204 SPARKS FROM A GEOLOGIST'S HAMMER. 



phere, and the period of least intensity during our sum- 

 mer. The effect must be to mitigate the extremes of both 

 seasons. As the southern hemisphere experiences the re- 

 fricreratinff effect of diminished distance during its winter, 

 the limits of the uncultivable and uninhabitable zone would 

 be removed considerably farther from the south pole than 

 they are from the north pole, were it not for the fact that 

 the larger proportion of watery surface in the southern 

 hemisphere prevents that hemisphere from accumulating 

 or losing heat as rapidly as the broad continental surfaces 

 of the northern hemisphere. In the course of some thou- 

 sands of years, however, all this will be reversed.* The 

 effects of such a cosmic change of climate upon the popu- 

 lations of the northern hemisphere must be literally of a 

 revolutionary character, like that of which a faint remi- 

 niscence is retained in the Zend Avesta. 



The foregoing considerations concern only the aggre- 

 gate amount of heat and lit^ht received by the earth as a 

 whole. The actual heatinsf and illuminatincr effects of the 

 sun at any particular spot on the earth's surface vary also, 

 with the angle at which the solar rays strike the spot. 

 This angle varies with the seasons and the hours of the 

 day. From whatever cause a variation in the altitude of 

 the sun is produced, his heating power is always propor- 

 tional to the perpendicular let-fall from the position of 

 the sun upon the horizon. 



Every one knows that the mid-day sun is less vertical 

 in winter than in summer. There is always some lati- 

 tude, however, at which the mid-day sun is exactly in 

 the zenith. About the 21st of June it is the tropic of 

 Cancer. From this time the sun recedes toward the south, 



* See " Geological Seasons." 



