140 THE BORDERLAND OF SCIENCE. 



is above each square mile of the earth's. Such an amount 

 of air would be twice as much as we should infer from 

 the mass of Mars, and we should require five times as 

 much air only to have an atmosphere as dense as our own 

 at the sea level. An atmosphere about twice as dense 

 as this would perhaps give a climate as mild, on the 

 average, as that of our earth. But it seems rather a 

 daring assumption to assign to Mars an atmosphere 

 exceeding ten times in quantity what we should infer 

 from the planet's mass. 



It seems, on the whole, safer to abandon the theory 

 that Mars is a suitable abode for such creatures as exist 

 on the earth ; and to try to explain observed appear- 

 ances unhampered by a theory which after all is not 

 in itself a probable one. For indeed we can employ in 

 a very effective way against this theory a mode of 

 argument which is commonly urged in its favour. It 

 is reasoned that since the earth, the only planet we 

 know, is inhabited, therefore probably the other planets 

 are so. But we have seen that, so far as the evidence 

 goes, all the other planets, save Mars alone, are prob- 

 ably not inhabited by beings such as those which exist 

 upon the earth. Therefore, even on a priori grounds, 

 it is more likely that Mars is similarly circumstanced ; 

 since there are six planets in favour of this inference, 

 and only one, our earth, against it. 



In resuming the inquiry, with the theory of Mars's 

 habitability abandoned for the nonce, we must recall 

 the facts which have been demonstrated respecting 

 Mars, only we may now view them in a new light. We 



