Who has seen the wind? Neither you nor I: (5-6)
Until we get that question answered, we're guessing the wind will continue in its awe-inspiring ways. When we can't see things we obviously feel, our imaginations tend to run away with themselves, and that's a great thing.
But when the trees bow down their heads, The wind is passing by. (7-8)
When we're feeling amazed, we tend to imbue nature with human characteristics in order to help explain what's happening. Nursery rhymes are no exception. Here the trees have heads like people and they even appear to have a silent understanding with the wind.
But when the leaves hang trembling, The wind is passing through. (5-6)
The trees are not only personified here as they tremble, but they also reflect the amazing force the wind appears to have. We can't see the force but we see the trees move in a way that tells us that being invisible doesn't make something less powerful. We think H.G. Wells would agree.