350 Scientific Intelligence. 



devoted to the theory of a solid isotropic earth, to the interpre- 

 tation of seismograms, to the determination of epicenter and 

 focus, and to statistical problems. The presentation of the subject 

 is clear and logical, the illustrations are well selected and neatly 

 reproduced, and the entire volume should be very useful, both as 

 an introduction to this important field of investigation and as a 

 practical guide to the use of seismographs. A pocket on the 

 inside of the back cover of the book contains three specimen 

 plates of seismograms recorded by Galitzin instruments at 

 Eskdalemuir at the times of the Dardanelles and Zante earth- 

 quakes, h. s. u. 



11. Alternating Currents and Alternating Current Machinery • 

 by Dugald C. Jackson and John Price Jackson. Pp. ix, 968 ; 

 526 figures. New York, 1913 (The Macmillan Co.).— Since the 

 appearance of the first edition in 1896 the subject has grown 

 rapidly so that the present volume has been rewritten and greatly 

 extended as compared with the earlier edition. (See this Journal, 

 vol. ii, page 455, 1896.) Some of the improvements of the text 

 may be briefly stated as follows: More attention is paid to the 

 transient state in electric circuits than formerly. A considerable 

 amount of related matter has been introduced in respect to vec- 

 tors, complex quantities and Fourier's series. The treatment of 

 power and power factor has been given great attention, and an 

 entire chapter is now devoted to the hysteresis and eddy current 

 losses which are developed in the iron cores of electrical 

 machinery. The discussion of synchronous machines and of 

 asynchronous motors and generators has been amplified and made 

 more complete. Finally, the treatment of the self-inductance 

 and mutual-inductance of line circuits and skin effect in conduc- 

 tors has been extended and supplemented by the consideration 

 of electrostatic capacity of lines and the influences of distributed 

 resistance, inductance, and capacity. 



The manner of presentation is intended to serve a twofold pur- 

 pose, namely, to fulfil the requirements of classes in engineering 

 schools and to serve as a reference book for electrical engineers. 

 Scattered throughout the volume may be found 208 problems for 

 solution by the student, and also a fairly large number of foot- 

 notes referring to other standard texts and original journal articles. 

 The book under consideration is apparently the best and most 

 up-to-date on the subject in the English language. h. s. tj. 



12. The mutual Repulsion of rigid Parallel Plates separated 

 by a Film of Air • by C. Babus. (Communicated.) — By the ap- 

 plication of displacement interferometry to the horizontal pen- 

 dulum,* I find that two parallel rigid plates whose distance apart 

 is of the order of l ram and less repel each other in air, with a force 

 far in excess of their gravitational attraction. This force increases 

 rapidly (certainly as fast as the inverse square) as the distance of 

 the plates decreases and vice versa, but can be recognized beyond 

 a millimeter of distance. For brass plates 20 cm in diameter and 



* The full method will be shown later in this Journal. 



