Lamarck, the celebrated naturalist, in the opening year of the present century, first proposed a classification of cloud forms. Two years later Luke Howard, a London merchant, published his epoch-making essay on _The Modifications of Clouds_. The theories there advanced and the nomenclature proposed have been accepted generally to our day, notwithstanding the more complete classifications devised by PoÎy, Ley, and others. Howard believed that clouds are formed by the aqueous vapour which rises from the earth, and that the globules which compose them are solid, and are not filled with hydrogen gas as had been maintained by Deluc and De Saussure. Howard classified the clouds as we do to-day, according to their appearance, into three principal types, viz. stratus, cumulus, and cirrus, which represented also low, middle, and high clouds. Stratus is the sheet of low-lying cloud which forms at night, and commonly rests on the earth; cumulus is the heaped-up cloud of the day-time; and cirrus is the curl cloud of the high atmosphere. These three types were further divided into four intermediate types, viz. nimbus, cumulo-stratus, cirro-stratus, and cirro-cumulus. Howard's nomenclature was used almost exclusively, until in 1889 the International Meteorological Conference that met at Paris recommended the adoption of another classification, based on Howard's, but modified by two experts, Abercromby of England and Hildebrandsson of Sweden. This classification also disregarded the origin of clouds, and was based only on their appearance. The next year an atlas, with coloured pictures of the clouds, separated according to the new nomenclature, with descriptive text, was prepared by Dr. Hildebrandsson, assisted by Drs. Neumayer and Kˆppen of the Deutsche Seewarte, or German National Meteorological Observatory. This atlas was adopted by the principal meteorological institutions on the continent of Europe for their observers. The preface contained the following statement: "The study of the forms of clouds is daily increasing in importance, both from the standpoints of theory and of weather prediction. Observations taken at the bottom of the atmospheric ocean are plainly insufficient to determine its circulation. The clouds, however, furnish information about the condition and motion of the air at various levels. But, a comparison of the observations of different observers is only possible when the same ideas are connected with the same expressions. It is hardly possible to give a sufficient verbal description of such indeterminate and changeable forms as those of the clouds; graphical representations are therefore necessary, with the help of a short description, in order to enable an observer to connect what he sees in the sky with what he finds in the instructions. In order that a cloud picture may be intelligible to non-specialists, the clouds and the blue sky must, at least, be plainly distinguishable from each other."