1874.] Climate of the Glaciai Period. 427 
and learning, and strengthened by laborious calculations, 
the theory that the cold of the Glacial period and the warmth 
of other geological epochs were due to great changes in the 
ellipticity of the earth’s orbit. As has long been known, 
the earth, in its annual course around the sun, does not 
describe a circle, but an ellipse, and is much nearer to the 
great luminary in some parts of its course than in others. 
Astronomers have also proved that the eccentricity of the 
orbit varies during vast periods of time, and that at its 
greatest eccentricity—one of which periods happened about 
200,000 years ago—the earth in aphelion was nearly 
98,500,000 miles distant, whilst now when in aphelion it is 
about 90,000,000 miles from the sun. 
One result of the eccentricity of the orbit, combined with 
the obliquity of the ecliptic, or the angle that the axis of the 
earth makes with the plane of its orbit, is, that at present 
the sun is north of the equator about 7+ days longer than 
it is south of it. But as at the time the sun ts south of the 
equator the earth is nearest the source of heat, the southern 
hemisphere receives just as much heat in its shorter summer 
solstice as the northern hemisphere does in its longer one. 
Astronomers have calculated the effect of a much greater 
eccentricity of the orbit, and have unanimously come to the 
conclusion that the absolute amount of heat received by the 
two hemispheres would be the same, however great that 
eccentricity might be. But as the total amount of heat re- 
ceived from the sun is inversely proportional to the shortest 
diameter of its orbit, it follows that during the periods of 
greatest eccentricity the absolute amount of heat received 
by the earth, and distributed equally to the two hemispheres, 
would be slightly in excess of that received when the eccen- 
tricity was much less. 
The general conclusion arrived at by astronomers before 
Mr. Croll examined the problem—including the eminent 
names of Humboldt, Arago, and Poisson—was that the 
climate of our globe could not be affected by any possible 
change in the ellipticity of its orbit. In this opinion 
Herschel—who at one time thought that great changes of 
climate might be so produced—appears afterwards to have 
coincided. Mr. Croll, however, states that in arriving at 
this conclusion a most important element of the enquiry 
had been omitted. Fully admitting that the absolute 
amount of heat received in the two hemispheres would be 
the same, however great the ellipticity might be, he yet 
urges that in that hemisphere in which the nights were 
longest there would be most heat lost by radiation, and 
