484 



NATURE 



[February 24, 1910 



omissions may be due to the restrictions imposed 

 upon the author by the object he had in view. 



For the rest, the booli is written in carefully con- 

 sidered sequence, the type is clear, and the diagrams 

 are excellent. There is a very useful warning in the 

 introduction to the inexperienced student who is apt 

 to attach exaggerated importance to the precision of 

 numerical results obtained from data which, in them- 

 selves, are liable to errors of observation of no slight 

 moment. The w-arning can hardly be over- 

 emphasised, for the writer of this article has fre- 

 quently noticed the tendency of students to pursue 

 the solution of problems to the third and fourth 

 decimal place when the integers alone cannot be 

 depended upon to the extent of lo, 20, or even 50 per 

 cent. Such fallacious exactitude is perhaps more 

 characteristic of the study of hydraulics than of any 

 other practical science. B. C. 



WORKS ON PHYSICS. 

 (i) L'Electricile coiisidcree comme Forme dc VEncrgie. 

 Electrostatique. Premiere Partic. By Lieut. - 

 Colonel E. Aries. Pp. 176. (Paris : A. Hermann 

 et Fils, igog.) Price 5 francs. 



(2) Lehrhuch der Physik. By E. Grimsehl. Pp. 

 xii+1052. (Leipzig: B. G. Teubner, 1909.) Price 

 15 marks. 



(3) Elements of Physics for Use in High Schools. 

 By H. Crew, revised by F. T. Jones. Pp. xiv+435. 

 (New York : The Macmillan Co. ; London : Mac- 

 millan and Co., Ltd., 1909.) Price 65. 



(4) Light. By Prof. R. C. Maclaurin. Pp. ix+251. 

 (New Yorli : The Columbia University Press, 1909.) 

 Price 1.50 dollars, net. 



(1) A S suggested by the title, the author in this 

 ■^*- pamphlet proposes to alter the present method 

 of measuring electricity. Usually the terms "charge 

 of electricity" and "quantity of electricity" are re- 

 garded as synonymous. Colonel Aries wishes to 

 draw a distinction between them. The former he re- 

 gards in the usual way, but defines the latter as pro- 

 portional to the electrostatic energy, i.e. half the 

 product of the charge and the potential. The con- 

 sequence of this definition is to arrive at the result 

 that the quantify of electricity associated with an m- 

 sulated conductor is not constant, but varies with 

 change of potential, although the charge is unaltered. 

 This the author explains by assuming that the elec- 

 tricity (meaning the quantity — in reality the energy) 

 streams to or fro, as the case may be, between the 

 conductor and the surrounding dielectric. 



To this change of nomenclature the objection may 

 be raised that it leads to no useful result. But 

 Colonel Aries suggests that in this passage of so- 

 called electricity is to be found the explanation of the 

 residual charge of condensers, and that it is identical 

 with ^L^xwel^s "displacement currents." Both sug- 

 gestions are absurd. However, the treatment of the 

 subject does not follow this point of view. Most of 

 the ordinary theorems in electrostatics are proved by 

 the ordinary methods, and it is a little diflficult to see 

 why the author makes the above suggestions at all. 

 The volume is not absolutely confined to electrostatics, 

 NO. 2104, VOL. 82] 



digressions being made into current electricity and 

 magnetism. 



(2) This book covers in some detail the whole ground 

 usually taken in a systematic study of physics. It 

 cannot be described as elementary, as generallv under- 

 stood. The standard is about that required for the 

 pass "degrees in the British universities. Were it in 

 English it could be thoroughly recommended to 

 students taking such courses. Recent experimental 

 work, particularly that concerning radio-activity and 

 electromagnetic radiations, finds a place in the section 

 on electricity, and a number of useful tables of physical 

 constants are given in the appendi.x. The notation of 

 the calculus is used only occasionally, and in most 

 cases, e.g. the estimation of certain moments of in- 

 ertia, simple integrations are performed by laborious 

 methods which avoid that notation. This, we think, 

 is undesirable in a book of this type. If a knowledge 

 of the calculus on the part of the student is assumed 

 — as undoubtedly it should be in this work — it should 

 be used consistently throughout the book. 



It is gratifying to find several chapters in the be- 

 ginning in which the fundamental principles of 

 mechanics are thoroughly dealt with. The greatest 

 difficulty encountered in the teaching of physics is in- 

 sufficient training of the students in this respect. We 

 take exception, however, to the somewhat illogical 

 order of this section. Weight and specific gravity are 

 defined before the questions of force and mass have 

 been considered, and the definition of the unit of mass 

 as " that which weighs one gram in the latitude 45° " 

 is entirely misleading. The book is well printed, and 

 possesses a profusion of diagrams illustrating the ex- 

 periments described in the text. 



(3) It is not often that we meet with a book 

 which so admirably fulfils the purpose for which it is 

 written as this one. It is intended for those who 

 are just commencing the study of physics, and it is 

 written in a way that must appeal to the student. 

 The reason, probably, is that reference is so frequently- 

 made to those common occurrences and mechanical 

 appliances in which the principles of physics are in- 

 volved. Also, although the treatment is quite ele- 

 mentary, phenomena such as the diffraction and 

 polarisation of light are not avoided, as is usually the I 

 case in te.xt-books of this class. A further novel: I 

 feature is the practice of frequently giving references 

 to books by other authors, in cases where the student 

 wishes for a more exhaustive treatment of a particular 

 section of the subject. Numerous worked examples 

 are given, and some four hundred questions are set 

 at the end of the book. 



There are only two faults we have to find. The 

 first is connected with the question of gravitation. 

 It is at least suggested, although not positivelv stated, 

 that the value of "^" should be greater at the bottom 

 of a mine than at the earth's surface. The second 

 fault is by no means peculiar to this book. It is the 

 erroneous view that a particle describing a circular 

 path is in equilibrium, and subject to two forces, one 

 centrifugal and the other equal and centripetal. One 

 may ask, " How is a student to reconcile this with 

 Newton's first law of motion?" 



(4) This volume consists of ten lectures on light. 



