The first seismoscope was developed by Chang Heng, the Royal Astronomer of the Bureau of Astronomy and Calendar in the year 132 A.D. during the Han Dynasty period. Initially made of bronze, with a diameter of about six feet, the seismoscope resembled a large urn, on the outside of which were eight dragon heads facing the eight principal directions of the compass. Below each dragon head was a toad with its mouth opened toward the dragon. The seismoscope would vibrate when an earthquake occurred, causing a ball to drop out of a dragon's mouth and be caught by a toad below. The noise produced by the dropping ball served notice of the earthquake, and the particular ball which fell gave the azimuthal direction of the earthquake. The internal mechanism of the seismoscope is unknown, but probably employed either a pendulum or inverted pendulum connected to levers that ejected the balls when the instrument moved. The instrument is reported to have detected an earthquake 400 miles away that was not felt at the location of the seismoscope.