The effect of the motion of a sounding body upon the length of its sonorous waves will be readily seen from the following illustration: Suppose a number of persons stationed at equal intervals in a line on a long platform capable of moving backward and forward. Suppose the men are four feet apart, and all walking forward at the same rate, and that the platform is stationary, and that, as the men leave the platform, they keep on walking at the same rate: the men will evidently be four feet apart in the line in front of the platform, as well as on it. Suppose next, that the platform is moving forward at the rate of one foot in the interval between two men's leaving the platform, and that the men continue to walk as before: it is evident that the men will then be three feet apart in the line after they have left the platform. The forward motion of the platform has the effect of crowding the men together a little. Were the platform moving backward at the same rate, the men would be five feet apart after they had left the platform. The backward motion of the platform has the effect of separating the men from one another. The distance between the men in this illustration corresponds to the length of the sound-wave, or the distance between its two ends. Were a person to stand beside the line, and count the men that passed him in the three cases given above, he would find that more persons would pass him in the same time when the platform is moving forward than when it is stationary, and fewer persons would pass him in the same time when the platform is moving backward than when it is stationary. In the same way, when a sounding body is moving rapidly forward, the sound-waves beat more rapidly upon the ear of a person who is standing still than when the body is at rest, and less rapidly when the sounding body is moving rapidly backward. Were the platform stationary, and were the person who is counting the men to be walking along the line, either towards or away from the platform, the effect upon the number of men passing him in a given time would be precisely the same as it would be were the person stationary, and the platform moving either towards or away from him at the same rate. So the change in the rapidity with which pulsations of sound beat upon the ear is precisely the same whether the ear is stationary and the sounding body moving, or the sounding body is stationary and the ear moving.