o another by the
conducting power; but every particle of air must come in contact with
the earth in order to receive heat from it.

EMILY.

Wind then by agitating the air should contribute to cool the earth and
warm the atmosphere, by bringing a more rapid succession of fresh strata
of air in contact with the earth, and yet in general wind feels cooler
than still air?

MRS. B.

Because the agitation of the air carries off heat from the surface of
our bodies more rapidly than still air, by occasioning a greater number
of points of contact in a given time.

EMILY.

Since it is from the earth and not the sun that the atmosphere receives
its heat, I no longer wonder that elevated regions should be colder than
plains and valleys; it was always a subject of astonishment to me, that
in ascending a mountain and approaching the sun, the air became colder
instead of being more heated.

MRS. B.

At the distance of about a hundred million of miles, which we are from
the sun, the approach of a few thousand feet makes no sensible
difference, whilst it produces a very considerable effect with regard to
the warming the atmosphere at the surface of the earth.

CAROLINE.

Yet as the warm air rises from the earth and the cold air descends to
it, I should have supposed that heat would have accumulated in the upper
regions of the atmosphere, and that we should have felt the air warmer
as we ascended?

MRS. B.

The atmosphere, you know, diminishes in density, and consequently in
weight, as it is more distant from the earth; the warm air, therefore,
rises only till it meets with a stratum of air of its own density; and
it will not ascend into the upper regions of the atmosphere until all
the parts beneath have been previously heated. The length of summer even
in warm climates does not heat the air sufficiently to melt the snow
which has accumulated during the winter on very high mountains, although
they are almost constantly exposed to the heat of the sun's rays, being
too much elevated to be often enveloped in clouds.

EMILY.

These explanations are very satisfactory; but allow me to ask you one
more question respecting the increased levity of heated liquids. You
said that when water was heated over the fire, the particles at the
bottom of the vessel ascended as soon as heated, in consequence of their
specific levity: why does not the same effect continue when the water
boils, and is converted into steam? and why does th