It now also became clear how the moon, whose attractive power upon the earth is 160 times smaller than that of the sun, is yet able to occasion a stronger tide, since, from her proximity to the earth, she attracts the surface more forcibly than the centre with the thirtieth part of her power, while the distant sun occasions a difference of attraction on these two points equal only to one twelve-thousandth part of her attractive force.
Now also a full explanation was first given why the highest tides take place at new and full moon: that is, when the moon stands between the sun and the earth; or the latter between the sun and the moon; as then the two celestial bodies unite their powers; while at half-moon the solar tide corresponding with the lunar ebb, or the lunar tide with the solar ebb, counteract each other.
But even Newton explained the true theory of the tides only in its more prominent and general features, and the labours of other mathematicians, such as MacLaurin, Bernoulli, Euler, La Place, and Whewell, were required for its further development, so as fully to explain all the particulars of the sublime phenomenon.
The reproach has often been made to science, that she banishes poetry from nature, and disenchants the forest and the field; but this surely is not the case in the present instance, for what poetical fiction can fill the soul with a grander image than that of the eternal restlessly-progressing tide-wave, which, following the triumphant march of the sun and moon, began as soon as the primeval ocean was formed, and shall last uninterruptedly as long as our solar system exists!
Were the whole earth covered with one sea of equal depth, the tides would regularly move onwards from east to west, and everywhere attain the same height under the same latitude. But the direction and the force of the tide-wave are modified by many obstacles on its way, such as coast-lines and groups of islands, and it has to traverse seas of very unequal depth and form. Flat coasts impede its current by friction, while it rolls faster along deep mural coasts. From all these causes the strength of the tides is very unequal in different places.
They are generally low on the wide and open ocean. Thus the highest tides at Otaheiti do not exceed eleven inches, three feet at St. Helena, one foot and a half at Porto Rico.
But when considerable obstructions oppose the progress of the tide-waves, such as vast promontories, long and narrow channels, or bays of diminishing width, and mouths of rivers directly facing its swell, it rises to a very great height. Thus, at the bottom of Fundy Bay, which stretches its long arm between Nova Scotia and New Brunswick, the spring-tides rise to sixty, seventy, or even one hundred feet, while at its entrance they do not exceed nine feet, and their swell is so rapid as frequently to sweep away cattle feeding on the shore.
The Bristol Channel and the bay of St. Malo in Brittany, are also renowned for their high tides. Near Chepstow, the flux is said sometimes to reach the surprising height of seventy feet, and at St. Malo the floods frequently rise to forty and fifty feet. When the water is low, this small sea-port town appears surrounded on all sides by fantastically shaped cliffs covered with sea-weeds and barnacles. Pools of salt water interspersed here and there among the hollowed stones, or on the even ground between them, and harbouring many curious varieties of marine animals, are the only visible signs of the vicinity of the ocean, whose hoarse murmurs are heard resounding from afar. But an astonishing change takes place a few hours after, when the town, surrounded by the sea, would be a complete island, but for a long, narrow causeway called "the Sillon," which connects it with the mainland. On the side fronting the open sea, the tide breaks with tremendous rage against the strong buttresses that have been raised to oppose its fury, rises foamingly to a height of thirty or forty feet, and threatens the tardy wanderer as he loiters on the narrow causeway. The cliffs that erewhile were seen to surround the town are now hidden under the waters, some few excepted, that raise their rugged heads like minute islands above the circumambient floods. The opposite side of the causeway is also washed by the sea: but here its motions are less tumultuous, for after having broken against numberless rocks and made a vast circuit, it scarce retains a vestige of its primitive strength. On this side lies the vast, but deserted harbour of St. Malo, completely dry at ebb-tide; a wide sea during the flood.
Two eminent French authors, Chateaubriand and Lamennais, were born at St. Malo, and there can be no doubt that the imposing spectacle I have briefly described must have greatly contributed to the widening of their intellectual horizon. Daily witnesses from their early childhood of one of the grandest phenomena of nature in all its wild sublimity, the boundless and the infinite soon grew familiar to their mind, enriching it with splendid imagery and bold conceptions.
Although the sun and the moon exert some attraction upon the smaller and inclosed seas, yet the development of a powerful flood-wave necessarily requires that the moon should act upon a sufficiently wide and deep expanse of ocean. Even the Atlantic is not broad enough for this purpose, as its equatorial width measures no more than one eighth of the earth's circumference: and the Pacific itself, notwithstanding its vast area, is so studded with islands and shallows, that it presents a much more obstructed basin for the action of the tide-wave than might be expected, from its apparent dimensions and equatorial position.