If we then administer a little pat to the first ball, you will see a wave-motion run along the line of balls. Each ball in turn moves to and fro a little way, and its movement is handed on to its neighbours. We have here an example of a longitudinal wave-motion which resembles that of the air when it is traversed by a sound wave. Another model which is of a more elaborate character shows us the sort of motion made in a tube when a sound wave due to a continuous musical sound is passing along it. It consists of a glass disc which is blackened, and has the paint removed along certain excentric circular lines. This disc is made to revolve in front of a wide slit in a piece of metal. By means of an optical lantern we project on to the screen an image of the slit, which you see is crossed by certain bright bars of light, crowded together at some places and more spaced apart at others. When the disc revolves, these bars of light each move to and fro successively, and the result is that the crowded place moves along, or is displaced. A wave of compression is propagated along the slit, and the localities where the bars of light are compressed or expanded continually change their place. If we imagine the air in a tube to be divided into slices, represented by these bars of light, the motion of the model exactly represents the motion of the air in the tube when it is traversed by a series of sound waves. The distance from one place of greatest compression to the next is called the wave-length of the sound wave. Hence, although a sound such as that of an explosion may consist in the propagation of a single layer of compression, the production of a continuous musical note involves the transference of a series of equidistant compressional zones, or waves. These models will have assisted you, I trust, to form a clear idea of the nature of a sound wave in air. It is something very different, in fact, from a wave on the surface of water, but it is characterized by the same general qualities of wave-motion. It is a state of longitudinal periodic motion in a row of particles, which is handed on from one to another. Each particle of air oscillates in the line of propagation of the wave, and moves a little way backwards and forwards on either side of its undisturbed position. It will be seen, therefore, that a solitary sound wave is a state of air-compression which travels along in the otherwise stationary air. The air is squeezed more tightly together in a certain region, and successive layers of air take up this condition. In the case of water-surface waves the wave is a region of elevation at which the water is raised above the general or average level, and this elevated region is transferred from place to place on otherwise stationary water. In the case of an air-wave train we have similar regions of compression following each other at distances, it may be, of a fraction of an inch or of several feet.