1.Suddenly, by the light of the lightening, we saw something in the middle of the river.

突然间，借助闪电的光亮，我们看到河中间有东西。

句子中的by意思是“借助”。

Something we go on working after dark by the lights of tractors.

by 还有其他一些意思和用法

1）close to; next to (接近，靠近)

the window by the door.

2）with the use or help of; through(借助于；通过)

We came by the back road.