A WHEWELL1TE ESSAY ON MAES. 139 



But when we apply this consideration to the case 

 of Mars we find a difficulty in the disproportionate 

 amount of atmosphere which must be assigned to this 

 small planet. It seems a very natural and probable 

 assumption that every planet would have an atmo- 

 sphere proportional in quantity to the quantity of 

 matter in the planet. Thus since the mass of Mars is 

 but one-ninth of the earth's mass, we should infer that 

 his atmosphere amounted in quantity to but one-ninth 

 part of the earth's atmosphere. Of course we could 

 not lay any stress on such an assumption ; but it must 

 be regarded as more probable, on a priori grounds, 

 than any other. This would leave Mars with much 

 less air over each square mile of his surface than there 

 is over each square mile of the earth's surface : for the 

 surface of Mars is much greater than a ninth part of 

 the earth's ; it is, in fact, between a third and a fourth 

 of the earth's surface. But this is not all; not only 

 (on the assumption we are dealing with) would there 

 be much less air over each mile of the surface of Mars 

 but this smaller quantity of air would be much less 

 strongly attracted towards the surface of the planet. 

 For, owing to his small bulk and the comparative 

 lightness of the materials of which he is constructed, 

 Mars exerts less than two-fifths of the attractive force 

 which our earth exerts. A mass which, on our earth, 

 would weigh a pound, would on Mars weigh little more 

 than six ounces ; and the atmospheric pressure would be 

 correspondingly reduced, even though Mars had as 

 much air above each square mile of his surface as there 



