WAVE-ACTION IN NATURE. 



Fig. 2. 



Now, in order that all these multifarious and diversified tremblings 

 of natural objects may be brought into relation with animate creatures 

 a common medium of communication is necessary. The air around us 

 is such a medium. It possesses the marvellous power of taking up 

 the numberless and ever-varying thrills of material 

 objects, and conveying them through space with all 

 their peculiarities. The sensitiveness of the air (if 

 we may so speak) to the faintest tremors in material 

 objects, and its power of transmitting their indi- 

 vidual qualities, are most wonderful. It drinks up 

 the infinitesimal motions of things, and diffuses them 

 swiftly, simultaneously, and in countless myriads in 

 all directions around. 



That air is the medium of sound is proved by the 

 fact that, when vibrations occur in space void of air, 

 the silence is not broken. If a bell suspended by a 

 string in a vacuum be struck, nothing is heard, al- 

 though, if it is in contact with the jar, the vibra- 

 tions are communicated to the outer air, and sounds 

 produced. That air transmits the kind of motion 

 that it receives is also proved by the fact that it will 

 take up vibrations at one point and communicate 

 them to a distant object that is capable of vibrat- 

 ing in the same way. 



The velocity of impulses in the air which pro- 

 duce sound has been well established, and all kinds 

 of shocks the firing of a gun, notes of a musical 

 instrument, or the voice, whether high or low, harsh 

 or soft all move at the same rate. The velocity 

 is not affected by changes in atmospheric pressure or 

 moisture, or by rain or snow, but it is affected by wind 

 and by temperature. The speed of sound is 1,090 

 feet per second at the freezing-point, and increases 

 about one foot per second for each degree of ascent 

 on the Fahrenheit scale. It, therefore, takes longer 

 to hear in winter than in summer. In many parts 

 of the country the change of temperature is so great that the velocity 

 of sound will vary more than 100 feet a second in the different seasons. 

 Sound moves in air with about the speed of a cannon-ball, and at a 

 rate ten times greater than the swiftest motion of air in a hurricane. 

 The sound produced in the open air tends to move in all directions 

 with equal speed, but this tendency may be disturbed by various con- 

 ditions. If the whole mass of air is moving in one direction, sound 

 will travel faster with it than against it. In still air the sound of a 

 musket-shot will be heard farthest in the direction of the impulse. Ex- 

 periments have shown that a person speaking in the open air can be 





Tube fracttteed bt 

 Vibration. 



