"Some of them have, I am sure," said Mary. "But now we are running along
too fast, and I must tell you about our own sun first, and its nearest
planet Mercury. Well, Mercury is a very warm little world, and it gets
so near the sun that sometimes it is about nine times as warm as here,
and at other times it is only four times as warm. You see, Mercury does
not go round the sun in a perfect circle, so at times it is farther away
than at others. Now, the sun is like a great fire in the sky, and the
nearer we go to it the warmer we are. How would you like to live on a
little world where it is nine times warmer than it is here?"
"I should not like it at all, would you, dollie?" said Nellie; "we would
roast if we went to world Mercury."
"But we don't know whether there are any people there," continued Mary,
"and if there are, they might not mind the heat at all. You can get used
to the heat, just as Uncle Robert did when he went to India. Don't you
remember how he felt the change when he came home, and how he shivered?
He missed the heat just as we would suffer from it if we went to India
for the first time."
[Illustration: COMPARATIVE SIZE OF SUN AS SEEN FROM THE PLANETS.]
"Then Uncle Robert would not mind going to Mercury," said Harry, laughing,
"if he is getting to like the heat in India. But I do not want him to
go yet, as he might never come back again; and what would we do without
him?"
"What would we?" said Nellie mournfully, her eyes filling with tears at
the very thought.
"Is a planet made of earth and stones and trees and flowers, just like
planet Earth?" asked Harry.
[Illustration: COMPARATIVE SIZE OF THE PLANETS.]
"Yes, dear," replied his sister; "only some planets, like Jupiter and
Saturn, are still wrapped up in a blanket of clouds and steam, and we
cannot see them yet. They are very hot indeed, and all the water that
will make the oceans and seas and bays is now steam and clouds hiding
the true planet from view. Water could no more rest on the surface of
the planets Jupiter and Saturn than it could rest on red-hot iron. Don't
you remember, the other day, when nurse upset a cup of water on the hot
stove, how the water sizzled and turned into steam in a moment?
"Now planet earth, a long time ago, when it was a very young world, was
very hot like Jupiter. All the lakes and seas and oceans were turned into
steam and blankets of cloud. It would have been a very uncomfortable world
to live on then. But it became cooler and cooler, and the clouds changed
into the oceans and seas and lakes that make our earth so beautiful.
"Some day this little world will grow old, and the oceans will get
smaller and smaller, and the earth colder and colder. Then there will be
scarcely any air to breathe, and we would gasp, and die just like that
poor fish that Uncle Robert caught last week and threw in the bottom of
the boat. Don't you remember, Nellie, how the poor little thing gasped
and jumped around? It could not live out of the water, so it died. Now,
we cannot live without air, and if this earth had not any air we would
die. But this will not happen for a very long time."
"Are you quite sure?" asked Harry, with an anxious look on his face;
"because I don't want to die yet, sister."
"Quite sure, my little brother," she said, kissing him tenderly; "for
hundreds and hundreds of years must pass away before anyone will have
any idea that the earth is growing old."
"And what will become of the poor little fishes when the oceans dry up?"
asked Nellie sadly, as she clasped her dollie closely in her arms, as
though to protect it from the coming trouble.
"I expect they will all die," said Harry wisely; "because you know,
Nellie, they can't live out of water. Can they?"
"Or else that fish Uncle Robert caught would have lived," said Nellie.
"But please tell us a story about Mercury, Cousin Mary, and the other
little planets."
"Well, Mercury is a very little planet, and instead of taking a year of
three hundred and sixty-five days, it goes around the sun in eighty-eight
days. That is, it goes round the sun four times while we go round it
only once. Some think Mercury always keeps the same side turned to the
sun, so that it is always day on one side and night on the other, but
we are not quite sure about this yet."
"I should like to live on Mercury, wouldn't you, Harry?" said Nellie,
clapping her hands with glee. "Just think of day all the time, and never
having to go to sleep!"
"But you would get very tired of that," said Mary, "and long for the
night to come. And, besides, would you not miss seeing the moon and the
beautiful stars?"
"I would live on the edge of Mercury," said Harry thoughtfully, "so that
when I was tired of day I might slip around it and have night. It must
be very cold on the other side, where the sun does not shine, if Mercury
gets all its heat from the sun."
"I suspect it is," said Mary, "and I don't believe we should like to live
on Mercury, after all; so let us try the next planet, which is called
Venus."