654 LOWELL—MARS ON GLACIAL EPOCHS. [Nov. 16, 
180° journey out will see as many calories reach it as the subse- 
quent journey of 180° in. For 
v2 dd = hd. 
whence 
ad0=hdadt 
r 
That is the angle swept over is at all points proportional to the 
amount of heat received, since this amount is always inversely as 
the square of the radius vector. 
16. Thus the heat received through any angle is independent of 
the eccentricity. But it is not independent of the axial tilt. The 
amount of heat received at any point, in consequence of the tilt, 
depends upon the position of the point. At the pole it varies from 
nothing for the six months about the winter solstice to | 
Lae real a \ 
a a 
for the other six about the summer one. 
For a hemisphere, taken as a whole, the total summer insolation 
much exceeds the winter one. (Weiner, Ueber die Starke der 
Bestrahlung, Zeztschrift der Oesterreichischein Gesellschaft fiir 
Meteorologie, 1879; also Sir Robert Ball, Zhe Cause of an Ice 
Age, 1892). 
Let 2H be the amount of heat falling on a section equal to the 
earth at unit distance in unit time, and let 6 be the declination of 
the sun. 
Theh the amount received by one hemisphere at distance 7 in 
the time d will be 
# (sin d) at 
5 
and by the other 
— (1 — sino ) dt 
x? 
but 
727 d0= hat 
whence the above 
sae + sind ) at 
Bac -+ sin J) @6 
