请将下面这段话翻译成英文：

地动仪

地动仪是东汉科学家张衡制作的测量地震的仪器。这也是世界上的第一架地动仪。地动仪由精铜铸成，仪器内部中央立着一根铜质都柱;仪体外部周围铸着八条龙，按八个方向布列。每个龙头嘴里衔有一粒小铜珠。地上对准龙嘴处，蹲着八个铜蟾蜍。当某处发生地震，发生地震方向的龙张嘴吐出铜珠，落到铜蟾蜍嘴里。

　　参考译文

Seismometer

A seismometer is a machine that measures motion in the ground made by earthquakes. The very first seismometer was invented by Zhang Heng who was a scientist during the Eastern Han dynasty. His seismometer was in the shape of an urn with eight bronze tubes (one for each of the eight directions) in the shape of a dragon on the outside and a swinging pendulum on the inside. Once motion was felt, a bronze ball would drop through the dragon tube into the awaiting mouth of a bronze toad indicating which direction the earthquake had just occurred.

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