Page:History of the Royal Astronomical Society (1923).djvu/208

 i8o HISTORY OF THE [1870-80 southern observer would see a longer transit than the northern be- cause of his position, and if he were in the Antarctic circle the rotation of the earth would increase the duration as seen by him, and the addition would be to the advantage of the determination. This has led to the view that Halley's method is not specially suitable for the first of a pair of December transits (and is similarly unsuit- able for the first of a pair at the Descending Node in June). In his paper read before the Society in 1868 December " On the preparatory arrangements which will be necessary for efficient observation of the Transits of Venus in the years 1874 and 1882," Sir George Airy said " that for reasons he had given in earlier papers (1857 May and 1864 June) the method by observation of the inter- val in time between ingress and egress at each of two stations at least, on nearly opposite parts of the earth (on which method, exclusively, reliance was placed in the treatment of the observa- tions of the transit of Venus in 1769) fails totally for the transit of 1874, and is embarrassed in 1882 with the difficulty of finding a proper station on the almost unknown southern continent." He therefore relied for the 1874 transit at least on the method known as Delisle's, the principle of that method being that whereas at some stations the ingress is seen accelerated, compared with geo- centric ingress, at others, differing from the first in longitude, it is retarded, and similarly for the egress. The determination there- fore depends on comparison of the time of either phase as seen from two stations, the difference of longitude of which must be precisely determined, the record being made in local time. For the transit of 1874, Airy selected the neighbourhood of Honolulu, Rodriguez or Mauritius, Auckland Islands or New Zealand, and Alexandria ; Kerguelen's Island and Crozet's Island were mentioned as being favourable so far as astronomical circum- stances were concerned, but objections were raised to these by the naval authorities who were consulted. In 1869 March, Mr. Proctor brought before the Society reasons to show that the method of duration of transit, or Halley's method, was not inapplic- able to the transit of 1874, because as Venus was far north of the sun's centre and the chord therefore short, the shortening by parallax from a southern station would be so great that this would outweigh considerations of an adverse kind. He suggested Enderby Land (66 south latitude, longitude 50 E.) as a suitable station, but failing this that either Kerguelen's or Crozet's Island would be sufficiently suitable. In the same communication he also proposed alternative stations to those chosen by the Astro- nomer Royal for the observations by Delisle's method. Crozet's Island and Kerguelen's Land, with some other places, being considered suitable also for this method.