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 166 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 87. But this never happens. In the 
short summer of 166 days we should receive the usual 63 degrees, 
and the 87 degrees would be spread over the long winter of 199 
days. This does happen at recurring 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 as 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. 6 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 log 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. 
