In an old book (*) I found an advert with a puzzle I had never seen before.

[![Original copy][1]][1]

Unfortunately the book is very rare and it is hard to make out in the scanned photocopy above. I have redrawn it:

[![Goldman's transformation puzzle][2]][2]

It looks a bit like the classic Tangram puzzle, but it consists of three identical trapezoid/trapezium shapes and three triangles of different sizes.
>With these pieces **you have to form a square, and then move only two pieces to form a triangle**.


*) The book was *The Arithmachinist* by Henry Goldman, from 1898. It was a self-published book promoting the [mechanical calculator][3] that he invented. A pdf file of the book can be found on [this page][4]. I have not been able to find a patent, and assume that the patent application was rejected.


  [1]: https://i.sstatic.net/aMfet.png
  [2]: https://i.sstatic.net/aGF6g.png
  [3]: https://youtu.be/_JYz_Q_HEgk
  [4]: http://djm.cc/dmoews.html