88 



E. WALTEK MAUNDER, F.K.A.S., ON THE 



These are the two chief factor:-; legulating the condition 

 of a planet ; the amount of light and heat received by it, and 

 the density and distribution of its atmosphere. Within the limits 

 of the solar system the first depends upon its distance from 

 the sun ; the second upon the size and density of the planet 

 itself. 



There is a simple way by which we may take a first step 

 towards appreciating the result oi' the greater distance of Mars. 

 If we take the earth at one of the equinoxes we shall find that 

 as much Hght and heat from the sun falls upon three square 

 yards at the equator as falls upon seven in latitude 64-|°. 

 This ditference is, of course, due to the angle on whicli the 

 higher latitude is presented to the sun, and we find that while 

 the mean temperature at the equator is about 80 degrees Fahr. 

 that of latitude of 64-|° is quite 50 degrees lower. As the 

 mean temperature of the earth as a whole is about 60 degrees, 

 we should from this way of looking at the problem take the 

 mean temperature of Mars as about 10 degrees, that is to say, 

 22 degrees below freezing point. So far then Mars would seem 

 to be as much worse oft" than the earth, as a place within the 

 Arctic Circle is worse oft' than the equator, but we have to add 

 the I'urtlier drawback that, owing to the thinness of the 

 atmosphere of Mars, we should ha^e to select within the 

 Arctic Circle the top of a mountain ten miles high to compare 

 with a station on the sea level at the equator. 



But we have omitted as yet a number of considerations all 

 of wiiich tend in the same direction, and all against the 

 habitability of the planet. Five-sevenths of the surface of the 

 earth is covered with water, and water is the great equaliser 

 of temperature. The atmospheric circulation of the earth, too, 

 is quick and efficient, so that our equatorial regions are much 

 cooler, our polar regions much warmer than they would be if 

 the air and water of the earth were stagnant. It is probable 

 that the difference in temperature between the equator and 

 latitude would be quite doubled if it were not for the 



equalising mfluences of our atmosphere and seas, and that we 

 ought to put the mean temperature of Mars as 100 degrees 

 below that of the earth. Professor Poynting, by another 

 method, has reached the same figure, and puts it as -iO degrees 

 below zero, the freezing point of mercury. 



Hardly less important than the mean temperature of a planet 

 is the range of temperature. At Greenwich the mean maximum 

 day temperature for the middle of July is about 75 degrees, the 

 mean minimum night temperature for the middle of January is* 



