The most conspicuous service which our satellite performs for us is that of raising the tides. The complete statement of the manner in which she does this would be too long for our pages; but the general outline of it will be seen from the accompanying rough diagram (Fig. 21), which, it must be remembered, makes no attempt at representing the scale either of the bodies concerned or of their distances from one another, but simply pictures their relations to one another at the times of spring and neap tides. The moon (M in Fig. 21, A) attracts the whole earth towards it. Its attraction is greatest at the point nearest to it, and therefore the water on the moonward side is drawn up, as it were, into a heap, making high tide on that side of the earth. But there is also high tide at the opposite side, the reason being that the solid body of the earth, which is nearer to the moon than the water on the further side, is more strongly attracted, and so leaves the water behind it. Thus there are high tides at the two opposite sides of the earth which lie in a straight line with the moon, and corresponding low tides at the intermediate positions. Tides are also produced by the attraction of the sun, but his vastly greater distance causes his tide-producing power to be much less than that of the moon. His influence is seen in the difference between spring and neap tides. Spring tides occur at new or full moon (Fig. 21, A, case of new moon). At these two periods the sun, moon, and earth, are all in one straight line, and the pull of the sun is therefore added to that of the moon to produce a spring tide. At the first and third quarters the sun and moon are at right angles to one another; their respective pulls therefore, to some extent, neutralize each other, and in consequence we have neap tide at these seasons. [Illustration: PLATE XI. The Moon, April 5, 1900. Paris Observatory.]