124 



ISLAND LIFE. 



[rART I. 



eight and a half millions of miles between our distance from the 

 sun in aphelion and perehelion (as the most distant and nearest 

 points of the earth's orbit are termed). At a hundred and fifty 

 thousand years back it had decreased somewhat — to six millions 

 of miles; but then it increased again, till at two hundred 

 thousand years ago it was ten and a quarter, and at two hundred 

 and ten thousand years ten and a half millions of miles. By 

 reference to the accompanying diagram, which includes the 

 last great period of excentricity, we find, that for the immense 

 period of a hundred and sixty thousand years (commencing 



jCPROBABLE DURATION OF THE GLACIAL CPOCH>, 



THOUSAND YEARS ACQ FROM 

 A.D.I800. 



DIAGRAM OF EXCENTRICITY AND PRECESSION. 



The dark and light bands mark the phases of precession, the dark showing short mild winters, 

 and the light long cold winters, the contrast being greater as the excentricity is higher. 

 The horizontal dotted line marks the present excentricity. The figures show the maxima 

 and minima of excentricity during the last 300,000 years from Dr. Croll's Tables. 



about eighty thousand years ago) the excentricity was very 

 great, reaching a maximum of three and a half times its 

 present amount at almost the remotest part of this period, at 

 which time the length of summer in one hemisphere and of 

 winter in the other would be nearly twenty-eight days in excess. 

 Now, during all this time, our position would change, as above 

 described (and as indicated on the diagram), every ten thousand 

 five hundred years ; so that we should have alternate periods 

 of very long and cold winters with short hot summers, and short 

 mild winters with long cool summers. In order to understand 

 the important effects which this would produce we must ascertain 



