The meeting of the directors of the meteorological institutions in different parts of the world, which was held at Munich in 1891, decided to adopt the classification of Abercromby and Hildebrandsson, and a committee was appointed to prepare an Atlas of Clouds, which should be cheaper than the preceding one. This committee, of which the writer has the honour to be the American member, met at Upsala in 1894. It defined the various forms of clouds, selected typical pictures to illustrate them, and drew up instructions for observing. This atlas, which was published in 1896, is the recognized authority on cloud forms. Meanwhile the United States Weather Bureau had issued a plate of clouds, printed in one colour, to familiarize its observers with the new system. The Navy Department has also an interest in clouds, for several thousand seamen in various parts of the world send their special logs to the United States Hydrographic Office. The Hydrographer, a few years ago, was Captain Sigsbee, who, long before he became known to the public as commander of the ill-fated _Maine_, had achieved scientific reputation from his investigations upon the depths and the currents of the ocean. Captain Sigsbee desired to render comparable the observations of clouds which were being made all over the world, and to this end he resolved to publish a coloured atlas of the international cloud types which should be intelligible to seamen, and yet not too costly for his office to supply. After two years of experimenting, during which the writer and his assistant, Mr. Clayton, were frequently consulted, the _Illustrative Cloud Forms_, with and without descriptive text, were issued in 1897 by the Hydrographic Office, and in several respects this atlas is the best. Still, it is impossible for anything but a photograph from the cloud itself to show the extreme delicacy of certain forms. Perhaps it should be explained, however, that as the blue sky and the white clouds act with almost equal actinic effect upon the sensitized plate, in order to obtain the proper contrast between sky and cloud it is necessary either to polarize the light from the sky, or, as is most commonly done, to separate the coloured rays by allowing them to pass through a yellow screen, and to fall upon autochromatic plates. Before defining the ten principal types of cloud it should be explained that two general classes of clouds are distinguished, separate or globular masses, which are most frequently seen in dry weather, and forms which are widely extended or completely cover the sky, which are typical of wet weather. Both these classes of clouds are found at all heights. =Cirrus= are thin, fibrous, detached, and feather-like clouds formed of ice-crystals. They are the highest of all the clouds, and move with the greatest velocity. =Cirro-stratus= form a thin whitish veil, more or less fibrous, which often produces halos around the sun and moon and other optical phenomena.