he somewhat vague statement by Sir William
Thomson, that "if a drop of water could be magnified so as to be as
large as the earth, and have a diameter of eight thousand miles,
then a molecule of this water in it would appear _somewhat larger
than a shot_." (What kind of shot?) "_and somewhat smaller than a
cricket-ball_"!

And as I finally review the common accounts given of cloud
formation, I find it quite hopeless for the general reader to deal
with the quantity of points which have to be kept in mind and
severally valued, before he can account for any given phenomena. I
have myself, in many of the passages of 'Modern Painters' before
referred to, conceived of cloud too narrowly as always produced by
_cold_, whereas the temperature of a cloud must continually, like
that of our visible breath in frosty weather, or of the visible
current of steam, or the smoking of a warm lake surface under
sudden frost, be above that of the surrounding atmosphere; and yet
I never remember entering a cloud without being chilled by it, and
the darkness of the plague-wind, unless in electric states of the
air, is always accompanied by deadly chill.

Nor, so far as I can read, has any proper account yet been given of
the balance, in serene air, of the warm air under the cold, in
which the warm air is at once compressed by weight, and expanded by
heat, and the cold air is thinned by its elevation, yet contracted
by its cold. There is indeed no possibility of embracing the
conditions in a single sentence, any more than in a single thought.
But the practical balance is effected in calm air, so that its
lower strata have no tendency to rise, like the air in a fire
balloon, nor its higher strata to fall, unless they congeal into
rain or snow.

I believe it will be an extreme benefit to my younger readers if I
write for them a little 'Grammar of Ice and Air,' collecting the
known facts on all these matters, and I am much minded to put by my
ecclesiastical history for a while, in order to relate what is
legible of the history of the visible Heaven.

[Footnote A: I trust that Dr. Rae will forgive my making the reader
better aware of the real value of this communication by allowing
him to see also the following passage from the kind private letter
by which it was supplemented:--

"Many years in the Hudson's Bay Company's service, I and my men
became educated for Arctic work, in which I was five different
times employed, in two of which expe