warning piece meaning in Chinese

报警部件

The third day in the morning , the wind having abated over night , the sea was calm , and i ventur d ; but i am a warning piece again , to all rash and ignorant pilots ; for no sooner was i come to the point , when even i was not my boat s length from the shore , but i found my self in a great depth of water , and a current like the sluice of a mill : it carry d my boat a long with it with such violence , that all i could do , could not keep her so much as on the edge of it ; but i found it hurry d me farther and farther out from the eddy , which was on my left hand
如果我把船开进这股急流，船就会被它冲到外海去，可能再也回不到岛上了。说真的，假如我没有先爬上这座山观察到这股急流，我相信一定会碰到这种危险的。因为，岛的那边也有一股同样的急流，不过离海岸较远，而且在海岸底下还有一股猛烈的回流即使我能躲过第一股急流，也会被卷入回流中去。