A kind of stereographic projection on the plane of the equinoctial; the eye is supposed in one of the poles, so that the tropic, ecliptic, and horizon form the arches of the circles, but the hour-circles are all curves, drawn by means of several altitudes of the sun, for some particular latitude, for every day in the year. The use of this instrument is to find the hour of the day, the sun's azimuth, and other common problems of the globe; as also to take the altitude of an object in degrees.