in Geological Climatology . 251 
before 1850, and to 1,260,000 years after that date ; or, in 
other words, over a period of no fewer than 4,520,000 years*, 
thus showing that Professor XewconuVs objection falls to the 
gronnd. 
Influence of Winter in Aphelion. — I have maintained that 
at a time when the eccentricity is high and the winter occurs 
in aphelion, the great increase in the sun J s distance and in the 
length of the winter would have the effect of causing a large 
increase in the quantity of snow falling during that season. 
This very obvious result follows as a necessary consequence 
from the fact that the moisture which now falls in the form 
of rain would then fall as snow. But Professor Xewcomb 
actually states that he cannot accept the conclusion that this 
would lead to more snow. 
Influence of a Snow-covered Surface. — I have argued that 
this accumulation of snow would lower the summer tempera- 
ture, and tend to prevent the disappearance of the snow, and 
have assigned three reasons for this conclusion : — 
First. — Direct radiation. The snow, for physical reasons 
well known, will cool the air more rapidly than the sun's 
rays will heat it. This is shown from the fact that in Green- 
land, a snow- and ice-covered country, a thermometer exposed 
to the direct radiation of the sun has been observed to stand 
above 100°, while the air surrounding the instrument was 
actually 12° below the freezing-point. Professor JSTewcomb 
and also Mr. Hill f regard the idea that this could in any way 
favour the accumulation of snow as absurd. They think that 
in fact it would have directly the opposite effect. They have 
perceived only one half of the result. It is quite true, as they 
affirm, that the cooling of the air by the snow will not prevent 
the melting of the snow, bat the reverse. There is, however, 
another and far more important result overlooked in their 
objection. If the snow- and ice-covered surface keeps the 
temperature of the air, in summer, below the freezing-point, 
which it evidently does in Greenland and in the Antarctic 
continent, the moisture of the air will fall as snow and not as 
rain. ISo doubt this is the chief reason why in those regions, 
even in the middle of summer, rain seldom falls, the precipita- 
tion being almost always in the form of snow, although at that 
very season the direct heat of the sun is often as great as in 
India. Were the snow and icy mantle removed a snow-shower 
* In this laborious undertaking Professor McFarland computed, by 
means of botb formula?, tbe eccentricity of tbe earth's orbit and the 
longitude of the perihelion for no fewer than 485 separate epochs. See 
American Journal of Science, vol. xx. p. 10-5 (1880). 
t ' Geological Magazine ' for January 1880, p. 12. 
