324 THE CANADIAN NATURALIST. [Oct. 



system, excepting the diameter of the earth and the distance and 

 diameter of the moon, may be corrected (e). 



The light and heat which are emitted from the sun reach the 

 earth without great diminution by the absorptive action of the at- 

 mosphere ; but the waste of heat from the surface of our planet 

 through radiation into space is prevented, or rather lessened, by 

 this same atmosphere. Many transparent bodies admit freely heat- 

 rays derived from a source of high temperature, but stop the rays 

 which emanate from bodies only slightly warmed. The atmosphere 

 possesses this quality in a remarkable degree, and owes it to the 

 presence of diffused water and vapor ; a fact whish Dr. Tyndall 

 has placed in the clear light of complete and varied experiment (/). 

 The application of this truth to the history of the earth and 

 of the other planets is obvious. The vaporous atmosphere 

 acts like warm clothing to the earth. By an augmented quantity 

 of vapor dissolved, and water suspended in the air, the waste of 

 surface-heat of the earth would be more impeded ; the soil, the 

 water, and the lower parts of the atmosphere would grow warmer ; 

 the climates would be more equalized ; the general conditions 

 more like what has been supposed to be the state of land, sea, and 

 air during the geological period of the Coal-measures. 



Such an augmentation of the watery constituents in the at- 

 mosphere would be a natural consequence of that greater flow of 

 heat from the interior, which, by many geologists, mathematicians, 

 and chemists, is supposed to have happened in the earlier periods 

 of the history of the earth. 



By the same considerations we may understand how the planet 

 Mars, which receives not half so much heat from the sun (g) as 



(e) Estimates of the earth's distance from the sun have varied much. 

 Cassini and Flamsteed, using observations of the parallax of Mars ; 

 ascribe to it ten or eleven thousand diameters of the earth=79 or 89 

 millions of miles. Huyghens estimated it at twelve thousand=95 millions 

 of miles. In 1745, Buffon reported it as the common opinion of astrono- 

 mers at 30 millions of leagues (Fr.)=90 millions miles (Engl) ; but after 

 the transit of Venus in 1769, he allowed 33 millions. Such was the 

 effect of that now supposed erroneous experiment on the opinions of as_ 

 tronomers. (Epoques de la Nature.) 



(/) Proc. of Roy. Soc. 1861. The Rumford Medal was adjudged to 

 Dr. Tyndall in 1864. 



(?) The proportion is about ^1 according to the received measure of 

 the mean distance. 



