Page:Popular Science Monthly Volume 1.djvu/247

Rh must pass in very high latitudes, not to speak of the greater prevalence of cloud in regions round the Pole. A truer cause of climatic change Is to he sought in the effect of precession of the equinoxes, the revolution of the apsides, and, above all, the eccentricity of the earth's orbit. The great cycle of change due to precession would cause the different seasons of the Northern and Southern Hemispheres to coincide in turn, within 25,868 years, with all the points through which the earth passes in its. orbit round the sun. Combining with this movement, that of the revolution of the apsides or "motion of the aphelion," as Herschel named it, reduces this term of years to about twenty-one thousand. Sir C. Lyell's explanation, aided by a new diagram, renders sufficiently clear the effects which would be produced upon climate by the successive phases of precession, especially when combined with increased eccentricity or distance from the sun. The difference between winter in aphelion and perihelion—the range of eccentricity extending, as he has shown, to 14,000,000 miles at some periods, instead of 3,000,000, as now—is set down by Mr. Croll as not less than one-fifth of the entire heat received from the sun. Some slight change in this direction since the year 1248 A.D. has been thought capable of actual proof by M. d'Adhémar, and of being verified by the observations of M. Venetz upon the decrease of Swiss glaciers prior to the tenth century, and their subsequent increase. An admirable table compiled by Mr. Stone shows the variations in eccentricity for a million years before 1800 A.D., with the number of days which would be added to winter by its occurrence in aphelion, which has been followed up for a million years more by Mr. Croll and Mr. Carrick Moore. From these figures there might appear to be a possibility of approximating to a date for the Glacial epoch; and Sir C. Lyell holds it "far from startling" that 200,000 years back might be fixed upon as about the period of greatest cold, when the excess of winter days amounted to 27.7. He had in his tenth edition speculated upon 800,000 or 1,000,000 years as nearer the Glacial epoch, but he feels compelled to narrow the time within the limit at which the principal geographical features of the continents and oceanic basins were approximately assuming their present form. Were the astronomical theory, however, to be relied upon as the basis for the solution of the problem, we ought to meet in the course of palæontological research with a series of Glacial periods perpetually recurring in the Northern Temperate Zone; supposing a large eccentricity by itself sufficient apart from the cooperation of terrestrial causes, to intensify the cold of high latitudes. But no such evidence of violent revolutions is to be found in the flora and fauna of earlier periods. The continuity of forms, particularly in the class of reptiles, from the Carboniferous to the Cretaceous period, is an obvious fact opposed to the intercalation of intense glacial epochs. Another fact is, that many great cycles of eccentricity must have been gone through in the long centuries of the Carboniferous period, in which no break in the order of life is manifested.