Grotesqueness of Mercury considered as a world--Its dimensions, mass, and movements--The question of an atmosphere--Mercury's visibility from the earth--Its eccentric orbit, and rapid changes of distance from the sun--Momentous consequences of these peculiarities--A virtual fall of fourteen million miles toward the sun in six weeks--The tremendous heat poured upon Mercury and its great variations--The little planet's singular manner of rotation on its axis--Schiaparelli's astonishing 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 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