Conditions favorable to Glaeiation. 



9j 



each great season, the winter would always be the long season, 

 and therefore winter would occur in both hemispheres at the 

 same time ! It appears from the paragraph following that in 

 which the mistake occurs, that Herschell was thinking partic- 

 ularly of tropical climates. Now at the equator it is literally 

 true that the heat received in summer and in winter is the 

 same, as will be shown in the second part of this paper, 

 and the application which Herschell makes of his equal 

 division is to explain the alleged great intensity of summer 

 heat in the tropical regions of Australia as compared with those 

 of northern Africa. 



"While at or close to the equator the heat received in each 

 season is the same, at the poles all of the heat is received in 

 summer. Hence while it is true that with the present obli- 

 quity only about three-eighths of the entire heat received by a 

 hemisphere is received in winter, this fact helps but little 

 towards an explanation of climate. The distribution of heat 

 between the seasons varies with the latitude, and to form any 

 just idea of the effect of eccentricity on climatic conditions one 

 must know the heat received per unit area in any latitude. 



The method of finding the amount of sunshine per unit area 

 between equinoxes is very simple in principle. The great cir- 

 cle bounding the illuminated half of the earth is called the 

 circle of illumination, and is represented by the right hand 

 circle in figure 1. Any parallel of latitude projected onto the 



Figure 1. — Circle of Illumination. 



circle of illumination becomes an ellipse, e. g., the ellipse r, s, t. 

 The heat received between this parallel and the pole while the 

 sun remains in the same position will be proportional to the 

 crescentic area marked W. If one supposes a second parallel 

 very close to the first, say at the unit distance from it, the 

 narrow interval between their projections will be proportional 

 to the heat received upou a zone of the earth's surface. These 

 conditions apply only to an instant of time because the declina- 



