No. 3.—Di Vico's Comet of 1846 (iv.) was discovered on Feb. 28, and though his name is commonly attached to it, it seems to have been found by Bond at Cambridge, U.S., two days previously. It was under observation for more than two months, and there does not appear to be any reason for doubting that its period is much about what is stated in the Table, or between 72 and 73 years.
No. 4.—Olbers's Comet of 1815 was discovered by him at Bremen on March 6, and was last observed on Aug. 26. Bessel made the periodic time in 1815 to be 74.04 years, whilst Nicolai made it 74.79 years. Bessel calculated the perturbations onward to the next perihelion, and found that the comet's return would be so expedited that the perihelion passage would take place about February 9, 1887. This forecast was not, however, borne out by the result, for the comet did not pass its perihelion till Oct. 8. The comet was discovered by Brooks in America on Aug. 24 and remained visible for 2 months.
No. 5.—Brorsen's long-period Comet was found on July 20, 1847, at Altona, and was observed for 8 weeks. Its orbit was investigated by several astronomers, and there seems no reason to doubt the accuracy of the period assigned by D'Arrest, namely, nearly 75 years.
The 6th and last comet enumerated in the Table prefixed to this chapter, namely, "Halley's", is one of such extreme interest, and has such a long history, extending back as it does for nearly or quite 2000 years, that it must have a chapter to itself; and this chapter will be completed by a brief enumeration of some of the comets whose periods have, with some reasonable probability, been estimated at hundreds or thousands of years. But after all said and done, however near the truth these figures may, or may not, be, it is obvious that they have little practical interest for us except as showing the possibilities of calculation as applied to comets. However, for what they are worth here are some of them:—
