AN EMBELLISHED BRONZE PORTABLE SUNDIALCHINA, QING DYNASTY, EARLY 19TH CENTURY

The time is shown when, using the compass to point northwards, the shadow of the perpendicular openwork arm is cast on the dial.
Compare with two similar compass-sundials illustrated in The Complete Collection of the Treasures of the Palace Museum - Scientific and Technical Instruments of the Qing Dynasty, 1998, p.25, no.18 and p.27, no.20