C roll's Theory Redivivus. 177 
As to the cause of these differences between the south 
polar cap and its counterpart on the north pole Mr. LoweU 
shows that, under the law of the radius vector, to which both 
gravitation and light, or heat, are amenable, eccentricity can 
have no effect; nor axial tilt, since that brings the two hemis- 
pheres in turn under the same though varying conditions — as 
variant for one as for the other. 
"Xot the amount of heat but the manner of its reception, 
then, is responsible for the differences we observe," between 
the maxima and minima of the Martian snow^ caps. The differ- 
ence between the maxima is rationally attributed to the sur- 
passing length of the Antarctic winter, which is 75 of our days 
longer than the northern. The total heat received from the sun 
by the two hemispheres is the same, but it is intensified in a 
short season in the northern. The contrast between the minima 
of these snow caps is not so easily explained. The southern 
snow cap is more reduced in its short summer than the north- 
ern in its longer summer. Mr. Lowell mentions two factors that 
spring from the planet itself which may account for this (a) 
The more intense diurnal heat of the southern summer pro- 
vokes greater volumes of water during its prevalence, and this 
adds to the readiness with which the snow cap as a whole could 
shrink. (b)The cloudiness at night (for every day is perfectly 
clear) is increased by greater daily dissolution of the cap, and 
hence the proportion of watery vapor is increased in the south- 
ern latitudes of Mars during the summer months. This in- 
creased blanket conserves the heat of the day before so that the 
following day, and every following day, is reinforced by the 
greater proportion of conserved heat. The two conspire to 
waste the Antarctic snows more rapidly than the northern. It 
appears then that on Mars eccentricity has no tendency to form 
a northern ice-cap, or about the pole of that hemisphere that 
has its summer solstice near perihelion, but that the permanent 
accumulation there is actually less tlian at the south pole. 
In transferring this argument to the earth we are confront- 
ed at once with a differing total amount of moisture and the 
presence of oceans. This involves greater precipitation, and 
the formation of a larger amount of winter ice. Mr. Lowel! 
shows that under increasing precipitation the Antarctic minima 
increase, relatively to the Arctic, faster than the Arctic, an.' 
