A parallel example is recorded in the twin art of telegraphy. At first it was believed that two wires were indispensable for a circuit. Steinheil showed that a single wire suffices if its terminals are soldered into plates buried in the ground. Thus, at a stroke, by impressing the earth into the service of electrical communication, he reduced the cost of telegraphic lines by one half. In another field the electrician has given himself a good deal of trouble in vain. As it originally streamed from voltaic batteries, the electric current had always a single direction; it was, to use a familiar phrase, a direct current. But when Faraday invented the first dynamo, and produced electricity from mechanical motion instead of from more costly chemical energy, the current was not direct but alternating; that is, its pulses came at one instant from the positive pole, the next instant from the negative. Inventors took great pains in devising apparatus to convert these alternating pulses into a direct current such as that yielded by a voltaic battery. To-day the alternating current for many important purposes, including transportation, is employed just as it leaves the dynamo. Such a current usually has comparatively high tension, at which transmission is much more economical than at low tension, small conductors serving instead of large ones. This advantage in many cases more than offsets the loss entailed by reversal of the magnetic field at each alternation; a loss but small when iron for the electro-magnets is well chosen. Short Cuts in Engineering. Rock may be so hard as to withstand a drill of the hardest steel; then the engineer pours an acid of the necessary dissolving power. A water pipe may freeze at a point difficult of access; it is thawed by the warmth created by an electric current. A surveyor has to reduce to square feet the irregular area of a factory site or a garden plot; around the edge of his diagram he runs a planimeter, it tells him automatically what surface it has surrounded in its excursion. If he has no planimeter, a delicate balance will serve just as well. Let him take a piece of paper, uniform in thickness, and cut it into the shape of the area in question. In weighing the diagram with care he learns its superficies because he knows the weight of each square inch or foot of the paper. Pumps for ages have exercised the wit of inventors who have devised wheels, screws, pistons, and scoops of every imaginable form. M. Giffard boldly discarded all moving parts whatever and in his injector, actuated directly by a blast of steam, provided a capital means of sending water into a boiler.