ratively small core of liquid, the greater part of the planet being
made up of seething vapor. So you see it would be about as difficult to
live on Jupiter as in a steam-boiler, or a caldron of molten lead. Since
last summer a great red spot has been noticed on the surface of the
planet, which has attracted much attention. Some think it is an immense
opening, large enough for our earth to be dropped through."

"Are the other planets such dreadful places?" asked May.

"Saturn seems to be in about the same condition as Jupiter. Mars is
thought to be solid, and to have land, water, and air. It has also two
brilliant white spots on opposite sides, which are supposed to be vast
fields of ice and snow. But the water seems to be disappearing; and the
time when the planet could be inhabited is thought to be long gone by."

"Where does the water go?" asked Joe.

"Probably it sinks into the cracks or fissures which form in the crust
of the planet when it begins to shrivel up with the cold."

"Then it must be like a great frozen grave-yard," said May. "But is
there no other planet that is pleasanter to think about?"

"The one that seems on the whole to be most like our own is Venus, and
so Professor Proctor calls it our sister planet. It is so close to the
sun that it is hidden most of the time, being only seen for a while
before sunrise, and at other times a while after sunset. In the one case
it is called the morning, and in the other the evening star. Also there
is Mercury, still nearer the sun, and hidden almost all the time."

"Then," said May, "there seems to be no way of knowing anything about
there being people like us in other worlds; and the more we look into
it, the more uncertain we become."

"That is about the way the case stands," said the Professor. "But if
science continues to make as rapid progress as it has lately done, we
may hope that it will yet throw more light on the question."

"How many planets are there?" asked Joe.

"Until quite recent times there were supposed to be only the five we
have mentioned. Since the beginning of the present century about two
hundred little planets, called asteroids, have been discovered between
the orbits, or paths, of Mars and Jupiter. Then there are Uranus and
Neptune, very far off from the sun and from us, so much so that the
latter was mistaken for a fixed star."

"Professor," said May, "you mentioned the moon as being near to us. Can
you explain to us how its di