78 



E. WALTER MAUNDER^ F.R.A.S.^ ON THE 



On subjects such as the conditions of existence on planetary- 

 bodies altogether inaccessible to direct observation it is imperative 

 that we should distinguish between (1) known and established facts, 

 (2) inferences of high probability, based on established facts, but still 

 made subject to various assumptions, and (3) speculations as to facts 

 which may possibly be the result of highly hypothetical conditions. 

 Most of those who deal with this subject are unable to keep these 

 three categories distinct, and stumble in the half-light of imperfect 

 knowledge. Mr. Maunder walks with a sure step in the light of 

 clear and definite knowledge, and we are therefore fortunate in 

 having him for our guide. 



The following paper was then read by E. Walter Mal^'dlr, Esq., 

 F.R.A.S. :— 



THE CONDITIONS OF HABITABIIITY OF A PIANET ; 



with Special Reference to the Planet Mars. 



rr^HE first thought which men had concerning the heavenly 

 I bodies was an obvious one : they were lights. There was 

 a greater light to rule the day, a lesser light to rule the night, 

 and there were the stars also. 



But with the acceptance of the Copernican theory, this world 

 on which we live, while losing its pride of place as the centre 

 of the universe, from another point of view received a pro- 

 motion, in that itself it became a heavenly body of the same 

 order as some of those that shine down upon us. And, as the 

 earth is an inhabited world, the question naturally arises " May 

 not these briglit lights of heaven also be, like it, inhabited 

 worlds ? " There is a strong and natural desire to obtain an 

 affirmative answer to the question ; all men would greatly 

 delight to be able to recognize the presence of races similar to 

 our own upon other worlds in the depths of space. 



What do we mean by an " inhabited " world ? We know 

 quite well what we mean by an " inhabited " island. When an 

 explorer in his voyage lights upon a land hitherto unknown, no 

 richness of vegetation, no fullness and complexity of animal life 

 will warrant him in describing it as inhabited. He can only 

 give it that title if he should find men there. Similarly, if we 

 speak of a planet as being habitable, we mean that it is suitable 



