98 OUTLINES OF FORESTRY. 



The influence of the vapor screen that is placed 

 between any surface and the sun on the climate of 

 the surface is thus referred to by Tyndall in his 

 " Heat as a Mode of Motion," * on page 417 : 



" A few years ago a work possessing great charms of style 

 and ingenuity of reasoning was written to prove that the more 

 distant planets of our system were uninhabitable. Applying 

 the law of inverse squares to their distances from the sun, the 

 diminution of temperature was found to be so great as to pre- 

 clude the possibility of human life in the more remote mem- 

 bers of the solar system. But in those calculations the influ- 

 ence of an atmospheric envelope was overlooked, and this 

 omission vitiated the entire argument. An atmosphere may 

 act the part of a barb to the solar rays, permitting them to 

 reach the earth, but preventing their escape. A layer of air 

 two inches in thickness, saturated with the vapor of sulphuric 

 ether, would offer very little resistance to the passage of the 

 solar rays, but I find that it would cut off fully thirty-five per 

 cent, of the planetary radiation. It would require no inor- 

 dinate thickening of the layer of vapor to double this ab- 

 sorption ; and it is perfectly evident that, with a protecting 

 envelope of this kind, permitting the heat to enter, but 

 preventing its escape, a comfortable temperature might be 

 obtained on the surface of the most distant planet." 



* Eeprinted, by permission, from " Heat as a Mode of Mo- 

 tion," by John Tyndall, LL.D., F.K.S. New York : Appleton 

 and Company, 1883. Pp. 591. 



