314 DISCOVERY CH. 



from time to time to heights of tens or hundreds of 

 thousands of miles. The manifestation of such tremen- 

 dous energy would produce fear rather than the calm 

 assurance which at present exists as to the constancy of 

 the Sun's condition. 



Because the total amount of heat received annually 

 from the Sun has not varied appreciably for the past 

 two thousand vears or so, we are accustomed to assume 



p 



that this uniformity will be maintained. But as almost 

 nothing is definitely known as to the source of the Sun's 

 heat, this belief in a continuity of the present conditions 

 is purely a matter of faith that the future will be as the 

 past. Atomic, or other changes may occur at any time 

 which would raise or lower the Sun's temperature 

 sufficiently to have a profound influence upon the 

 Earth. Any large variation of this kind would com- 

 pletely change the face of the world ; and a decrease 

 of ten per cent, in the amount of solar heat received 

 would convert the temperate regions into frozen wastes. 



Observations made in recent years have shown that 

 at times the Sun does radiate more heat than at others, 

 the amount fluctuating by about ten per cent, during 

 irregular periods of from five to ten days' duration. 

 These changes are possibly due to local differences of 

 temperature on the Sun, but they may become general 

 and progressive in one direction or another. In fact, 

 though theoretical considerations suggest that the Sun's 

 heat will remain practically constant for millions of 

 years, no one can state decisively that it will do so for a 

 hundred years. 



Astronomy thus gives no support to the mind that 

 uses the knowledge existing at any epoch as if it were a 

 complete inventory of the universe. We can reason 

 about things known to exist, but we must remember 



