NATURE 



289 



THURSDAY, JANUARY 24, i{ 



MR. GRANT ALLEN'S NOTIONS ABOUT 

 FORCE AND ENERGY. 



I'^orce and Energy : a Theory of Dynamics. By Grant 

 Allen. Pp. 161. (Longmans, Green, and Co., 1888.) 



THERE exists a certain class of mind, allied perhaps 

 to the ancient Greek Sophist variety, to which 

 ignorance of a subject offers no sufficient obstacle to the 

 composition of a treatise upon it. It may be rash to 

 suggest that this type of mind is well developed in 

 philosophers of the Spencerian school, though it would 

 be possible to adduce some evidence in support of such a 

 suggestion. 



In the volume before us Mr. Grant Allen sets to work 

 to reconstruct the fundamental science of dynamics, an 

 edifice which, since the time of Galileo and Newton, has 

 been standing on what has seemed a fairly secure and 

 substantial basis, but which he seems to think it is now 

 time to demolish in order to make room for a newly 

 excogitated theory. The attempt is audacious, and the 

 result— what might have been expected. The performance 

 lends itself indeed to the most scathing criticism ; blunders 

 and misstatements abound on nearly every page, and the 

 whole structure is simply an emanation of mental fog. 



Thus much it is necessary to say in order to give an ade- 

 quate idea of the nature of the book ; but, having said as 

 much as this, it is possible to speak quite otherwise of the 

 friendly tone and apparently candid modesty in which the 

 preface has been written. The preface, indeed, almost 

 disarms a critic, or at any rate it causes him to use the 

 blunt end of his lance ; and were it not necessary to call 

 attention to the erroneousness of such of the doctrines 

 as are new, and expose their hoUowness to a number of 

 unlearned persons who are always eager to see some flaw 

 found in a universally accepted theory the difficulties of 

 which they have never mastered, and which they therefore 

 vainly hope will turn out incorrect, one would gladly 

 accept the apology and explanation of the preface and let 

 the work sink into oblivion unnoticed. 



Its pretentious form, however, renders criticism neces- 

 sary ; and indeed criticism is expected by the author, 

 though, as he naively confesses, it will not undermine his 

 own opinion of the truth and value of his work. 



The book is in two parts : Part I. "Abstract or 

 Analytic"; Part II. "Concrete or Synthetic." The second 

 part consists of more or less popular illustrations of the 

 doctrines inculcated in the first part : it need not there- 

 fore much concern us. The first part starts with chapters 

 on " Power," " Force," " Energy " ; later on it has three 

 chapters called respectively, " The Persistence of Force,'' 

 "The Conservation of Energy," "The Indestructibility of 

 Power." These last headings are not very promising ; and 

 the performance does not belie the promise. 



Chapter II. leads off with the following definition : — 



"A Force is a Power which initiates or accelerates 

 aggregative- motion, while it resists or retards separative 

 motion, in two or more particles of ponderable matter 

 (and possibly also of the ethereal medium)." 

 Vol. xkxix.— No. 1004. 



In other words, the author agrees to limit the term 

 " force" to that which is commonly known as attraction. 

 Very good. Now take Chapter III. : — \ 



" An Energy is a Power which resists or retards aggre- 

 gative motion, while it initiates or accelerates separative 

 motion, in two or more particles of ponderable matter, or 

 of the ethereal medium." 



In other words, he is going to denote that which is com- 

 monly known as repulsion by the name " energy.'' He 

 by no means always adheres to the limitations imposed 

 by this definition, and frequently he means by the term 

 energy the same as is ordinarily understood by the term, 

 though he does so most happily in cases where the result 

 can be looked upon as a separation of some bodies. 



Thus, for instance, the upward motion of a cannon-ball 

 he would style an energy, because it separates the ball 

 from the earth ; the strain in a bow he would call an 

 energy, because it can separate the arrow from the bow ; 

 heat he would call energy, because it usually expands 

 things ; but the horizontal motion of a cannon-ball or a 

 railway train he will have a difficulty in calling an energy, 

 and, in fact, is unable to do so without a flagrant mistake 

 of principle in the case of the ball, and some manifest 

 special pleading in the case of the train. Still more un- 

 able is he properly to apply the term energy to the 

 downward motion of a falling body. We shall see what 

 he makes of this obvious difficulty shortly. Nevertheless, 

 this is the main doctrine in the book — viz. that anything 

 tending towards aggregation is a force, while anything 

 tending to disgregation is an energy. 



Passing, then, to the next chapter, " The Species of 

 Force," he subdivides force into attraction between large 

 masses, or gravitation — between molecules, or cohesion — 

 between atoms, or chemical affinity — and, lastly, between 

 electric charges. It may as well be said, once for all, 

 that throughout the book there are many lame and hesi- 

 tating references to electricity and ether, which are so 

 vague as to be quite harmless, and which it will be the 

 most charitable plan to simply ignore. We had better 

 ignore most of the chemical and molecular statements 

 also, for the same reason, and attend only to those which 

 deal with ordinary lumps of matter. But even here it 

 will not do to criticize the language too closely, or one 

 would have enough to do. This, for instance, is one of 

 the first sentences about gravitation : " When an aerolite 

 comes within the circle of the earth's attraction, it is 

 Gravitation which makes them leap towards one another." 

 " The circle of the earth's attraction " and the sudden 

 "leap" of the aerolite when it comes within this circle, 

 are phrases which scarcely express quite accurately the 

 facts of the case ! 



The next sentence" throws a flood of light upon the 

 state of the writer's mind when he formed his conception 

 of the difference between force and energy, and explains 

 also, I venture to surmise, why his doctrine found so 

 blithe an acceptance with Mr. Edward Clodd, as Mr. 

 Clodd's own book and the preface to this one inform us. 

 It is the orbital energy of the moon which counteracts the 

 aggregative power of gravity. " If the moon were to lose 

 its orbital Energy, Gravitation would pull it to the earth." 



This sentence is not indeed untrue, but it is significant 

 as showing that it is the old puzzle of centrifugal force or 



O 



