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river at the bottom of the town, and which are explained by the 
air on the hills, when cooled by radiation at night, sliding down 
into the valley, where it becomes mixed with the moister air 
below. ‘This causes a depression of the mean temperature of the 
mixture below its resultant dew-point and fog ensues. The fog 
will be more or less dense, according as the air in the valley is 
more or less nearly approaching to saturation before the mixture 
takes place. Once formed in this way, the fog is further increased 
by the radiation of heat from its own particles, “water in the 
liquid state being a good radiant of caloric.”* 
The appearance of these fogs, as seen from the upper part of 
the town the following morning a little before sunrise, is that of a 
mist occupying the lower part of the valley, but separated, if the 
sky is clear, by a well-defined line from the slopes of the hills 
above, which stand out as distinct as at other times. After sun- 
rise, the mist is seen slowly rolling upwards in large volumes like 
smoke, the hills becoming more and more obscured till they are 
quite hidden from view, the mist being in the end diffused 
through the atmosphere to a higher or lower elevation according 
to circumstances. If the sky is clouded or not quite clear, the 
mist is seen diffused from the beginning, its density being nearly 
the same everywhere. 
Sometimes this mist clears off as the day advances, being taken 
up with the ascending current of air caused by the increasing 
temperature of the earth’s surface. It then either passes into 
drier strata higher up and disappears entirely, or collects into 
“cumuli (the form of cloud often so noticeable about midday) above 
‘the vapour-plain of the cloud region. If the mist is generally 
‘diffused at sunrise, with a cloudy sky over head, it may continue 
through the day, or turns to rain as it happens. 
_ Amist that rises to higher and higher elevations previous to 
‘its disappearance, often shows itself on the tops of the hills, after 
* Herschell’s Meteorology, p. 93. 
