I was looking at building a tree house for my son and was wondering how to build a zip line for him. I ran across a few zip line kits that allow you set up a zip line between trees. Some of these kits include everything you need, and some of them include everything except for the steel cable. Some of the zip lines come with harnesses and others just with a handle. There is no need to go into instructions on how to install the zip line, they are...