DOPPLER' S PRINCIPLE.
Suppose a source of sound, a whistle for example, is giving out a note of a certain number of vibrations per second. If it be u" the frequency will be about 1,536. The velocity of this disturbance would be 330 m. per second. It has been proved that sounds of different pitch travel at the same rate. If this whistle be attached to a train travelling at the rate of about a mile a minute or about 37 m. per second towards the listener, the vibration at the end of each second will have 37 m. less distance to travel than the vibrations emitted at the beginning of the same second ; and as the velocity of the sound is identical throughout, the vibration emitted last will reach the observer a little earlier than if the train had been still. To the observer, the interval be tween the reception of these vibrations will be less than a second, or, putting it another way, he will hear more of them per second, and the note will be higher in pitch. The same reasoning will show that
if the source of sound be travelling away in the opposite direction with an equal velocity, the frequency of the note will be lower than that emitted by the source by exactly the same interval as it was for merly raised. The frequency will be altered in the ratio of the altered relative velocity of the sound to the unaltered, i.e. X, = where X stands :Or the original wave-length, and v the original velocity of the disturbance. The veloci ties under different conditions from the two simple cases considered, must not be added arithmetically, but by the Parallelogram of Velocities. In the case where for the instant the source is travelling past the observer at right angles to the direction the sound is tak ing, his distance from the source remains practically constant over an extremely short period of time, and the note will be unaltered.