worked at the problem very assiduously, thought there ought to be no hesitation in accepting the idea, subject to the limitation that the medium does not extend farther from the Sun than the orbit of Mercury.
The 1838 return is also noticeable for an important discovery in physical Astronomy which it indirectly was the cause of evolving. In August 1835 the comet passed very near the planet Mercury—so near in fact that Encke shewed that if Laplace's value of Mercury's mass was correct the planet's attractive power would diminish the comet's Geocentric R. A. on Nov. 2. 1838, by 58', and increase its Declination by 17'. As the observations indicated no such disturbance of the comet's orbit it was obvious that the received mass of the planet was much too great, and as a matter of fact a much lower value has since been adopted.[1]
Passing over the returns of 1842 and 1845 as offering no points of particular interest we find that in 1848, on Sept. 24, the diameter of the comet's head was 8' and that it was just visible to the naked eye on Oct. 6, and for some weeks subsequently. The adjacent illustration [Fig. 31, Plate IX.] will convey a good idea of the telescopic appearance of the comet during the month of September 1848. Early in November it had a tail about 1° long turned as usual from the Sun, and another and smaller one pointing towards the Sun. On Nov. 22 the comet was within 3,600,000 miles of Mercury.
Since 1848 Encke's Comet has been observed so many times that it would be monotonous and unprofitable to detail all the several appearances. I shall therefore only make a selection of apparitions which yielded some observations of interest and importance, more or less.
In 1871 the comet was well seen and numerous observations made. Some physical peculiarities were noted which deserve mention. In October, soon after its first discovery, the comet was a nearly round and faint nebulosity without apparent condensation anywhere. By the beginning of November it had
- ↑ In Hind's Comets, p. 65 et seq. the general principles upon which these inquiries are conducted are laid down with the clearness of language for which that distinguished astronomer was noted.
