CHAPTER IV.

OUTLINE OE THE PHYSICAL AGENCIES WHICH LEAD TO SECULAR CHANGES OF CLIMATE.

Eccentricity of the Earth’s Orbit; its Effect on Climate.—Glacial Epoch not the direct Result of an Increase of Eccentricity.—An important Consideration overlooked.—Change of Eccentricity affects Climate only indirectly.—Agencies which are brought into Operation by an Increase of Eccentricity.—How an Accumulation of Snow is produced.—The Effect of Snow on the Summer Temperature.—Reason of the Low Summer Temperature of Polar Regions.—Deflection of Ocean-currents the chief Cause of Secular Changes of Climate.—How the foregoing Causes deflect Ocean-currents.—Nearness of the Sun in Perigee a Cause of the Accumulation of Ice.—A remarkable Circumstance regarding the Causes which lead to Secular Changes of Climate.—The primary Cause an Increase of Eccentricity.—Mean Temperature of whole Earth should be greater in Aphelion than in Perihelion.—Professor Tyndall on the Glacial Epoch.—A general Reduction of Temperature will not produce a Glacial Epoch.—Objection from the present Condition of the Planet Mars 54