196 



NATURE 



{Dec. 29, 1887 



Now the mathematician noticed that in these equations 

 the quantity w occurs divided by ^, so he said, Let us call 



the quotient — the "mass" of the body, and denote it 



by the letter in, so that w = mg, equivalent to taking 

 g pounds as the unit of mass. 



Unfortunately, in this way the "mass" of a body, 

 which is the measure of an unalterable quantity, is now 

 measured by a variable unit, while the " weight " of a body, 

 which is now defined by the mathematician as the force 

 with which the earth attracts the body, depending on the 

 local value of g, is, although a variable quantity, always 

 represented by the same number— namely, the number of 

 pounds in the body. 



This confusion is entirely obviated if, following the 

 engineers, we discard the word " mass " altogether ; if 

 we measure, as is customary in ordinary life, weight 

 in pounds, and if we change the unit of force to the abso- 

 lute unit, called by Prof James Thomson the " poundal." 

 Now, if a force of p poundals acts on a weight of w 

 pounds, it will produce acceleration a, such that p — wa, 

 and then pt = wv, and ps = ^wv'^ so that ps, the work 

 done in foot-poundals, has the dynamical equivalent |wz/^ 

 foot-poundals of energy ; not, as the footnote to p. 248 



would imply, that ^'nv"- 



because 



but 



TVV- 



because \mv'^ is the kinetic energy in foot-poundals of 

 ni pounds moving with velocity v ; while the impulse pi 

 second-poundals, has the dynamical equivalent of wv 

 units of momentum. 



.■ The unit of momentum has not yet received a name, 

 but the Committee on Dynamics of the Association for 

 the Improvement of Geometrical Teaching is preparing 

 to suggest a distinctive name. 



Supposing then that w represents the weight of a body 

 in pounds, how is it possible, as [asserted on pp. 219, 220, 



that ^ remains constant when 'the body is moved about 



to parts of the earth where g has different values ? and 

 where is the practical value of estimating the effective 

 inertia in terms of the variable unit of mass, as in § 48, 

 when the constant unit of weight would be simpler and 

 practically more intelligible ? 



These theoretical questions of units of force and 

 weight have been discussed here at some length, as it is 

 important that Prof. Kennedy in his next edition should 

 carefully revise this part of the subject, which will best be 

 done if he disregards the discussions on "mass "of the 

 ordinary text-books, and if he writes always in the ordinary 

 vernacular language used by engineers. 



In Chapter XII., on " Friction in Machines," the true 

 laws of friction are given for the first time in any treatise 

 in this country, Morin's illusory laws as usually taught 

 being entirely discarded. With proper lubrication of 

 machinery the question of friction is properly a ques- 

 tion of viscous liquid motion. Some interesting appli- 

 cations, with graphical solutions to such problems as 

 friction-brakes and pulley-tackle, are appended, which 

 ought immediately to be incorporated into academical 

 text-books. Of the same nature are the problems on 

 train-resistance in Chapter IX.: a slip on p. 328 of intro- 

 ducing an extraneous factor, tt, need only be mentioned 

 here, as the author himself has already corrected it. 



We have discussed the dynamical part of the book first, 

 but it is the kinematical part, which treats of mechanism, 

 which forms the greater half of the book. Here the 

 author has analyzed the classification of machines and 

 their elementary parts with great skill and clearness, and 

 illustrated the theory with excellent diagrams. The idea 

 of the "centrode" is largely used in the book, the inven- 

 tion of which is originally due to Belanger. While 

 analyzing fully the centrodes of valve mechanism, the 

 author has mysteriously stopped short of the discussion 

 of valve diagrams, which, in the steam-engine, is the most 

 important practical application of kinematics. Peaucellier's 

 parallel motion is fully described, with Kempe's ampli- 

 fications : it would be instructive to see a diagram of 

 Peaucellier's motion as applied to an actual steam-engine. 

 Proll's velocity and acceleration diagrams are carefully 

 explained, with extensions due to the author : this subject 

 has received considerable development of late from 

 German writers, and is capable of solving very elegantly 

 such difficult and important practical problems as, for 

 example, the determination of the bending moment at 

 any point of a connecting rod. 



A very useful table of moments of inertia concludes 

 the volume, but here we should prefer to see k the radius 

 of gyration, called in this book the radius of inertia, or 

 rather k'-, the square of the radius, tabulated, side by 

 side of the corresponding area A or volume V. 



In conclusion. Prof. Kennedy's students are to be con- 

 gratulated on the possession of such an admirable text- 

 book, and it is to be hoped that the style and influence 

 of its teaching will make itself widely felt outside of 

 professional circles. A. G. Greenhill. 



THE SOLOMON ISLANDS. 

 The Solomon Islands and their Natives. By H. B. 

 Guppy, M.B., F.G.S,, late Surgeon R.N. (London : 

 Swan Sonnenschein, Lowrey, and Co., 1887.) 



THE Solomon Islands, whether we consider the 

 romantic narratives of their discovery and redis- 

 covery, the comparatively unsophisticated character of 

 their inhabitants, their faunistic and floral relationships, 

 or their remarkable geological structure, are of more than 

 common interest to the scientific world, and it is a matter 

 for congratulation that their description has been under- 

 taken by a traveller and historian so eminently qualified 

 for the task as is Dr. Guppy. The book which he has 

 produced is a rich storehouse of interesting and important 

 observations, and will henceforth be an indispensable 

 work of reference to every student of the races inhabiting 

 the Pacific islands. It is worth while to lay stress upon 

 this fact for the sake of encouraging future travellers to 

 give their observations to the world, because Dr. Guppy 

 did not at first intend to make any special investigation 

 of the habits and manners of the inhabitants, but was led 

 to do so by the want of interest displayed by those who 

 seemed to have so much better opportunities. 



The Solomon Islanders seem to be of various types in 

 different parts of the group, but their prevailing charac- 

 ters are distinctly Melanesian or Papuan. A circumstance 1 

 is pointed out which seems to indicate the Indian Archi- : 

 pelago as having been the route by which the Eastern 

 Polynesians reached the Pacific. This circumsta 



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