"They usually have a bright golden head, sometimes as large as the earth,
and as they approach the sun they adorn themselves with a glittering train
millions of miles in length. Some of the comets are regular visitors,
and we know just when to expect them; others come, and do not return
for hundreds of years, while a few visit the sun never to return again."
"Where do they come from?" asked Harry.
[Illustration: A COMET.]
"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?"
[Illustration: 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."
"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.'"