180 The Planet Mars in 1873. [April. 



It is hardly necessary to inform the reader that -the point 

 of chief interest in the study of Mars is the determination 

 of the degree in which he resembles or differs from our 

 earth. And in one respect the discussion of this question 

 is more interesting than it would be in the case of a planet 

 like Venus, which is the equal (or nearly so) of the earth 

 in size. Mars is so much smaller than the earth, that 

 although he belongs to the same family — that is to the 

 inner or so-called terrestrial family of planets — the question 

 might well arise whether he belongs in reality to the same 

 order. He is, in fact, nearer to our moon in volume than 

 to the earth, and comes about midway between the earth 

 and moon as respects mass.* Now we know that the moon 

 is totally unlike the earth in all the circumstances which we 

 associate with the requirements of living creatures ; and 

 therefore it might well be believed that Mars is as likely to 

 resemble the moon as the earth in this respect, and is even 

 more likejy to occupy a position in the scale of creation 

 utterly unlike that which is occupied by either the moon or 

 the earth. It is the discussion of this question, in the light 

 of the evidence obtained by observation, which renders the 

 study of the planet Mars so full of interest. 



I will briefly recapitulate what is known about Mars, 

 noting that the matter is more fully dealt with in my " Other 

 Worlds," and in a paper on Mars in my " Essays on 

 Astronomy." It is necessary for me to allude to these prior 

 discussions of my subject, since otherwise the present paper 

 might seem wanting in completeness. My purpose is to 

 treat as brief!}" as possible of those matters on which I 

 have already touched, in order that as much as possible of 

 the present essay may deal with new matter. 



We know first that the surface of Mars is divided into 

 land and water; and that the continents and oceans of 

 Mars have the shapes depicted in the projections which 

 (for another primary purpose, however) illustrate the present 

 essay. The land has a tint suggesting the idea that the 

 chief constituent of the soil may be a substance resembling 

 our sandstones ; though on this point it would be unsafe to 



* By this I do not mean that his mass is nearly the arithmetical mean 

 between the mass of the earth and that of the moon, but nearly the geometrical 

 mean. Thus the mass of the earth being taken as unity, that of the moon is 

 0-0114. and the arithmetical mean of these quantities is 0-5057. Now the mass 

 of Mars is o-iiS, or less than a quarter of this arithmetical mean. But the 

 geometrical mean between 1 and 0-0114 is about 0-107, which approaches 

 nearly enough to the value of the mass of Mars to justify the remark in the 

 text. ' It must not be forgotten, however, that as respects the actual quantity 

 of matter he contains, Mars resembles the moon more nearly than the earth. 



