184 
TERRESTRIAL VAPOUR.—FOGS. 
and by the density of the air at this season, kept 
floating near the ground, and that when the sun 
withdraws his heat, and night ensues, this vapour 
falls like dew, rendering substances damp and as it 
were greasy to the touch. This moisture soon 
freezes and forms a coating of ice to vegetation, and 
nnmerous icicles appear clinging to every part of 
trees and shrubs. At another time of the year too, 
this terrestrial heat eliminates such abundance of 
moisture during day, that at night an atmosphere 
of vapour rests on the ground and proves during the 
heat of summer of vast utility to vegetation; at other 
periods again, namely during the spring and au¬ 
tumnal months whenrain arrives from the south-west 
so freely, the amount of floating vapour (different in 
kind however from the former) is proportionately 
large, and by its weight or the influence of winds, 
it rolls into the vallies direct from the sea, or down 
the sides of the hills to which it had been driven, 
and there settles. On surveying the country on the 
mornings after these very common pccurrences, 
the vallies are indicated solely by the dense aqueous 
atmosphere above them, and on descending to them 
we find the air moist, the trees dripping, and re¬ 
storing part of the vapour to the rivers, while the 
remainder is evaporated when the sun attains 
sufficient altitude above the horizon. 
Since it is a law of of Nature, that water in be¬ 
coming ice should expand in bulk, we see effects 
demonstrative of this, not only on the large scale of 
ice swimming on lakes, rivers, seas, &c. but also on 
a small scale on very many occasions. The moisture 
accumulated in old walls and hedges during autumn 
passes into ice during frosts, and thus assuming a 
larger bulk, the mould, stones, &c. of which they 
are composed become displaced; for a while the 
congelation of the water holds the materials together 
but when a thaw occurs and the ice is dissolved, the 
