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Ch. XIII.] 



ON THE CLIMATE OF THE EARTH. 



271 



Under what conditions extreme excentricity may exaggerate 

 cold. — Mr. James Croll, in an able memoir published in 

 1864, ' On the Physical Cause of the Change of Climates 

 during Geological Epochs,' * made an important suggestion, 

 explanatory of the manner in which a maximum excentri- 

 city would tend to exaggerate the cold in that hemisphere 

 in which winter occurred in aphelion. The difference, he 

 observes, (in conformity with a remark of Sir J. Herschel f) 

 of the sun's heat, whether in winter or summer, as con- 

 trasted with that which obtains in those same seasons, in 

 the present state of the ellipticity of the orbit, would not 

 be expressed merely by the differences in distance, before 

 alluded to, namely, 14^- millions of miles for the extreme, 

 and 3 millions for the present excentricity, but it would 

 amount to no less than one-fifth of the entire heat received 

 from the sun, because that heat would vary inversely as 

 the squares of the distance. In consequence, therefore, of 

 the lowering of the temperature by one-fifth in that hemi- 

 sphere in which winter occurs when the earth is farthest 

 from the sun, all the moisture precipitated from the air 

 in high latitudes would fall in the form of snow instead of 

 rain; and the heat of summer, 

 than what we now experience, would be insufficient to re- 

 move the accumulation of winter snow. The direct power 

 of the sun's # rays would be greatly intensified when the sky 

 was cloudless, but the constant melting of so much ice 

 would, in a great measure, neutralise their force, by giving 

 rise to fogs and an overcast sky. The rain of summer, he 

 says, would not melt one-eighth part of the snow in winter, 

 for it takes nearly eight tons of water at 50° F. to melt one 

 ton of snow at 32° F. Mr. Croll contends, therefore, that 



this maximum excentricity 

 would be enduring the extreme cold of a lengthened winter, 

 and having its summer heat chilled by the melting of ice, 

 the other hemisphere would be enjoying a perpetual spring ; 

 the polar winter occurring in perihelion, when the tempera- 

 ture was one-fifth greater than at present, and there being 

 no accumulation of snow beyond what the sun's rays could 



although one -fifth greater 



while one hemisphere during 



* Croll, Phil. Mag., August 1864. 



t Herschel' s Astronomy, Art. 3G8 a. 





