Tomahawk trisect an angle

A geometric implement discovered in a 19th century book, and whose inventor is unknown. It essentially consists of a semicircle, a segment which prolongs its diameter and is equal to the radius, and a segment perpendicular to it.

It can be used to trisect an angle, an operation impossible with straightedge and compass. If it is adjusted to an the angle so that

1. The line passes through .

2. lies on line .

3. The line is tangent to the semicircle.

Then angle is equal to one third of .

The interesting fact about the tomahawk is that it can be easily constructed with straightedge and compass. Hence these tools are, from a merely practical point of view, sufficient to trisect an angle. This does not contradict what is known from mathematical theory, since the procedure of shifting a figure on the paper until its parts fall in given positions is not an Euclidean construction.
Wolfram