The early railroad at Menlo Park has gone to ruin and decay, but the crude locomotive built by Edison has become the property of the Pratt Institute, of Brooklyn, New York, to whose students it is a constant example and incentive. Down to the present moment Edison has kept up an active interest in transportation problems. His latest work has been in the line of operating street-cars with his improved storage battery. During the time that this book has been in course of preparation he has given a great deal of time to this question. Some years ago there were a number of street-cars in various cities operated by storage batteries of a class entirely different from the battery invented by Edison. We refer to storage batteries containing lead and sulphuric acid. These were found to be so costly to operate and maintain that their use was abandoned. Mr. Edison's new nickel and iron storage battery with alkaline solution has been found by practical use to be entirely satisfactory for operating street-cars, not only at a low cost, but also with ease of operation and at a trifling expense for maintenance. Of course there have been many problems, but he has surmounted the principal difficulties, and there are now quite a number of street-cars operated by his storage battery in various cities. These cars are earning profits and their number is steadily increasing. XVIII GRINDING MOUNTAINS TO DUST On walking along the sea-shore the reader may have noticed occasional streaks or patches of bluish-black sand, somewhat like gunpowder in appearance. It is carried up from the bed of the sea and deposited by the waves on the shore to a greater or lesser extent on many beaches. If a magnet be brought near to this "black sand" the particles will be immediately attracted to it, just as iron filings would be in such a case. As a matter of fact, these particles of black sand are grains of finely divided magnetic iron in a very pure state. Now, if we should take a piece of magnetic iron ore in the form of a rock and grind it to powder the particles of iron could be separated from the ground-up mass by drawing them out with a magnet, just as they could be drawn out of a heap of seashore sand. If all the grains of iron were thus separated and put together, or concentrated, they would be called concentrates. During the last century a great many experimenters besides Edison attempted to perfect various cheap methods of magnetically separating iron ores, but until he took up the work on a large scale no one seems to have realized the real meaning of the tremendous problems involved. The beginning of this work on the part of Edison was his invention in 1880 of a peculiar form of magnetic separator. It consisted of a suspended V-shaped hopper with an adjustable slit along the pointed end. A long electromagnet was placed, edgewise, a little below the hopper, and a bin with a dividing partition in the center was placed on the floor below.