Page:Elementary Text-book of Physics (Anthony, 1897).djvu/189

§ 150] same support, and one of them be made to vibrate, any others which have the same period of vibration will soon be found in motion, while those which have a diilerent period will show no signs of disturbance. The vibration of the first pendulum produces a slight movement of the support, which is communicated alike to all the other pendulums. Each movement may be considered as a slight impulse, which imparts to each pendulum a very small vibratory motion. For those pendulums having the same period as the one in vibration, these impulses come just in time to increase the motion already produced, and so, after a time, produce a sensible motion; while for those pendulums having a different period the vibration at first imparted will not keep time with the impulses, and these will therefore as often tend to destroy as to increase the motion. It is important to note that the pendulum imparting the motion loses all it imparts. This is not only true of pendulums, but of all vibrating bodies. Two strings stretched from the same support and tuned to unison will both vibrate when either one is caused to sound. A tuning-fork suitably mounted on a sounding-box will communicate its vibrations to another tuned to exact unison even when they are thirty or forty feet apart and only air intervenes. In this case it is the sound-wave generated by the first fork which excites the second fork, and in so doing the wave loses a part of its own motion, so that beyond the second fork, on the line joining the two, the sound will be less intense than at the same distance in other directions. Such communication of vibrations is called resonance.

Air-columns of suitable dimensions will vibrate in sympathy with other sounding bodies. If water be gradually poured into a deep jar, over the mouth of which is a vibrating tuning-fork, there will be found in general a certain length of the air-column for which the tone of the fork is strongly reinforced. From the theory of organ pipes, it is plain that this length corresponds approximately to a quarter wave length for that tone. In this case, also, when the strongest reinforcement occurs, the sound of the fork will rapidly die away. The sounding-boxes on which the