In order to understand this statement clearly let us consider the appended tabular diagram. On a calm day imagine a source of sound at $S$ in line a. This may be either a gun or a bell. Imagine an observer 1,100 feet distant, located at $O$. The velocity of sound in air is 1,100 feet per second. This velocity we will take as unity, as indicated in the third column, and the velocity with which the sound reaches the observer is also 1, as shown in the fourth. It will reach him in a unit interval of 1 second, as shown in the fifth. If the bell is struck, it will give its normal pitch or frequency, which we will also call unity, in the sixth column. Now imagine case b where the observer is on a train advancing toward $S$. When he is 1,100 feet distant, the gun is fired, but as he is advancing toward it, he hears it at $O$ in rather less than a second, as shown in the fifth column. The velocity of the sound with regard to him is rather more than unity, as shown in the fourth column. If the bell is sounded, the pitch, that is the frequency, is raised, because he receives more sound waves per second than before. In case c the observer is stationary, but the source of sound is receding. At a distance of 1,100 feet the gun is fired, and the observer hears it after an interval of just one second, as in case a. The velocities with regard to the observer and through the medium are also unity. If the bell is struck the pitch is lowered, since he receives fewer sound waves per second, the reverse of case b. Velocity Source in to Interval Frequency Observer Medium Observer Air a S 1 1 1 1 O b S 1 1+ 1- 1+ O c S 1 1 1 1- O d S 1 1+ 1- 1 O Ether A S 1 1 1 1 O B S 1- 1 1 1+ O C S 1 1 1 1- O D S 1 - 1 1 1 O In case d imagine the source and the observer 1,100 feet apart, and advancing on the same train. When the gun is fired, the velocity of the sound waves will be greater with regard to the observer, and he will hear the sound in less than a second, as in case b. When the bell is struck it will have the normal pitch, the same as in case a.