I know. The point is that the cross product of the field and anything--the direction of your shoelaces, if you like--is going to be perpendicular to the field, so it can't be used to travel along the field.
Did you get the book?
Yes! Thank you very kindly. It was a thoughtful gesture, and much appreciated. I sent a Freep note of thanks to your old screen name, as I didn't know you'd changed. It looks pretty dense, but I'll try to work at least the first chapter into my hopelessly long reading list.
Ask RadioAstronomer, he's seen my backlog. :-)