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on a vibrating body, give some weight to the supposition 

 of undulatory motion of sound in its transmission. Sound 

 is, however, transmitted directly from the vibrating body, 

 but the appearances of wave motion produced are not in 

 the direction of propagation, assumed in the undulatory 

 theory of light ; that is, that light is the result of motion 

 of eetherial particles at right angles to the direction in which 

 the light travels. 



In all the instances in which we find undulations on the 

 surface of a fluid, we shall also find that those undulations 

 are the eff'ect of gravity acting tangentially to the plane in 

 which the undulation takes place. And I do not know any 

 palpable instance of undulatory motion in which the influence 

 of a tangential force cannot be perceived ; as when a rope or 

 a chain is made to undulate, the undulation is first induced 

 by a force transverse to the direction in which the rope or 

 chain is stretched. But we cannot discover any like trans- 

 verse or tangential force necessary to the propagation of 

 sound; and certainly we are without proof that any transverse 

 motion does take place in the progression of a ray of sound. 

 I therefore think we may, as regards our present knowledge 

 of the nature of sound, reject every idea which would assume 

 a motion more complicated than such a direct motion or series 

 of impulses as will serve to explain the various observed 

 phenomena. 



In all the instances of vibration producing sound which 

 can be submitted to observation, we may perceive that the 

 motions of the vibrating body are such as to push away the 

 surrounding air by a series of regular impulses, and that the 

 particles of air might transmit such impulses through the 

 surrounding particles, in like manner as the suspended line 

 of marbles communicate impulses. And in examining the 

 motions of the light powders on the vibrating plates, we find 

 evidence of a direct motion of the air from the plate, but 



