1900.] LOWELL—MARS ON GLACIAL EPOCHS. 655 
again, since sin 6 sin @ sin ¢, where ¢ = obliquity of the ecliptic 
the above becomes from equinox to equinox 
A (1 -+sinesin@) d0= A(x + 2 sine) 
h h 
Oo 
and for the other hemisphere 
Hf c 
= —2Ss 
ae ine ) 
With «= 24° 52’, the present received value on Mars, we have 
the two in the proportion of 63 to 37. 
But though the summer and winter insolation thus differ, they 
are the same for each hemisphere in turn. Consequently the 
above cannot be the cause of the differences in question between 
the respective maxima and minima of the polar caps. 
17. Not the amount of heat but the manner of its reception, 
then, is responsible for the difference we observe. Looking at it 
in this light we shall detect certain diversities of position compe- 
tent possibly to the result. 
Of the opposite variations presented to us by the two caps, the 
one most difficult to detect is the easiest toexplain. The difference 
in the maxima seems to be due to the surpassing length of the 
antartic night. 
Owing to the eccentricity of the orbital ellipse and the position 
of the solstices, the southern hemisphere is both farther away from 
the sun during its winter and is so for alonger time. The arctic 
polar night is 306 of our days long; the antarctic 381. Thus for 
seventy-five more days than happens to its fellow, the southern pole 
never sees the sun. Now since the total sunlight from equinox to 
equinox is the same in both hemispheres, its distribution by days is 
different. In the northern hemisphere the same amount is crowded 
into a smaller compass in the proportion of 381 to 306; that being 
that hemisphere’s relative ratio of days. But since, during the 
winter, there is a balance of accumulation over dissipation of snow, 
each twenty-four hours must on the average add its tithe to the sum 
total. The northern days, being the warmer, each add less than 
the southern ones; and furthermore there are fewer of them, On 
both these scores the integral of the additions about the northern 
pole is less than about the southern one. Consequently the snow 
sheet is there the less developed. 
