Remarks on the Formation of Fogs. 47 



a thermometer, with the bulb blackened, exposed, would indicate 

 a still greater reduction for that of the earth ; while the comple- 

 ment of the dew-point of the preceding day was not more than 

 5° or 10°. If the temperature of the earth alone suffered diminu- 

 tion, the effect would be limited to the deposition of dew, but at 

 the same time that dew is being deposited upon the cooled sur- 

 face, thus reducing the temperature of the dew-point, the tem- 

 perature of the air is also diminishing, and it depends altogether 

 upon the relative amounts of the two concurrent decrements, 

 whether any of the atmospheric water will take the form of vis- 

 ible vapor in the air or not. My observations, though not suffi- 

 ciently numerous to enable me to make a satisfactory application 

 of mathematical formulae to the results, will still enable us to 

 come to some safe general conclusions on these points. When 

 the diminution of atmospheric temperature and of the tempera- 

 ture of the dew-point depends on the influence of a cold surface, 

 the former seems always to decline most rapidly. This may, I 

 think, be accounted for if we observe upon what the rapidity of 

 diminution in each case depends. The decrement of atmos- 

 pheric temperature depends for its value — first, upon a certain 

 amount or radiation to the cold surface ; second, upon the con- 

 ducting power of the air, that is, on its degree of humidity, or in 

 other words, upon the temperature of the dew-point ; third, upon 

 the difference between the temperature of the air, and that of the 

 refrigerating surface. The decrement of the dew-point temper- 

 ature will depend — first, upon the deposition of dew, that is, it 

 will be nearly as the amount of humidity; and second, upon 

 the difference between the temperature of the dew-point and 

 that of the cold surface.* If therefore we take c to represent the 

 temperature of the surface, and y and <$' to represent respectively 

 the difference between the temperature of the surface and that 

 of the air and dew-point, we shall have the temperature of the 

 air=c-f <p, and that of the dew-poiut=c-{-<p / ; and if /? be taken 

 to represent the effect of radiation, we shall have the decrement 

 of atmospheric temperature proportionate to <p(c+ 9') +£; while 

 the decrement of the dew-point temperature will be as <jp'(c-|- <*>'). 



* These suppositions, though not strictly accurate, are sufficiently so for the 

 present purpose, as the variations from the truth affect somewhat equally the two 

 decrements. 



