MR. RHODES' treatise on alternating currents can hardly be regarded as particularly elementary, since he certainly assumes in his readers a preliminary acquaintance with the principles of the subject. We are inclined to think that although the mathematical parts are good, “ihe treatment on the electrical side, especially in the simpler parts, leaves a good deal to be desired. Thus on p. 23 the quantity 2πnL is defined as the reactance, whereas, a few pages further on (p. 31) this name is used for the fuller expression including self-induction and capacity without any explanation as to why the same term is used in both cases; the same observations apply to the definition of impedance. One may be pardoned for laying stress on such faults as these, since the subject is at best a difficult one, and without a perfectly sound knowledge of the fundamental ideas, the student will never make much progress. An Elementary Treatise on Alternating Currents. By W. G. Rhodes. Pp. xii + 211. (London: Longmans, Green and Co., 1902.) Price 7s. 6d. net.