I'm pretty sure I own the copyright,

A gin pole consists of an upright spar that is
guyed at the top to maintain it in a vertical
or nearly vertical position and is equipped
with suitable hoisting tackle. The vertical
spar may be of timber, a wide-flange steelbeam
section, a railroad rail, or similar
members of sufficient strength to support
the load being lifted. The load may be
hoisted by hand tackle or by hand- or
engine-driven hoists. The gin pole is used
widely in erection work because of the ease
with which it can be rigged, moved, and
operated. It is suitable for raising loads of
medium weight to heights of 10 to 50 feet
where only a vertical lift is required. The
gin pole may also be used to drag loads horizontally
toward the base of the pole when
preparing for a vertical lift. It cannot be
drifted (inclined) more than 45 degrees from
the vertical or seven-tenths the height of
the pole, nor is it suitable for swinging the
load horizontally. The length and thickness
of the gin pole depends on the purpose for
which it is installed. It should be no longer
than 60 times its minimum thickness
because of its tendency to buckle under compression.
A usable rule is to allow 5 feet of
pole for each inch of minimum thickness.
Table 5-1, page 5-2, lists values when using
spruce timbers as gin poles, with allowances
for normal stresses in hoisting operations.

Off topic Tim, I was helping relevel a barn and had told my helper to put his thumb over the water level tube while we walked to opposite ends of the building. I couldn't get it to make any sense when I looked around the corner. He was following directions, his thumb was still firmly over the end of the tube.