CONDITIONS OF HABITABILITY OF A PLANET. 



89 



about 33 degrees, a range of 42 degrees. This range is not that 

 between the very liighest and lowest temperatures ever recorded, 

 but the average range between the hottest part of the day in 

 summer and the coldest part of the night in winter. Britain 

 is however an island, and the surrounding ocean tempers our 

 climate and contracts the range of temperature very greatly. 

 A continental climate in the same latitude would show a range 

 about twice as great. 



This range of temperature is, on the average, smallest at the 

 equator, greatest at the poles; the length of the day and night 

 being invariable at the equator, while at the poles there is but 

 one day and one night in the whole year. The range therefore 

 increases with the latitude. On Mars, where the year is nearly 

 twice the length of ours, the range from equator to pole must 

 be much greater than on the earth : the more so that the absence 

 of oceans and the sluggishness of tlie atmospheric circulation 

 would leave unmodified the full effect of a polar day and a 

 polar night each almost as long as a complete terrestrial year. 



The range in any particular latitude would also be greater 

 than on the earth. We know that during the night the earth 

 radiates into space the heat which it has received from the sun 

 on the previous day, and the rarer and drier the air, the more 

 rapid the fall of temperature. But the Martian air is so thin 

 that during the day it offers no hindrance to the heating effect 

 of the sun's rays upon the soil, and during the night little or 

 no hindrance to radiation; it cannot play the part fulfilled by 

 the earth's atmosphere of imparting lieat that it has gathered 

 during the day to the soil during the niglit. The conclusion 

 therefore reached by the late Professor Newcomb is generally 

 accepted by astronomers, that " during the night of Mars, even 

 in tlie equatorial regions, the surface of the planet probably 

 falls to a lower temperature than any we ever experience on 

 our globe. If any water exists it must not only be frozen but 

 the temperature of the ice must be far below the freezing 

 point." During the night of the polar regions, the temperature 

 of Mars must closely approach the absolute zero. 



But though this is the case, and the mean temperature of 

 Mars even in the equatorial regions is below the freezing point 

 of water, yet, owing to the wide range of temperature, due tu 

 the rarity of the atmosphere, it is probable that the maximum 

 temperature at noonday in summer time for any particular 

 latitude does not differ very greatly from that experienced in 

 similar latitudes here. And it is just those regions of the 

 planet which are enjoying noontide in summer which are most 



