Dry fogs are characterised by a dull opaque appearance of the atmosphere. They are most common in certain parts of North America, though they sometimes occur in Germany and in England. They are generally referred to the _electrical state of the atmosphere_, but the theory of them is still a matter of doubt. 368. _What is a mist?_ The term _mist_ is generally applied to vapours that rise over _marshy places_, or the surfaces of _water_, and roll or move over the land. 369. _What is the difference between a mist and a fog?_ Fogs, as they are known to us, generally arise over the _land_, and are usually mingled with the smoke of large towns. Mists generally arise over water, or wet surfaces. 370. _Why do mists and fogs disappear at sunrise?_ Because the condensed vapours are again _expanded_ and _dispersed_ by the heat of the sun's rays. 371. _Why do fogs frequently rise in the morning and fall again in the evening?_ Because, warmed by the sun's rays, they become more rarefied, and fly away at an altitude where they _appear_ to be altogether dispelled; but at night, when the earth _cools by radiation_, the vapours near the earth _again condense_, and settle in the _form of fog_. 372. _Why do fogs sometimes rest upon a given locality for several days together, and then disappear?_ They are probably kept near to the surface of the earth by a superstratum of cold air. A cold air lying _above_, or a cold air lying _below_, might equally contribute to keep a fog near the surface of a particular part of the earth, until a _flow of wind_, or a _fall of rain_, altered the atmospheric condition. [Verse: "He bindeth up the waters in his thick clouds; and the cloud is not rent under them."--JOB XXVI.] There are many interesting facts connected with the history of dew. It has attracted the attention of natural philosophers in all ages. But its true theory was never understood until recently. The ancients imagined that dews were shed from the stars; and the alchemists and physicians of the middle ages believed that the dew distilled by night possessed penetrating and wonder-working powers. The ladies of those times sought to preserve their beauty by washing in dew, which they regarded as a "celestial wash." They collected it by placing upon the grass heaps of wool, upon the threads of which the magic drops clustered. CHAPTER XVIII. 373. _What are clouds?_ Clouds are volumes of _vapour_, usually elevated to a considerable height. [Illustration: Fig. 4--CIRRO-CUMULUS, OR SONDER CLOUD.] 374. _Whence do clouds arise?_ From the _evaporation of water_ at the earth's surface. 375. _Why do we not see them ascend?_ We do, sometimes, in the form of what we call _mists_, but generally the vapours that rise and contribute to the formation of clouds are so thin that they are _invisible_.