If a stone is thrown into a weir, the disturbance is propagated over the surface of the water in the form of rings. The propagation of sound, light, or the signals used in wireless telegraphy and telephony, are the results of such motions. All these different waves vary with respect to one another: not only is the type of particle different but the nature of the restraining forces also varies according to the nature of the particular waves which are being propagated. There are, however, a whole series of laws which are common to all wave motions. Since the principle of independent motions is quite general, it follows that when two wave motions occur at any one place, the resultant effect will be due to both motions. The final wave form will depend upon the amplitude, wave-length, the difference in phase, and the direction of propagation, which characterize the waves arriving at the point under consideration.