VARIATIONS IN CLIMATE. 805 



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 deter- 

 mine the trend of climate on account of the short time they cover. 

 It has hapj)ened more than once during that time that a series of 

 seasons of a peculiarly marked character has been followed ab- 

 ruptly 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 suc- 

 cession 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 dif- 

 ferent constitutions or different temperatures, may be dismissed 

 as being purely hypothetical. No fact has been adduced in sup- 

 port 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 dis- 

 tance to which the sun departs from the equinoctial at midsum- 

 mer and midwinter. Its action is to heighten or reduce the con- 

 trast between those seasons according as its measure is greater or 

 less. By precession the equinoctial points shift their places back- 

 ward along the ecliptic, accomplishing a revolution in 21,500 

 years. It entails the revolution of the apsides, which is equiva- 

 lent 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 vari- 

 ation in the eccentricity of tlie earth's orbit these distances, called 

 the aphelion and perihelion distances, are lengthened and short- 

 ened, the difference between them is increased and diminished. 



