Physical Theory of Secular Changes of Climate. 193 



without influencing the other agents, or being influenced by 

 them, its real efficiency in bringing about either the glacial 

 condition of climate or the warm condition of climate, as the 

 case may have been, would not have been so great. 



The primary cause that set all those various physical agencies 

 in operation which brought about the glacial epoch, was a high 

 state of eccentricity of the earth's orbit. When the eccentricity 

 is at a high value, snow and ice begin to accumulate, from the 

 increasing length and coldness of the winter, on that hemisphere 

 whose winter solstice is approaching toward the aphelion. The 

 accumulating snow then begins to bring into operation all the 

 various agencies which we have been describing ; and, as we have 

 just seen, these, when once in full operation, mutually aid one 

 another. As the eccentricity increases century by century, the 

 temperate regions become more and more covered with snow and 

 ice, first on account of the continued increase in the coldness 

 and length of the winters, and secondly, and chiefly, from the 

 continued increase in the potency of those physical agents which 

 have been called into operation. This glacial state of things 

 goes on at an increasing rate, and reaches a maximum when the 

 solstice point arrives at the aphelion. After the solstice passes 

 the aphelion, a contrary process commences. The snow and ice 

 gradually begin to diminish on the cold hemisphere and to 

 make their appearance on the other hemisphere. The gla- 

 ciated hemisphere begins to turn warmer and the warm he- 

 misphere colder, and this continues to go on for a period of 

 ten or twelve thousand years, until the winter solstice reaches 

 the perihelion. By this time the conditions of the two hemi- 

 spheres are reversed; the formerly glaciated hemisphere has 

 now become the warm one, and the warm hemisphere the gla- 

 ciated. The transference of the ice from the one hemisphere 

 to the other continues so long as the eccentricity remains at a 

 high value. 



The Mean Temperature of the whole Earth should be greater 

 in Aphelion than in Perihelion. — When the eccentricity becomes 

 reduced to about its present value, its influence on climate is 

 but little felt. It is, however, probable that the present exten- 

 sions of ice on the southern hemisphere may, to a considerable 

 extent, be the result of eccentricity. The difference in the cli- 

 matic conditions of the two hemispheres is just what should be 

 according to theory : — (1) The mean temperature of that hemi- 

 sphere is less than that of the northern. (2) The winters of the 

 southern hemisphere are colder than those of the northern. 

 (3) The summers, though occurring in perihelion, are also com- 

 paratively cold ; this, as we have seen, is w r hat ought to be ac- 

 cording to theory. (4) The mean temperature of the whole 



Phil. Mag. S. 4, Vol. 39. No. 260. March 1870. O 



