It may be remarked, in passing, that there is a strong tendency in the human mind to seek for and be satisfied with what we called mechanical explanations. This probably arises from the fact that the only things which we can picture to ourselves in our minds very clearly are movements or changes in relative positions. If we can in imagination reduce any physical operation to some kind of movement or displacement taking place in some kind of material, we seem to arrive at a kind of terminus of thought which is more or less satisfactory. We invariably aim at being able to visualize an operation concerning which we are thinking, and it requires some mental self-control to be able to content ourselves with a general expression which does not lend itself readily to visualization. There are plenty of indications, however, that this mental method of procedure, and this endeavour to reduce all physical operations to simple mechanics and to movements of some kind, may in the end be found to be unjustifiable; and the time may arrive when we may be more satisfied to explain mechanical operations in terms of electrical phraseology rather than aim at dissecting electrical effects into mechanical operations. Thus, for instance, instead of speaking of electric inertia, it may be really more justifiable to speak of the inductance of ordinary matter. The final terms in which we endeavour to offer ourselves an explanation of physical events are in all probability very much a matter of convenience and custom. We may, however, for present purposes rest content by thinking of the electro-magnetic medium as in some sense like a heavy elastic substance which is capable of undergoing some kind of strain or distortion, the said strain relieving itself as soon as the distorting force is withdrawn; but, in addition, we must think of the medium as possessing a quality analogous to inertia, so that as distortion vanishes it overshoots the mark, and the medium only regains its state of equilibrium at the particular point considered, by a series of oscillations or alternate distortions, gradually decreasing in amount. Any medium which possesses these two qualities has, in virtue of explanations already given, the property of having waves created in it, and what we mean by an electric wave is a state of electric strain which is propagated through the æther with a velocity equal to that of light, just as an air wave consists of a state of compression which is propagated through the air with a velocity of 1100 feet a second. [Illustration: FIG. 72.—Electric-radiation detector (Fleming).]