12 ^ Sir H. Davy on the formation of mists. 
nearly to 4,0° F. the surface cannot be the coolest part. It 
follows, therefore, that wherever water exists in considerable 
masses, and has a temperature nearly equal to that of the 
land, or only a few degrees below it, and above 45 0 F. at sun- 
set, its surface during the night, in calm and clear weather, 
will be warmer than that of the contiguous land ; and the air 
above the land will necessarily be colder than that above the 
water ; and when they both contain their due proportion of 
aqueous vapour, and the situation of the ground is such as to 
permit the cold air from the land to mix with the warmer air 
above the water, mist or fog will be the result ; which will 
be so much the greater in quantity, as the land surrounding 
or inclosing the water is higher, the water deeper, and the 
temperature of the water, which will coincide with the quantity 
or strength of vapour in the air above it, greater. 
I shall detail some observations which appear to me to 
show the correctness of this view. June 9th, 10th, 11th, the 
temperature of the atmosphere and of the Danube was re- 
peatedly examined during a voyage that I made upon this 
river from Ratisbonne to Vienna, and on each of these days, 
the sky being perfectly clear, the appearance of mist above 
the river in the evening uniformly coincided with the diminu- 
tion of the temperature of the air from three to six degrees 
below that of the river, and the disappearance of fog in the 
morning with the elevation of the temperature of the air above 
that of the river. From Ratisbonne to Passau, the tempera- 
ture of the Danube was pretty uniform throughout the 24 
hours, being highest, 62° F. or 62^° F., between 12 and 
2 o’clock, and about one degree less before sun-rise, and the 
temperature of the air from 6i° F, to 73 ° F, during the day, 
