"Because it has a regular path marked out for it in the sky," replied Mary, "and it travels along that path unless something happens to it on the way. It may go too near giant planet Jupiter. Just like our tramp again. Let us suppose he has a regular path marked out and it takes him across Uncle Robert's farm and leads to our kitchen door. We may expect to see Mr. Tramp to-morrow, but as he crosses the farm a dog bites him and frightens him away. Perhaps then we may not see him again." "Poor old man," laughed Harry. "I hope that won't happen to him. Do the 'celestial tramps' travel very quickly through the sky?" "Not very quickly until they come close to the sun. Then they rush around it ever so much faster than an express train; but as they recede from the sun they go more slowly until they seem only to creep along, as if worn out by their long journey. They also lose their trains after they go away from the sun, and the train becomes shorter and shorter, till the comet looks like a round, fluffy ball, just as it did before it came too near the sun. It is the sun's heat that drives the particles from the head of the comet and forms a train." "What are comets made of?" asked Harry. "Of millions of tiny little particles covered with coats of glowing gas. These particles are made up of carbon, sodium, iron, and magnesium. You will find plenty of sodium in the sea, while common table salt is partly sodium. You know what magnesium is. Some of that medicine doctor gives you is made of it." "So if I get some iron and salt and coal and some of my medicine, and put them all together, I should have a bit of a comet," said Harry. "But you must remember the coal, iron, sodium, and magnesium must be very much heated, and don't forget the coat of gas. Sometimes a comet breaks into pieces, and the fragments travel along by themselves as meteors." "Sometimes the earth plunges through swarms of meteors, which journey in regular paths around the sun. At such a time, the bright masses seem to fall in showers from the sky. There are three great showers which we always know when to expect. Some come in August, some on the 13th or 14th of November, and there is another shower which always appears within a day or two of the 27th of November. "'If you November's stars would see, From twelfth to fourteenth watching be, In August too stars shine from heaven, On nights between nine and eleven.'" STORY OF METEORS. "What are meteors?" asked Harry. "Meteors are great masses of stone or iron which sometimes weigh several tons. Lieutenant Peary found one not long ago in the Arctic regions, and it weighed about eighty tons. It is lucky for us that many meteors do not fall on the earth, or we should have to walk about with iron umbrellas over our heads as a protection. When they do fall on earth, they are much prized and placed in our museums as curiosities.