And now some one will ask, perhaps impatiently, “What, after all, is a comet?” We have seen that many are continuously, and others casually, members of the solar system. What do they consist of? Spectrum-analysis shows that they consist chiefly of the chemical element carbon.[6] Though they have weight, and are attracted by the sun, yet they seem to be for all their size and terrifying shape and glare incredibly light and airy things. Herschel declared that the tail of a big comet probably consisted of but two or three pounds of solid matter—diffused, rarefied, and luminous. And the head or nucleus certainly does not weigh many hundreds of tons. In the eighteenth century astronomers observed a comet pass right in among the moons of the planet Jupiter. You might expect the moons to be terribly knocked about by such an impact. They were not; they were not deflected in the smallest appreciable degree from their position and regular movement! One is naturally inclined to look upon the tail of a comet as something like the smoke of a railway engine trailing behind the advancing “head.” As a matter of fact, it does not always trail behind, but is always turned away from the sun, so that when the comet is travelling away from the sun the tail is in front! It is now held that the tail is caused by the radiant energy (light and heat) of the sun, blowing, as it were, the lighter particles from the incandescent head, and causing them to spread out in a long track of variable shape. The photographs of the third comet of the year 1908 show that the tail can vary to an astonishing extent and with great rapidity—that is to say, in four or five hours. It is seen in those photographs as a scimitar-like curved blade, then with a second head or nucleus behind the leading one, then actually bent like the letter Z, and then divided into seven distinct diverging “plumes,” and then it returns to its former simple shape—all in the course of a few days. Astronomers have now shown that there is a close connection between comets and the showers of “shooting stars” or meteors which frequently strike the earth’s atmosphere. It is considered probable that comets eventually break down into streams of meteors, and that their “life” (if one may use that term) is, relatively to that of other heavenly bodies (which are all undergoing change and, in many cases, decay), not a very long one. But there are no facts at present known which enable us to tell whether a given comet is young or old, and it would have been a decided shock had it been found that Halley’s comet, which has so happily spent every seventy-sixth year with us for so many centuries, had “burst up,” or by “indisposition” had been unable to pay his usual visit as expected in 1910. FOOTNOTES [5] The pirates of the _Flowery Land_. [6] I am indebted to Mr. Rolston, of the Solar Physics Observatory, South Kensington, for some information on this matter.