142 LIGHT SCIENCE FOR LEISURE HOURS. 



the whole earth, insomuch that the belief seems forced 

 upon us that the cause, whatever it may be, is to 

 be sought outside the earth. And just now, when 

 this explanation of a meteorological phenomenon is 

 suggested, astronomical evidence comes in which seems 

 to show how the lowering of the earth's temperature 

 may be explained that Adolph Erman, in point of 

 fact, though wrong as regards nearly all the details of 

 his theory, was not so far wrong as regards the general 

 theory as had been supposed by some of his critics. 



Let us, in the first place, consider the evidence as 

 to the existence of these cold spells. 



Theoretically, there should be each year a gradual 

 rise of temperature from about the middle of January 

 to about the middle of July. At a first view it might 

 seem that the winter and summer solstice should be 

 the time of least and greatest heat ; but as a matter 

 of observation the greatest heat is never attained in 

 the case of any body or collection of bodies exposed to 

 the action of a source of varying heat, at the time when 

 the heat emitted from that source has its maximum 

 amount, nor, vice versa, the greatest cold when the 

 emitted heat is least, but always somewhat later. Each 

 day, for instance (in steady weather), the greatest 

 warmth is attained at about two in the afternoon, while 

 two in the morning is the time of greatest cold, on the 

 average, throughout the diurnal hours. 



Now we might fairly expect that if the average 

 daily temperature for a great number of years at any 

 given place were observed, the variation of temperature 



