and Propagation of Sound. 27 



motion of a certain velocity* ; but it is not simply this, for 

 the velocity of wind, which is much greater than that of most 

 initial soniferous impulses, does not suffice to produce sound, 

 unless it meets with an obstacle; and certainly the movements 

 of the earth and the heavenly bodies should, according to this 

 definition, develop sound, and realize the poetic idea of " the 

 music of the spheres." A more exact physical definition 

 would be, motion of a certain velocity resisted with a certain 

 force. The moving and the resisting forces, acting in op- 

 posite ways, constitute the vibrations of sound f . 



4. The motion of matter producing sound should be consi- 

 dered as molecular, although the result is the motion of a 

 mass. Let it be represented thus : an impulse being im- 

 pinged on certain molecules, momentarily overcomes the re- 

 sistance of their inertia, and causes them to start from their 

 place; that force of repulsion which, existing between the dif- 

 ferent molecules, more or less strongly resists the attempt to 

 approximate them, transfers the impulse from molecule to 

 molecule, and thus extends it throughout the mass. The im- 

 pulse that forced these molecules from their position being 

 overcome by the reaction of the elastic forces, (attractive and 

 repulsive,) these forces drive them back to beyond their pro- 

 per station, whence, from the same "cause, they again spring, 

 until by a series of these alternating vibratory motions, the 

 disturbing force is lostf. The assimilating or propagating 

 power, then, of these vibrations depends on the repulsive and 

 attractive forces (2.) of the molecules of the vibrating matter, 

 and in proportion as these are strong to resist or react on a 

 mechanical impulse, they will convert that impulse into a 

 sonorous vibration (3.). 



5. Uniformity or equality of molecular elasticity (2.) is 



* *' It appears that the only condition necessary for the production of a 

 simple sound is a sufficient degree of velocity in the motion or impulse 

 which occasions it." — Dr. Youngs Lectures^ vol. i. p. 378. Dr. Young 

 here considered sound in a physiological sense. The paragraphs 2, 4 and 5, 

 appeared in a chapter on Sound prefixed to my " Rational Exposition of 

 Physical Signs," Scc^published in 1828, some years before Sir John Herschel's 

 articles in the Encyclopcsdia MetropoUtana and Philosophical Magazine re- 

 ferred to in the Editors' notes below. 



[ t Sir John F. W. Herschel's implied definition {Encycl. Metrop., Essay 

 on Sound, art. 138,) is as follows: *' Every impulse mechanically communi- 

 cated to the air, or other sonorous medium, is propagated onward by its 

 elasticity as a wave or pulse ; but, in order that it shall affect the ear as an 

 audible sound, a certain force and suddenness is necessary :" this, we appre- 

 hend, is virtually the same with Dr. Williams's definition in the text. — Edit.] 



[X Illustrations of this subject will be found in Sir John F. W. Herschel's 

 paper on the Absorption of Light by Coloured Media, Lond. and Edinb. 

 Phil. Mag., vol. iii. p. 403— 404.— Edit.] 



E2 



