62 Mr. Harvey on the Formation of Mists. 



me from attending to it during the afternoon ;— but at half-past 

 seven, finding the mist rapidly disappearing, I went on the 

 water, and found, that as the temperature of the air increased, 

 so the mist diminished. The first observation found the tem- 

 peratures of the air and water the same, each being 62f° ; but 

 when the air increased to 63i-° and 64°, the mist melted rapidly 

 away. This phenomenon accords most perfectly in principle 

 with the observation made by Sir Humphry during his voyage 

 on the Danube, — that the disappearance of mist results from 

 an elevation of the temperature of the air. 



Examples have occurred during the past summer, of mists 

 existing in a very dense state, over water, in the morning, when 

 the difference in the two temperatures has only amounted to 

 two degrees ; and in one instance indeed, a remarkably dense 

 mist was examined, when its temperature was only one degree 

 below that of the water. To produce a mist, in the first in- 

 stance, it appears, from the experiments of Sir H., that the air 

 must be cooled from three to six degrees beloiv the temperature 

 of the water. After, however, it has once been formed, it may 

 exist for a considerable time, after the air has gained such in- 

 crements of heat, as to reduce the difference between the tempe- 

 ratures of the air and water to a very small quantity. Between 

 the first formation of a mist, and its final disappearance, it is evi- 

 dent, from the principles laid down, that a moment must exist, 

 when the temperatures of the air and the water will exactly coin- 

 cide. Before this period, the principle which promoted the forma- 

 tion of the mist, may sometimes continue in operation, but with 

 a diminished activity, until an equality of temperature is at- 

 tained ;— but after this, the mist will disappear, with a rapidity 

 proportional to the magnitude of the increments which the at- 

 mosphere may receive. The continuance of the mist (omitting 

 the consideration of the radiation of heat from the superficial 

 particles of water composing the mist) must be regulated by 

 the difference between the temperature of the air and water ; 

 and which, from the diversified nature of our atmospheric 

 changes, will be exceedingly varied and uncertain. 



