he cumulus
form, amounting to nearly 6,500 feet. In the morning, when the cirrus
clouds are at their lowest level, the frequency of their lowest
forms--the cirro-cumulus--is greatest; and in the evening, when the
height of the cirrus is greatest, the frequency of its highest
forms--the cirro-stratus--is also greatest. With regard to the
connection between the character of the weather and the height of the
clouds, the heights of the bases of the cumulus are nearly constant in
all conditions. The summits, however, are lowest in the vicinity of a
barometric maximum. They increase in the region of a depression, and
attain their greatest height in thunderstorms, the thickness of the
cumulo stratus stretching sometimes for several miles. The highest
forms of clouds appear to float at their lowest levels in the region
of a depression. The forms of clouds are identical in all parts of the
world, as has been shown in papers lately read by the Hon. R.
Abercromby before the English and Scottish Meteorological
Societies.--_Nature_.

* * * * *

ON THE CAUSE OF IRIDESCENCE IN CLOUDS.

By G. JOHNSTONE STONEY.

When the sky is occupied by light cirro-cumulus cloud, an optical
phenomenon of the most delicate beauty sometimes presents itself, in
which the borders of the clouds and their lighter portions are
suffused with soft shades of color like those of mother-of-pearl,
among which lovely pinks and greens are the most conspicuous. Usually
these colors are distributed in irregular patches, just as in
mother-of-pearl; but occasionally they are seen to form round the
denser patches of cloud a regular colored fringe, in which the several
tints are arranged in stripes following the sinuosities of the outline
of the cloud.

I cannot find in any of the books an explanation of this beautiful
spectacle, all the more pleasing because it generally presents itself
in delightful summer weather. It is not mentioned in the part of
Moigno's great _Repertoire d'Optique_ which treats of meteorological
optics, nor in any other work which I have consulted. It seems
desirable, therefore, to make an attempt to search out what appears to
be its explanation.

At the elevation in our atmosphere at which these delicate clouds are
formed the temperature is too low, even in midsummer, for water to
exist in the liquid state; and accordingly, the attenuated vapor from
which they were condensed passed at once into a so