Observations of the direction of motion, and apparent velocity of clouds at different heights, have been made at Blue Hill several times a day since 1886. To measure the motion of clouds the nephoscope (Fig. 1) is used. It consists of a horizontal circular mirror with a concentric circle of azimuths and an eye-piece _C_, movable in a plane _BD_ at right angles to the mirror and also around it, through which the image of the cloud is brought to the centre of the mirror _A_. It can be proved by geometry that the motion of the cloud-image is proportional to the movement of the cloud itself, so by noting in what direction and how far the image is displaced in a given time, we have the true direction of motion of the cloud itself and also its relative velocity, comparable with the velocity of all clouds having the same height. If the height is known, then the relative velocity can be easily converted into absolute velocity, and thus the velocity of currents at different heights in the atmosphere is accurately ascertained. [Illustration: FIG. 1.--Nephoscope at Blue Hill Observatory.] The height of clouds seems to have been measured trigonometrically from two stations as early as 1644 by Riccioli and Grimaldi, two Jesuits of Bologna, but notwithstanding these measurements and some conclusions derived from observations on mountains, and in balloons, the altitudes of the different clouds were not known with any accuracy until in 1884 Ekholm and Hagstrˆm made a series of trigonometrical measurements upon the different kinds of clouds at Upsala, Sweden. About the same time attempts were made at Kew Observatory to measure clouds by photography, and in 1885 probably the first trigonometrical measurements in America were made at Cambridge, Mass., by Professor W. M. Davis and Mr. A. McAdie. In 1890-91 the Swedish methods were employed at Blue Hill by Messrs. Clayton and Fergusson of the Observatory staff, and until recently the measurements there and at Upsala comprised all that was known accurately about the heights and velocities of the various species of clouds. [Illustration: FIG. 2.--Cloud Theodolite at Blue Hill Observatory.]