-1859] WRITINGS OF JOSEPH HENRY. 249 



conclusive experiments furnished a definite explanation of 

 all the phenomena. They are simply due to the cold pro- 

 duced in different bodies by radiation. As we have seen, the 

 earth is constantly radiating heat into celestial space, and is 

 constantly receiving it from the sun during the continuance 

 of that body above the horizon. As long as the heat from 

 the sun exceeds that radiated into space the temperature of 

 the surface of the earth and that of the air in contact with it 

 continues to increase; but when the two are equal the tem- 

 perature remains stationary for a short time and then begins 

 to decline as the heat of the sun, on account of the obliquity 

 of the rays, becomes less than the radiation into space. The 

 maximum of heat generally takes place between 2 and 3 

 o'clock in the afternoon, and the cooling from this point 

 goes on until near sunrise of the next morning. As soon as 

 the sun descends below the horizon the cooling of the surface 

 of the earth takes place more rapidly if the sky be clear; the 

 air in contact with grass and other substances which are 

 cooled by this radiation will deposit its moisture in a manner 

 analogous to that of the deposition of water on a surface of 

 a metallic vessel containing a cold liquid. Although the 

 atmosphere may contain the same amount of vapor, yet the 

 quantity of dew deposited during the night in different 

 places and on different substances is very unequal. It is 

 evident that it must depend to some extent upon the 

 quantity of moisture, since if the air were dry, no deposi- 

 tion could take place; and indeed it has been remarked that 

 on some parts of the plains west of the Mississippi dew is 

 never observed. It must also depend upon the clearness of 

 the sky; for if the heavens be covered with a cloud the 

 radiant heat from the earth will not pass off into celestial 

 space, but will be partly' absorbed by the cloud and radiated 

 back to the earth. This is not a mere hypothesis but has 

 been proved by direct experiment. The author of this article 

 while at Princeton some years ago placed a thermo-electric 

 apparatus in the bottom of a tube provided with a conical 

 reflector, and thus formed, if the expression may be allowed, 

 a thermal telescope, with which the heat of a cloud of the 



