288. =Clouds.=--The water which goes up in the air in evaporation is variously disposed of. Some of it is deposited as dew or frost. Some of it forms fog. Some of it also mounts far upward and forms the clouds, which are really collections of fog made high up in the air. In fog and in clouds the water which in its evaporation is invisible becomes visible. Let us see how this is. There is always more or less of water in clear air, but the particles are so minutely divided and so thoroughly mingled with the particles of the air that they can not be seen. But in a fog or cloud the particles of water are gathered together in little companies, as we may express it. And it is supposed, some think ascertained, that each of these companies of particles is globular and hollow. If so, then we may regard every cloud as a vast collection of minute bubbles or balloons careering through the air. [Illustration: Fig. 202.] [Illustration: Fig. 203.] 289. =Shapes of Clouds.=--Clouds have a very great variety of shape, the causes of which are for the most part not understood. They are generally divided into four classes: Cirrus, Cumulus, Stratus, and Nimbus. The _Cirrus_ is represented in Fig. 202 (p. 225). It is a light, fleecy cloud, having graceful turns like curls, and hence its name, which is the Latin word for curl. Such clouds are commonly very high up in the air. The _Cumulus_ (Latin for heap) you see in Fig. 203 (p. 225). Clouds taking this form appear as heaps rounded upward, and often appear like mountains of snow when they are illuminated by the sun. We see such clouds mostly in summer. The _Stratus_ (Latin for covering) is seen in the same figure under the Cumulus. Clouds of this form lie low in the horizon, stretched along like a sheet. They often form in the latter part of the day, and increase in the night, but the rising sun dissipates them. The _Nimbus_, or rain-cloud, is represented in Fig. 204 (p. 226). It has a uniform gray or dark color. We often have two forms of cloud mingled together. Thus in Fig. 205 (p. 226) we have a mixture of the Stratus and the Cirrus, termed _Cirro-Stratus_. This is commonly called the mackerel-sky, and is quite a sure prognostic of rain. Then we have the _Cirro-Cumulus_, Fig. 206 (p. 227), and the _Cumulo-Stratus_, Fig. 207 (p. 227). [Illustration: Fig. 204.] [Illustration: Fig. 205.]