246 DISCOURSE ON THE STUDY 



CHAP. II. 



OF THE COMMUNICATION OF MOTION THROUGH 

 BODIES. OF SOUND AND LIGHT. 



(269.) THE propagation of motion through all sub- 

 stances, whether of a single impulse, as a blow or 

 thrust, or of one frequently and regularly repeated, 

 such as a jarring or vibratory movement, depends 

 wholly on these molecular forces; and it is on such 

 propagation that sound and very probably light de- 

 pend. To conceive the manner in which a motion may 

 be conveyed from one part of a substance to another, 

 whether solid or fluid, we may attend to what takes 

 place when a wave is made to run along a stretched 

 string, or the surface of still water. Every part of 

 the string, or water, is in succession moved from its 

 place, and agitated with a motion similar to that of 

 the original impulse, leaving its place and returning 

 to it, and when one part ceases to move the next 

 receives as it were the impression, and forwards it 

 onward. This may seem a slow and circuitous pro- 

 cess in description ; but when sound, for example, is 

 conveyed through the air, we are to consider, 1st, 

 that the air, the substance actually in motion, is ex- 

 tremely light and acted upon by a very powerful 

 elasticity, so that the force which propagates the 

 motion, or by which the particles adjacent act on, 

 and urge forward, each other, is very great, compared 

 tvith the quantity of materials set in motion by it : 



