The early observers divided the stars into constellations, and traced upon the sky fanciful figures of chairs, dragons, sheep, crowns, bears, persons, each of these constellations having some few prominent stars and a number of smaller ones. While the outlines of these objects are entirely fanciful, the arrangement is of great use in helping us to find or describe stars. Distant in the starry space are vast shining clouds, which were once supposed to be whirling masses of vapor or “star dust,” and received the name of nebulæ, or clouds. Better telescopes have revealed the true character of the nebulæ, and we find that they are thickly crowded stars, so immensely distant that their light mingles as one mass, and the spaces between them are not seen. It is also true that there are some nebulæ which are really “cloud-masses of glowing gas.” Wandering among the stars are seen at times brilliant bodies with long trains or tails of glowing vapor. These are comets. Once their appearances caused great terror, as indicating disaster to the human race, and even now some persons are alarmed lest one of these sky wanderers should, in his frantic flight, run against our earth and dash the poor little globe to pieces! But these comets are not such vagrants as they seem. Law reigns in all the universe, and even comets are subject to it, and come and go on a distinct track. The motion of many comets has been determined, and their return into the range of our vision is accurately foretold. There are some comets which always remain in our own solar system; others come from the depths of space, pass about our sun, and dash away into infinite distance to return no more. Various opinions are held as to the material of comets. Some consider that they are composed of myriads of small bodies like tiny meteors; others consider them made of gas. The head or nucleus is doubtless more solid than the tail, and it has been found that tails of comets differ in composition; they contain hydrogen, carbon, iron, chlorine, and other kinds of matter, such as we have in our world. The tails of comets always turn _from_ the sun, and as the comet nears the sun in its circuit the tail expands; as it leaves the sun the tail decreases in size. Some comets have no tails, some have several. It is supposed that the tails of comets diminish and finally disappear, from destruction of their expanded material by the sun’s heat as they repeatedly draw near to the centre of our system. Some comets go and come in their orbits in three or four years, others roam for centuries before they sweep again into our range of sight.