http://en.wikipedia.org/wiki/Grasshopper_escapement
www.bhi.co.uk/update/Grasshopper.pdf
http://www.angelfire.com/ut/horology/grassh.html
http://www.woodworkforums.com/f168/grasshopper-escapement-100836/
http://www.bealltool.com/clockforum/index.php?board=2.0
http://www.vectric.com/forum/viewtopic.php?f=5&t=10596