seen in observing the great Comet of 1618. In the case of this comet Cysatus noticed an evident tendency in it to break up. When first seen it was a nebulous object, but some weeks afterwards it appeared to consist of a group of several small nebulosities. But the best authenticated instance of this character is that of Biela's Comet in 1845–6. When first detected, on November 28, it presented the appearance of a faint nebulosity, almost circular, with a slight condensation towards the centre: on December 19 it appeared somewhat elongated, and by the end of the month the comet had actually separated into two distinct nebulosities, which travelled
Fig. 9.
biela's comet, feb. 19, 1846. (O. Struve.)
together for more than 3 months: the maximum distance between the parts (about 160,000 miles) was attained on March 3, 1846, after which it began to diminish until the comet was lost sight of in April. At its return in 1852 the separation was still maintained, but the interval had increased to 1,270,000 miles. As I shall have a good deal to say about Biela's Comet in a later chapter no more need be said about it here.
Biela's Comet does not stand alone among modern comets as regards its duplicity. A comet seen in February and March, 1860, only by E. Liais, a French observer in Brazil, consisted when discovered of a principal nebulosity accom-
