under the difficult task of accounting for such change of climate on philosophical principles. It is here that the community of the sciences is beautifully illustrated. Astronomy comes to the aid of her younger sister, and Geology receives from her the solution she could not herself compass.
Astronomy makes known three great irregularities in the motions of the earth, requiring thousands of years each for their recurrence. They are—
1. The "precession of the equinoxes," combined with the revolution of the line of the apsides, produces a progression in longitude, of the aphelion place of the earth, bringing about a coincidence of the winter-solstice with the aphelion once in 21,356 years. At the present time our aphelion position occurs in the summer season.
2. Variation of the inclination of the earth's axis to the plane of its orbit. This passes through a double oscillation in about ten thousand years. Its effect is to carry the solstitial point through a small variation in latitude, and to that extent prolonging or withdrawing the influence of the sun's rays in the polar regions.
3. Change in the eccentricity of the earth's orbit. This irregularity passes from maximum to maximum once in about one hundred thousand years. Its effect is to lengthen those seasons nearest the earth's aphelion, and to shorten those nearest its perihelion. At the present time, our aphelion, occurring in the summer season, lengthens the warm months to that amount that, combined with the shortening effect of perihelion in winter, makes a difference of about eight days between the summer and winter months of the year.
The conditions favorable for polar glaciation are as follows:
1. Winter-solstice in aphelion.
2. Obliquity of the ecliptic at minimum.
3. Eccentricity of orbit in maximum.
The coincidence of these three causes would produce the greatest glaciation. The least multiple of their periods of recurrence is about forty-two million years. The second cause may combine a great many times with either of the others within that interval. Its effect being quite inferior to that of either of the other two, it may be disregarded, and the time required for the combination of the other two would then be 4,200,000 years, which would necessarily be preceded and followed by a number of approximations. These would occasion corresponding periods of increase and diminution in the degree of cold that would not reach the maximum cold incident to their coincidence. Probably the most powerful of the causes enumerated is the occurrence of the winter-solstice in aphelion, which alone may have produced the last glacial epoch. In that case changes in the prevalence of the ice would be due to the operation of cause No. 2, or the obliquity of the ecliptic. The earth is now 622 years past its period of south-polar glaciation in the operation of cause No. 1, and is entering
