[Illustration: CUMULUS. Cloud photography has become a special branch of photographic art, entailing not only the use of appropriate lenses and plates, but also of ray filters, or other special devices for sharpening the contrast between the cloud and the blue sky. Mr. Ellerman’s pictures, made on Mt. Wilson and elsewhere in California, stand in the front rank. (_Photographed by F. Ellerman._)] One more species of fog requires mention here, viz., the dirty, foul-smelling “painter” of the Peruvian coast, which deposits on vessels lying in the harbor of Callao and elsewhere a slimy brown substance known as “Peruvian paint.” This substance comes from the ocean and is probably due to the decomposition of marine organisms. The “painter” prevails during the months December to April. According to a plausible hypothesis a change in the temperature of the water at that season, resulting from a periodical shift of ocean currents, kills vast quantities of plankton, the decay of which would give rise to the phenomena observed. [Illustration: MAMMATO-CUMULUS. A rather rare cloud form, associated with thunderstorms and tornadoes. It is known in Scotland as the “pocky” (i. e. baggy) cloud and in parts of England as “rain balls.” (_Photographed by L. C. Twyford._)] [Illustration: CUMULO-NIMBUS. This is the thundercloud. (_Courtesy of the Naval Air Service._)] Clouds, though they are nothing more than masses of fog situated at some distance from the earth, are susceptible of a classification, according to shape and texture, that is not applicable to fog. Among the billions of human beings who, in all ages, have amused themselves by discovering pictures in the clouds it would be remarkable if a good many had not, from time to time, conceived the idea of reducing these pictures to a few general types. According to a note published a few years ago in the “Quarterly Journal” of the Royal Meteorological Society, there is some reason to believe that an elaborate classification of the clouds was in use among the ancient Hindus. A passage quoted from an Indian work of the fourth century B. C. says: “Three are the clouds that continuously rain for seven days; eighty are they that pour minute drops; and sixty are they that appear with the sunshine.” In the occidental world, however, we have no record of any attempt to classify the clouds prior to the year 1801, when the following classification was proposed by the French naturalist Lamarck: Nuages en balayures (cloud sweepings), Nuages en barres (clouds in bars), Nuages pommelés (dappled clouds), Nuages groupés (grouped or piled clouds), Nuages en voile (veil clouds), Nuages attroupés ou moutonnés (clouds in flocks).