MARS AS A WORLD. 151 



others. We are attended by a single moon ; and if in 

 this respect we are disposed to envy the superior endow- 

 ments of a planet which has two moons, like Mars, four 

 moons, like Jujriter, or eight, like Saturn, we should be 

 consoled by the reflection that though Venus is a globe as 

 large as we are, yet she has no attendant moons at all, 

 and that Mercury is in a similar solitary condition. 

 In physical constitution, also, it will shortly appear how 

 our earth is more solid than this planet or less solid than 

 that ; how it has more air and clouds than some of these 

 bodies, and less air and clouds than others ; how the earth, 

 viewed as a habitable world, seems to be in the meridian 

 of life, while there are other globes still in the phase of 

 early youth, or in a more advanced old age. All these 

 various circumstances give to the study of the worlds 

 which form the sun's family a quite exceptional interest 

 to us. We may expect to learn from this study some 

 facts which will throw light on that question so pregnant 

 with interest, as to the possible habitability of the other 

 globes in space. 



Of all the planets the one that comes into the most 

 favourable position for telescopic scrutiny from the earth 

 is unquestionably that globe known to the ancients by the 

 name of Mars. 



Mars revolves around the sun, and accomplishes its 

 journey in a period of 687 days. It moves in a path 

 which we shall not greatly misrepresent if we describe it as 

 a circle with a radius of about one hundred and fifty-one 

 millions of miles. The true shape of the path is, however, 

 oval or elliptical, so that the planet is sometimes about 

 one-tenth of the distance just stated nearer to the sun, and 



