In the previous chapters your attention has been directed to the subject of waves on water and waves in air, and we shall now proceed to discuss some of the more difficult matters connected with the production of waves in the æther. We shall find that this portion of our subject makes more demands upon our powers of comprehension, since much that we have to consider is not directly the object of sense perception, and the inferences which we have to make from observed facts are less simple and easy to follow. Nevertheless, I trust that if you have been able to grasp clearly the nature of a surface-wave on water and of a compressional wave in air, you will not readily allow yourselves to be discouraged from encountering a new class of ideas, but will be able to advance still further, and gain a more or less clear conception of the nature of an electric wave in the æther. In the first place, we must consider the medium in which these waves are created. We can see with our eyes a water-surface, and we are able to understand without much difficulty that the surface can be thrown into humps and hollows, or become wrinkled, and also that these elevations and depressions can change their position, thus creating a surface wave which moves forward. The movement of the water wave is, therefore, only the result of a local elevation of the surface which travels along or takes place progressively at different places on the surface. Then, again, in the case of an air wave, although we cannot see the air, we are able, with some little assistance from experiments, to present to ourselves a clear mental picture of a progressive movement through the air of a region of compression, that is to say, a certain slice, layer, or zone of the air is more compressed than the neighbouring portions, and this region of compression changes its place progressively. It has been carefully explained that the production of a wave of any kind implies, therefore, two things—first, a medium or material in which the wave exists; and, secondly, some kind of periodic change or movement which is experienced by the various portions of this medium at different places successively. If, therefore, we are given any medium, say water or air, and asked to explain the production of a wave in it, we have first to consider what kind of changes can take place in it, or on it, which can appear progressively at different parts. In the case of the water-surface, some parts may be heaped up higher than the rest, and the heaping up may occur at successive places in such fashion that when it disappears at one place it reappears at a contiguous or neighbouring place. In the case of air, some portion may be compressed more than the rest, and the place of compression may move forward, so that as the compression is released in one place it makes its appearance in an adjacent one. In the first case, we have a wave of elevation on water; in the second case, a wave of compression in the air.