Brother and sister solve mystery involving chambered nautilus

This a kids book I read in the late 1970s, early 1980s. The kids are in a house possibly by a beach, and the mystery might have something to do with Ralph Waldo Emerson. Or maybe a book called The Chambered Nautilus.
I remember borrowing the book from the library. It was a hardback, and might have had a small drawing at the head of each chapter.

Sounds like this earlier search: http://www.librarything.com/topic/172461

That's it! Thank you so much!

I loved Diamond in the Window as a kid. If that's the book you mean. I was immersed. I did not realize until recently it had overtones on transcendentalism.

Touchstone added The Diamond in the Window