The _cause of the tides_ is the combined action of the sun and moon. The relative effects of these two bodies on the oceanic waters are directly as their mass and inversely as the square of their distance; but the moon, though small in comparison with the sun, is so much nearer to the earth that she exerts the greater influence in the production of the great _tide wave_. Thus the mean force of the moon, as compared with that of the sun, is as 21/4 to 1. The attractive force of the moon is most strongly felt by those parts of the ocean over which she is vertical, and they are, consequently, drawn toward her. In the same manner, the influence of the luminary being less powerfully exerted on the waters furthest from her than on the earth itself, they must remain behind. By these means, at the two opposite sides of the earth, in the direction of the straight line between the centers of the earth and moon, the waters are simultaneously raised above their mean level; and the moon, in her progressive westerly motion, as she comes to each meridian in succession, causes two uprisings of the water--two high tides--the one when she passes the meridian above, the other when she crosses it below; and this is done, not by drawing after her the water first raised, but by raising continually that under her at the time; this is the _tide wave_. In a similar manner (from causes already referred to) the sun produces two tides of much smaller dimensions, and the joint effect of the action of the two luminaries is this, that instead of four separate tides resulting from their separate influence, the _sun merely alters the form of the wave raised by the moon_; or, in other words, the greater of the two waves (which is due to the moon) is modified in its height by the smaller (sun's) wave. When the summit of the two happens to coincide, the summit of the combined wave will be at the highest. When the hollow of the smaller wave coincides with the summit of the larger, the summit of the combined wave will be at the lowest.