TUTOR. It is that of a dusky colour, somewhat like tarnished copper.—I have one thing more to remark before we quit this subject, which is, that the moon’s nodes have a retrograde or backward motion, in a direction contrary to the earth’s annual motion, and go through all the signs and degrees of the ecliptic in little less than nineteen years, when there will be a regular period of eclipses, or return of the same eclipses for many ages. PUPIL. Pray, Sir, what do you propose for our next subject? TUTOR. The ebbing and flowing of the sea, or cause of the tides. ------------------------------------------------------------------------ DIALOGUE XII. TUTOR. In order to explain the cause of the tides, I have since I saw you last prepared a little drawing for you, (Plate IV. fig. 3.) where S represents the sun, M the moon at change, E the center of the earth, and A B C D its surface, covered with water. It is obvious, from the principles of gravitation, that if the earth were at rest the water in the ocean would be truly spherical, if its figure were not altered by the action of some other power. But, daily experience proves that it is continually agitated. PUPIL. What is the cause of this agitation? TUTOR. The attraction of the sun and moon, particularly the latter: for, as she is so much nearer the earth than the sun, she attracts with a much greater force than he does, and consequently raises the water much higher, which, being a fluid, loses as it were its gravitating power, and yields to their superior force. PUPIL. What proportion does the attractive power of the sun bear to that of the moon? TUTOR. As three to ten. So when the moon is at change, the sun and moon being in conjunction, or on the same side of the earth, the action of both bodies is on the surface of the water, the moon raising it ten parts,[17] and the sun three, the sum of which is thirteen parts, represented by B _b_. Now it is evident, that if thirteen parts be added by the attractive power of those bodies, the same number of parts must be drawn off from some other part, as A _a_, C _c_. It will now be high-water under the moon at _b_, and its opposite side _d_, and low-water at _a_ and _c_. [Footnote 17: By part here I do not mean any specific measure.] PUPIL. That the attraction of the sun and moon must occasion a swelling of the waters on the side next them, I can readily conceive, and that this swell must cause a falling off at the sides: but that the tide should rise as high on the side opposite to the sun and moon, in a direction contrary to their attraction, is what I am not able to account for.