January 30, 1902 J 



NATURE 



293 



would not move, and the observation was effected by 

 noticing the circle on the diagram which came under 

 the intersection of the cross wires. It is not very clear 

 how the motion in the line of sight was measured, or 

 how the effects of foreshortening were removed. There 

 was no illumination, and the observations were confined 

 to daylight. The general effect of the wind is to make 

 the top of the Tower describe an ellipse, and several 

 diagrams are reproduced showing the effect of the greatest 

 storms. The maximum displacement occurred during 

 the storm of December 20, 1893, when the major axis 

 of the ellipse was o'lo m., and the minor axis 006 m. 

 The time occupied in the description of the ellipse would 

 have been interesting, but is not given. The measured 

 velocity of the wind at the moment of observation is re- 

 corded as 3r8 m. per second, or 71 miles per hour. 

 During this storm a velocity of 44 m. per second, 

 or 98 miles per hour, was recorded, but at that 

 moment the major axis of the ellipse was only 

 006 m. This seems to have occasioned M. Eiffel 

 some surprise, but fortunately these excessive gusts 

 are generally operative over a very small area, and the 

 total wind force on the Tower is not to be measured by 

 that experienced at a point very near the anemometer. 

 The same apparatus has served for the measurement of 

 the effects of temperature. The curves traced are 

 generally of a complex character, depending on the 

 position of the sun, and consist mainly of small excursions 

 into the north-west and west quadrants. An example is 

 given of the motion on a very hot day in August, when 

 the centre of the diagram practically traced an elongated 

 ellipse, 24 cm. in length, parallel to the east and west 

 direction. 



The researches summarised in the volume appear to be 

 rather unequal in value, and a very small connection 

 with the Tower affords a sufficient warranty for their 

 introduction. Thus we get some account of the recent 

 balloon ascent of M. Santos Dumont, because it was a 

 part of the scheme for testing the capacity of giving 

 definite direction to such an apparatus that the Eiffel 

 Tower should be included in the closed curve to be de- 

 scribed by the aeronaut. Perhaps, however, one under- 

 estimates the part played by the Tower in this instance, 

 for M. Emmanuel Aime, slightly changing the well-known 

 aphorism of Voltaire, assures us if the Tower did not 

 exist it would be necessary to invent it for the necessities 

 of aerostation. U seems, however, that M. Dumont 

 prefers to pursue his experiments where he gets no assist- 

 ance from the lofty structure. This tendency to stray 

 from the subject is still more noticeable in the appendix, 

 where we get a chapter "renfermant une notice sur les 

 travaux executees par mes etablissements industriels de 

 1867 a 1890." We have no desire to quarrel with M. 

 Eiffel on this ground. He has carried out many great 

 and difficult works in various parts of the world, and is 

 to be congratulated on the success that has generally 

 attended them. In forming our estimate of what he has 

 accomplished for engineermg science he should not be 

 judged simply by the most popular or conspicuous 

 example of his talent, but by the work of his whole 

 career, which he may contemplate with complete 

 satisfaction. 



NO. 1683, VOL. 65] 



VOIGTS ELEMENTARY MECHANICS. 

 Elemcntare Mechaiiik ah Einleifmig in lias Studiiim der 



theorclischen Physik. \'on W. Voigt. Zweite umgear- 



beitete Auflage. Pp. x -(- 578. (Leipzig : Veit, 1901.) 



Price Mk. 14. 

 T^HE object of this book is to provide the student of 

 -L physics with a working knowledge of theoretical 

 mechanics. With this view the reader is introduced suc- 

 cessively to dynamics of a particle, dynamics of rigid 

 bodies, attractions, hydrodynamics, elasticity ; in each 

 department statics holds a subordinate position, equili- 

 brium being treated as a particular case. The design of 

 presenting, within the compass of a volume of moderate 

 size, an account of the things that are fundamental in the 

 mechanics of bodies, whether solid or fluid, rigid or de- 

 formable, is entirely laudable. It brings into prominence 

 the essential unity of subjects which are frequently treated 

 as independent of one another ; it imposes a selection of 

 the topics to be discussed, and thus results in the elimina- 

 tion of much that is artificial and conventional though 

 sanctioned by tradition. 



A critical discussion of the principles of mechanics 

 would perhaps have been out of place in a work of this 

 character ; at any rate it is not attempted by the author. 

 His standpoint, so far as it is indicated, would appear to 

 be nearer to that of Thomson and Tait's " Natural 

 Philosophy" than to that of Kirchhofit's " Vorlesungen 

 liber mathematische Physik, Mechanik." As regards 

 methods, it is noteworthy that the author makes compara- 

 tively little use of the conception of energy, and that he 

 does not introduce Lagrange's equations. Accordingly, 

 the stability of floating bodies is discussed geometrically 

 after the manner of Dupin, and the small oscillations of 

 a system with a finite number of degrees of freedom are 

 not discussed at all. On the other hand, space is 

 found for an account of " vector fields " and " tensor 

 fields." The distribution of velocity in a fluid affords an 

 example of a vector field, the distribution of strain in a 

 body affords an example of a tensor field ; with a vector 

 field there is associated at each point a directed linear 

 segment, with a tensor field there is associated at each 

 point a certain surface of the second degree. Most 

 recent continental writings on physical mathematics 

 treat of vector fields. The chapter devoted to the 

 dynamics of rigid bodies is made unusually interesting 

 by the use of the theories of several pieces of apparatus — 

 the balance, bifilar suspension, Atwood's machine, F"ou- 

 cault's pendulum — as illustrations of the mode of forma- 

 tion and solution of equations of equilibrium or motion 

 the theory of the application of the pendulum to the 

 determination of the acceleration due to gravity is also 

 given. The treatment of rolling friction, of which two 

 accounts, apparently conflicting with each other, are 

 given in two separate articles, leaves something to be 

 desired. An e.xcellent feature of the book is the emphasis 

 laid on the "dimensions" of physical quantities; no 

 quantity is introduced without an explicit statement of 

 its dimensions in terms of the units of mass, length and 

 time. 



The plan and purpose of the book require that the 

 reader should not be assumed to possess a knowledge 



