GEOLOGICAL SEASONS. 185 



winters and summers alternating with those of the north- 

 ern hemisphere.* 



Whatever the climatic effect of this astronomical cause, 

 it is now generally regarded as insufficient. Mr. Croll 

 indeed pronounces its efficiency null.f 



Lastly, let us consider the effects of increased eccen- 

 tricity of the earth's orbit on the climates of the northern 

 hemisphere, understanding increased eccentricity signi- 

 fies an elongation of the earth's orbit, so as to bring the 

 perihelion point nearer the sun, and remove the aphelion 

 point to a greater distance. This subject was investigated 

 by Sir John Herschel,^: and after him by Arago and 

 Humboldt ; but their conclusion showed that neither in- 

 crease nor diminution of eccentricity could directly influ- 

 ence, to any material extent, the amount of heat received 

 by the two hemispheres respectively in the course of a 

 year ; or so disturb the annual distribution over either 

 hemisphere as to result in a permanent and general 

 glaciation. This results from the fact that just in pro- 

 portion as the earth's perihelion distance from the sun is 

 diminished, the earth's orbital velocity in that part of its 

 orbit is accelerated, and thus the perihelion effect upon 

 climate is shortened in duration; and just as the aphelion 

 distance is increased the earth's aphelion velocity is re- 

 tarded, and the diminished solar intensity is continued 



* Adh'Jmar, Revolutions de la mer, 2d. ed., 1860 ; Le Hon, Periodicite dts 

 Grandes Deluges, 1858; A. R. Wallace, Island Life. Mr. J. J. Murphy maintains 

 that the occurrence of the summer solstice in perihelion would tend to increase 

 northern glaciation (Quar. Jour. Geol. Soc., xxv, 350). 



t Croll, Climate and Time, 83 ; Phil, Mag., Sept. 1869. See, also, Arago, 

 Edinb. New Phil. Jour., vi, 1834. 



JSir J. Herschel, Geological Transactions, 1832; Treatise on Astronomy, 315; 

 Outlines of Astronomy, 368; Arago, Annuaire du Bureau des Longitudes, 1834, 

 p. 199; Edinb. New Phil. Jour., April 1834, p. 244; Humboldt, Cosmos, iv, 459, 

 Bohn's ed. ; Phys. Descrip. Heavens, 336. 



