1. He made the stick in the shape of an arc.
    他把这根棍子做成了弧形。

2. After switching on powerful arc lights, they saw great stalagmites.
    他们打开高功率弧光灯，看到了巨大的石林。

3. After switching on powerful arc lights, they saw great stalagmites---- some of them over forty feet high ---- rising up like tree-trunks to meet the stalactites suspended from the roof.
    他们打开强力弧光灯，看见一株株巨大的石笋，有的高达40英尺，像树干似地向上长着，与洞顶悬挂下来的钟乳石相接。