1884. 91 Trans. A\ V. Ac. Set. 



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 now is. 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 for a 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 4^ 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 Archaean 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 



