In the Atlantic the marginal wave travels towards the north, and impinges upon the coasts of North America and of Europe. In the Indian Ocean it also pursues a northerly course, and finally washes the shores of Hindostan, the Bay of Bengal, and the Arabian Gulf: while in the Pacific, on the contrary, the waves diverge from the equator towards the poles—but in all they partake also of the westerly course of the moon. Although such are the directions in which the tides unquestionably proceed along the _shores_ of those seas, yet observations at islands in the open sea and towards the centres of the oceans contradict the idea of corresponding progressive waves throughout the entire area of those seas. Upon the coasts of Britain and New Brunswick the tides are high, from the local circumstances of the coast and bottom of the sea; while in the centre of the ocean, where they are due to the action of the sun and moon only, they are remarkably small. The spring-tides rise more than 40 feet at Bristol, and in the Bay of Fundy, in Nova Scotia, they rise upwards of 50 feet; the general height in the North Atlantic is 10 or 12 feet, but in the open and deep sea they are less; and at St. Helena they are not more than 3 feet, whilst among the islands in the Pacific they are scarcely perceptible. The mean height of the tides will be increased by a very small quantity for ages to come, in consequence of the decrease in the mean distance of the moon from the earth; the contrary effect will take place after that period has elapsed, and the moon’s mean distance begins to increase again, which it will continue to do for many ages. Thus, the mean distance of the moon, and the consequent minute increase in the height of the tides, will oscillate between fixed limits for ever.[110] The tidal wave extends to the bottom of the ocean, and moves uniformly and with great speed in very deep water, variably and slow in shallow water; the time of propagation depends on the depth of the water as well as on the nature and form of the shores. Its velocity varies inversely as the square of the depth—a law which theoretically affords the means of ascertaining the proportionate depth of the sea in different parts; it is one of the great constants of nature, and is to fluids what the pendulum is to solids—a connecting link between time and force. The great oceanic wave that twice a-day brings the tides to our shores, has occupied a day and a half in travelling from the place where it was generated. The wave first impinges on the west coast of Ireland and England, and then passes round the north of Scotland, up the North Sea, and enters the Thames, having made the tour of Great Britain in about 18 hours.