The word comet means a "long-haired" star. The star-like nucleus of the comet is called the head and the long nebulous streamer which trails behind, the tail. The head may be colossal, or it may be a mere shred; the head of Donati's comet would have more than filled the space between the earth and moon, while that of the comet of 1811 was considerably larger than the sun itself. The length of a comet's tail is equally amazing for it extends often for millions of miles. The comet of 1882 had 100,000,000 miles of tail while that of 1483 had one twice as long. The comet of 1680 extended below the horizon when its head touched the zenith. The mean density of a comet is very small. Young states that it is nothing more than a sandbank--a swarm of solid particles of unknown size and widely separated. This light and airy object darts through space at a most hectic speed, often at the rate of a number of hundred miles per second, when close to the sun. It is of such flimsy construction that its light does not even obscure the stars although there have been comets so bright that they were visible in the daytime. It is now believed that a comet derives its light, except that portion which is reflected sunlight, from electrical discharges between its particles. By some influence emanating from the sun, gas is emitted from the scattered swarm of meteoric particles which compose the head, and these stream out "like smoke from a locomotive" in a magnificent luminous trail. This only occurs when in the vicinity of the sun. Each time the comet comes back to the sun, it loses some of its beauty and its mass decreases--yet like a moth to a flame, it always returns. Newcomb describes the way in which a comet gradually loses itself in the following manner: "Altogether a good idea of the operations going on in a comet may be obtained if we conceive of a nucleus (or head) to be composed of water or other volatile fluid which is boiling away under the heat of the sun, while the tail is a column of steam rising from it. We do not see the same tail of the comet all the time because the matter which makes up the tail is constantly streaming outwards and constantly being replaced by new vapor rising from the nucleus." In the course of time, the whole comet becomes disintegrated and its pathway scattered with debris. Astronomers now look upon meteors as the remains of worn out comets, for meteoric swarms follow along the same orbits as many of the comets. When the earth crosses the orbit of a disintegrated comet, or meteoric swarm, the fragments come in contact with our atmosphere where they blaze into incandescence and are seen as shooting stars.