Page:The story of the comets.djvu/133

VII. 1872, but bad weather and the advance of twilight prevented any further observations. Pogson described his comet as circular; 75" in diameter and having a bright nucleus with a bright but distinct spreading tail 8' in length. This description would not seem to fit in with the description given of Biela's Comet at previous apparitions, but not much stress should be laid upon that fact. However, on other grounds it was the opinion of Bruhns that the comet seen by Pogson could not possibly be Biela's, but an unknown comet, which by a remarkable coincidence was in or near the place where Biela's Comet ought to have been seen.

The question stood and still stands for consideration, "Why has Biela's Comet disappeared? "The answer to this question seems now to belong not to the subject of cometary, but to that of meteoric astronomy, and will be discussed in a separate chapter.

This also is a comet which has a mysterious history, as to which questions are constantly cropping up which cannot be answered.

On Aug. 22, 1844, Di Vico at Rome discovered a telescopic comet which, towards the end of the following month, became visible to the naked eye. With a telescope a bright stellar nucleus and a short tail were seen. It soon became apparent that the comet was travelling in an elliptic orbit, to which Brünnow assigned a period of 1993 days. He calculated that the comet's next return to perihelion would occur in the spring of 1850, but that owing to the position of the comet in its orbit relatively to the Sun for some months, it would be impossible to see it at a sufficient distance clear of the Sun's rays.

The next return to perihelion was fixed for Aug. 6, 1855, and as theory suggested that the comet would be favourably situated for observation, hopes were entertained that it would be detected. They were, however, doomed to disappointment; and as a matter of fact, the comet has never for a certainty