426 Scientific Proceedings, Royal Dublin Society. 
the entire duration of geological time, the following equation ; 
Mean radiation of heat at 80° N. lat. in excess of present radia- 
tion— 
_ 19°36 x 417 26°41 x 21-1 a 52°06 x 37:2 
200 200 200 
= 28:°988 feet of ice. 
From this we calculate the entire duration of geological time, 
by means of equation (12) to be §48,540 years ; or somewhat over 
half a million of years. But, in reality, the duration of geological 
time, could not have been nearly so great as’ this, if the earth 
were the sole source of heat, for in the foregoing we have supposed 
the heat given freely to the surface from the interior, as if the 
conductivity of the earth were infinite. In reality the heat would 
be transmitted slowly to the surface, which latter would cool 
rapidly, making geological time very short, although a large store 
of heat might remain in the central parts of the earth, though not 
available to mitigate sensibly the rigour of the surface climate. 
It is highly probable that the earth cooled down to a condi- 
tion in which the central heat had but little effect upon climate, 
long before the commencement of geological time; so that cli- 
mates always depended, chiefly, on sun-heat, modified by at- 
mospheric conditions. 
Let us now consider the influence of the latter— 
(C.) Atmospheric conditions considered as the sole cause of 
Geological Climates, the Sun-heat and Earth-heat being the 
same as wt present. 
In the equation (9), or 
=a(0--a) H (0+ A)? (9) 
a is a coefficient independent of temperature, depending on the 
surface conditions of the place of observation. 
a, or the control temperature of the inclosure, depends on the 
atmosphere and on star-heat ; or on atmospheric conditions only, 
if star-heat remained as now through all geological time. 
HT depends of sun-heat and earth-heat only. 
