CLOUDS. 407 
under the cloud, and all semblance to the original cumulus or later nimbus will 
have disappeared—all that remains being a flat cloud, a true cirrus or cirro-stra- 
tus, which may dissolve in a short time and leave no trace of the storm or shower 
to which it once belonged. I have seen many instances of all these transforma- 
tions, from cumulus to nimbus, from nimbus to cirro-stratus, from cirro-stratus to 
cirrus, and often the time required for these changes did not exceed an hour. It 
is rmuch more common for the cirro-stratus to exist for several, hours then change 
to the cirrus in which form it will float for days, moving in an easterly direction, 
at an immense height. The little white films of cirri, which pass over us nearly 
every day, especially in hot, dry weather, are the ashes of storm-clouds whose 
force was spent on the peaks of the Rocky mountains, or possibly on the briny 
waves of the distant Pacific. | 
The ascending currents which form the cumuli and carry them to great 
heights, sometimes impart to them sufficient inertia to cause their entrance into 
currents of air having a temperature considerably below 32° Fah. When this 
takes place vesicles of vapor in the upper portions of the cloud become suddenly 
converted into buoyant frost-crystals, many of which speed away on the wings of 
the wind—the cold current moving much more rapidly than that containing the 
lower portions of the cloud. A large number of these frost-crystals and spherules 
of ice, descend into the lower and denser portions of the cloud, diminishing its 
temperature, thereby tending to produce precipitation; and no doubt in many in- 
stances rain does result from these conditions ; though a far greater amount is 
caused by the cooling of ascending currents of humid air. 
With regard to their direction of motion, clouds must, of course, take the di- 
rection of the current of air in which they float. In the different parts of the 
earth the direction of the prevailing wind varies. The direction of the cumu- 
lus and the stratus will usually be the same as the surface wind, while the direc- 
tion taken by the other forms, is more or less independent of it. Of these the 
cumulo-stratus and nimbus, being formed from the cumulus, usually (at least dur- 
ing the day-time) take the direction of the surface wind; but the cirro-stratus and 
cirrus almost invariably move in an an easterly direction. 
The cumulus, proper, as stated heretofore, is an original form of cloud, but 
there is occasionally a cloud which greatly resembles it, though it is a transforma- 
tion. The cirro-stratus is a frozen cloud but sometimes becomes reduced to ves- 
icular vapor, and soon after it collects into little, rounded, fleecy masses called - 
cirro-cumuli. When this process continues for a considerable time all the distin- 
guishing characteristics of the cirro-stratus will become obliterated, and the cloud 
assumes the exact appearance of the cumulus. It never attains very great size 
and owing to its immense height, appears almost motionless. It forms only in 
hot weather; and is quite often the harbinger of a storm. 
The velocity with which clouds move depends mainly upon the velocity of 
the air-current in which they are suspended. The force of gravitation has a ten- 
dency to bring scattered clouds together ; and when they have a common altitude, 
