Page:Popular Science Monthly Volume 40.djvu/827

Rh within which some observers suppose a round of meteorological changes is accomplished for a single locality. Popular opinions are founded most largely on hap-hazard recollections of vague impressions that can not be depended upon; and even if we had accurate records in place of these, they could not be used to determine the trend of climate on account of the short time they cover. It has happened more than once during that time that a series of seasons of a peculiarly marked character has been followed abruptly by a series of opposite character, nullifying the conclusions that may have been taking shape from the former series. The speculations concerning a decrease of rainfall in the United States in consequence of the removal of the forests have been disturbed by the recent prevalence, in part of the disforested area, of a succession of seasons of heavy and continuous rains.

Cosmical revolutions and changes taking place on the surface of the earth have been mentioned as causes by which climates may be permanently modified, and have been brought in to account for the changes which geology shows have taken place in the past.

Among the theories of cosmical causes, one, which supposes the solar system to be carried through parts of space having different constitutions or different temperatures, may be dismissed as being purely hypothetical. No fact has been adduced in support of it, and no valid reason has been presented for supposing that there are differences in the parts of space. Other theories, which refer climatic changes to astronomical cycles affecting the earth's orbit and its position therein, have a more substantial basis. They have been considered by sober authors and have a hold on the minds of intelligent students; and the cycles have a real existence and are capable of producing effects that can be calculated. They comprise a secular variation in the obliquity of the ecliptic; the precession of the equinoxes, with the attendant revolution of the apsides; and an oscillation in the eccentricity of the earth's orbit—all conforming to regular and well-defined periods.

The variation in the obliquity of the ecliptic affects the distance to which the sun departs from the equinoctial at midsummer and midwinter. Its action is to heighten or reduce the contrast between those seasons according as its measure is greater or less. By precession the equinoctial points shift their places backward along the ecliptic, accomplishing a revolution in 21,500 years. It entails the revolution of the apsides, which is equivalent to a displacement in relation to the seasons of the points of the earth's greatest and of its least distance from the sun. By the variation in the eccentricity of the earth's orbit these distances, called the aphelion and perihelion distances, are lengthened and shortened, the difference between them is increased and diminished.