is sometimes seen cutting the moon's disc
with a sharp line. The Cumulo-Stratus, or Twain Cloud, is denser than
the Cumulus, and more ragged in its outlines. It overhangs its base in
folds, and often bears perched on its summit some other form of cloud,
which inosculates itself with it. Sometimes a Cirro-Stratus cloud
comes along and fastens itself to it parasitically. It is one of our
most picturesque forms of clouds.

Within the last two years we have twice observed in the city of New
York, during the summer afternoons, large masses of clouds coming over
from the southwest, and hanging rather low, which could not be well
placed in any of the classes already described, or recognized as such
by meteorologists. They consisted of a great number of hemispherical
forms of large diameter, hanging vertically from a Stratus cloud or
plane above them, and to which they appeared attached. They were
regular in shape, and very distinct; they barely touched each other,
and were of a gray color. They might be compared to a hay-field turned
upside down, with innumerable hay-cocks hanging below it.
Unfortunately, the circumstances under which the spectacle was
observed did not; admit of any resort to the barometer, thermometer,
or anemometer. Should further observations verify these remarks, it
might perhaps be proper to style this variety the Hemispherical.

* * * * *

Dew is another atmospheric product. It is the condensation of the
warmer vapor of the atmosphere, in calm and serene nights, and in the
absence of clouds, by the cold surface of bodies on which it rests. In
some countries it is copious enough to supply the want of rain. The
earth radiates its own acquired heat, grows colder than the
atmosphere, and so condenses it.

What is thermometrically called the dew-point is that degree at which
the moisture present in the atmosphere, on being subjected to a
decrease of temperature, begins to be precipitated or condensed. It is
the same as the point of saturation. Daniell calls it "the constituent
temperature of atmospheric vapor." It is our criterion for
ascertaining how much moisture there is in the air, and at what degree
of heat or cold it would be precipitated. When the air is saturated, a
dry bulb and a wet bulb will read alike.

The dew-point has been a puzzle to most persons. Very few treatises
explain it satisfactorily. The definition just given, though explicit,
is not quite enough.