Experiments of a very definite kind have not yet been made as to the nature of the arc produced by induced currents developed in alternating current machines; but, from the experiments made with electric candles, we are forced to admit that the current reacts as if it were alternately reversed through the arc, since the carbons are used up to an equal degree; and, moreover, Mr. Pilleux's experiments show that effects analogous to those of induction coils are produced by the reaction of magnets upon the arc. There is, then, here a doubtful point that it would be interesting to clear up; and we believe that it is consequently proper to introduce in this place Mr. Pilleux's note: "Having at my disposal," says he, "a powerful vertical voltaic arc of 12 centimeters in length, kept up by alternately reversed currents, and one of the most powerful permanent magnets that Mr. De Meritens employs for magneto-electric machines, I have been enabled to make the following experiments: "1. When I caused one of the poles of my magnet to slowly approach the voltaic arc, I ascertained that, at a distance of 10 centimeters, the arc became flattened so as to assume the appearance of those gas jets called 'butterfly.' The plane of the 'butterfly' was parallel with the pole that I presented, or, in other words, with the section of the magnet. At the same time, the arc began to emit a strident noise, which became deafening when the pole of the magnet was brought to within a distance of about 2 millimeters. At this moment, the butterfly form produced by the arc was _greatly spread out, and reduced to the thickness of a sheet of paper_; and then it burst with violence, and projected to a distance a great number of particles of incandescent carbon. "2. The magnet employed being a horseshoe one, when I directed it laterally so as to present successively, now the north and then the south pole to the arc, the 'butterfly' pivoted upon itself so as not to present the same surface to each pole of the magnet." By referring to the accompanying figure, which we extract from our note on the Ruhmkorff apparatus, it will be seen that the aureola which developed as a circular film from right to left at D, on the north pole of the magnet, N.S. (Fig. 1), projected itself in an opposite direction at C, upon the south pole, S, of the same magnet; but, between the two poles, these two contrary actions being obliged to unite, they gave rise in doing so to a very characteristic helicoid spiral whose direction depended upon that of the current of discharge through the aureola, or upon the polarity of the magnetic poles. On the contrary, when the discharge took place in the direction of the equatorial line, as in Fig. 2, the circular film developed itself in the plane of the neutral line above or below the line of discharge, according to the direction of the current and the magnetic polarity of the magnet.