OJJ NATURAL PHILOSOPHY. 89 



stood, before we can have a true and complete 

 knowledge of sound:* 1st, The excitement and 

 propagation of motion. 2dly, The production of 

 sensation. These, then, are two other phenomena, 

 of a simpler, or, it would be more correct to say, of 

 a more general or elementary order, into which 

 the complex phenomenon of sound resolves itself. 

 But again, if we consider the communication of 

 motion from body to body, or from one part to 

 another of the same, we shall perceive that it is 

 again resolvable into several other phenomena. 

 1st, The original setting in motion of a material 

 body, or any part of one. 2dly, The behaviour 

 of a particle set in motion, when it meets another 

 lying in its way, or is otherwise impeded or influ- 

 enced by its connection with surrounding particles. 

 Sdly, The behaviour of the particles so impeding 

 or influencing it under such circumstances; besides 

 which, the last two point out another phenomenon, 

 which it is necessary also to consider, viz. the phe- 

 nomenon of the connection of the parts of material 

 bodies in masses, by which they form aggregates, 

 and are enabled to influence each other's motions. 



(80.) Thus, then, we see that an analysis of the 

 phenomenon of sound leads to the enquiry, 1st, of 

 two causes, viz. the cause of motion, and the cause 

 of sensation, these being phenomena which (at least 

 as human knowledge stands at present) we are 

 unable to analyse further; and, therefore, we set 

 them down as simple, elementary, and referable, 

 for any thing we can see to the contrary, to the 

 immediate action of their causes. 2dly, Of several 

 questions relating to the connection between the 



