BY A. B. BIGGS. 23 



It must be evident that, at enormously long intervals (as 

 we judge of time), the combined effects of the conditions 

 above enumerated must be at a maximum as regards the 

 relative intensity (as distinguished from total amount) of 

 summer and winter heat. This will be when the eccentricity 

 of the orbit is at its maximum, and the relative movements 

 indicated in Nos. 6 and 7 have brought the line of the 

 equinoxes at right angles with the major axis, thus cutting the 

 orbit into its shortest possible perihelion, and longest 

 aphelion divisions. That hemisphere, then, which has the 

 perihelion summer will have its summer portion of heat 

 (see JN'o. 3) concentrated into a short and intensely hot 

 summer; whilst its winter portion will be spread over a 

 long, cold winter. At the same time the reverse of this will 

 prevail in the other hemisphere. These conditions will 

 alternate between the two hemispheres in the mid interval of 

 10,500 years. 



I must now add, as No. 9, a further condition as laid down 

 by Sir Eobert Ball, which is, that " the sunbeams in the 

 brief and fiercely hot summer of the glacial period fail to 

 melt as much ice as had heen accumulated during the preceding 

 winter'' If this statement is correct, it must be admitted, I 

 think, that the theory is established beyond question. If, on 

 the other hand, it be not correct, and that the intensity of 

 summer heat compensates the prolonged coldness of the winter 

 (and this is the only point that is not quite clear to my mind), 

 the theory breaks down utterly, even though all the other 

 conditions remain intact. 



The point on which Sir John Herschel is challenged by 

 Sir Robert Ball is contained in the concluding sentence of 

 condition 8. Sir Eobert Ball puts the case as against 

 Herschel thus (I cannot well shorten the quotation) : — 



" Suppose that the total heat received from the sun on one 

 hemisphere of the earth during the course of a twelvemonth 

 be represented by 100, we proceed to consider how these 

 parts are shared between the seasons. I mean by ' summer * 

 in the Northern Hemisphere the interval from the vernal 

 equinox to the autumnal, and by ' winter ' the interval from 

 the autumnal equinox to the vernal. With this understanding 

 63 paits of heat are received on each hemisphere during its 

 summer, and the remaining 37 parts during the winter. 

 . . . Herschel' s erroneous statement was to the effect that 

 theheat was equally distributed^ so that 50 parts were received 

 in summer and 50 'parts in winter^ (All italics are mine 

 unless otherwise indicated.) 



Now, the question is : — Is this just what Sir John Herschel 

 said or meant to say ? The passage on which Sir Robert 

 Ball founds his charge can be only that on page 333 of the 



