278 



COMPARATIVE HEAT OF THE SUN 



[Ch. XIII 



nearest approach, to the sun now occurs eleven days after the 

 shortest day. M. Venetz had previously pointed out as an 

 historical fact, that the Swiss glaciers were greater than they 

 are now before the 10th century, and that then, after retreating 

 for four centuries, they advanced again and have been slowly 



former 



In other words, at that 

 period when the sun was nearest the earth in midwinter or 

 for two centuries before and two after 1248, there was the 



& 



melting of ice in the northern hemispl 



astronomical 



It may 



more 



Fahrenheit between the cold of the present winter and that 

 of 1248, would be appreciable in the course of six hundred 

 years ; but the observation may help the reader to understand 

 in what direction the precession of the equinoxes, if capable 

 of producing a sensible change, would now be affecting 

 climate. 



Measurement of 



i 



But it will be 



asked, how does the physicist arrive at the determinate quan- 

 tity, 23° Fahr., by which, as above asserted on theoretic 



m 



one another ? For though it is known that the sun's radiation 

 varies as the inverse square of the distances, this law would 

 not conduct us to the absolute quantity, unless we also knew 

 from what starting-point the heat is to be measured. 



The degrees of our thermometers are arbitrary quantities, 



measured 



many 



from arbitrary points. The question is, how 

 egrees indicated by one of these scales (say 

 Fahrenheit's) are due to the action of the sun ; that is, what 

 would that scale indicate, supposing there were no sun — in 



from 



tempe 



ature of space due to radiation 

 m sources ? Upon this point, 

 namely, what would be the temperature of space if the sun 

 did not exist, though there is not an absolute agreement 

 among philosophers, yet the amounts arrived at by totally 

 different methods are so nearly the same, as to inspire con- 

 siderable confidence in their accuracy, or to lead us to regard 

 them as making at least some approximation to the truth. 

 From the relation of the temperature to the pressure 



• 



