n.击穿；飞弧；严重打火
动词arc的现在分词.
The sun appears to move in an arc across the sky. 太阳看起来在天空以弧线运行。
An early instrument for measuring altitude of celestial bodies, consisting of a90= graduated arc with a movable radius for measuring angles. 象限仪用来测量天体纬度的一种早期工具，由一个有90度刻度的圆弧和一个用来测量角度的可移动的半径构成
