"We scarcely know," replied Mary, "except that it is from outer space, just like tramps on earth. We do not know where tramps come from, nor do we expect to see them again. If they do revisit us, however, we can usually recognize them. Do you remember the old man who came to the kitchen door the other day and begged for food? You felt so sorry for him. You would know him if you saw him again on account of his long white beard, white hair, and shabby clothes.
"When a celestial tramp returns, however, it is not so easy to recognize it. When it first greeted us it may have had a large head and a gorgeous train millions of miles in length. Next time we see it, how it has changed! Its head may be small, its train may have vanished, or it may be the proud owner of three or four trains. A comet usually changes its appearance at every visit. Just as if the old man we saw the other day were to cut off his beard, dye his hair black, and wear Uncle Robert's dress-suit. We should not know him, should we, Harry?"
OLD PICTURE OF A COMET.
"I should think not," said Harry, laughing at the very idea. "Then how can you tell when the same comet visits us again?"
"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."