jfiDie 14, 1877] 



NATURE 



121 



drop the old. And we are not without hope that Clerk- 

 Maxwell's book may effect the complete abolition of the 

 older methods, which are already sadly shattered. Per- 

 petual "distinctions without a difference," like the three 

 classes of levers above alluded to, can only confuse and 

 irritate the student — often making him doubt whether he 

 really understands the gist of an explanation or no. And 

 to give rules for calculating results without explaining 

 how to obtain these rules, or what they imply physically, 

 is, in the vernacular, simply cra.m : — call it euphemistically 

 what you will. To learn how to do this is not, in any 

 sense, to acquire knowledge. 



Clerk-Maxwell's book is not very easy reading. No 

 genuine scientific book can be. But the peculiar charac- 

 teristic of it is that (while any one with ordinary abilities 

 can read, understand, and profit by it) it is the more 

 suggestive the more one already knows. We may boldly 

 say that there is no one now living who would not feel 

 his conceptions of physical science at once enlarged, and 

 rendered more definite by the perusal of it. Short and 

 (on the whole) simple as it is, it is one of the most sug- 

 gestive works we have ever met with. The following 

 extract needs no comment of ours: let us see how the 

 metaphysicians will digest it : — 



" But, as there is nothing to distinguish one portion o' 

 time from another except the different events which 

 occur in them, so there is nothing to distinguish one part 

 of space from another except its relation to the place of 

 material bodies. We cannot describe the time of an 

 event except by reference to some other event, or the 

 place of a body except by reference to some other body. 

 All our knowledge, both of time and place, is essentially 

 r^a'ive. When a man has acquired the habit of putting 

 words together, without troubling himself to form the 

 thoughts which ought to correspond to them, it is easy 

 for him to frame an antithesis between this relative know- 

 ledge and a so-called absolute knowledge, and to point 

 out our ignorance of the absolute position of a point as 

 an instance of the limitation of our faculties. Any one, 

 however, who will try to imagine the state of a mind 

 conscious of knowing the absolute position of a point will 

 ever after be content with our relative knowledge." 



We can afford space for only one other quotation ; but, 

 as it is very important, we quote in extenso : — 



Article cxiv. — Centrifugal Force. 



" This is the force which must act on the body M in 

 order to keep it in the circle of radius v, in which it is 

 moving with velocity V. 



" The direction of this force is towards the centre of 

 the circle. 



" If this force is applied by means of a string fastened 

 to the body, the string will be in a state of tension. To 

 a person holding the other end of the string this tension 

 will appear to be directed towards the body M, as if the 

 body M had a tendency to move away from the centre of 

 the circle which it is describing. 



" Hence this latter force is often called Centrifugal 

 Force. 



" The force which really acts on the body, being 

 directed towards the centre of the circle, is called Cen- 

 tripetal Force, and in some popular treatises the centri- 

 petal and centrifugal forces are described as opposing and 

 Ijalancing each other. But they are merely the different 

 aspects of the same stress." 



This is one of the few passages in the work to parts of 

 which exception may fairly be taken. Of course, the 



physical statements are correct, and they are very clearly 



put. But it is hardly fair to a junior reader to begin by 

 telling him that Centrifugal Force means the force which 

 must act on a body in order to keep it moving in a circle, 

 and then to say that the force which really acts is called 

 Cenlr'.petal Force ! That force is required to produce 

 change of direction of a body's motion, and that when this 

 is applied by means of a string held in the hand the im- 

 pression on our " muscular sense " is the same as if the 

 body were pullins; at the other end of the string, are 

 facts : but they no more justify the use (however guarded) 

 of the word ''centrifugal" than the tension of the couplings, 

 just before a train starts, proves that the carriages have a 

 tendency to run backwards. 



There is one very great blemish in Clerk-Maxwell's 

 book, from which those of his rivals are comparatively 

 free. Some of the woodcuts are simply atrocious. This 

 must be looked to in future editions, for passages of great 

 importance are at prc5ent rendered totally unintelligible 

 to the beginner : and from this cause alone. 



Clerk-Maxwell's work, then, is simply Nature itself, so 

 far as we understand it. The peaks, precipices, and cre- 

 vasses are all there in their native majesty and beauty. 

 Whoso wishes to view them more closely is free to roam 

 where he pleases. When he comes to what he may fear 

 will prove a dangerous or impassable place, he will find 

 the requisite steps cut, or the needful rope attached, 

 sufficiently but not obtrusively, by the skilful hand of one 

 who has made his own roads in all directions, and has 

 thus estabUshed a claim to show others how to follow. 



In the rival elementary works the precipice and the 

 crevasse are not to be seen : there are, however, many 

 pools and ditches ; for the most part shallow, but vc}y 

 dirty. You are confined to the more easily accessible por- 

 tions of the region. In the better class of such books 

 these are trimly levelled — the shrubs and trees are clipped 

 into forms of geometrical (/,6' , unnatural) symmetry like a 

 Dutch hedge. Smooth straight walks are laid down lead- 

 ing to old well-known " points of view," — and, as in Trinity 

 of ormer days, undergraduates are warned against walking 

 on the grass-plats. 



These "royal roads" to knowledge have ever been 

 the main cause of the stagnation of science in a country. 

 He would be a bold man indeed who would venture to 

 assert that the country which, in times all but within the 

 memory of many of us, produced such mighty master- 

 minds as Lagrange, Fourier, Ampere, and Laplace, does 

 not now contain many who might well have rivalled the 

 achievements even of men like these. But they have no 

 chance of doing so ; they are taught, not by their own 

 struggles against natural obstacles, with occasional slight 

 assistance at a point of unexpected difficulty, but by being 

 started off in groups, "eyes front" and in heavy marching 

 order, at hours and at a pace determined for all alike 

 by an Official of the Central Government, along those 

 straight and level (though perhaps sometimes rough) 

 roads which have been laid down for them ! Can we 

 wonder that, whatever their natural fitness, they don't 

 now become mountaineers 'i 



I still vividly remember the horror with which I 

 v.-atched the the struggles of a former class-fellow of my 

 own, whose friends had just sent him to another school 

 that he might learn geometry a little earlier than was the 

 custom wi^h u;. For him th?re was no longer any pli> — 



