129 
Sir H. Davy on the formation of mists. 
After mists have formed above rivers and lakes, their in- 
crease seems not only to depend upon the constant operation 
of the cause which originally produced them, but likewise 
upon the radiation of heat from the superficial particles of 
water composing the mist; which produces a descending 
current of cold air in the very body of the mist, whilst the 
warm water continually sends up vapour : it is to these cir- 
cumstances, that the phenomena must be ascribed of mists 
from a river or lake, sometimes arising considerably above 
the surrounding hills. I have often witnessed this appearance 
during the month of October, after very still and very clear 
nights, in the Campagna of Rome above the Tiber, and on 
Monte Albano over the lakes existing in the ancient craters 
of this extinguished volcano, and, in one instance, on the 17th 
of October, before sun-rise, there not being a breath of wind, 
a dense white cloud of a pyramidal form was seen on the site 
of Alban lake, and rising far above the highest peak of the 
mountain, its form gradually changed after sun-rise, its apex 
first disappeared, and its body, as it were, melted away in the 
sun beams. 
Where rivers rise from great sources in the interior of 
rocks or strata, as they have the mean temperature of the 
climate, mists can rarely form upon them except in winter, 
or late in autumn, or early in spring. In passing across the 
Apennines, October 1st, 2d, and 3d, 1818, there having been 
much rain for some days preceding, and the nights being very 
clear, I observed the beds of all the rivers in the valleys filled 
with mist, morning and evening, except that of the Clitumnus 
near its source, in which there was no mist, and this river 
rises at once from a lime-stone bed, and when I examined it, 
MDCCCXIX. S 
