October 5, 1S93] 



NA TURE 



539 



portant respects. In the first place, the mechanical 

 stresses are proportional to the squares of the quantities 

 that represent the strains ; and in the second place, 

 they depend on the absolute strain, and not on the 

 relative displacement of the parts of the medium. 

 Solid structures can be invented that have laws of this 

 kind. The change of longitudinal stress in a stretched 

 string is proportional to the square of the transverse dis- 

 placement, and, if the ends of the string are fixed, this 

 stress depends on the absolute value of the displacement. 

 Upon a foundation of a somewhat similar kind a theory 

 as to the structure of the ether being like a solid in ten- 

 sion may be founded, which gets over many of the diffi- 

 culties of the simple elastic solid theory of the ether. 

 We are, however, still a good way off any really satis- 

 factory theory as to the structure of the ether, but the 

 leading idea of Maxwell's theory, that electromagnetic 

 attractions and repulsions are due to some sort of strain 

 in the ether, is the direction in which scientific men are 

 at present seeking for a dynamical explanation of electro- 

 magnetism and for a structure of the ether. Prof. Hertz, 

 however, seems content to look upon Maxwell's theory 

 as the series of Maxwell's equations. This is hardly fair. 

 Maxwell has done much more than produce a series of 

 equations that represent electromagnetic actions. Weber 

 and Clausius went very close to that without revolution- 

 ising our ideas as to the nature of these actions. Any 

 exposition of Maxwell's theory which does not clearly 

 put before the reader that energy is stored in the ether 

 by stresses working on strains, is a very incomplete re- 

 presentation of Maxwell's theory. The bulk of Prof. 

 Hertz's work is, however, not concerned with any theory, 

 but with the practical study of electromagnetic propaga- 

 tion along conducting wires and throughout space. This 

 is the work for which Prof.Hertz is so justly famous, and on 

 account of which Hertzian oscillators, Hertzian receivers, 

 Hertzian waves have become in the few years since iSSS 

 the objects of universal attention. No physical experi- 

 ments since those by which Joule founded the theory of 

 the conservation of energy have produced as great an 

 effect on science as these experiments here described by 

 their author. The subject is brought down to last year, 

 and the experiments of others are mentioned and dis- 

 cussed. In this connection it may be worth while re- 

 marking that the observation that the waves emitted by 

 a Hertzian oscillator are of all sorts of wave-lengths was 

 clearly stated by Prof. Hertz himself when he explained 

 how rapidly they died out. For what is a rapidly dying 

 out oscillation except a Fourier series of all sorts of waves ? 

 There is consequently no essential difference between 

 these two statements. The first states more than the 

 second, for it explains the character of what in the other 

 statement is described by the vague term, "all sorts of 

 waves." 



The whole work is most interesting, and well deserves 

 the best attention of all interested in the greatest scien- 

 tific advance of the last quarter of the nineteenth 

 century, a century that has seen thermodynamics 

 founded by Carnot and Clausius, conservation of energy 

 by Joule, bacteriology by Pasteur, the origin of species 

 by Darwin, and the functions of the ether by Faraday, 

 Maxwell, and Her;z. 



NO. 1249, VOL. 48] 



OUR BOOK SHELF. 



Helps to the Study of the Bible. By Henry Frowde. 

 (London, 1893 ) 



The publisher of this useful volume of Helps is to be 

 congratulated on the production of a work which is far 

 in advance of any other book of the same kind published 

 in England. It consists of six parts, which comprise 

 a brief history of the Bible and its most ancient versions, 

 including terse remarks on its canon and authenticity ; 

 a summary of the contents of the books of the Old and 

 New Testaments ; an account of the Apocrypha, together 

 with historical and chronological notices of the period ; 

 a series of chapters on the history, geography, geology, 

 botany, zoology and ornithology of the country of 

 Palestine, on the Jewish Calendar, weights, measures, 

 money and time, and on the musical instruments of the 

 Bible ; and a concordance, atlas, list of obsolete 

 English words, glossary of antiquities and customs, &c., 

 referred to in the Bible. The book represents the col- 

 lected learning of many eminent specialists and scholars, 

 arranged in a handy form and most convenient for 

 reference. The evidence relating to Bible history which 

 may be derived from the recently established sciences 

 of Assyriology and Egyptology, is illustrated by a series 

 of beautiful plates, which cannot fail to be appreciated 

 by every thoughtful reader of the Bible, and are worth 

 more for purposes of explanation than many disserta- 

 tions could ever have been. In the first plate the 

 connection of the Hebrew alphabet with the hieratic 

 writing of Egypt is shown, and from this we are led to 

 the Latin and Greek alphabets and to the Rosetta and 

 Moabite Stones. Facsimiles of the oldest Hebrew and 

 Syriac MSS. of the Bible are next given, together with 

 specimens of the text of the Vaticanus, Sinaiticus and 

 Alexandrinus codices. The funeral customs of the 

 Egyptians are explained by reproductions from bas-reliefs, 

 papyri, iS:c., and from the monuments of Assyria and 

 Babylonia a large number of important illustrations 

 have been selected to throw light upon the various 

 occasions upon which the Israelites came in contact with 

 the "great king." The busts of the Roman emperors 

 referred to in the New Testament, and the Temple of 

 Diana, are the subjects of the plates inserted to illustrate 

 the New Testament. At the foot of each plate is a 

 brief description, which, we must hope, may in some 

 cases be lengthened in future editions of this excellent 

 book. 



Differential Calculus for Beginners. By Joseph 



Edwards, M.A. (Macmillan and Co., 1893.) 

 Mr. Edwards has put together in a handy form for 

 schoolboys the elementary parts of his large treatise on 

 the Differential Calculus. The subject is here presented 

 in a clear and interesting manner for beginners, and it is 

 to be hoped that the book will be useful in leading to a 

 more general study of this indispensable subject than has 

 hitherto been customary in this country. 



The French schoolboy learns the elementary ideas as 

 part of his Algebra, but with us it has been thought right 

 that " calculus dodging " should precede the study of the 

 calculus itself, under a mistaken application of the pro- 

 verb — Principiis enim cognitis, multo facilius extremain- 

 tcllii^etis. 



Geometrical applications are very judiciously intro- 

 duced at an early stage, but considering that the first 

 differential coefficient invented was for the expression of 

 a Velocity, these applications would be rendered more 

 instructive by the introduction of the notion of Time as 

 the primary independent variable. 



But " this is Dynamics " the schoolmaster will say, and 

 so must be kept separate by a sort of water-tight bulk- 

 head. G. 



