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
density is 73 per cent of the earth's density. Gravity on its surface is