Q: Looking for a book about a young boy that jumps into a whirlwind portal Looking for the title of a book, Book was in school and public libraries in the US in the early to mid 1970's.  SF / Young Adult.
Story line was a young boy, out in the woods that sees a dust devil/whirl wind and jumps into it.  He finds himself in another world.   It's similar to "A Connecticut Yankee in King Arthur's Court", but instead of an engineer, it's a young adult.  I seem to remember it was 15th 16th century based story line. 
The main character feels that he's been there for days/weeks but when he finally gets to the wall and jumps, he finds the dust devil is just a short distance from where he jumped in.  
