Despite all this change and restlessness, we talk of the ocean having a “level” surface. We picture it to our minds as being in outer shape the same as that of the Earth—a sphere. But this is not strictly true to fact.
If we could look upon the Earth, with large far-seeing eyes, from a few thousands of miles off, we should find curious irregularities in the watery outline. Instead of showing all round a smooth surface, the ocean would be found to rise here and sink there, to be in one part higher, in another part lower. A man roving over the ocean, all about the Earth, would have in places to ascend undulations like hills, almost high enough sometimes to be called mountains, in other parts to descend declivities.
Most of us have noticed in a cup filled with water, that the water-surface is not perfectly flat. Close to the sides of the cup may be noticed a distinct rise. It is the same in a tumbler, in a basin, in a slender glass tube. For the sides of the cup or tumbler or tube attract the water, drawing it upward; and this is known as Capillary Attraction.
With the ocean the very same thing is seen. If high land borders on deep water, the extra attraction of mountain-masses will act just as the sides of a cup or tumbler will act. They draw upward the water of the ocean to a higher level. When I say that this is “seen,” I do not mean that any careless looker-on will be aware of the fact. It has to be discovered by careful measurement.
In some cases a marked difference has been found. The enormous masses of the Himalayas, for instance, exert a powerful drawing upon the neighbouring sea; and at the delta of the Indus the ocean-level, in consequence of that attraction, is actually three hundred feet higher than on the coast of Ceylon.
Besides land attraction, winds have an extraordinary power to heap up waters in one place more than in another. To some slight extent this may be seen upon English shores, when a strong gale happens to blow landward at high tide. On such occasions the waters often rise far beyond their usual mark.
Mention was made earlier of those Phenicians who, having known an eastern ocean with tides, and a Mediterranean Sea without tides, must have been perplexed to find a western ocean which corresponded with the eastern in its ebb and flow.
We all know for ourselves in these later days, how the tides rise and fall around our coast, twice in twenty-four hours. Each high-water is twelve hours and twenty-five minutes later than the last; so each succeeding day sees a difference of fifty minutes in the time of high or of low tide.
To a very large extent Tides are due to the attractive power of the moon. They are due also to the sun, but in a much less degree, which at first sight seems singular, since the attraction of the sun, by reason of its greater size, far exceeds that of the moon. From the fact, however, that the powerful drawing of the sun comes from an immense distance, it follows that it has much less effect than the small attraction of the moon, which comes from very near at hand.
Her influence over our earth is exerted far more strongly with respect to those ocean-waters lying just under herself, and far less with respect to those waters on the farther side of the globe. The effect of these different pullings is to raise a double wave or swell,—one on the surface of the ocean just below the moon, and one on the opposite side of the earth. The waves mean high tides; and low tides occur at places half-way between them.