The motion of the particles of the water in a deep-sea wave resembles that of the white knobs in the model described. Those who swim will recall to mind their sensations as a sea wave surges over them. The wave lifts up the swimmer, then pushes him a little forward, then lets him down, and, lastly, drags him back. It is this dragging-back action which is so dangerous to persons who cannot swim, when they are bathing on a steep coast where strong waves are rolling in towards the shore. Two other kinds of wave-motion may be illustrated by the model shown in Fig. 3. In this appliance there are a number of eccentric wheels fixed to a shaft. Each wheel is embraced by a band carrying a long rod which ends in a white ball. The wheels are so placed on the shaft that, when at rest, the balls are arranged in a wavy line. Then, on turning round the shaft, each ball rises and falls in a vertical line, and executes a periodic motion, lagging behind that of its neighbour on one side. The result is to produce a wave-motion along the line of balls. By slightly altering the model, each ball can be made to describe a circle in a direction at right angles to the line of the balls, and then we have a sort of corkscrew wave-motion propagated along the line of balls. [Illustration: FIG. 4.] Again, another form of wave-motion may be illustrated by the model shown in Fig. 4. In this case a number of golf-balls are hung up by strings, and spiral brass springs are interposed between each ball. On giving a slight tap to the end ball, we notice that its to-and-fro motion is handed on from ball to ball, and we have a wave-motion in which the individual movement of the balls is _in the direction_ of the wave-movement, and not across it. The kind of wave illustrated by the model in Fig. 3 is called a _transverse_ wave, and that shown in Fig. 4 is called a _longitudinal_ wave. At this stage it may be well to define the meaning of some other expressions which will be much used in these lectures. We have seen that in a wave-motion each part of the medium makes some kind of movement over and over again; and of its neighbours on either side, one is a little ahead of it in its performance, and the other a little in arrear. If we look along the line, we shall see that we can select portions of it which are exactly in the same stage of movement—that is, are moving in the same way at the same time. The distance between these portions is called _one wave-length_. Thus, in the case of sea waves, the distance between two adjacent crests, or humps, is one wave-length. When we use the expression, _a long wave_, we do not mean a wave which is of great length _in the direction_ of the ridge, but waves in which the crests, or humps, are separated far apart, measuring from crest to crest _across_ the ridges.