On Rossetti’s Law of Cooling. 423 
Substituting in equation (9), we find— 
H= 41:90 feet of ice, . . Present. 
H= 61:26 ~ - Miocene, 
H= 75°36 * : . Jurassic. 
H=112:56 ” : - Albumen coagulation. 
or as follows :— 
Comparative Table of Sun-heats at 80° N. lat. at various 
Geological Periods. 
(1.) Present time, . . - 100:00 
(2.) Miocene time, . - . 146:20 
(3.) Jurassic time, . 179°85 
(4.) Time of coagulation of aleurich: 268-60 
When we consider that the whole of geological time is as in- 
significant in comparison with astronomical time, as the human 
period is in comparison with geological time ; and that in astro- 
nomical time the sun-heat has been reduced to many thousandths 
of its original value; it will not appear a great effort of the 
imagination to explain the phenomena of geological climates by 
the opens of a sun which has cooled down, during geological 
time, to about one-third of what its value was se, life nese 
to appear. 
We may approximate to the relative durations of Geological 
periods, by calculating the times of cooling of the sun, to the 
amounts represented in the foregoing table. Properly speaking 
this should be done by a formula similar to (9) in which a@ would 
denote the “heat-inclosure” of the sun, and aa coefficient depend- 
ing on the properties of the sun’s surface, both of which quantities 
are completely unknown. 
We may, however, obtain an approximate result by calculat- 
ing the times of cooling of the sun, on the supposition that it 
radiates directly into space, neglecting the influence of the solar 
atmosphere. 
This supposition reduces equation (8) to the following— 
os =a (6+ A) (10) 
* We use the negative sign, because as ¢ increases 9 diminishes, 
