1884. 91 Trans. IV. Y. Ac, Sct. 
Mr. MEECH has shown that with a perpendicular axis polar regions 
would receive during the year less heat than they do at present; hence 
Dr. CROLL infers that with such an axis polar climate would be less 
genial than it nowis. This would be true if temperature depended 
solely on the amount of heat received. But, as everybody knows, it 
depends far more upon the amount of heat retained. Greenhouses 
are often uncomfortably warm when the temperature without is near 
freezing. The solar rays readily enter through the glass, and are ab- 
sorbed by the floors, walls, etc., while the heat which they radiate 
back is unable to escape. Professor TYNDALL has shown that many 
substances possess this property, and among them are aqueous vapor 
and carbonic acid gas. 
The amount of carbonic acid in the atmosphere must have been far 
greater in ancient times than now, for all the graphite, coal, lignite, 
etc., now in the earth’s crust once existed in that form. With the 
beginning of plant life a process of elimination commenced. It con- 
tinued till near the end of the Tertiary, when the amount taken out 
by living forms and that restored by their decay became equal—a con- 
dition which still exists. 
The carbonic acid was then, as now, uniformly diffused, and it 
acted as glass does in a greenhouse. It kept the heat in, and, conse- 
quently, the atmosphere itself grew warmer. This increased its capa- 
city for moisture, and that, in its turn, helped to retain the heat. 
In this, I think, lies the cause of the warm polar climate, those 
otherwise cold regions being protected by this warm ‘‘ double blanket.” 
Prof. TYNDALL says: ‘‘ The removal fora single summer night of the 
aqueous vapor which covers England would be attended by the de- 
struction of every plant which a freezing temperature could kill.” 
Besides the carbonic acid and water vapor, there were probably 
other gases and vapors in the atmosphere. Ammonia produces 13 
times the effect of CO’, and marsh gas 44 times. Whatever there 
were of these, they tended to increase the potency of that ‘* warm 
blanket.” 
The amazingly slow change of temperature—many millions of years 
to reduce it from that of the Archean to that of the later Tertiary— 
finds reasonable explanation in the effect of these gases and vapors. 
Prof. TYNDALL has shown that, commencing with a vacuum, and add- 
ing a small number of very small increments, the absorption is sensi- 
bly proportional to the increments, but as the quantity increases, the 
deviation from proportionality augments (Heat a Mode of Motion, 
page 356); at length a condition is reached in which further incre- 
ments produce very little effect. 
The converse of this is very important. Commencing with a large 
