322 SOUND. 



In order to illustrate this subject, let us suppose our- 

 selves standing on an elevated rock near the sea-shore ; 

 a cannon is fired from a distant ship, and we are desirous 

 to know the number of miles the ship is below us. We 

 observe the moment when we see the flash of the cannon, 

 and then by a stop watch or by some accurate instrument 

 count the number of seconds which elapse before we hear 

 the report. Suppose it to be 30 seconds. Now, as it has 

 been ascertained, that sound travels at the rate of 1 142 feet 

 in a second, we must in this case, multiply 114:2 by 30. 

 We shall thus find that the ship is 34,^60 feet from us. 

 Dividing 34,'<2(iO by 5280 (the number of feet in a mile,) 

 we find that the ship is six miles and 2580 feet from us. 



It is evident that these calculations of distances cannot, 

 in any case, be very accurate. It is difficult to ascertain 

 the time within half a second, and in half a second the 

 sound would move five hundred feet. But it may be of 

 some service occasionally to know the distance of an ob- 

 ject within a thousand feet. If chased at sea by a pirate, 

 it would be a satisfaction to know that his ship is many 

 miles from it, even if we were not sure whether it was 

 seven miles or seven and a half. In a thunder storm, 

 also, perfect accuracy in regard to the distance of the 

 cloud is not necessary. In the same manner, the distance 

 of a thunder cloud in a storm, may be easily calculated 

 by noticing the time which elapses between the lightning 

 and the thunder. It will be recollected by our readers, 

 that every flash of lightning is attended by a clap of 

 thunder ; and that they both really take place at the same 

 instant, though we see the flash usually at the moment it 

 occurs,* while we must wait for the sound of the thun- 

 der to traverse the intermediate distance. In case a 

 stop watch is not at hand, the distance may be calculated 

 with some success, by counting the pulsations of any 

 healthy individual. These pulsations are generally about 

 once a second. 



* Light travels a great distance almost instantaneously, it re- 

 quiring only 8 seconds to cross the orbit of the earth, a distance ot 

 190,000,000 of mile*. Consequently in a passage of a/etc miles, the 

 lape of time is not perceptible. 



