ther point: even here on
earth very strong winds are caused by the heating of the tropics; the
hot air, being lighter than the cold air, rises, and the colder air from
the poles rushes in to supply its place. This causes wind, but the winds
which would be raised on Venus by the rush of air from the icy side of
the planet to the hot one would be tornadoes such as we could but
faintly dream of. It is, of course, useless to speculate when we know so
little, but in a subject so intensely interesting we cannot help
guessing a little.

Venus is only slightly smaller than the earth, and her density is not
very unlike ours; therefore the pull of gravity must be pretty much
there what it is here--that is to say, things will weigh at her surface
about the same as they do here. Her orbit is nearly a circle, so that
her distance from the sun does not vary much, and the heat will not be
much greater from this cause at one time of the year than another.

As her orbit is tilted up a little she does not pass between us and the
sun at each revolution, but occasionally she does so, and this passing
is called a transit. Many important facts have been learned by watching
these transits. Mercury also has transits across the sun, but as she is
so much smaller than Venus they are not of such great importance. It was
by the close observation of Venus during her transits that the distance
from the earth to the sun was first measured. Not until the year 2004
will another transit of Venus occur.

It is not difficult to imagine that the earth must appear a splendid
spectacle from Venus, whence she is seen to great advantage. When
nearest to us she must see us like a little moon, with markings as the
continents and seas rotate, and these will change as they are obscured
by the clouds rolling over them. At the North and South Poles will be
glittering ice-caps, growing larger and smaller as they turn toward or
away from the sun. A brilliant spectacle!

[Illustration: ORBITS OF MARS, THE EARTH, VENUS, AND MERCURY.]

We might say with a sigh, 'If only we could see such a world!' Well, we
can see a world--not indeed, so large as Venus, yet a world that comes
almost as near to us as Venus does, and which, unlike her, is outside us
in order from the sun, so that when it is nearest to us the full
sunlight is on it. This is Mars, our neighbour on the other side, and of
all the fascinating objects in the sky Mars is the most fascinating, for
there, if a