This is a type of non-planar clock, constructed using a hollow cylinder. The hour lines and the calendar are drawn on the interior surface. The gnomon is a thin ring not removed from the cylinder. The most common way to read the hour is by means of a hole in the gnomon, through which a narrow beam of sunlight passes to fall on the clock surface. The cylinder must be partially open to allow the gnomon shadow to fall on the interior surface.
To use it correctly the long axis of the cylinder must be placed in an east-west direction, perpendicular to the local meridian..
