Page:Popular Science Monthly Volume 40.djvu/575

Rh an extreme ellipticity of the earth's orbit. The sun is now about three million miles nearer us in the winter than in summer, and the winter (that is, the time from the autumnal to the vernal equinox) is seven days shorter than the summer. In about eleven thousand years from now the condition of things will be reversed, and the northern hemisphere will have a summer seven days shorter than the winter, occurring while the earth is three million miles nearer its source of heat. About two hundred and fifty thousand years ago the eccentricity of the earth's orbit was so great that the difference in these seasons was thirty-three days, and the difference between the distance of the earth from the sun at perihelion and that at aphelion was seven or eight million miles.

These facts served as the basis for Mr. Croll's theory, who assumed, on the strength of Herschel's authority, that the absolute amount of heat received by the earth during the season which occurred in perihelion was the same as that received during aphelion. He reasoned, therefore, that when the winters occurred in aphelion both their increased length and the greater distance from the sun would favor the radiation of heat to such an extent that a glacial period would be produced, especially in those periods when the eccentricity of the earth's orbit was greatest. Dr. Ball comes to the aid of Mr. Croll by showing that the distribution of heat between summer and winter is not in equal quantities, as supposed by Mr. Croll, but that sixty-three per cent of the annual heat received by a hemisphere of the earth falls upon it during the summer—that is, from the vernal to the autumnal equinox—and only thirty-seven per cent during the winter. If, therefore, there was any truth in Croll's original theory. Dr. Ball's discovery will greatly increase the efficiency of the cause.

But the accumulating objections urged by geologists against the theory of Mr. Croll must still apply with all their force. For after Dr. Ball's amendment there is even greater demand than before for geological evidence of a long succession of glacial periods, especially during the later geological eras. But it is the universal opinion of geologists that the Tertiary period was throughout one of great mildness of climate, even up to the vicinity of the north pole; yet the Tertiary age doubtless stretched over more than one period of extreme eccentricity of the earth's orbit. Furthermore, the point of glacial radiation in North America is not the north pole, but the region south of Hudson Bay. So clearly is this the case, that President Chamberlin (who has charge of the glacial department of the United States Geological Survey) has adopted the theory that the cause of the glacial phenomena of North America was an actual change of the position of the pole; while others, who can not give their adherence to so improbable a cause, are laying renewed emphasis upon the changes of level in the earth's surface which occurred toward the close of the Tertiary period.

While, however, we are not convinced of the adequacy of Croll's hypothesis, even as amended by Dr. Ball, we can speak most highly of Dr. Ball's work in bringing clearly before our minds a possible astronomical cause for the Glacial period with which all students of this attractive subject must reckon. The defect in the theory lies not in the mathematical calculations, but in our real lack of knowledge concerning the causes which distribute the heat over the surface of the earth. Meteorology is the science to which we look with most expectancy for further light upon the cause of the Glacial period. The astronomical causes suggested by Dr. Ball's discussion may be so readily masked by slight changes in the direction of oceanic and atmospheric currents produced by relatively slight changes of land level as to be almost entirely ruled out of account.

volume aspires to fill a unique place in the literature of mineralogy. As the author notes in his preface, there is no lack of treatises on the science, both determinative and descriptive. Still, to a naturalist familiar with the methods of nomenclature in the animal and vegetable kingdoms, the names of mineral species are barbarous, trivial, and unmeaning. This state of affairs