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704, and termed a primer of primers. A feature of the work is the appending to each primer of one or more extracts from current electrical works on the subject matter of the primer.

In electricity and magnetism the author deals with the sources and phenomena of static and current electricity and magnetism. His statement of the theories of magnetism is a particularly clear and concise summing up of the present views of the subject, and it is to be regretted that he did not undertake to do the same with the theories of the electric current. In the primer on atmospheric electricity our quite limited knowledge of the subject is presented concisely, though it is to be noted that the author follows the accepted views of lightning protection, and gives no hint of the recent important experiments and theories of Prof. Lodge on this subject.

The second of the books takes its name from the first three primers, which are devoted to the measurement of electric currents, electro-motive force and resistance, and are concerned with an account of how these measurements are made.

The voltaic cell forms the subject of one primer, and thermo-electric batteries of another. The distribution of electricity by continuous currents and the arc and the incandescent light are considered in three primers. In the primer devoted to the alternating current a brief account is given of the modern theory of such a current; and in a primer on alternating currents of high frequency there is an excellent summary of the remarkable experiments of Tesla with such currents. A primer is devoted to induction coils and transformers, one to dynamos, another to the electric motor, and another to the electric transmission of power. Other primers are on electro-dynamics, electro-dynamic induction, and alternating current distribution. The books are printed on good paper, in clear type, and are of convenient size.

researches of Dr. Hopkinson on electro-technical subjects, more especially those upon the dynamo, have long been recognized as of the highest importance, both for their theoretical interest and for their value in the bearing they have upon the work of the practical constructor. The papers in which these researches have been described have heretofore been accessible only in the proceedings of scientific societies and in the technical journals, and are now for the first time collected in the present volume. The collection consists of eleven papers, five of which are devoted to the dynamo, in which are developed the theory and use of what has come to be known as the "characteristic curve of the dynamo."

This curve expresses the relation between the current and electro-motive force of a dynamo at a given speed—the horizontal distances or abscissas representing the amount of current, and the ordinates the electromotive forces—and in the hands of Dr. Hopkinson has been found capable of giving a solution to all the complicated questions of practical dynamo construction. Other papers are: Some Points in Electric Lighting, the Theory of Alternating Currents, the Theory of the Alternate-Current Dynamos, and a report upon the Westinghouse transformers. In the first of these a very interesting mechanical illustration is given of the facts of electrical induction by means of a model, first suggested by the late Prof. Clerk Maxwell, and in the second the proper method of coupling up alternating dynamos in a supply circuit is pointed out, and the conditions for the most efficient action determined. Alike to the student and the practical dynamo designer these papers will prove of the greatest value, and will form a desirable if not essential addition to his technical library.

author of The Naturalist in La Plata gives us in this volume some further account of his wanderings in South America. He calls himself an "idler" here, being made such by an accidental pistol-shot which kept him for some time from active exploration. Yet, though unable to go far afield, Mr. Hudson gathered many curious observations and much store of entertaining anecdote during his idle days. The reader will learn from these chapters that Patagonia is not