GEOLOGICAL SEASONS. 183 



helion and aphelion points (apsides) in reference to the 

 equinoctial points; 3. The eccentricity of the earth's orbit. 

 These elements are all changing. They do not, however, 

 change indefinitely in one direction. They pass through 

 a cycle of values. Each has, in the course of ages, its 

 maximum and its minimum. All these astronomical 

 causes were long ago considered, but were successively 

 pronounced inadequate. The search for an adequate as- 

 tronomical cause was undertaken by Humboldt, Arago, 

 Lyell, Sir John Herschel, and others, but without success. 

 It is not difficult to perceive that the three causes named 

 must produce severally some effect upon the climates of 

 the northern and southern hemispheres respectively; but 

 in each case it has been generally considered unimportant. 

 As to increased obliquity of the axis, it is obvious that 

 it would render the sun's rays more vertical in the hemi- 

 sphere turned toward the sun, that is, the width of the 

 torrid zone would be increased. During summer this in- 

 creased vertically of the rays would diminish polar glacia- 

 tion. During winter the sun would be permanently below 

 the horizon ; but that is its condition with the present 

 obliquity. When below the winter horizon, it is imma- 

 terial whether one or many degrees below, the solar in- 

 fluence is simply wanting. Therefore, increased obliquity 

 would not increase glaciation during the winter, though it 

 would diminish glaciation during the summer. The re- 

 sultant annual effect would be a diminution of glaciation; 

 and, correspondingly, diminished obliquity would cause an 

 increase of glaciation. This cause would not produce alter- 

 nating effects in the two polar regions during a succession 

 of secular intervals, but would operate alike in both re- 

 gions during the whole cycle of changes in the obliquity. 



