filled with mistrust on observing that this thought, which appeared to me so simple and natural, was not mentioned by any of the numerous natural philosophers who occupy themselves so assiduously with electro-magnetic experiments. I could not believe but that these notions must have struck them; but I was forced to suppose that they had either seen immediately the impracticability of them, or that, even if they had made some experiments upon the subject, they had met with insurmountable difficulties in its application. This long deterred me from making any experiments; but in the lecture which I gave before this Society on the 10th of December 1832, I could not refrain, when speaking of the powerful electro-magnets of Henry and Ten Eyck, from asking the question, "whether such a considerable power as that which is obtained by interrupting the electric current and then restoring it, could not be applied with advantage to mechanical science." After that lecture I considered the subject again, and thought I had convinced myself of its practicability; but that even if it were so, the result could not be very important, because the motion of the keeper must necessarily be very small. Notwithstanding, I had a more powerful electro-magnet made than any I had hitherto possessed, with which I intended to try the experiment; and I regarded the expense the less, as this apparatus appeared to me at the same time to be very appropriate for the evolution of the currents observed by Faraday; for this purpose the armature also must be covered with copper wire, and then each time the poles of the electro-magnet are reversed, a magneto-electric current circulates through this wire. In the mean time I found another method whereby the object I had in view might be effected, and which would allow a greater degree of motion to the armature: I thought I could effect this in the following manner. I placed on the table two cylindrical soft-iron horseshoes bound round with similar wires; so that when the electrical current was transmitted through both wires, the similar poles should lie opposite to each other: between these, and at a small distance from either, I placed a cylinder of soft iron, serving for a keeper; and then I expected to see the armature play to and fro between the two electro-magnets when I sent the electric current first round the one and then round the other electro-magnet. After several fruitless rough experiments it succeeded at last, and I therefore then instructed a turner to make an apparatus, that I might be able to repeat, by means of it, in a more easy and perfect manner, these yet very imperfect experiments. I had proceeded so far, when, on the 4th of January, I received the latest part of Baumgärtner's Zeitschrift, published at Vienna on the 17th of November 1832. I there observed a treatise, intitled, "Electro-magnetic Experiments of Salvatore Dal Negro, Professor of Natural Philosophy in the Imperial University at Padua," (translated from the Italian). The author says in the Introduction:
"Philosophers have already known for some time the power of elec-
