d, the watery
particles approach each other, and form themselves into drops of water,
which being heavier than the atmosphere, descend to the earth. There are
also other circumstances, and particularly the variation in the weight
of the atmosphere, which may contribute to the formation of rain. This,
however, is an intricate subject, into which we cannot more fully enter
at present.

EMILY.

In what manner do you account for the formation of dew?

MRS. B.

Dew is a deposition of watery particles or minute drops from the
atmosphere, precipitated by the coolness of the evening.

CAROLINE.

This precipitation is owing, I suppose, to the cooling of the
atmosphere, which prevents its retaining so great a quantity of watery
vapour in solution as during the heat of the day.

MRS. B.

Such was, from time immemorial, the generally received opinion
respecting the cause of dew; but it has been very recently proved by a
course of ingenious experiments of Dr. Wells, that the deposition of dew
is produced by the cooling of the surface of the earth, which he has
shown to take place previously to the cooling of the atmosphere; for on
examining the temperature of a plot of grass just before the dew-fall,
he found that it was considerably colder than the air a few feet above
it, from which the dew was shortly after precipitated.

EMILY.

But why should the earth cool in the evening sooner than the atmosphere?

MRS. B.

Because it parts with its heat more readily than the air; the earth is
an excellent radiator of caloric, whilst the atmosphere does not possess
that property, at least in any sensible degree. Towards evening,
therefore, when the solar heat declines, and when after sunset it
entirely ceases, the earth rapidly cools by radiating heat towards the
skies; whilst the air has no means of parting with its heat but by
coming into contact with the cooled surface of the earth, to which it
communicates its caloric. Its solvent power being thus reduced, it is
unable to retain so large a portion of watery vapour, and deposits those
pearly drops which we call dew.

EMILY.

If this be the cause of dew, we need not be apprehensive of receiving
any injury from it; for it can be deposited only on surfaces that are
colder than the atmosphere, which is never the case with our bodies.

MRS. B.

Very true; yet I would not advise you for this reason to be too
confident of escaping all the ill effects which may arise from ex