Explain the use of irony when gulliver is explaining what a cannon is to the king?

What book is this? Tho, you know irony is when something unexpected happens. And I don't know what book this is, but it is a bit ironic that the king doesn't know what a cannon is, and he is expected to rule a kingdom. You wouldn't expect a king to not know the simplest of things.

You could make your answer based on what I said. Hope this helps!