254 WRITINGS OF JOSEPH HENRY. [1855- 



rating surface the fog will be produced, and will increase in 

 density and height so long as a difference of temperature 

 continues. If however a wind be blowing at the time, so 

 that successive portions of unsaturated air are brought over 

 the place, no fog will be produced, A still atmosphere there- 

 fore is a necessary condition to the accumulation of fog. 



The foregoing is the usual method in which fog is pro- 

 duced, for it is well known that in cold weather the surfaces 

 of lakes and rivers are much warmer than the stratum of air 

 which rests upon them. 



It is however frequently observed that fogs are formed 

 during still nights in low places when the surface of the 

 ground is colder than the stratum of the atmosphere which 

 rests upon it, and indeed we have shown that the tempera- 

 ture of the surface of the earth on a still and clear night is 

 always lower than that of the air which is immediately in 

 contact with it ; and it is not easy, without further explana- 

 tion, to see the reason why fogs should not always be pro- 

 duced in this case as well as dew. When the atmosphere is 

 still the condensation of the vapor by the coldness of the 

 surface is so gradual that the air is not disturbed, and the 

 stratum immediately above the grass has relatively less mois- 

 ture in it than that a few yards higher; hence no fog ought to 

 be produced in this case, since all the precipitation produced 

 is that which has settled directly upon the grass in the form 

 of dew. In this case we may define the dew to be a fog 

 entirely condensed into drops of water. The question still 

 arises how under these circumstances can a fog really be pro- 

 duced. The answer is that another condition is required, 

 namely that the surface cooled by radiation should slope 

 to a lower level, as in the side of a hill or the concave surface 

 of the sides of a hollow. In this case the superincumbent 

 stratum of air of which the temperature has been lowered 

 by contact with the cold earth, flows down the declivity by 

 its greater weight into the valley below, and there mingling 

 with the damp air which generally exists in such places, 

 precipitates a part of its transparent vapor into visible fog. 

 In this manner large hollows are sometimes seen in the morn- 



