BY A. B. BIGGS. 25 



the summer of one hemisphere and winter of the other alter- 

 nately, and he can only be referring to that portion of this 

 " supply " which pertains to the summer or winter of either 

 hemisphere. That he could have meant nothino^ else is still 

 more evident from a further quotation (page 230), in which 

 he admits the unequal distribution of heat between summer 

 and winter : — " Whenever, then, the sun remains more than 

 12 hours above the horizon of any place, and less beneath, 

 the general temperature of that place will be above the 

 average ; when the reverse, below ;" that is, the summer and 

 winter portions of the year respectively. 



The extent of the inequality of the distribution of the 

 annual supply of heat between the summer and winter por- 

 tions of the year in either hemisphere depends (as I have 

 said) wholly and solely upon the inclination of the eartVs 

 axis, and no one could be more cognisant of this fact than 

 Sir John Herschel. Assuming^ extreme conditions, the incli- 

 nation of the axis, instead of being some 66^ deg., might 

 have been zero, that is, coincident with the plane of the 

 -ecliptic, in which case the distribution of summer and winter 

 heat (the yearly total), in either hemisphere (employing Sir 

 Eobert Ball's formula), would be as 818 to 182. On the 

 other hand, the inclination might have been 90 deg., that is 

 vertical, when there would have been no inequality, and the 

 distinction of summer and winter would vanish. The incli- 

 nation being actually between these two extremes (that is, 

 66h deg.), the relative summer and winter heat supplies must 

 lie between the aforesaid ratios, and is no doubt correctly 

 stated by Sir Eobert Ball as 63 to 37. 



Of the geological aspect of the question I do not deem 

 myself competent to speak ; but, admitting the deductions of 

 geologists as to the alternations of extreme heat and cold in 

 the geological history of the earth, and the correctness of Sir 

 Eobert Ball's dictum with regard to the inefficiency of the 

 extreme summer hear to dissipate the glacial effects of the 

 preceding winter's cold (No. 9), the theory discussed by Sir 

 Eobert Ball is, I think, the only one yet propounded that will 

 bear investigation. 



