With small effort and expense, very important information concerning the atmosphere can be obtained by means of studies of clouds; growing awareness of this, within recent years is indicated by the appearance of new techniques and new forms of instruments—nephoscopes—for the measurement of the heights, motions, and other phenomena of clouds. The larger number of these instruments, accompanied by brief descriptions, have been produced in Europe, and few details of new or older nephoscopes have appeared in America since Abbe's “Treatise on Meteorological Apparatus and Methods” in 1888.

In the present paper are comparisons of the more important nephoscopes in use since the beginning of accurate observations, descriptions of installations found satisfactory, and a bibliography of published researches concerning nephoscopes and observations made with them. The nephoscope of the widest range of use and the most economical to produce is of the reflecting form; (1) the mirror is a disk of black glass, both sides of which are reflectors, so that (2) it may be reversed when one side becomes dulled through use; (3) the diameter of the mirror should be at least 20 centimetres, to permit measurement of rapidly-moving or indistinct clouds; (4) the eyepiece—a small disk in which is a hole about 3 mm in diameter through which the cloud-image is followed—is carried at any desired height above or distance from the mirror, on a flexible, adjustable arm extending from the mirror, or on a separate support or stand.

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