There is a difference in the tides, depending on the moon, of a complicated nature, and, first, as to the period of seven days. For the tides are of moderate height from the new moon to the first quarter; from this time they increase, and are the highest at the full: they then decrease. On the seventh day they are equal to what they were at the first quarter, and they again increase from the time that she is at first quarter on the other side. At her conjunction with the sun they are equally high as at the full. When the moon is in the northern hemisphere, and recedes further from the earth, the tides are lower than when, going towards the south, she exercises her influence at a less distance[612]. After an interval of eight years, and the hundredth revolution of the moon, the periods and the heights of the tides return into the same order as at first, this planet always acting upon them; and all these effects are likewise increased by the annual changes of the sun[613], the tides rising up higher at the equinoxes, and more so at the autumnal than at the vernal; while they are lower[614] about the winter solstice, and still more so at the summer solstice; not indeed precisely at the points of time which I have mentioned, but a few days after[615]; for example, not exactly at the full nor at the new moon, but after them; and not immediately when the moon becomes visible or invisible, or has advanced to the middle of her course, but generally about two hours later than the equinoctial hours[616]; the effect of what is going on in the heavens being felt after a short interval; as we observe with respect to lightning, thunder, and thunderbolts. But the tides of the ocean cover greater spaces and produce greater inundations than the tides of the other seas; whether it be that the whole of the universe taken together is more full of life than its individual parts, or that the large open space feels more sensibly the power of the planet, as it moves freely about, than when restrained within narrow bounds. On which account neither lakes nor rivers are moved in the same manner. Pytheas[617] of Massilia informs us, that in Britain the tide rises 80 cubits[618]. Inland seas are enclosed as in a harbour, but, in some parts of them, there is a more free space which obeys the influence[619]. Among many other examples, the force of the tide will carry us in three days from Italy to Utica, when the sea is tranquil and there is no impulse from the sails[620]. But these motions are more felt about the shores than in the deep parts of the seas, as in the body the extremities of the veins feel the pulse, which is the vital spirit, more than the other parts[621]. And in most estuaries, on account of the unequal rising of the stars in each tract, the tides differ from each other, but this respects the period, not the nature of them; as is the case in the Syrtes. CHAP. 100.—WHERE THE TIDES RISE AND FALL IN AN UNUSUAL MANNER.