130 Sir H. Davy on the formation of mists. 
at half past six o’clock, A. M. October 3, was lower than 
the atmosphere. 
Great dryness in the air, or a current of dry air passing 
across a river, will prevent the formation of mist, even when 
the temperature of the water is much higher than that of the 
atmosphere : thus on the l^th of June, near Mautern, though 
the Danube at five in the morning was 6i° F. and the air only 
54 0 , yet there was no mist ; but a strong easterly wind blew, 
and from the rapidity with which water evaporated it, it was 
evident that this wind was in a state of extreme dryness. 
The Tiber has furnished me with a number of still more 
striking examples. October 13th, the night having been very 
clear, on arriving at the Ponte Molle, at half past six in the 
morning, I found no mist on the river, yet the temperature of 
the air immediately above it was 48° F. and that of the river 
56° F., a strong north wind blew, which indicated, by the 
hygrometer, a degree of dryness of 55 0 , and this part of the 
river was exposed to it ; but the valley above, where the 
river was sheltered from the wind, was full of mist, and the 
mist in rising to the exposed level might be seen, as it were, 
dissolving, presenting thin strias which never reached above a 
certain elevation, and many of which disappeared a few 
seconds after they rose. From the 13th to the 25th of Oct. 
during which time the tramontane or north wind blew, I 
witnessed repeatedly the same phenomenon, and in the whole 
of this time there was only one morning when there was no 
mist in the sheltered valleys, and the cause was perfectly 
obvious ; the night had been very cloudy, and the thermome- 
ter, before sun rise, indicated a difference of only one degree 
in the atmosphere below that of the river. 
