Meteorological Observations. 45 



Mists over tanks have been ascribed by some writers to 

 the condensation of the moisture in the air, by the colder 

 water in the tank, the superior radiating power of water 

 being supposed to account for its being colder than the 

 surrounding soil ; but I have always found that the water of 

 the tank was warmer than the adjacent air, and the mist 

 was produced by the condensation of the vapour arising 

 from the former. 



On high elevated pinnacles of granite I have had op- 

 portunities of trying the temperature of isolated masses of 

 mist, which were carried past me floating with the wind, and 

 have invariably found their temperature to be below the 

 dew-point at the time, and generally two degrees at least 

 cooler than the surrounding air. Daniel, at page 101 of 

 his Meteorological Essays makes an assertion opposed to the 

 above, but there can be but little doubt he must have been 

 led into error by reasoning on false principles. The 

 mechanical condensation of the volume of a vapour will of 

 course produce the evolution of heat, but it is also as plain 

 that the condensation of vapour into mist must always have 

 been produced by the temperature of the vapour having 

 been lowered. 



Caps of mist settled on the summits of mountains have 

 been ascribed by Daniel (Essays, page 123) to the condensa- 

 tion of moisture produced by the contact of the cold moun- 

 tain, and this opinion has been very generally assumed. As 

 these settled caps of mist appear to be stationary even where 

 the wind blows strong, this hypothesis would seem not very 

 probable, more particularly as had the mountain been colder 

 than the surrounding air, the vapour would have been 

 deposited on it in the form of dew ; and mist would not have 

 been formed. Having had opportunities of being on conical 

 hills of granite while thus capped with mist, 1 have always 

 found that the rock was much warmer than the air, and 

 that it was the vapour arising from the warm and damp 



