-1859] WRITINGS OF JOSEPH HENRY. 253 



Since radiation from the surface of the earth is most in- 

 tense on clear nights, when the moon is visible, many of the 

 effects which are due to this cause have been referred to 

 lunar influence; for example, a piece of fresh meat exposed 

 to the moonlight is said to become tainted in a few hours; 

 this may arise from the deposition of moisture on the surface 

 of the meat due to the cooling from radiation. The moon 

 itself however acts as a cloud and radiates back to the earth 

 a portion of the heat which it received from the earth as 

 well as a portion of that which it received from the sun; 

 and hence Sir John Herschel has referred to this cause, with 

 apparent probability, the origin of an assertion of the sailors 

 that "the moon eats up the clouds." He supposes that they 

 may be dissipated by the radiant heat from that body, which 

 being of low intensity and but feebly penetrating the lower 

 stratum of the atmosphere may serve to dissipate the clouds. 

 Though a wrong explanation is generally given by the pop- 

 ular observer of natural phenomena, and though effects and 

 causes are frequently made to change places in his explana- 

 tions, yet it is true, as Biot has properly said, that the scien- 

 tist who devotes himself assiduously to investigate the sub- 

 ject of popular errors will find in them a sufficient amount 

 of truth to full}^ repay him for his labor. 



Formation of fogs. — The difference between a fog and a 

 cloud relates principally to the conditions under which they 

 are severally formed. A fog has been aptly called a cloud 

 resting on the earth and a cloud a fog suspended in the 

 atmosphere. The circumstances under which a fog is usually 

 produced are the following: Either the surface of the earth 

 or water is warmer than the air or it is cooler. If the tem- 

 perature of a river or of a damp portion of ground is higher 

 than that of the atmosphere which rests upon it the warmer 

 surface will give off vapor of an elastic force due to its tem- 

 perature. Should the superincumbent air be extremely dry 

 the vapor will diffuse itself up through it in an invisible 

 form without condensation, and no fog will be formed until 

 by the continuation of the process the air becomes completely 

 saturated ; and then if an excess of heat remain in the evapo- 



