194 HISTORY OF THE [1870-80 Though it was clear that in some cases the appearance was merely that of a sun-spot, five such observations made respectively in the years 1802, 1839, 1849, 1859, and 1862 were thought by Le Verrier to have represented passages of a single planet across the sun's disc, and he deduced from the observations the elements of the orbit of a planet which would, if it actually existed, be in transit on 1877 March 21, 22, or 23. Mr. Hind supported this by showing that a sixth observation that made by Stark on 1819 October 9, was also consistent with the same orbit. M. Le Verrier sent a letter to our Society inviting their co-operation in testing the predictions by observation, and the Astronomer Royal, in con- sequence, sent telegrams to observatories in India, Australia, New Zealand, and North and South America, with a view to keeping a continuous watch on the sun on these days, photographic if possible, and he made a statement on the matter at the meeting on March 9. The total failure of the observations both in the opposite hemisphere and in our own, renders it certain that no such object crossed the sun's disc at the predicted time. The Rev. Stephen Perry at Stonyhurst and Mr. Rand Capron at Guild- ford kept careful and continuous watch during the three days, the weather being very favourable at both places, but saw no trace of the object they were seeking. The observation of Mars, for determination of the solar parallax, by Mr. David Gill, from the Island of Ascension, at its opposition in 1877, was an important event of this session, and though the story and the connection of the Society with the undertaking are briefly told by Gill in his autobiography in the History of the Cape Observatory, it may be not out of place to give here a sketch of the incident and others in the early career of this famous astronomer, who had recently begun his association with the Society. In 1872, Lord Lindsay, the only son of the Earl of Crawford and Balcarres, established an Observatory at the family seat at Dun Echt, of which he put Gill in charge. For observation of the Transit of Venus in 1874, Lord Lindsay organised an expedition to the Island of Mauritius at his own expense, the greater part of the arrangements and work, which involved the determination of the longitude of Mauritius, being assigned to Gill. The observa- tions of Venus in Transit were made by heliometer, and it was proposed to take advantage of the opportunity afforded by having the instrument in position at Mauritius, to make observations of the planet Juno, which was in opposition on 1874 November 5, for determination of the solar parallax. Owing to unforeseen delay in the journey the first observation of the planet was not made until November 12, but sufficient observations were obtained to
