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THE POPULAR SCIENCE MONTHLY.

conductors, and also to carry the conductor in a conduit laid between the rails, connection being made between this conductor and the moving car by a contact arm carried by the car. The numerous difficulties apparently inherent in all forms of surface or underground conductors have led to their total abandonment in this country in favor of the overhead conductor familiarly known as the trolley system. In this system the conductor carrying the current to supply the moving cars is strung on poles eighteen or twenty feet from the ground, and connection is made between it and the moving car by means of a long arm affixed to the roof of the car, and carrying at its upper end a contaot, generally in the form of a grooved disk. This arm is held against the conductor by springs and is controllable from the car by a rope connection so that it can be pulled down out of connection with the conductor or readily replaced when jolted out of position by the motion of the car. Although this system has encountered much hostility on account of its supposed danger, it has steadily made its way on account of its capacity to adapt itself to all manner and conditions of service, and handle the varying traffic of a street railway expeditiously, economically, and reliably. Inventors are still busy working upon other methods of applying the current, among which probably the most promising is the storage battery. Despite the fact that this method is now in practical service upon two lines in this country, it can not be said to have passed the experimental stage, but it is very generally recognized that this would constitute a final method of electric traction if it can be worked out so as to have the reliability and economy of the trolley system.

This brief epitome of the work in electric traction is set forth in detail by the authors of the present volume. As they state, their work is the first systematic presentation of the subject, and it is therefore a welcome addition to the current literature of the applications of electricity, the more so that it is an extremely well executed one. One of the editors is the editor of the Electrical World, and has, therefore, been in a position to keep himself informed on the history of the subject and to realize what the practical difficulties have been in the development of the art. The authors begin their exposition with a brief consideration of the general electrical principles involved in the dynamo and electric motor, and then pass to a consideration of prime movers, in which they devote considerable space to the theory of the steam engine as well as to a description of the best types of engine to perform the work required in electric traction. They also consider the theory and best forms of water-wheels available. The forms of motors suitable, and the method of mounting them upon the car so as to apply the motion of the armature to the driving of the wheels, are given an amount of space commensurate with their importance, as it is here that the largest amount of detail labor has been necessary in working out the practical problems of the system. A chapter is given to the line, the track, and the power stations, which deal with the practical considerations to be taken into account in this part of the equipment. In the chapter on the efficiency of electric traction there is a very excellent discussion of the subject, ranging from the efficiency of the engine to that of the complete system. A chapter is given to storage-battery traction, in which the authors describe the best forms of battery which have been devised, and point out clearly the difficulties encountered with this form of apparatus, and the large amount of work that yet remains to be done before the storage battery can take its place in economic competition with the trolley. In a chapter on high-speed service an account is given of the experiments conducted by Mr. Crosby at Laurel, Md., with an electric car driven at a speed of over a hundred miles an hour. The authors believe that it is quite practicable to establish an electric railway service in which speeds of one hundred and fifty miles an hour may be maintained, and give calculations of the power required and designs of the electric apparatus and cars. The authors conclude their volume with a chapter on historical notes in which they detail the early experiments in electric traction in which the broad principles were worked out, at a time when the electric battery was the only source of current. Owing to a lack of any economical source of electricity, these early attempts resulted in nothing practical in the way of the establishment of actual roads, but they served the purpose of laying the subject open to future workers and pre-