In 1825 the comet was again observed by Valz, passing perihelion on September 16, and in 1828 it was seen by Struve. Encke now made a very remarkable discovery. Determining its period with great accuracy, in 1832 he found that his comet returned to perihelion two and a half hours before the predicted time. As this repeatedly happened, Encke put forward the theory that the acceleration was due to the existence of a resisting medium in the neighbourhood of the Sun, too rarefied to retard the planetary motions, but quite dense enough to make the comet’s path smaller, and to eventually precipitate it on the Sun. The theory was widely accepted, but after 1868 the acceleration began to decrease, diminishing by one-half; besides, no other comet is thus accelerated, and the hypothesis has accordingly been abandoned. The second comet recognised as periodic was that discovered on February 27, 1826, by an Austrian officer, _Wilhelm von Biela_ (1782-1856), and ten days later by the French observer, _Gambart_ (1800-1836), both of whom, in computing its orbit, noticed a remarkable similarity to the orbits of comets which appeared in 1772 and 1805. Accordingly, they concluded it to be periodic, with a period of between six and seven years. The comet returned in 1832. In 1828 Olbers had published certain calculations showing that portions of the comet would sweep over the part of the Earth’s orbit a month later than the Earth itself. This gave rise to a panic that the comet would destroy the Earth, which did not subside till it was announced by Arago that the Earth and the comet would at no time approach to within fifty million miles of each other. The comet returned again in the end of 1845. It was kept well in view by astronomers in Europe and America. On December 19, 1846, Hind noticed that the comet was pear-shaped, and ten days later it had divided in two. The two comets returned again in 1852 and were well observed; but they were never seen again, at least as comets. Their subsequent history belongs to meteoric astronomy. A comet discovered by Faye at Paris in 1843 was found to have a period of seven and a half years. It has returned regularly since its discovery, true to astronomical prediction. Its motion was particularly investigated for traces of a resisting medium, by _Didrik Magnus Axel Möller_ (1830-1896), director of the Lund Observatory, who reached a negative conclusion.