Page:The story of the comets.djvu/30

8 The dimensions may be taken as typical of those of many other comets.

Few things are more remarkable to witness, and more paradoxical to explain, than the changes of bulk which the head of a comet generally undergoes in approaching to, or receding from, the Sun. One might expect, reasoning from terrestrial analogy, that as a comet approaches the Sun the increased heat to which it is submitted would expand its head, whereas the effect observed is the contrary; it grows smaller as it grows hotter. And when receding from the Sun the observed changes are of a converse character; the comet's head seems to expand as it gets farther away and grows cooler.

No satisfactory explanation of this anomaly has been given unless it is permissible to accept Sir J. Herschel's idea that the change of bulk is due to some part of the cometary matter remote from the nucleus being evaporated, as it were, under the influence of the Sun's heat, just as a morning mist is evaporated and disappears as the sun rises in the heavens and its radiant heat becomes more potent.

History informs us that some comets have shone with such splendour as to have been distinctly seen in the day-time. The comets of B.C. 43, A.D. 575(?), 1106, 1402 (i.), 1402 (ii.), 1472, 1532, 1577, 1618 (ii.), 1744, 1843 (i.), 1847 (i.), 1853 (iii), 1861 (ii.), 1882 (i.), are the principal ones which have been thus observed. Perhaps we might assume that about 4 or 5 comets are so visible in every century. The Comet of 1853 (iii.) was seen on June 10 at Olmütz only 12° distant from the Sun, and again, after perihelion, on Sept. 2, 3, and 4 at noon.

What is the colour of a comet? Have comets ever any colour? From my own observations, extending over many years (and I suppose I have telescopically examined more