The United States Weather Bureau classifies a fog as “dense” if it hides objects at a distance of 1,000 feet; otherwise it is described as “light.” British meteorologists record fogs on a scale of five degrees. During the ice patrol of the _Seneca_ in 1915 samples of foggy air were examined for the purpose of calculating the amount of water and the number of drops they contained per unit volume, as well as the size of the drops. A block of dense fog 3 feet wide, 6 feet high, and 100 feet long was found to contain less than one-seventh of a glassful of water, distributed in 60,000,000,000 drops. During the densest fog of the voyage the diameter of the fog particles averaged 0.0004 inch; just about the limit of visibility with the naked eye. In spite of the extremely attenuated state of the water in fogs, as indicated by these figures, the moisture they deposit on terrestrial objects is great enough to be of considerable agricultural importance in some parts of the world. Thus along the coast of Peru, where the rainfall is negligible (though not, as often stated, nonexistent), a wet fog known as the “garúa” suffices to maintain a luxuriant vegetation during several months of each year. There are frozen fogs as well as frozen clouds. The “frost smoke” that rises over the Norwegian fjords and over ice-free spots in the polar seas is generally composed of icy particles or snowflakes. An ice fog that sometimes forms in mountain valleys in the western United States is known as the “pogonip”--a name derived from the Shoshonean language. This fog often appears very suddenly, even in the brightest weather. The minute needles of ice of which it consists are said to be extremely injurious to the lungs. There are tales of a whole tribe of Indians perishing from its effects. Whatever truth there may be in such stories, it is greatly dreaded by both the Indians and the whites. The mountains of Nevada appear to be the favorite home of the pogonip. What meteorologists call “dry fog” is a haze of dust or smoke, sometimes very dense. We have already described the prevalence of this turbid state of the atmosphere following volcanic eruptions, the burning of forests and moors, and desert dust storms. Under the head of dry fog many writers include a sort of heat haze, which does not necessarily involve the suspension of either solid or liquid matter in the air, but is due to the mixing of local air currents of different densities, especially when evaporation is proceeding rapidly from moist ground under strong sunshine. The _callina_ of Spain and the _qobar_ of the upper Nile region are probably due partly to this cause, and partly to dust. [Illustration: ALTO-CUMULUS CLOUDS. These clouds always occur in roundish fleecy masses or in elongated fleecy rolls, with blue sky between. A score of different types have been distinguished and named by certain cloud specialists. (_Photographed by A. J. Weed._)]