A nephoscope is a 19th-century instrument for measuring the altitude, direction, and velocity of clouds, using transit-time measurement. This is different from a nephometer, which is an instrument used in measuring the amount of cloudiness. Description A nephoscope emits a light ray, which strikes and reflects off the base of a targeted cloud. The distance to the cloud can be estimated using the delay between sending the light ray and receiving it back: Mirror nephoscope Developed by Carl Gottfrid Fineman, this instrument consists of a magnetic compass, the case of which is covered with a black mirror, around which is movable a circular metal frame. A little window in this mirror enables the observer to see the tip of the compass needle underneath. On the surface of the mirror are engraved three concentric circles and four diameters; one of the latter passes through the middle of the little window. The mirror constitutes a compass card, its radii corresponding to the cardinal points. On the movable frame surrounding the mirror is fixed a vertical pointer graduated in millimeters, which can be moved up and down by means of a rack and pinion. The whole apparatus is mounted on a tripod stand provided with leveling screws. To make an observation, the mirror is adjusted to the horizontal with the leveling-screws, and is oriented to the meridian by moving the whole apparatus until the compass needle is seen through the window, to lie in the north-south line of the mirror (making, however, allowance for the magnetic declination). The observer stands in such a position as to bring the image of any chosen part of a cloud at the center of the mirror. The vertical pointer is also adjusted by screwing it up or down and by rotating it around the mirror until its tip is reflected in the center of the mirror. As the image of the cloud moves toward the circumference of the mirror, the observer moves his head so as to keep the tip of the pointer and the cloud image in coincidence. The radius along which the image moves gives the direction of the cloud's movement, and the time required to pass from one circle to the next its relative speed, which may be reduced to certain arbitrary units. This instrument is, however, not very easy to use, and gives only moderately accurate measurements. Comb nephoscope Developed by Louis Besson