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Art. X. — Observations on the formation of Mists , and the 
deposition of Dew. By George Harvey, Esq., M. G. S., &c. 
Communicated by the Author. 
F OR the formation of mists, it is necessary that the tempe- 
rature of the water should be greater than that of the air ; 
but for the deposition of dew, that the heat of the body on 
which it is to be deposited, should be less than that of the at- 
mosphere. For the knowledge of the former of these principles 
we are indebted to the sagacity of Sir Humphry Davy ; and 
for the latter to the late lamented Dr Wells. 
Previous to the time of Dr Wells, it was supposed by Mr 
Wilson and Mr Six, that the cold connected with the formation 
of dew ought always to be proportional to the quantity of that 
fluid ; and the first mentioned philosopher had the credit of 
showing, that <fi the same degree of cold in the precipitating 
body, may be attended with much , with little , or no dew, ac- 
cording to the existing state of the atmosphere, in regard to 
moisture. As the difference of temperature* therefore, between 
the precipitating body and the air, is not to be regarded as a 
proper measure of the quantity of dew deposited on a clear and 
tranquil night ; neither is the difference between the tempera- 
tures of water and air to be considered as at all proportional to 
the density of a mist. The quantity of aqueous vapour con- 
tained in the atmosphere, is as necessary for the copious forma- 
tion of mist, as for the abundant deposition of dew. 
There are three principal conditions to be considered, when 
the formation of dew is taken in conjunction with that of mist. 
In the first place, the temperature of the water, and the law 
according to which it cools ; secondly, the temperature of the 
land, and the law by which it radiates its heat to the sky ; and, 
thirdly, the quantity of vapour contained in the air. The va- 
ried character of these conditions must necessarily impart a 
corresponding diversity to the results. The same circumstan- 
ces of temperature, which in one case would produce a copious 
deposition of dew, and a mist of considerable density, in ano- 
ther case would produce only a moderate formation of each ; 
and even if we suppose the quantity of atmospheric moisture 
to remain constant, alterations of temperature will occasion in- 
numerable varieties of both. 
