* The blue mist. l. 20. Mists are clouds resting on the ground, they generally come on at the beginning of night, and either fill the moist vallies, or hang on the summits of hills, according to the degree of moisture previously dissolved, and the education of heat from them. The air over rivers during the warmth of the day suspends much moisture, and as the changeful surface of rivers occasions them to cool sooner than the land at the approach of evening, mists are most frequently seen to begin over rivers, and to spread themselves over moist grounds, and fill the vallies, while the mists on the tops of mountains are more properly clouds, condensed by the coldness of their situation.
On ascending up the side of a hill from a misty valley, I have observed a beautiful coloured halo round the moon when a certain thickness of mist was over me, which ceased to be visible as soon as I emerged out of it; and well remember admiring with other spectators the shadow of the three spires of the cathedral church at Lichfield, the moon rising behind it, apparently broken off, and lying distinctly over our heads as if horizontally on the surface of the mist, which arose about as high as the roof of the church. There are some curious remarks on shadows or reflections seen on the surface of mists from high mountains in Ulloa's Voyages. The dry mist of summer 1783, was probably occasioned by volcanic eruption, as mentioned in note on Chunda, Vol. II. and therefore more like the atmosphere of smoke which hangs on still days over great cities.
There is a dry mist, or rather a diminished transparence of the air, which according to Mr. Saussure accompanies fair weather, while great transparence of air indicates rain. Thus when large rivers two miles broad, such as at Liverpool, appear narrow, it is said to prognosticate rain; and when wide, fair weather. This want of transparence of the air in dry weather, may be owing to new combinations or decompositions of the vapours dissolved in it, but wants further investigation. Essais sur L'Hygromet, p. 357.
[ return to text ]