Date of the Glacial and the Upper Miocene Period. 147 



longitude of the perihelion. In Table IV. the excentricity and 

 the longitude of the perihelion of the six periods marked with 

 an S are copied from a letter of Mr. Stone to Sir Charles Lyell, 

 published in the Supplement of the Phil. Mag. for June 

 1865 ; the eight periods marked L are copied from M. Lever- 

 rier's Memoir in the Connaissance des Temps for 1843. For 

 the correctness of everything else, both in this Table and in the 

 other three, I alone am responsible. 



Column IV. gives the number of degrees passed over by the 

 perihelion during each 10,000 years. From this column it will 

 be seen how irregular is the motion of the perihelion. At four 

 different periods it had a retrograde motion for 20,000 years. 

 Column V. shows the number of days by which the winter ex- 

 ceeds the summer when the winter occurs in aphelion. Column 

 VI. shows the intensity of the sun's heat during midwinter, 

 when the winter occurs in aphelion, the present midwinter 

 intensity being taken at 1000. These six columns comprehend 

 all the astronomical part of the Tables. Regarding the correct- 

 ness of the principles upon which these columns are constructed, 

 there is no diversity of opinion. But these columns afford no 

 direct information as to the character of the climate, or how 

 much the temperature is increased or diminished. To find this 

 we pass on to columns VII., VIII. and IX., calculated on 

 physical principles. Now, unless the physical principles upon 

 which these three columns are calculated be wholly erroneous, 

 undoubtedly change of excentricity must very seriously affect 

 climate. Column VII. shows how many degrees Fahrenheit the 

 temperature is lowered by a decrease in the intensity of the 

 sun's heat corresponding to column VI. For example, 850,000 

 years ago, if the winters occurred then in aphelion, the direct 

 heat of the sun during midwinter would be only J* *J of what 

 it is at present at the same season of the year. Column VII. 

 shows that this decrease in the intensity of the sun's heat would 

 lower the temperature 45 0, 3 F. 



The principle upon which this result is arrived at is this : — - 

 The temperature of space, as determined by Sir John Herschel, 

 is -239° F. M. Pouillet, by a different method, arrived at 

 almost the same result. Taking the midwinter temperature of 

 Great Britain at 39°, consequently, 239° + 39° = 278° represent 

 the number of degrees of rise due to the sun's heat at mid- 

 winter ; in other words, it takes a quantity of sun-heat which 

 we have represented by 1000 to maintain the temperature of 

 the earth's surface at Great Britain 278° above the temperature 

 of space. Were the sun extinguished, the temperature of our 

 island would sink 278° below its present midwinter temperature, 

 or to the temperature of space. But 850,000 years ago, if the 



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