From AMS Glossary
A direct-vision nephoscope that consists of a grid work of bars mounted horizontally on the end of a vertical column and made free to rotate about the vertical axis.
The observer rotates the grid and adjusts his position until some feature of the cloud appears to move along the major axis of the grid. The azimuth angle at which the grid is set is taken as the direction of cloud motion.
Middleton, W. E. K. 1969. Invention of the Meteorological Instruments. Johns Hopkins Press, Baltimore, . 270– 271.