Page:The Scientific Monthly vol. 3.djvu/548

 542 THE SCIENTIFIC MONTHLY

nent, disappeared entirely and remained invisible until the comets had receded to a considerable distance from the sun and the sodium lines were no longer in evidence. The carbon light could scarcely be gen- erated by heat action^ for if so the carbon bands should have been in evidence during the time that the comet was passing nearest to the sun. Much more probably the bright-line spectra of the head and tail are of electrical origin^ or fluorescent. This phase of the subject is technical, and to some extent speculative, and we can not profitably pursue it further on this occasion.

A certain proportion of the light of many comets is slightly polar- ized. The interpretation of this phenomenon is that a fraction of the light of the heads and of the inner tails of comets is sunlight difibracted by minute dust particles or gas molecules in the comet structure.

Returning to the subject of the disintegration and disappearance of comets:

A small comet was discovered by Montaigne in 1772. A comet was discovered by Pons in 1805. A comet was discovered by Biela in 1826. Biela computed the orbit of his comet and found it to be moving in an ellipse of period six and a half years, and he proved that the three comets discovered respectively by Montaigne, Pons and himself were identically the same comet. Biela's comet was rediscovered in 1832, almost precisely in its expected place. The next return was missed because the body was not in good position for observing. It was redis- covered in 1845, when it was seen to consist of two comets moving side by side on orbits almost identical. In 1852 both comets were re- observed, but farther separated than they had been in 1845. The comet was searched for at the proper times for several later returns, but it was never seen ai^ain."

Kirkwood published in 1872 a list of eight comets which had divided in a similar manner and disappeared.

A number of other comets have completely disappeared, though their orbits were very well determined.

This brings us to another interesting phase of our subject:

The Perseid meteors are with us at this time of the year. Many of them have been seen every year for several decades. They are usually most numerous on the nights of August 9, 10 and 11. Predictions concerning meteors are somewhat risky, but so faithfully have the Perseids come every August that I have no doubt an observer to-night, to-morrow night and the next night, from midnight on to daylight, would see dozens of meteors whose paths traced backwards would pass through a small area in the constellation of Perseus. In 1866 Schia- parelli computed the orbit of the Perseid meteors and noticed that it

B One of tlie components of the Biela comet may have been observed for a few hours from Madras in 1872.

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