What you see here is neither a machine nor a scientific experiment; it is the reading of a letter written by Benjamin Franklin to the Royal Society, that august English scientific institution in London.

This was on 6 June 1751. In the late 1740s Franklin had formed the theory that lightning could be an electrical phenomenon. He sent a letter to the Royal Society presenting a ridiculously dangerous experiment aimed at capturing atmospheric electricity: the electric kite. When outlined to the assembled members of the society, the idea was given a withering reception, but after several more receptive French scientists carried out the experiment successfully, Franklin set about designing the lightning rod.

Because of the enthusiasm it triggered in scientific circles, this letter marks the beginning of the mastery of atmospheric electricity, which no doubt explains why Dufy chose to depict it in his fresco.  Franklin himself can be seen just to the right of the lectern.