402 JOSEPH JOHN MUKPHY ON THE GLACIAL 



the northern hemisphere, which under those circumstances I believe 

 to be the glaciated one. The parallel columns describe the climates 

 of the world at the two solstices : — 



When the Earth is in When the Earth is in 



Perihelion. Aphelion. 



The northern hemisphere -The northern hemisphere 



has a mild winter. has a cool summer. 



The equatorial region The equatorial region 



has its hot season. has its cool season. 



The southern hemisphere The southern hemisphere 



has a hot summer. has a cold winter. 



It is thus seen that the southern hemisphere will have an ex- 

 treme annual range of temperature, while that of the northern is 

 very small ; and not only so, but the effect of the equatorial hot 

 season will spread far into the northern hemisphere, making the 

 winter of at least the lower latitudes of that hemisphere warmer 

 than the summer. This reversal of the seasons will probably in 

 no case extend to the pole ; in the immediately circumpolar regions 

 the summer will always be warmer than the winter ; but the line 

 of no annual range, where the temperatures of midsummer and of 

 midwinter are the same, instead of being, as now, near the equator*, 

 will be perhaps near the Arctic Circle. If this is the case, the 

 eccentricity is a little too great for the maximum of glaciation. 

 The condition most favourable to glaciation will probably be that 

 where the line of no annual range is at or near the margin of the 

 polar ice-cap. But if the maximum of eccentricity is too great for 

 the maximum of glaciation, the degree of eccentricity which will 

 produce the maximum of glaciation will be attained at some time 

 while the eccentricity is approaching, and again while it is receding 

 from, its maximum. 



The statements in the above tabular form would probably be ac- 

 cepted by every one as self-evident, if they applied to a globe having 

 its surface all land, so that the subject of climate was not compli- 

 cated by the thermal effects due to evaporation, condensation, 

 freezing, and melting ; and Mr. Croll in that case would not have 

 propounded his strange paradox, that the mean temperature of the 

 whole earth at maximum eccentricity is higher when in aphelion 

 than when in perihelion. The principles stated in the above table, 

 however, are not physical, but purely astronomical ; and we have 

 now to trace their physical results. 



Round the north pole there is a considerable area, including 

 Greenland and great part of the Asiatic and American continents, 

 where the mean temperature of the year is below the freezings 



* See the paper by Mr. Keith Johnston, jun., already referred to. 



