March 24, 192 1] 



NATURE 



99 



the more equivocal result of the applause of a 

 crowd of allies and sympathisers, members per- 

 haps of the same university, pupils and admirers, 

 supporters of all kinds. As a rule, a halo-wearer 

 can do no wrong-. This royal prerogative is some- 

 times mildly disputed, but the disputer generally 

 gets the worst of the discussion, and, unless he is 

 pachydermatous, is duly sorry for himself. 



One of the most reverenced of halo-wearers is 

 Clerk Maxwell, who holds his great place by 

 patent given from the highest source of all such 

 dignity. In his writings originality of thought 

 was accompanied always by felicity of phrase 

 and expression, tempered with a savour of wit 

 which is found only in men of subtle and pene- 

 trating humour, that wit which is, above all 

 things, a saving grace in literature, and especially 

 in science. Read his address " On the Mirror Gal- 

 vanometer," supposed to be delivered to a pupil 

 in an alcove with drawn curtains ; in spite of the 

 somewhat unpromising subject, it is as good as — 

 nay, some would say it is much better than — its 

 prototype, "The Splendour Falls on Castle 

 Walls," or, indeed, almost any other lyric in 

 Tennyson's "Princess." 



From time to time Clerk Maxwell wrote on 

 elementary science in a way which attracted the 

 attention and enchained the admiration of every- 

 one. The first of these writings was " The Theory 

 of Heat"; the second, published in 1877, was 

 " Matter and Motion." Both were unique. In 

 various respects — e.g. in the question of entropy — 

 the book on heat was open to objection, but as a 

 presentation of thermodynamic theory it was, and 

 has remained, unrivalled. We prefer the thermo- 

 dynamic relations in the form which they take 

 when the steps of temperature, pressure, volume, 

 etc., are infinitesimal, and the notation (easily 

 •explained and understood) of infinitesimals is 

 used ; but this is a detail of no great importance. 

 Nothing could exceed the elegance of the dis- 

 cussion, the importance of the semi-graphical, 

 semi-analytical treatment of the energetics of the 

 subject, and the theme of available energy. 



In " Matter and Motion " the subject was really 

 Newtonian dynamics, a theme which, in spite 01 

 the silly exaltation^, by the popular Press of Ein- 

 stein above Newtonj still remains supreme in 

 dynamics. The first edition had poor and ex- 

 asperating diagrams, and was not well printed ; 

 the present edition has been issued under the 

 editorship of Sir Joseph Larmor, who has given 

 the work everywhere, and in all details, the utmost 

 ■care and attention. When we consider that the 

 reprint of the original edition is contained in the 

 small compass of 136 of the new pages, it appears 

 marvellous that a view of dynamics so complete in 

 NO. 2682, VOL. 107] 



itself in many respects could be compressed into 

 so few pages of print. 



On various interesting topics, such as Gravita- 

 tion and Light and "The Principle of Least 

 Action," Sir Joseph Larmor has added appen- 

 dices, while he has inserted as chap. ix. a dis- 

 cussion of "The Equations of Motion of a Con- 

 nected System," which increases the size of the 

 book by only thirty-nine of the present pages. 

 Needless to say, these additions are models of 

 condensation, and at the same time of absolute 

 clearness and accuracy. The new view of the 

 gravitational field, which Einstein's theory of space 

 and time affords, leads to an explanation of an 

 outstanding discrepancy of observation with 

 theory in the motion of the planet Mercury. This 

 involves a certain warping of the reference frame 

 which must be set up for these motions, and this 

 has been verified by the observations of the solar 

 eclipse of 1919, by the fact that rays of light 

 passing near the sun have been found to be 

 deflected by a certain amount predicted beforehand 

 towards that luminary. 



It is difficult also to pick out what were the 

 peculiarly interesting parts of Clerk Maxwell's 

 " Matter and Motion." Every bit of it was 

 distinctive and distinguished, but in some 

 ways the discussion of the hodograph, and the 

 question of absolute velocity of rotation, im- 

 pressed us most. The chapter on the latter subject 

 was read again and again and pondered continu- 

 ally. There came afterwards the discussions by 

 Love and Mach, which, however rigidly logical 

 and silencing, seemed to us far from convincing. 

 Mach's book was no doubt very valuable, but 

 the touch of the writer, if precise, seemed to 

 lack lightness and, as compared with Maxwell's, 

 that distinction which the magic of genius alone 

 can confer. One might weary of Mach's excel- 

 lent treatise ; of Maxwell one never tired. 



It is now possible to make a wider survey of the 

 whole subject. The elegance of the hodographic 

 theory appears very vividly in Maxwell's treat- 

 ment. It is a great thing to say, but there is 

 scarcely anything among the numerous discoveries 

 of Hamilton in dynamics which so signally 

 illustrates his penetrating genius. The hodograph 

 was hit upon some four or five years before 

 Hamilton by Mobius, as may be verified by con- 

 sulting his treatise on physical astronomy, "Die 

 Mechanik des Himmels," But application of the 

 idea Mobius makes little or none. With Hamil- 

 ton the applications are everything ; the idea is 

 used to obtain all kinds of beautiful results. That 

 Mobius had anticipated him Hamilton was fully 

 aware, and acknowledges (see the Life by 

 Graves) that Mobius might have claimed the 



