NATURE 



361 



I 



THURSDAY, AUGUST 16, 1900. 



A STANDARD TEXT-BOOK OF PHYSICS. 

 Miiller-Pouillefs Lehrbuch der Physik und Meteorologie. 

 Neunte umgearbeitete und vermehrte Auflage von 

 Dr. Leopold Pfaundler. In drei Banden. Erster 

 Band, Mechanik, Akustik. Pp. xxi + 888 (1886). 

 Zweiter Band, unter Mitwirkung des Dr. Otto 

 Lummer, Erste Abtheilung, Optik. Pp. xx+1192 

 (1894-1897). Zweite Abtheilung, Warme. Pp. xiv 

 + 768 (1898). Dritter Band, Elektrischen Erschein- 

 engen. Pp. xvi + 1062 (1888-1890). (Braunschweig: 

 Friedrich Vieweg und Sohn.) 



THE appearance of the second part of the second 

 volume of this work marks the completion of the 

 ninth edition of an important treatise on experimental 

 physics which has for many years been widely used in 

 Germany. The importance of the work lies in the fact 

 that it aims at giving a full description of physical appa- 

 ratus and experimental methods, no attempt being made 

 to expound mathematical theories, and none but the 

 most elementary mathematics being employed or 

 assumed as one of the reader's acquirements. 



Herein the work differs from most of our English 

 text-books of physics, in which the tendency has latterly 

 been to combine a certain amount of mathematical 

 theory with short accounts of experiments in illustration 

 of the theory, both the mathematical and experimental 

 portions being of necessity very incomplete. This 

 tendency, probably necessitated by our examination 

 system, will, as long as it continues, prevent our having 

 in English such complete works on experimental physics 

 as that now before us. 



Any work on physics, however, in several volumes 

 produced at different times, must, when completed, 

 present some lack of uniformity among its parts, 

 especially if the part dealing with that branch of the 

 subject which varies most rapidly is not produced last. 

 This is the case in the present instance. The volume 

 on magnetism and electricity was published some ten or 

 twelve years ago, several years before the appearance of 

 the volumes on light and heat. The reason given is that, 

 on account of the rapid advance made in electricity, the 

 volume dealing with this branch in the previous edition 

 appeared much more out-of-date than the other volumes, 

 and therefore had more need of revision. For the same 

 reason, on now reviewing the whole of the present 

 edition, one cannot help being struck with the fact that 

 the volume dealing with electricity and magnetism far 

 less adequately represents the present state of the subject 

 in this branch than do the other volumes in their own 

 regions. 



In the first volume of the present edition, dealing 

 with mechanics and sound, after an introductory chapter 

 on fundamental notions and a short discussion of 

 uniform and uniformly accelerated motion of a point 

 in a straight line, the subject of mass and force is im- 

 mediately taken up, further treatment of kinematics 

 being postponed to a later stage. It seems to the writer 

 to be preferable, especially in an elementary book on the 

 subject, to deal more fully with kinematics before going 

 NO. 1607, VOL. 62] 



on to dynamics proper. The student should first become 

 well acquainted with the notions of velocity, acceleration, 

 their composition and resolution, and should give special 

 attention to cases in which the acceleration is not in the 

 same direction as the velocity. In this way he is enabled 

 to acquire a much clearer idea about acceleration as a 

 quantity with a direction of its own, and is therefore 

 much better prepared to make the transition from his pre- 

 vious vague notion of force to the more accurate dynamical 

 meaning of the term. 



The subject of mass and its measurement is discussed 

 at some length, and in a very instructive manner. 

 The action of a force in producing acceleration in 

 a body is .finally adopted as the basis of the 

 dynamical measurement of mass. This system in- 

 volves the definition of force. A definition of mass 

 (due to Mach) independent of the definition of force 

 is referred to in a footnote on p. 85, viz. : bodies 

 which (by gravitation) produce equal but opposite 

 accelerations in each other are said to have equal masses. 

 This includes the definition of the ratio of the masses 

 of two bodies as the ratio of the accelerations which 

 they produce in each other, and when a unit of mass 

 is chosen, the mass of any other body is measured 

 by the acceleration given to the unit divided by the 

 acceleration experienced by the body itself. 



The phraseology is sometimes not as accurate as one 

 could wish ; thus on p. 92 we find the expression " an 

 acceleration of one metre," and in the following sentence, 

 "a velocity of one metre" ; and again, the kilogramme is 

 stated to be both the unit of mass and the gravitational 

 unit of force. .Although explanations follow, it must lead 

 to some confusion in the mind of a beginner to find that 

 a kilogramme means sometimes a mass and sometimes a 

 force. It is of the greatest importance in an exposition 

 of the principles of dynamics that one meaning only 

 should be attached to every technical term. A similar 

 confusion arises in connection with the term " weight," 

 about which there is a lengthy discussion on pp. 96-99. 

 The difficulty might have been much diminished by re- 

 serving the word kilogramme to mean a mass and 

 weight to mean a force — viz. the resultant force acting 

 on a body falling freely near the earth. The common 

 use of the terms should be explained afterwards. 



On pp. 326-333 a short account is given of the be- 

 haviour of spmning tops and gyroscopes, with a general 

 explanation of the couples called into play by a deflec- 

 tion of the axis of rotation. The " drift " of a shell fired 

 by a cannon is ascribed mainly to gyrostatic action. 

 The constantly increasing angle between the axis of 

 rotation and the direction of motion causes the air in 

 front of the shell to exert a force tending generally to 

 raise the head of the shell with respect to the centre 

 of mass ; this produces a deflection of the point of the 

 shell to the right, and the increased pressure thus intro- 

 duced on the left side causes a deflection to the right. 

 It is possible that, with a shell of suitable shape, the 

 pressure of the air would tend to raise the rear end, and 

 the gyrostatic deflection would in this case be to the 

 left. As is remarked in a footnote, however, the greater 

 friction on the under side of the shell probably plays 

 an important part, and this always causes a drift to the 

 right. 



R 



