CHAPTER IV. OUTLINE OF 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. _Primary cause of Change of Eccentricity of the Earth’s Orbit._—There are two causes affecting the position of the earth in relation to the sun, which must, to a very large extent, influence the earth’s climate; viz., the precession of the equinoxes and the change in the eccentricity of the earth’s orbit. If we duly examine the combined influence of these two causes, we shall find that the northern and southern portions of the globe are subject to an excessively slow secular change of climate, consisting in a slow periodic change of alternate warmer and colder cycles. According to the calculations of Leverrier, the superior limit of the earth’s eccentricity is 0·07775.[27] The eccentricity is at present diminishing, and will continue to do so during 23,980 years, from the year 1800 A.D., when its value will be then ·00314. The change in the eccentricity of the earth’s orbit may affect the climate in two different ways; viz., by either increasing or diminishing the mean annual amount of heat received from the sun, or by increasing or diminishing the difference between summer and winter temperature. Let us consider the former case first. The total quantity of heat received from the sun during one revolution is inversely proportional to the minor axis. The difference of the minor axis of the orbit when at its maximum and its minimum state of eccentricity is as 997 to 1000. This small amount of difference cannot therefore sensibly affect the climate. Hence we must seek for our cause in the second case under consideration.