Mr. S. T. Preston on the Kinetic Theory of Gravitation. 307 



the more is simplicity indispensable, and superfluity incom- 

 patible with precision and certainty in the results. To assume 

 a constitution for the aether that could not be realized or 

 clearly explained would surely be futile, since the explanation 

 or clear conception forms the logical support of any theory, 

 without which the theory resembles a mere dogmatic state- 

 ment incapable of being sustained by reason. 



13. There is one other point which we would notice in con- 

 nexion with this subject. The idea would appear to be to a 

 certain extent prevalent that the aether must have a constitu- 

 tion essentially different from the air, because the vibrations 

 producing light are transverse, while those producing sound 

 are longitudinal. It seems to be sometimes inferred from 

 this that the vibrations of the aether are only transverse, and 

 those of the air only longitudinal. There would be no warrant 

 for this conclusion ; and we think that it has done harm and 

 greatly hindered any rational idea from being formed of the 

 nature of the aether. According to the kinetic theory, which 

 is known to represent the constitution of the air, the vibrations 

 of the particles of air disturbed by a vibrating body and pro- 

 pagated in the form of waves, are not only longitudinal ; for 

 since according to the kinetic theory the particles of air in 

 their normal state are moving equally in all directions, it 

 follows that these particles are accelerated and retarded both 

 in transverse and in longitudinal directions at the passage of 

 waves. It is true that the transverse component of the motion 

 probably may not affect the ear, on account of its special 

 structure. It would be wrong, however, to infer from this 

 that the transverse component of the motion did not exist. 

 So in the case of the aether, it would be unwarranted to infer 

 that the longitudinal component of the motion did not exist, 

 because this component was incapable of affecting the eye. 

 The eye and the ear may be very differently constituted ; and 

 a motion that affects the one might not affect the other. Sir 

 John Herschel says regarding this point in his essay " On 

 Light " (' Popular Lectures on Scientific Subjects,' page 

 358) : — Ci According to any conception we can form of an 

 elastic medium, its particles must be conceived free to move 

 (within certain limits greater or less according to the coercive 

 forces which restrain them) in every direction." He then 

 goes on to explain how the efficacy of the transverse component 

 of the movement in the case of light, and the longitudinal 

 component of the movement in the case of sound, may be ac- 

 counted for by the diverse structure of the eye and ear. Any 

 inference which is not valid, invariably does some harm ; and 

 this idea of a forward movement being propagated in a medium 

 by only transverse vibrations, being almost inconceivable, has 



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