magnitude. The outline of the coma was slightly oval with the minor axis (on one occasion at least) pointing towards the Sun.
The comet returned in 1832 but was only seen by one European observer, Harding at Göttingen, owing to its path lying chiefly in the Southern hemisphere.
Passing over the return of 1835, when the comet was seen both in Europe and at the Cape, we come to that of 1838. As the comet's apparent path would allow of observations being made in Europe under very favourable conditions it was looked for with much interest. Boguslawski discovered it on Aug. 14, but it was not generally seen till the middle of
Fig. 30.
ENCKE'S COMET: NOV. 30, 1828.
(W. Struve.)
October. In the first week in November it was visible to the naked eye in Draco. With a telescope a rather bright nucleus was seen, and the general form of the coma was that of a broad parabola. It was this return which brought into prominence a peculiarity of the comet's motion which raised a question which still continues open for discussion. Encke found that, notwithstanding every allowance being made for planetary influences, the comet always attained its perihelion distance about 212 hours sooner than his calculations led him to expect. In order to account for this gradual diminution of the period of revolution, which in 1789 was nearly 1213d, but in 1838 was scarcely 1211110d, Encke conjectured the existence of a thin ethereal medium, sufficiently dense to produce an effect on a body of such extreme tenuity as the comet in question, but incapable of exercising any sensible influence on the movements of the planets. Hind thus soliloquised on the subject:—"This contraction of the orbit must be continually progressing, if we suppose the existence of such a medium; and we are naturally led to inquire, What will be the final con-
