ASTKONOMTC CAUSES OF CLIMATIC CHANGES. 345 



mg a period of 13,000 years. But there is also a still 

 slower change in the direction of the axis major of the 

 Earth's orbit ; from which it results that the alternation we 

 have described is completed in about 21,000 years. That 

 is to say, if at a given time the Earth is nearest to the sun 

 at our mid-summer, and furthest from the sun at our mid- 

 winter : then, in 10,500 years afterwards, it will be furthest 

 fi'ora the sun at our mid-summer, and nearest at our mid- 

 winter. 



Now the difference between the distances from the sun 

 at the two extremes of this alternation, amounts to one- 

 thirtieth ; and hence, the difference between the quantities 

 of heat received from the sun on a summer's day under 

 these opposite conditions amounts to one-fifteenth. Esti- 

 mating this, not with reference to the zero of our thermome- 

 ters, but with reference to the temperature of the celestial 

 spaces, Sir John Herschel calculates "23° Fahrenheit as 

 the least variation of temperature under such circumstances 

 which can reasonably be attributed to the actual variation 

 of the sun's distance." Thus, then, each hemisphere has 

 at a certain epoch, a short summer of extreme heat, fol- 

 lowed by a long and very cold winter. Through the slow 

 change in the direction of the Earth's axis, these extremes 

 are gradually mitigated. And at the end of 10,500 years, 

 there is reached the opposite state — a long and moderate 

 summer, with a short and mild winter. At present, in con- 

 sequence of the predominance of sea in the southern hem- 

 isphere, the extremes to which its astronomical conditions 

 subject it, are much ameliorated ; while the great propor- 

 tion of land in the northern hemisphere, tends to exagge- 

 rate such contrast as now exists in it between winter and 

 Bummer : whence it results that the climates of the two 

 hemispheres are not widely unlike. But 10,000 years hence, 

 the northern hemisphere will undergo annual variations of 

 temperature far more marked than now. 



