Page:Popular Science Monthly Volume 7.djvu/29

Rh Prof. Wheatstone's experiments were immediately connected with two large insulated metallic surfaces exposed to the air, so that the primary act of induction—after making the contact for discharge—might be in part removed from the internal portion of the wire at the first instant, and disposed for the moment on its surface jointly with the air and surrounding conductors, then I venture to anticipate that the middle spark would be more retarded than before. And if those two plates were the inner and outer coatings of a large jar or Leyden battery, then the retardation of the spark would be much greater." The experiment was not made for sixteen years. It was then shown as the explanation of the retardation of the current in our subterraneous and submarine wires.

Sir Francis Ronalds, with wonderful prescience, had in 1823—fifteen years before Faraday—suggested "the probability that the electrical induction which would take place in a wire inclosed in glass tubes of many miles in length (the wire acting like the interior coating of a battery) might amount to the retention of a charge, or at least might destroy the suddenness of the discharge." Faraday's prophetic vision and Ronalds's far-sighted knowledge are verified in every working cable. The accuracy with which our cable-repairers are directed by our electricians to the spot where the wire is broken, the exactitude with which the working speed of a cable is predicted, the unfelt and invisible supervision which is exercised over the care and maintenance of our telegraphs—even though they pass through distant countries and different climes—are evidences that electricity, in this particular field, is approaching the last and prophetic stage of its growth. This field is resistance, and Ohm is its prophet.—Telegraphic Journal.



ITH that proneness to go wrong, which we notice in most things human, and which crops out in science as well as elsewhere, the art of making pictures by the chemical action of radiant forces has got a false name. This is all the worse, as it was at first correctly designated, and that too by him who had the clearest right to give the process a title. Davy and Wedgwood, early in the century, had labored to produce sun-pictures by means of the camera-obscura but had met with little success. In 1814 M. Neipce, of Chalons, in France, took up the subject, and, in the course of ten years' assiduous work, he succeeded in a method of forming sun-pictures on chemically-prepared copper, pewter, and glass plates, by which the lights, semi-tints, and shadows, were represented as in Nature, and he also succeeded in 