The important question of the distance of the Sun was thoroughly investigated in 1824 by _Johann Franz Encke_ (1791-1865), then of Seeberg, near Gotha, who, from a discussion of the transits of Venus in 1761 and 1769, found a parallax of 8″·571, corresponding to a mean distance of 95,000,000 miles. This value was accepted for thirty years, until _Peter Andreas Hansen_ (1795-1874), in 1854, and _Urban Jean Joseph Le Verrier_ (1811-1877), in 1858, found from mathematical investigations that the distance indicated was too great. Preparations were accordingly made for the observation of the transits of Venus, which took place respectively on December 8, 1874, and December 6, 1882. On the first occasion many expeditions were sent to view the transit, consisting of French, German, American, English, Scottish, Italian, Russian, and Dutch astronomers, and it was hoped that the solar parallax would be accurately measured once for all. However, the transit, although favoured with good weather, was not successful, owing to the difficulty of making exact measurements, by reason of the illumination and refraction in the atmosphere of Venus. Accordingly the values deduced for the parallax were far from unanimous. The transit of 1882 was not observed so extensively, as astronomers had found the transit of Venus to be by no means the best method. In 1877 Sir _David Gill_ (born 1843), the great Scottish astronomer, determined the solar parallax successfully from measures of the parallax of Mars in opposition. His value was 8″·78, corresponding to 93,080,000 miles. Some years previous to this _Johann Gottfried Galle_ (born 1812), the German astronomer, had, from measurements of the parallax of the asteroid Flora, deduced a solar parallax of 8″·87. Gill’s work at the Cape in 1888, on the Asteroids, was successful in giving a more accurate value than the transit of Venus: in 1900 and 1901 measures of the parallax of the asteroid Eros, the nearest minor planet, were made by many different observatories, and agree with the other results. The values which have been derived from the velocity of light, and from the constant of aberration, are fairly in agreement with those derived from direct measurement. On the whole, the most probable value of the parallax is about 8″·8, indicating a mean distance of about 92,700,000 miles, with a “probable error” of about 150,000 miles.