Interesting tidbit from a literary perspective--
In Chapter 51 "The Spirit-Spout" there is an interesting footnote on "the tell-tale that swung from a beam in the ceiling."
The cabin-compass is called the tell-tale, because without going to the compass at the helm, the Captain, while below, can inform himself of the course of the ship.
What intrigues me is that I now understand more behind the name of Edgar Allen Poe's short story "The Tell-Tale Heart." I never looked it up before, never thought to. I thought it was a fancy compound adjective that Poe was using, like the tale that the heart would tell, also like a tattle-tale.
In fact, my Webster's dictionary gives "TALEBEARER, INFORMER" as the first definition of telltale, without the hyphen that comes inbetween the words in the short story.
However, this makes even more sense than just being a plain "informer." If the tell-tale is to show the true direction without even leaving the cabin, in a sense an inner compass, Poe's character shows the true direction of a person's conscience. (Even though he killed the guy anyway in the story--too bad his tell-tale didn't kick in until after he killed the guy.)