Fig. 7.—Illustrating the Tide-producing Influence of the Moon
Now, from what has been previously said with regard to the tidal influence of the moon, we see that the tide must necessarily be high under the moon, as well as on the side of the earth directly opposite this body, and that the high tides must follow the moon in its regular motion. But we must not forget that the earth itself is continually turning on its axis, making a complete rotation in about twenty-four hours; while the moon, which revolves round the earth in about twenty-eight days, describes only a small portion of its orbit in the same time; thus, while the tidal wave slowly follows the moon as it travels in its orbit, the earth slips round, as it were, under the tidal wave, causing four changes of tide in approximately the period of one rotation. Suppose, for example, the earth to be performing its daily rotation in the direction indicated by the arrow (fig. 8), and the tide high at the place markedÛuccessively, where the tide is high and low respectively. Hence the daily changes are to a great extent determined by the rotation of the earth.
But we have already observed that each change of tide occupies a little more than six hours, the average time being nearly six hours and a quarter, and so we find that the high and low tides occur nearly an hour later every day. This is due to the fact that, owing to the revolution of the moon round the earth in the same direction as that of the rotation of the earth itself, the day as measured by the moon is nearly an hour longer than the average solar day as given by the clock.
Fig. 8.—Illustrating the Tides
There is yet another point worth noting with regard to the relation between the moon and the tidal movements of the water, which is that the high tides are never exactly under the moon, but always occur some time after the moon has passed the meridian. This is due to the inertia of the ocean, and to the resistance offered by the land to its movements.
Now, in addition to these diurnal changes of the tide, there are others, extending over longer periods, and which must be more or less familiar to everyone who has spent some time on the coast. On a certain day, for instance, we observe that the high tide flows very far up the beach, and that this is followed, a few hours later, by an unusually low ebb, exposing rocks or sand-banks that are not frequently visible. Careful observations of the motions of the water for some days after will show that this great difference between the levels of high and low-water gradually decreases until, about a week later, it is considerably reduced, the high tide not flowing so far inland and the low-water mark not extending so far seaward. Then, from this time, the difference increases again, till, after about two weeks from the commencement of our observations, we find it at the maximum again.
