316 SOUND. 



struclion of violins, harps, guitars, &c, and of sounding 

 boards, generally is governed by the same law. In the 

 dancing-master's kit or small fiddle, which he carries in 

 his pocket, there are the same strings, and the same bow 

 as for a violin, but it has very little sound because the 

 extent of its surface is so small. A heavy piece of metal, 

 called a sourdine, when fixed upon the bridge of a violin, 

 damps the sound, because it is a dead mass resisting the 

 motion of the elastic wood.' 



2. Motion of Sound through the Air. 



Every one has noticed that sound does not pass from 

 place to place, through the air instantaneously. We see 

 the flash and the smoke of a distant cannon some seconds 

 before we hear the report. When a cloud is remote 

 from us, we see its lightning, and a pause, sometimes of 

 minutes, intervenes, before the rumbling of the thunder 

 reaches our ears. We see the flashes, in these cases, at 

 the instant of their occurrence, and hear the sound after 

 a sufficient interval has passed to allow of the transmission 

 of the vibrations through the intervening distance. 



The velocity with which sound passes through the 

 air, has been ascertained by careful experiments. The 

 method is obvious. If a ship stationed at a known dis- 

 tance from the spectator, fires a gun, and the distance 

 between the flash and the report is accurately noted by 

 means of a stop watch, it is easy to calculate the velocity 

 per second. For example, suppose that by means of the 

 stop watch, we find that thirty seconds elapse after seeing 

 the flash, before hearing the report. We next ascertain 

 exactly the distance of the ship. Suppose we find it to 

 be 6 miles and 860 yards. This reduced to feet, is 

 34,260 ft. which divided by 30, the number of seconds, 

 gives us for a quotient 1142. The result of the calcula- 

 tion therefore is, that the sound of the cannon travels at 

 the rate of 1 142 feet in a second. 



This is the velocity which is now generally assigned 

 to sound. Though different experiments have given 

 very different results, as the following table will show. 

 The first number, 968 feet, given by Sir Isaac Newton, 

 was obtained not by experiment but by mathematical 

 calculation. 



