ular, and her orbital
motion so nearly invariable, she has but a very slight libration with
reference to the sun, and the east and west lunes on her surface, where
day and night would alternate once in her year of 225 days, would be so
narrow as to be practically negligible.

But, owing to her extensive atmosphere, there would be a very broad band
of twilight on Venus, running entirely around the planet at the inner
edge of the light hemisphere. What the meteorological conditions within
this zone would be is purely a matter of conjecture. As in the case of
Mercury, we should expect an interchange of atmospheric currents between
the light and dark sides of the planet, the heated air rising under the
influence of the unsetting sun in one hemisphere, and being replaced by
an indraught of cold air from the other. The twilight band would
probably be the scene of atmospheric conflicts and storms, and of
immense precipitation, if there were oceans on the light hemisphere to
charge the air with moisture.

It has been suggested that ice and snow might be piled in a vast circle
of glaciers, belting the planet along the line between perpetual day
and night, and that where the sunbeams touched these icy deposits near
the edge of the light hemisphere a marvelous spectacle of prismatic
hills of crystal would be presented!

It may be remarked that it would be the inhabitants of the dark
hemisphere who would enjoy the beautiful scene of the earth and the moon
in opposition.

CHAPTER IV

MARS, A WORLD MORE ADVANCED THAN OURS

Mars is the fourth planet in the order of distance from the sun, and the
outermost member of the terrestrial group. Its mean distance is
141,500,000 miles, variable, through the eccentricity of its orbit, to
the extent of about 13,000,000 miles. It will be observed that this is
only a million miles less than the variation in Mercury's distance from
the sun, from which, in a previous chapter, were deduced most momentous
consequences; but, in the case of Mars, the ratio of the variation to
the mean distance is far smaller than with Mercury, so that the effect
upon the temperature of the planet is relatively insignificant.

Mars gets a little less than half as much solar light and heat as the
earth receives, its situation in this respect being just the opposite
to that of Venus. Its period of orbital revolution, or the length of its
year, is 687 of our days. The diameter of Mars is 4,200 miles, and its