26 



NATURE 



[November 8, 1900 



It seems idle to pursue the author further in his specu- 

 lations. Not content with formulating a theory of the 

 operation of nature on the molecular scale, he devotes 

 the last part of his essay to the exposition of his views on 

 the subject of cosmogony. In this he adopts the nebular 

 theory of Laplace as a general scheme, and applies to it 

 the ideas which he has previously developed. He has 

 an explanation to offer of the cases of retrograde motion 

 in the solar system and of other phenomena which have 

 been thought to present difficulties. On these points 

 he apparently satisfies himself, though others, in the 

 absence of rigorous proof, may remain sceptical. It is 

 in reference to a subject of some debate, the ultimate fate 

 of the solar system, in fact, that he offers a novel sug- 

 gestion, based on his peculiar conception of the atom. 

 As others have supposed, the end must come with the 

 slow dissipation of energy ; but, in the system of M. 

 Despaux, this implies the loss, not only of heat, but also of 

 gravitational power in the atom, or, in other words, of 

 weight. The energy passes to the ether, and matter 

 deprived of its chief property becomes immaterial. The 

 world, then, no longer remains as an inert mass, but 

 reaches its dissolution and leaves " not a rack behind." 

 It is true that M. Despaux contemplates a return of the 

 energy to the atom, and the whole process from the birth 

 to the death of the system repeated in an endless cycle. 

 But even he does not venture to describe in any detail 

 how this is to come about. 



Enough has been said as to the scope of this book, 

 and some reason has been assigned for considering it 

 unphilosophical in design and unenlightened in execution. 

 But it is not to be thought that the author is an unfavour- 

 able example of the class to which he belongs. He 

 expresses clearly and concisely what he has to say ; he is 

 respectful to the great workers who have adhered to the 

 recognised laborious paths ; and it should not be thought 

 that there are no ideas to be found in his work which are 

 true, though such as are true may generally appear not to 

 be original. Yet, when all that is said, it is difficult to 

 conceive the utility of such a work. 



A NEW TEXT- BOOK ON SOUND. 

 A Text-Book of Physics. Sound. By J. H. Poynting, 



Sc.D., F.R.S., and J. J. Thomson, M.A., F.R.S., 



Hon. Sc.D. (Dublin), Hon. D.L. (Princeton). Pp. 



x + 163. (London : Chas. Griffin and Co., Ltd., 



1899.) 

 'T^HIS volume will be welcomed by those interested in 

 J- the teaching of physical science, not only on 

 account of its individual merits, but also as the first in- 

 stalment of a complete treatise on physics now in pre- 

 paration by the authors. It is intended for the use of 

 students who lay most stress on the study of the experi- 

 mental part of physics, and who have not yet reached the 

 stage at which the reading of advanced treatises on 

 special subjects is desirable. For this class of students 

 it is important that the mathematics used should be of 

 the simplest. So far as concerns those who are un- 

 acquainted with the calculus, this is self-evident ; it is, 

 further, none the less true with regard to those possessing 

 some knowledge of the higher mathematics. Unless great 

 care is exercised, the use of the calculus is apt to become 

 so far mechanical that the student may possibly miss 

 NO. 16 19, VOL. 63I 



many tacit assumptions which it would be advantageous 

 for him to clearly recognise. To all students it is alike 

 of importance that each stage in the reasoning employed 

 should be brought into view as clearly as possible, and 

 subjected to the most searching scrutiny. This can be 

 done, sometimes by the use of comparatively simple ana- 

 lytical and geometrical devices, often by the application 

 of the principles of the calculus developed ab initio. The 

 volume under consideration comprises many most suc- 

 cessful attempts to apply simple mathematical methods 

 to the solution of important, and sometimes fairly intri- 

 cate, problems. The investigation of the modes of 

 vibration of a stretched string, on pp. 86-88, is perhaps 

 the least successful effort in this direction ; few students 

 would, it may be feared, be able to keep the essential fea- 

 tures of the problem clearly distinguished from the 

 number of geometrical and analytical assumptions and 

 approximations involved. The investigation of the same 

 problem from another standpoint, as given on p. 93, is 

 much to be preferred in this respect. 



In the first chapter a good account is given of the 

 general nature and characteristics of sound. A simple 

 experiment, due to Prof. Boys, in which the vibratory 

 character of sound in air is made manifest, might per- 

 haps have been mentioned with advantage. A Bunsen 

 flame is burnt near the end of an open organ pipe, and 

 when the latter is sounded the sinusoidal paths of dust 

 particles traversing the flame are readily seen. From a 

 mathematical point of view, it is to be regretted that a 

 solution is not here given of the problem of the motion 

 of a heavy particle, attracted toward a point with a 

 force proportional to its displacement therefrom. If we 

 suppose the particle to revolve in a circular orbit about 

 the point, then it is easily seen that the centrifugal force 

 must be equal to the central attraction exerted. Resolv- 

 ing the instantaneous displacement, velocity and central 

 force parallel and perpendicularly to any given axis, then 

 we have two harmonic motions executed under the 

 actions of forces proportional to the displacement from 

 the centre. Considering only one of these harmonic 

 motions, the value of the kinetic and potential energies 

 at any point may easily be written down, when it 

 becomes evident that their sum is constant. Equating the 

 potential energy at the extremity of an excursion to the 

 kinetic energy at the point of equilibrium, the well-known 

 expression for the time of vibration is readily obtained. 

 It is interesting to note that this graphical solution 

 corresponds to assigning the real part of Ae'"' as the 

 value of X which satisfies the differential equation 



The second chapter commences with a simple theo- 

 retical investigation of the velocity of sound in a fluid, 

 and is followed by an interesting and valuable account of 

 the experimental aspect of the same question. In con- 

 nection with the reflection of sound, the curious musical 

 ring, often heard to follow each footfall when one is 

 walking near palisading, is simply explained, and Lord 

 Rayleigh's theory of whispering galleries is described. 

 The introduction in ensuing editions of a few reproduc- 

 tions of Prof. Wood's photographs of sound waves would 

 enhance the interest of this section. Refraction of sounds 

 by winds and air-layers of different densities is also 



