THE REVOLUTIONS OF THE CRUST OF THE EARTH. 343 



Wheu. according to the calculation of Mr. Croll,* 850,000 years before 

 1800, the eccentricity was nearly at its lowest limits, the difference be- 

 tween the temperature of the summer at the perihelion, and of the 

 winter at the aphelion, was certainly very great ; but by virtue of the 

 €ompeusatiou of intensity by duration, the glacial periods could not 

 have occurred. If, on the contrary, we consider what might be the in- 

 fluence of these extremes of temperature upon the movement of the 

 fluid envelope, and upon the circulation of the liquid matter at the sur- 

 face of the globe, we must come to the conclusion that, on account of 

 the aerial and oceanic currents, and on account of the preponderant 

 precipitation of humidity upon one hemisphere, the terrestrial globe is 

 unequally heated on the two sides of the equator. 



Before entering further into these considerations and showing that 

 the eccentricity of the orbit and the precession of the line of the apsides 

 are the principal causes of the geological cold periods, especially evi- 

 dent if we examine their influence upon meteorological phenomena, we 

 will endeavor to show how causes the most insensible may often, by 

 long accumulation of their influence, produce sensible effects, the origin 

 of which very often escapes us. Theory tells us that all surfaces inter- 

 cei)ting light and heat must reflect a certain quantity. The moon 

 reflects light, but the best electro-thermometer does not indicate with 

 any certainty the presence of reflected heat. This does not prevent 

 scientists from believing that the earth's satellite reflects an amount of 

 heat in proportion to the light reflected, and that this heat influences 

 the mean temperature of the earth. Analysis of about 238,000 observa- 

 tions made during twenty-six years at Prague, by M. Ch. Zenger, shows, 

 first, that when the obliquity of the lunar orbit attains its maximum, 

 the mean of the annual temperature reaches its minimum (the baromet- 

 ric pressure in this case attains its greatest value) j second, that the 

 variations of temperature (as well as of pressure) are periodical; the 

 duration of their period being half a lunar year (9.5 solar years); third, 

 the variations of temperature are more sensible during the winter than 

 during the summer mouths, for in the first case they are six times 

 greater than in the second. t 



From the time when the perfect equilibrium of the temperature of 

 the two hemispheres was first broken until our day, we must attribute 

 to slow and almost insensible causes the periodicity and alternation of 

 conditions favorable and unlavorable for the organisms of the earth. 

 In order to appreciate the action of the cosmic causes we are consider- 

 ing, let us suppose that the eccentricity of the terrestrial orbit is at its 

 maximum, and that the winter of the northern hemisphere coincides 

 with the aphelion. In this case the length of the winter will be 199.5 

 days, and that of the summer 165.5 days. The reverse will be the case 



*PIuL Magaz., February, 1S67, p. 120. 



t C. V. Zeuger, On the perioak change of clmaie canned by the moon. Phil. Mao;az,, June, 

 1868, p. 433-439. 



