张衡和地动仪

　　在北京中国历史博物馆的展览大厅里，陈列着世界上第一架地动仪的复原模型。这架地动仪的发明者是中国东汉时著名的科学家张衡。

　　张衡(公元78——140年)，河南南阳人。他勤学好问，博览群书，特别爱好天文、历法和数学，是一位博学多才的科学家。

　　公元132年，张衡在京城洛阳制成了可以测定地震方向的“候风地动仪”。地动仪全部用精铜铸成，外形像一个带盖的大茶杯。仪器表面铸有八条垂直向下的龙，龙头分别对准东、南、西、北、东南、东北、西南、西北八个方向，每条龙的嘴里都含着一个铜球。在对着龙嘴的地上，蹲着八个仰着头、张着嘴的銅蟾蜍(chan chu)。地动仪的内部结构非常精细巧妙，当某个方向发生地震时，仪器上对着那个方向的龙嘴就会张开，铜球就会掉进铜蟾蜍的嘴里，自动报告发生地震的方向。公元138年的一天，地动仪西边的龙嘴吐出了铜球。果然，远在千里之外的陇西(今甘肃省)在这一天发生了地震。这是人类第一次用仪器测报地震。在欧洲，直到1,700多年后，才发明了同类的仪器。

　　张衡还制造出了世界上第一架测量天体位置的水运浑天仪，凡是已知的重要天文现象，都刻在这架仪器上。人们可以通过浑天仪观测到日月星辰运行的情况。张衡又是一位机械工程师，制造过能飞的“木雕”和能计算里程的“计里鼓车”等。

　　人们非常敬重张衡这位1,800多年前的大科学家，经常举行纪念活动，表示对他的敬意。月球上有一座环形山是以他的名字命名的，这也是世界人民对他的纪念。

　　In the Exhibition Hall of the Museum of Chinese History in Beijing, there is a restored model of the first seismograph. The inventor was Zhang Heng, a famous scientist in the Eastern Han Dynasty (25-220 AD).

　　Zhang Heng (78-140 AD) was from Nanyang of Henan Province. He studied diligently, and was especially fond of astronomy, calendar and mathematics.

　　In 132 AD, in the then national capital of Luoyang, Zhang Heng made the ancient seismograph to determine the direction of an earthquake. It was made of fine copper, and looked like a big cup with a lid. The instrument was cast with eight dragons on the surface (whose heads pointed in eight directions -east, south, west, north, southeast, northeast, southwest, and northwest), and each dragon had a copper ball in the mouth. On the ground below the dragons there were eight copper toads raising their heads and opening their mouths opposite the dragons'mouths. The inner side of the seismograph was ingeniously constructed: when an earthquake occurred, the dragon facing that direction would open its mouth, and the ball would fall into the toad's mouth, automatically indicating the direction of the earthquake. One day in 138 AD, the dragon in the west expelled its ball. As expected, an earthquake had occurred that day in Longxi (present-day Western Gansu Province) a thousand kilometers away. It was the first time that mankind had used an instrument to detect an earthquake. It was over 1,700 years later that a similar instrument was invented in Europe.

　　Zhang Heng also made the first water-driven celestial globe in the world to measure the position of celestial bodies, which was carved with known important astronomical phenomena. People could observe the movement of the sun, moon and stars. Zhang Heng was also a mechanical engineer, and made a flying "wooden eagle"and a "mileage-counting drum-cart".

　　People highly esteem Zhang Heng, a great scientist living more than 1,800 years ago, and they often hold commemorative activities to show respect for him. A ring of hills on the moon was named after him.