tacks the lead plate to which it goes, thus forming peroxide of lead, and the hydrogen reduces any oxide that may be on the other lead 
Fig. 3.plate, thus producing pure lead, some of the surplus hydrogen forming as a film upon the surface. The charging current is then reversed, so that the latter plate is now attacked, and is then reversed again; the effect of these operations being to render the surfaces of both lead plates porous so that they present a large surface, and can therefore hold a great deal of peroxide of lead. When the charging current is broken, the oxygen, which has been forcibly separated from the liquid, seeks to recombine in the same way that a stone which has been forcibly separated from the earth seeks the earth when liberated. If now the two lead plates be joined with a wire, the effect of the oxygen in the peroxide of lead trying to recombine is to generate an electrical current in the opposite direction to the original one; and this is the current which is utilized. The value of accumulators would be much increased if this return current could be made greater, and if the weight and cost of the accumulators themselves could be made less. At present, however, their use is restricted by reason of their great cost and weight, and by the small ratio (about fifty per cent in practice) of the electrical energy returned to that expended in charging them. Nevertheless, the fact that the accumulator system of electric railroading obviates the necessity for any conductors, which sometimes are inconvenient and expensive, and which themselves occasion great loss of electrical energy, leads many to believe that for short routes, as upon street-car lines of cities, accumulators will be very efficient.
At the Chicago Exposition of Railway Appliances, which has just closed, the system of Messrs. T. A. Edison and S. D. Field, of New York, was tried, and with undeniable success. By this system a third conductor is used; but it is not placed upon poles, as in the Siemens system (for this would not be practicable in the streets of a city), but lies in a long sunken trough which runs between and parallel to the rails. The trough is covered, and a long and very narrow slit runs the whole length of the cover. Through this slit extends a strong metallic rod which is connected mechanically with a contact-carriage lying upon the conductor, and which is mechanically and electrically connected with the car.
It is claimed that by means of a scraper, carried by the contact-carriage, there will be no trouble occasioned by any accumulation on the conductor of ice, snow, or mud, but that the car can be satisfactorily run in all kinds of weather.
Fig. 4 represents the generator and track as arranged at the Chi-
