TUTOR. Light, like material bodies, if it meet with no obstruction, will move in right lines; now, the rays of the sun in coming to the earth must pass through a great quantity of the atmosphere, which being a fluid, refracts or bends the rays of light, by which refraction it is that we are favoured with the sight of the sun 3-1/4 minutes every morning before he rises above the horizon, and every evening after he sinks below it, which in one year amounts to more than 40 hours. This refraction is greatest near the horizon, and ends in the zenith. PUPIL. Pray, Sir, can you make this clearer by an experiment? TUTOR. I have just thought of one. Take a bason filled with water, and a strait stick or piece of wire; put it perpendicularly into the water, that is, that it lean neither way, and there will be no refraction; incline it a little towards the edge of the bason and it will appear a little bent at the surface of the water; incline it still more, and the refraction will be greater. PUPIL. I have often seen this appearance when I have put my stick into water, but did not before know the cause. TUTOR. You may try one more experiment. Pour the water out of the bason, and set the bason on the floor; put a guinea into it, and let it represent the sun.—Why do you smile? PUPIL. Because I have not the sun’s representative to try the experiment with. TUTOR. Well, well, put a shilling into the bason and call it the moon, and it will answer the same purpose:—Walk backward till you just lose sight of it, then the right line from your eye continued over the edge of the bason must pass beyond the money at the bottom of it. PUPIL. That is evident. TUTOR. Keep your position, and desire some friend to pour the water gently into the bason so as not to remove the money, and you will clearly distinguish it. Now, if you call the edge of the bason the horizon, the water the atmosphere, and the shilling the moon, is it not clear that you will see it above the horizon, when it is really below it? PUPIL. I think so, Sir. TUTOR. Well, try the experiment, and let me know the result when I next see you. ------------------------------------------------------------------------ DIALOGUE IX. TUTOR. I presume, Sir, you have made the experiment I recommended to you. PUPIL. I have, Sir; and am so well convinced of what you told me, that nothing farther need be said on the subject. TUTOR. As that is the case, I shall proceed.—I dare say you do not forget what the plane of the ecliptic is. PUPIL. I do not, Sir; but have a perfect recollection of it. TUTOR. Now, remember, that the axis of the earth is not upright or perpendicular to the plane of the ecliptic, but inclines to, or leans towards it, 23-1/2 degrees, and makes an angle with it of 66-1/2 degrees. PUPIL. An angle signifies a corner; but that cannot be the meaning here.