338 



OUR PHYSICAL WORLD 



officer stationed here notes these times. Suppose that the instru- 

 ment at b registers the reception of the sound a half-second 



after it is received at a 

 and the instrument at c 

 one second after it is 

 received at a. Suppose, 

 further, that the atmos- 

 pheric conditions are such 

 that sound is traveling at 

 the rate of 1,100 feet per 

 second. Then evidently 

 b is 550 feet and c 1,100 

 feet farther from the gun 

 than is a. The officer at 

 the central station has 

 a diagram showing the 

 relative positions of a, b, 

 and c, and their distances 

 from each other laid out 

 to a scale. On this same 

 scale he draws about b a 

 circle with a radius of 550 

 feet and about c a circle 

 with a radius of 1,100 

 feet. The gun is located at the center of a circle which passes 

 through a and is tangent to the circles about b and c. The 

 mathematics involved in the determination of this center is too 

 complicated to be briefly explained. This method was found so 

 efficient that a gun miles away could be located within 50 feet 

 of its exact position. 



FIG. 175. Diagram to show method of 

 locating a gun g, by sound. 



