184 The Planet Mars in 1873. [April, 



air polewards (whence, necessarily, winds blowing towards 

 the equator may be inferred) ; and there must, in fact, not 

 only be general climatic relations resembling those on our 

 earth, but also similar meteorological phenomena. 



It is not so easy as has been sometimes supposed (by my- 

 self amongst others) to decide between these two solutions. 

 All that the telescope reveals in Mars has been held to show 

 that the latter solution must be accepted. We actually 

 appear to see the clouds, which are formed in Martial tem- 

 perate regions, showing that great quantities of aqueous 

 vapour are commonly present in the atmosphere over these 

 regions. We know that more heat than that which would 

 evaporate aqueous vapour near the arctic regions must 

 necessarily be expended on the great oceans of Mars, and 

 that therefore aqueous vapour must be raised into the air 

 over these oceans. And we have seen that spectroscopic 

 analysis confirms this conclusion, or rather establishes it as 

 a demonstrated fact. 



But we are thus brought into the presence of somewhat 

 serious difficulties. 



In the first place, let us remember that the direct supply 

 of heat from the sun is certainly that which has been men- 

 tioned above. In other words, the surface of Mars receives, 

 mile for mile, less than 4-aths of the heat which our earth 

 receives. This heat may be treasured up (as it were) more 

 completely, or owing to some cause unknown may act more 

 efficiently ; but there can be no question that no greater 

 amount of heat is actually received. So that we have this 

 first difficulty to encounter, that regarding Mars as a 

 whole, he seems to be more than twice as well warmed 

 as in the nature of things he would be, supposing the con- 

 dition of his surface and of his atmosphere resembled what 

 we are acquainted with on earth. 



But now as to his atmosphere. Let us suppose that it is 

 constituted like the earth's atmosphere, and let us enquire 

 what must be its density and pressure under such and such 

 conditions. But first it may be asked whether we may not 

 be justified in forming some such opinion as to the quantity 

 of air around Mars which is indicated in Mr. Williams's 

 work "The Fuel of the Sun." Here, as is probably known 

 to many of my readers, the assumption is made that every 

 celestial body has' a certain proportion of air around it, a 

 proportion somewhat artificially determined by Mr. Wil- 

 liams, as depending on a numerical relation, the necessity of 

 which is not demonstrated by the evidence. Nevertheless, 

 it seems a reasonable assumption that the larger bodies 



