June 23, 1898J 



NATURE 



171 



The letters m and W are the modern dynamical 

 ■equivalents of the B and tt, the ^ea)p»;Tt(c»} and irpaKTiioj, 

 •embroidered on the hem of the robe of the vision which 

 appeared to Boethius in his dungeon, to inspire his 

 Consfllatiott of Philosophy. 



Let the letter W still continue to denote the number of 

 pounds of matter in the body, and let m denote the 

 number of grammes ; let us adopt the method of Prof. T. 

 \V. Wright's " Mechanics," reviewed by Prof Perry, a new 

 edition of which has just appeared, and employ the ab- 

 )lute system with Metric units only, so that the "poundal" 

 ^ merely mentioned once to point out its uselessness. 

 Now Prof Perry can denote W -r- 32-1912 by the letter 

 M, so that the unit of M is a 32-1912 pound shot ; and 

 if he calls M the mass of the body, in opposition to 

 Mr. Love, he is only following the custom which can 

 be traced back through the treatises of Todhunter, Par- 

 kinson, Earnshaw, Whewell, Poisson, Lagrange, &c., up 

 to Euler. 



Thus M. de Freycinet writes, in his Essais sur la 

 ■hhilosophie des Sciences : — 



"•II ne suffit pas d'avoir la notion claire de la masse. 

 II faut aller plus loin. Pour les besoins de la Dynamique 

 il est necessaire de savoir chiffrer les masses. — Une 

 quantity d'eau peu inferieure a 10 decimetres cubes, soit 

 9 litres, 8088, . . . le nombre habituellement designe par 

 la lettre ^, voilk I'unite de masse." 



With these writers we find that the gravitation unit 

 ot force alone is employed, and, contrary to Mr. Love's 

 classification in § 294, the unit of mass is a derived unit, 

 being that quantity of matter which will receive unit 

 acceleration from the gravitational unit of force. The 

 same method is employed in all engineering treatises, 

 but we are inclined to agree with Mr. Love in thinking 

 it might be abandoned with advantage, as being a mere 



lazy device to avoid writing — ; and coming back to 



Euler, we find him explaining at length, in some six 

 pages of his "Dynamics," 1760, that the acceleration 



/; = A — , due to a force P acting on a mass M, and that 



we must take X = 2^, where ^ is taken by Euler to measure 

 the distance a body falls from rest in one second. 



Students will be grateful to the author for the two 

 elegant and complete chapters on two-dimensional 

 Motion of a Rigid Body, a great desideratum. A very 

 large and valuable collection of illustrative examples are 

 brought together, most of which are capable of experi- 

 mental verification in our field of gravity ; and in such 

 cases it would increase the instructiveness to employ the 

 gravitation measure of force, the only one capable of 

 exact measurement. 



" When, as in astronomy, we endeavour to ascertain 

 (these) causes by simply watching their effects, we ob- 

 serve ; when, as in our laboratories, we interfere arbi- 

 trarily with the causes or circumstances of a phenomenon, 

 we are said to experiment " (Thomson and Tait). 



In recording theoretical results of astronomical observ- 

 ation, absolute units are certainly appropriate, but they 

 are all susceptible to the probable error in the determin- 

 ation of the gravitation constant y. 



The author has performed a useful service in § 277, 

 in calling attention to the looseness of the ordinary 

 NO. 1495, VOL. 58] 



school-book definitions, that " the weight of a body is 

 the force with which it is attracted by the Earth." 



But we must return to the charge again, and protest 

 against the assumption that the addition of the word 

 " weight " to " pounds " is required to connote the 

 idea of force. Architects may measure the pressure on 

 foundations in cwt/ft", but there is no such thing in ex- 

 istence as a pressure gauge graduated in Ibs-wt/in* ; it 

 is always in lbs/in^ ; more than that, we doubt the ex- 

 istence of any gauge graduated in dynes/cm^, or barads ; 

 and the stock of instruments at present in use is suffi- 

 ciently large to resist this innovation. The centesimal 

 measurement of time, required for the completeness of 

 the metric decimal system, never came into use, if only 

 because of the number of clocks, watches and chrono- 

 meters in existence ; so that the C.G.S. system is a 

 mongrel one, involving the sexagesimal second of time. 



In the careful examination of the ultimate axioms of 

 Dynamics which he has set himself for reconsideration, 

 the author has thrown down a challenge to the Meta- 

 physicians, in the theory of the relativity not only of 

 motion, of rotation as well as of translation, but also of 

 time, matter, force, &c., which we trust will not pass 

 unnoticed. 



This minute survey of the foundations of Dynamics 

 has, like a visit to the dentist, revealed so many unsus- 

 pected flaws, that it seems doubtful if Dynamics can 

 remain an exact Science. Considering that the gravi- 

 tation of a body varies with the velocity relative to the 

 Earth, how are we justified in accepting the sacred defi- 

 nitions of the C.G.S. units, which may be affected by 

 similar defects ? A spirit of dynamical scepticism is in 

 the air, as testified by the treatises of Mach and his 

 disciples. Hertz, Boltzmann, and by Poincare on Hertz in 

 the Revue g^nirale des Sciences. Maxwell's and Clifford's 

 work does not appear to have influenced the author. 



According to the Preface, "The foundations of Me- 

 chanical Science were laid by Newton" ; but we think 

 that the claims of Galileo are passed over, not to mention 

 Archimedes. Galileo appears throughout this treatise as 

 Galilei ; both forms of the name are correct, according 

 to the German student song — 



" Auch ging er wohl mitunter 

 Zur Kirche als frumber Mann ; 

 Doch beten und singen nicht kunnt er, 

 Schaut lieber zur Decke hinan : 

 Was sah er da in der Hohi ? 

 Tschahi, tschaheia ho — 

 Die Ampel sah Galilei 

 Und auch der Galileo ! " &c. 



Some novelties in the way of nomenclature are wel- 

 come, such as "frame of reference" for "coordinate 

 axes," "localised vector," "kinetic reaction" (due to 

 Mr. Larmor, we believe) for d'Alembert's reversed 

 effective force " ; but when the writer proposes to upset 

 the well-established use of common words, and teach us 

 a new language of recent invention, he might as well set 

 to work to change the names of the stars and planets ; 

 and we are compelled to protest, in the words of Biron, 



" These earthly godfathers of heaven's lights. 

 That give a name to every fixed star, 

 Have no more profit of their shining nights 

 Than those that walk, and wot not what they are." 



A. G. Greenhill. 



