ther 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
white polar caps--Peculiar surface markings--Schiaparelli's
discovery of the canals--His description of their appearance
and of their duplication--Influence of the seasons on the
aspect of the canals--What are the canals?--Mr. Lowell's
observations--The theory of irrigation--How the inhabitants
of Mars are supposed to have taken advantage of the annual
accession of water supplied by the melting of the polar
caps--Wonderful details shown in charts of Mars--Curious
effects that may follow from the small force of gravity
on Mars--Imaginary giants--Reasons for thinking that
Mars may be, in an evolutionary sense, older than the
earth--Speculations about interplanetary signals from
Mars, and their origin--Mars's atmosphere--The question of
water--The problem of temperature--Eccentricities of Mars's
moons

CHAPTER V

_THE ASTEROIDS, A FAMILY OF DWARF WORLDS_ 129

Only four asteroids large enough to be measured--Remarkable
differences in their brightness irrespective of size--Their
widely scattered and intermixed orbits--Eccentric orbit of
Eros--the nearest celestial body to the earth except the
moon--Its existence recorded by photography before it was
discovered--Its great and rapid fluctuations in light, and
the curious hypotheses based upon them--Is it a fragment of
an exploded planet?--The startling theory of Olbers as to
the origin of the asteroids revived--Curious results of the
slight force of gravity on an asteroid--An imaginary visit
to