

14 



DYNAMICAL GEOLOGY. 



1. SOURCES OF HEAT. 



The Earth has three prominent sources of heat: (1) The Sun; 

 (2) The heat of the Earth's interior ; (3) Chemical and mechanical 

 action. 



1. The Sun The heat received by the earth from the sun va- 

 ries : — 



1. With the Seasons. — The earth, owing to the obliquity between 

 its equatorial plane (at right angles to the axis of rotation) and the 

 plane of the ecliptic (or that of its orbit) gives more light and heat 

 for about six months to the northern hemisphere than to the southern, 

 making thereby a northern summer with a southern winter ; and the 

 reverse for the other six months. 



Further, the time of the equinoxes, or that of crossing the equator, 

 northward and southward, is slowly changing backward in the series 

 of months, and in less than six centuries the vernal equinox, now on 

 March 23d, will be in the month of February ; thus the summer 

 months after a while will become those of the winter. The rate of 

 the precession of the equinoxes is about 50*1 seconds a year, or a de- 

 gree in about 71-6 years, which corresponds nearly to a month in 2,158 

 years and a complete revolution in 25,868 years. 



2. With the Time of the Perihelion and Aphelion. — The sun is now 

 in aphelion during the northern summer and southern winter. With 

 aphelion in winter, the winters are colder and the summers are warmer 

 than with perihelion in winter. The position of the major axis of the 

 earth's orbit (the extremities of which are the aphelion and perihelion 

 points) is changing, and a complete revolution takes 110,000 years ; 

 and since this change is in the opposite direction from that of the 

 precession of the equinoxes, above stated, the cycle of the seasons is 

 shortened from 25,868 years to about 21,000 years; for supposing 

 the perihelion and either equinox to coincide, and then the precession 

 to move in its direction and the perihelion in the opposite, at their re- 

 spective rates, they would again be in conjunction, in consequence of 

 these rates, in 21,000 years. Hence, every 10,500 years, the seasons 

 become reversed, that is, the months of our winter become the sum- 

 mer months. Another consequence of this aphelion cycle is, that the 

 winter and summer intervals between the equinoxes vary in relative 

 lengths, the aphelion side being the longer. At the present time the 

 aphelion comes in summer, and the summer-half of the year is eight 

 days longer than the winter. 



3. With Changes in the Eccentricity of the Earth's Orbit. — The 

 earth's elliptical orbit varies slowly in eccentricity, — that is, in the 

 length of its major axis, — making the aphelion distance greater and 



