25 



and winter will be the same length. But consider their positions. 

 When the line joining them is at right angles to the major axis ; 

 we have then the extreme case, the greatest inequality between the 

 portions of the orbit, the greatest difference between summer and 

 winter, which as I said, may (but not necessarily always be 33 days. 

 There might then be a summer of 199 and a winter of 1H6 or a winter 

 of 199, and a summer of 166. Take the latter case, and you have 

 again the conditions necessary for an Ice Age. 



Now, how does the intensifying condition put forth by Sir R. 

 Ball affect this ? — viz., the unequal distribution of yearly sun 

 heat in each hemisphere, 63 degrees in summer, and 37 degrees in 

 winter, which never varies under any conditions. A fair distribution 

 (if we may say so), would be that the loug winter should get the 63 

 and the short summer the 37. But this never happens. In the 

 short summer of 166 days we should receive the usual 63 degrees, 

 and the 37 degrees would be spread over the long winter of 199 

 days. This does happen at r>^curring periods, and evidently assists 

 in producing an Age of Ice. 



You notice that during the period of long cold winters, our sum- 

 mers would be intensely hot. Heat is a3 necessary as cold to 

 produce a Glacial Age ; since heat is the agent by which rain and 

 snow are formed, out of which glaciers are produced ; and the long 

 winter would produce ice in such quantities that the short summer 

 would melt a comparatively small portion of it. 



To sum up then what has been brought forward : We may expect 

 a Glacial Epoch when these two conditions occur at the same time. 

 a When the ellipticity is greatest and the difference in the sun's 

 distance in summer and winter is greatest, h When winter occurs 

 in aphelion and is therefore much longer than the summer. Along 

 with these we must place the constant fact of the northern (or 

 southern) hemisphere, receiving the largest amount of heat in the 

 short summer, and the smaller quantity in the long winter. 



As regards the date of the last Ice Age, if we accept the Astro- 

 nomical Explanation, the above conditions were fulfilled during a 

 period of 160,000 years, from 240,000 years ago to about 80,000 

 years ago. But the ice and its effects would last lo ig after that 

 before they disappeared, or rather before the ice retreated to its 

 present position round the North Pole. 



The Lecture was well illustrated by Lantern Slides. 



