These tables of factors have been constructed at the
Greenwich Observatory, and are generally used.
The Hygrometer, invented by Mr. Daniell, gives the dew-point by
inspection.
It is an error to suppose that dew falls like rain from the air; it
forms on the body which is cooled down below the temperature of the
air. It differs in quantity with the radiating or cooling surface;
that which has absorbed and retained the most heat during the day
radiates the most at night and furnishes the most cold in return.
Hoar-frost, such as we find on our window-panes, or on the grass, is
the moisture of the warm air cooled down and frozen, and is produced
when the cold at the surface is below the freezing-point. What we in
common parlance call the action of frost, and which in this climate is
well known to be very powerful, is not particularly injurious to
organized bodies.
Mists are the vapor near the ground rendered visible by the
temperature of the air falling below that of the vapor. When we see
our breath in a cold morning, we see a mist. Where the surface is
comparatively warm and damp, and the air is cooler, we have mists,
which, if dense, are called fogs.
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