A much better writeup is here: http://sometimes-interesting.com/2013/12/04/concrete-arrows-and-the-u-s-airmail-beacon-system/ http://sometimes-interesting.com/

A much better writeup is here: http://sometimes-interesting.com/2013/12/04/concrete-arrows-and-the-u-s-airmail-beacon-system/ http://sometimes-interesting.com/2013/12/04/concrete-arrows-...