te
observations which he has had the kindness to set down with their
dates for me, in the following letter:--

"4, ADDISON GARDENS, KENSINGTON, _4th Feb., 1884._

DEAR SIR,--I have looked up my old journal of thirty years ago,
written in pencil because it was impossible to keep ink unfrozen in
the snow-hut in which I passed the winter of 1853-4, at Repulse
Bay, on the Arctic Circle.[A]

On the 1st of February, 1854, I find the following:--

'A beautiful appearance of some cirrus clouds near the sun, the
central part of the cloud being of a fine pink or red, then green,
and pink fringe. This continued for about a quarter of an hour. The
same was observed on the 27th of the month, but not so bright.
Distance of clouds from sun, from 3 deg. to 6 deg..'

On the 1st February the temperature was 38 deg. below zero, and on the
27th February 26 deg. below.

'On the 23d and 30th (of March) the same splendid appearance of
clouds as mentioned in last month's journal was observed. On the
first of these days, about 10.30 a.m., it was extremely beautiful.
The clouds were about 8 deg. or 10 deg. from the sun, below him and
slightly to the eastward,--having a green fringe all round, then
pink; the center part at first green, and then pink or red.'

The temperature was 21 deg. below zero, Fahrenheit.

There may have been other colors--blue, perhaps--but I merely noted
the most prominent; and what I call green may have been bluish,
although I do not mention this last color in my notes.

From the lowness of the temperature at the time, the clouds _must_
have been frozen moisture.

The phenomenon is by no means common, even in the Arctic zone.

The second beautiful cloud-picture shown this afternoon brought so
visibly to my memory the appearance seen by me as above described,
that I could not avoid remarking upon it.

Believe me very truly yours,
JOHN RAE." (M.D., F.R.S.)

Now this letter enables me to leave the elements of your problem
for you in very clear terms.

Your sky--altogether--may be composed of one or more of four
things:--

Molecules of water in warm weather.
Molecules of ice in cold weather.
Molecules of water-vapor in warm weather.
Molecules of ice-vapor in cold weather.

But of the size, distances, or modes of attraction between these
different kinds of particles, I find no definite information
anywhere, except t