I always carry a spare compass because as this story demonstrates with the little details available, people can and do lose compasses and can find themselves requiring assistance when they would least suspect it.

Judging by some of the comments here, the overall idea and concepts of carrying a spare compass is not worth it. Sorry for posting the link...
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Earth and sky, woods and fields, lakes and rivers, the mountain and the sea, are excellent schoolmasters, and teach some of us more than we can ever learn from books.

John Lubbock