﻿Astronomical Theory of Ice Ages and Genial Ages. 543 



sun-heat on terrestrial temperatures in the latitudes with which 

 glaciation is concerned shall be great compared with that of 

 the heat transferred from latitude to latitude by air and ocean 

 currents. That this is not the case in regard to the changes 

 of temperature from summer to winter, we have a ready means 

 of showing ; indeed it appears that the direct effect of winter 

 sun-heat in northern latitudes is but a trifling factor in 

 keeping the temperature above the "natural zero" when 

 compared with the heat transferred from place to place. For 

 consider the equator : it receives nearly the same daily sun- 

 heat during winter and summer, and its temperature stands 

 continuously about 380° F. above Ball's u natural zero." Hence, 

 if we calculate and then plot on a diagram the average daily 

 summer and winter sun-heats for each latitude, on such a 

 scale that the equatorial sun-heat is represented by 380, then 

 the ordinates giving the summer and winter sun-heats for 

 the different latitudes should also, if Herschel's principle 

 were applicable, give the summer and winter excesses of their 

 temperatures above the natural zero. In fig. 1 this has been 

 done for latitudes 40° to 90°, the only ones we are concerned 

 with in dealing with the Glacial Period. I have also shown 

 by the dotted lines the mean of the i Challenger's ' January 

 and July temperatures for the meridians 90° E. and 90° W. 

 passing through the middle of the great Asiatic and American 

 continents up to latitude 70°, beyond which reliable informa- 

 tion cannot be obtained. The scale of sun-heat is shown at 

 the left side, and the scale of Fahrenheit temperatures at the 

 right of the centre line. By examining the figure we see 

 that even in the Continental areas, where the most extreme 

 variations are observed, the midwinter temperatures bear 

 no relation whatever to the very small amount of winter 

 sun-heat; so that we infer that the transference of heat 

 from one latitude to another is the chief factor in winter 

 temperatures. 



If we were to follow Croll's method, and take the mid- 

 summer and midwinter sun-heats instead of the daily average, 

 the discrepancy shown in the figure between actual tempera- 

 tures and sun-heats would be enormously increased. 



To complete the criticism, let us examine the most extreme 

 of all climates, that of Yakutsh in Siberia. Here the mid- 

 summer temperature rises to about 370° over the " natural 

 zero" and the midwinter sinks to 260°. But the daily 

 average sun-heat in summer is to the daily average in winter 

 as 1300 is to 260, or as 370 is to 74. Hence if the mid- 

 winter temperature be right, the midsummer ought to be 

 1000° F., or if the midsummer be right, the midwinter ought 



