Date of the Glacial and the Upper Miocene Period. 151 



winters occurred in aphelion, the heat of the sun at midwinter 

 would only equal 837 instead of 1000 as at present. Consequently, 

 if it takes 1000 parts of heat to maintain the temperature 278° 

 above the temperature of space, 837 parts of heat will only be 

 able to maintain the temperature 232°* 7 above the temperature 

 of space ; for 232°'7 is to 278 as 837 is to 1000. Therefore, 

 if the temperature was then only 232°' 7 above that of space, it 

 would be 45°*3 below what it is at present. 



This method of calculating how much the temperature is 

 lowered by a given reduction of the sun's heat is that given by 

 Sir John Herschel in his ( Outlines of Astronomy/ § 369 a. 

 About three years ago, in an article in f The Reader/ I endea- 

 voured to show that this method is not rigidly correct. It 

 results from the principles of the dynamical theory of heat, 

 and is also supported by experiments made by Professor Draper, 

 of New York, and others, that the rate at which a body radiates 

 its heat off into space is not directly proportionate to its abso- 

 lute temperature. The rate at which a body loses its heat as 

 its temperature rises, increases more rapidly than the tempera- 

 ture. As a body rises in temperature, the rate at which it 

 radiates off its heat increases ; but the rate of this increase is not 

 uniform, but increases with the temperature ; consequently the 

 temperature is not lowered in proportion to the decrease of the 

 sun's heat. But this error is probably neutralized by one of an 

 opposite nature, to which I shall now refer. 



We know that absolute zero is at least 493° below the melt- 

 ing-point of ice ; consequently, if the heat derived from the 

 stars is able to maintain a temperature of —239° or 222° of 

 absolute temperature, then nearly as much heat is derived from 

 the stars as from the sun. But if so, why do the stars give so 

 much heat and so very little light ? If the radiation from the 

 stars could maintain a thermometer 222° above absolute zero, 

 then space must be far more transparent to heat-rays than to 

 light-rays, or else the stars give out a great amount of heat but 

 very little light, neither of which suppositions is probably true. 

 The probability is, I venture to presume, that the temperature 

 of space is not much above absolute zero. In this case, by 

 adopting — 239° as the temperature of space, we make the 

 values given in column VII. too small. But as the two errors 

 tend to neutralize each other, these values may in the meantime 

 be accepted as not very far from the truth, or at least as near 

 as can be arrived at in the present state of science on this point. 

 But whether these values be too high or too low, one thing is 

 certain, that a very slight increase or a very slight decrease in the 

 quantity of heat received from the sun must affect temperature to 



