discovery--A day side and a night side--Interesting effects
of libration--The heavens as viewed from Mercury--Can it
support life?


CHAPTER III

_VENUS, THE TWIN OF THE EARTH_ 46

A planet that matches ours in size--Its beauty in the
sky--Remarkable circularity of its orbit--Probable
absence of seasons and stable conditions of temperature
and weather on Venus--Its dense and abundant atmosphere--Seeing
the atmosphere of Venus from the earth--Is the real face of the
planet hidden under an atmospheric veil?--Conditions of
habitability--All planetary life need not be of the terrestrial
type--The limit fixed by destructive temperature--Importance of
air and water in the problem--Reasons why Venus may be a
more agreeable abode than the earth--Splendor of our globe
as seen from Venus--What astronomers on Venus might learn
about the earth--A serious question raised--Does Venus, like
Mercury, rotate but once in the course of a revolution about
the sun?--Reasons for and against that view


CHAPTER IV

_MARS, A WORLD MORE ADVANCED THAN OURS_ 85

Resemblances between Mars and the earth--Its seasons and its