Observing a light across the water,
the captain had his signalman instruct the other vessel to change her course ten degrees south.

The response was prompt:
Change your course ten degrees north."

"I am a captain," he responded testily.
"Change your course ten degrees south."

The reply: "I'm a seaman first class--change your course north."

The captain was furious.
"Change your course now.
I'm on a battleship."

"Change your course ten degrees north, sir--
I'm in a lighthouse!"