refraction near the horizon, was one of the results of Henderson's short stay at the Cape Observatory. With the mural circle he observed the apparent zenith distances of stars culminating within 5 of the horizon, on both sides of the zenith. The result was that the observations, except in the case of four or five stars, agreed better with the tables of Ivory than with those of Bessel.f Of other fundamental determinations we find what is one of the most important of all, the position of the ecliptic, investigated by Airy from his Cambridge observations in the years 1833-35.$ The constant of Nutation was determined by Robinson from 6023 zenith distances of fifteen stars, observed by Pond in the years 1812-35 with the Greenwich mural circle. A new value of the lunar parallax was another fruit of Hender- son's Cape observations. He deduced it from observations of the moon's declination made with the mural circle at the Cape in 1832 and 1833, combined with corresponding observations made at Greenwich and Cambridge. || But valuable as these results of what might be called Hender- son's expedition to the Cape undoubtedly are, they are thrown into the shade by his great achievement, the first reliable determination of the annual parallax of a fixed star. The astronomical world had grown rather tired of announcements of annual parallax found from meridian observations. Brinkley's parallaxes had been vigorously assailed by Pond ; and though the question remained in doubt for some years, it was gradually recognised that they were imaginary.^ Henderson's paper was laid before
- Memoirs, 4, 517-530. Summary in M.N., 1, 193.
t Ibid., 10, 271-282. J Ibid., 8, 105 ; 9, n ; 10, 235. Ibid., 11, 1-19 ; M.N., 4, 133- II Ibid., 10, 283-294. Tf Chandler found in 1892 that Brinkley's observations indicated a rotation of the pole in about a year, and that this would to some extent account for his strange results.
