312 SOUND. 



at a distance from each other of ten or fifteen feet, and 

 the bow be drawn across a string of one, so as to produce 

 a loud sound, the corresponding string in the other in- 

 strument will be thrown into vibrations, and a similar 

 sound will proceed from it. This can arise from no other 

 circumstance than that the vibrations of the string, across 

 which the bow is drawn, are communicated to the air, 

 and through the air to the same string in the other viol. 



These vibratory motions in the air, are very different 

 in their nature froi^progrcssivc motions, and it is im- 

 portant here to explam them. They are usually illustrated 

 by the undulations of water. 



If a piece of wood is lying still, on the surface of water, 

 and a stone be thrown beyond it, a series of undulations 

 are produced, which recede from the place where the 

 stone fell, and seem to be a motion of the water itself to- 

 wards the shore. This is, however, not the fact. The 

 motion seems to be progressive, but is really only vibra- 

 tory. The stone falling into the water, produces a de- 

 pression in that place, the surrounding water presses in 

 to fill it, and by its momentum an elevation is produced. 

 The water thus elevated falls by its weight, and produces 

 an elevated ring around it. This by its descent produces 

 another and another, larger in diameter, and nearer the 

 shore ; but in such rapid succession, that it seems as if 

 the teave first produced, itself moiled towards the shore. 

 That this is not really the case is evident from this fact : 

 If, before the stone was thrown in, apiece of wood had 

 been floating on the water, we should have observed that 

 the wood would approach but very little, if at all, nearer 

 the shore. It would rise and fall with each successive 

 circle, but still make no visible progress. Now, as the 

 wood would move with the water, if there was really a 

 progressive motion of the latter, we can safely infer that 

 all the motion of the water is simply a rising and falling, 

 or a vibratory motion, not a progressive one, towards the 

 shore. The appearance is delusive. 



The undulations of the air are in the same manner vi- 

 bratory, that is to and fro, not progressive. This may be 

 illustrated by the annexed diagram. 



