The true position of the stick is marked by A B, and the apparent position, by A C. We see the rising sun some minutes before he has risen above our horizon, and the setting sun after he has sunk below it, because his rays are refracted by our atmosphere. [Illustration] If the sun be at A, he will appear to a spectator on the surface of the earth at C, as if he were at B, because the rays will be refracted at D, which represents the limit of the atmosphere, towards C. Perhaps you knew all this before. Well, the knowledge of these laws will very nearly enable you to understand the wonders of which I have told you. I will try to explain to you the manner in which it seems that the appearances are produced by the operation of the laws. If a ray pass through a body that refracts it from its original course, it will go on when it has got through, in a line parallel with its first direction. Thus, let A B be a thick piece of glass, and C D, a ray of light passing through it, which would be refracted from _c_ to _d_; C _c_ would be exactly parallel to _d_ D, and the point C would be seen from D as if it were at E. [Illustration] If you think a little upon this, you will see that nothing is necessary to account for objects appearing nearer to a given point at one time than another, or objects upon the earth appearing high up in the air, except different degrees of refraction. Thus in the instance of the coast of France seeming to approach Hastings, it is evident that the effect would be produced by an extraordinary degree of refraction in a stratum of air over the sea, through which the rays of light producing vision must come. [Illustration] The explanation will be the same as that of the diagram of the rising sun in a former page; if the ray from A took the direction D, as under ordinary circumstances it would do, it is evident that a spectator at C would see nothing. But instead of this, the ray was refracted near the middle to the point C, which represents the town of Hastings, and therefore the object A appeared as if it had been placed at B. You shall now hear what causes are likely to alter the refracting power of the air at times. When air, water, or any other substance, is made hot, it becomes rarified, and its refracting power is thereby diminished. Have you ever watched, while you have been on one side of a stove, or of any heated body, the appearance of things on the other side through the air above it? If so, you must have seen how strangely they seem to tremble. Now this is caused by a stream of irregularly heated air rising from the stove, and enabling you every instant to see the things beyond it more nearly in their true position than you can through air of the ordinary temperature.