was removed by Powalky's
and Stone's rediscussions, in 1864 and 1868 respectively, of the transit
observations of 1769. Using improved determinations of the longitude of
the various stations, and a selective judgment in dealing with their
materials, which, however indispensable, did not escape adverse
criticism, they brought out results confirmatory of the no longer
disputed necessity for largely increasing the solar parallax, and
proportionately diminishing the solar distance. Once more in 1890, and
this time with better success, the eighteenth-century transits were
investigated by Professor Newcomb.[764] Turning to account the
experience gained in the interim regarding the optical phenomena
accompanying such events, he elicited from the mass of somewhat
discordant observations at his command, a parallax (8.79") in close
agreement with the value given by sundry modes of recent research.

Conclusions on the subject, however, were still regarded as purely
provisional. A transit of Venus was fast approaching, and to its
arbitrament, as to that of a court of final appeal, the pending question
was to be referred. It is true that the verdict in the same case by the
same tribunal a century earlier had proved of so indecisive a character
as to form only a starting-point for fresh litigation; but that century
had not passed in vain, and it was confidently anticipated that
observational difficulties, then equally unexpected and insuperable,
would yield to the elaborate care and skill of forewarned modern
preparation.

The conditions of the transit of December 8, 1874, were sketched out by
Sir George Airy, then Astronomer-Royal, in 1857,[765] and formed the
subject of eager discussion in this and other countries down to the very
eve of the occurrence. In these Mr. Proctor took a leading part; and it
was due to his urgent representations that provision was made for the
employment of the method identified with the name of Halley,[766] which
had been too hastily assumed inapplicable to the first of each
transit-pair. It depends upon the difference in the length of time taken
by the planet to cross the sun's disc, as seen from various points of
the terrestrial surface, and requires, accordingly, the visibility of
both entrance and exit at the same station. Since these were, in 1874,
separated by about three and a half hours, and the interval may be much
longer, the choice of posts for the successful use of the "method of
durations