WATER-WAVES AND SOUND-WAVES. 



167 



not with uncertain speed, but so regularly that all the waves all 

 round are all parts of circles and of concentric circles. 



Fig. 2. Showing the Formation of Waves by the Circular Motion of Each Particle 

 of Water in a Vertical Plane. Eight positions in each revolution are shown I. One 

 Particle in Motion. II. Two Particles in Motion. III. Three Particles in Motion. IV. Com- 

 plete Wave and Motion of its Elements. 



Let us drop two stones in at some little distance apart. What hap- 

 pens then ? We have two similar systems each working its way out- 

 ward, to all appearance independently of the other. We get what is 

 represented in Fig. 1. 



Fig. 3. Graphical Method of observing the Mode of Vibration of a Tuning-Fork. 



