The Han court was ultimately responsible for the major efforts of disaster relief when natural disasters such as earthquakes compromised the lives of commoners. To better predict and prepare for calamities, Zhang Heng invented a seismometer in 132 CE which provided instant alert to authorities in the capital at Luoyang that an earthquake had occurred in a location indicated by a specific cardinal direction. Even though the earthquake itself could not be felt in the capital when Zhang demonstrated his device to the court by predicting that an earthquake had just occurred in the northwest, a messenger arrived soon after to report that an earthquake had indeed struck 400 km to 500 km northwest of Luoyang. Zhang named his device the "instrument for measuring the seasonal winds and the movements of the Earth"', named because he and others at the time erroneously believed that earthquakes were caused by the enormous compression of trapped air. As described in the Book of the Later Han, the frame of the seismometer was a domed bronze vessel in the shape of a wine jar 1.8 m in diameter and adorned with scenes of mountains and animals. An inverted pendulum was the trigger mechanism, for if disturbed by the ground tremors of both nearby and faraway earthquakes, would release a metal ball from one of eight mobile directions, which would drop into the mouth of one of eight metal toads below arranged like the points on a compass, thereby indicating the direction of the earthquake. The Book of the Later Han states that when the ball fell into any one of eight toad mouths, it produced a loud noise which gained the attention of those observing the device.