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the depth of the water over which they travel. Thus it has been calculated by Professor Airy that a wave 100 feet in breadth and in water 100 feet deep travels at the rate of about 15 miles per hour; one 1000 feet broad and in water 1000 feet deep, at the rate of 48 miles; whereas another, 10,000 feet in breadth and in water 10,000 feet deep, will sweep forward with a velocity of not less than 154 miles per hour. This relation between the breadth of a wave, its velocity of progress, and the depth of the water in which it travels, has been embodied by Mr Airy in the following table:

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Breadth of the Wave in Feet.

100 1000 10,000 100,000 1,000,000 | 10,000,000

Corresponding Velocity of Wave per Second in Feet.

1

2.262 5.320

10

2.262 7.154

100

1,000

5.667 5.671 5.671 16.883 17.921 17.933 2.262 7.154 22.264 53.300 2.262 7.154 22.264 71.543 7 154 22.264 71.543 7.151 22.264 71.543 226.260 715.430 1688.300 1793.300

5.671

5.671

5.671

17.933

17.933

17.933

56.672 56.710
168.830 179.210
226.260 533.900 566.720

56.710

56.710

179.210

179.330

567.100

10,000 2.262

100,000 2.262

128. The force with which a wave strikes against any opposing barrier depends, in like manner, upon its bulk and velocity; and in the case of huge waves this impact is enormous. From experiments made at lighthouses and breakwaters, their effective pressure has been estimated as high as 6000 lb. per square foot and one has only to observe the breaches occasionally made in seawalls, and the distance to which blocks of stone, several tons in weight, have been hurled forward, to be convinced of their great propulsive power. Of course the force with which a wave simply strikes is not to be altogether estimated by its propulsive power, for substances submerged in water lose a certain portion of their weight, which greatly facilitates their displacement and transport.

Tides-their Origin and Influence.

129. The next, and perhaps the most important and persistent of oceanic movements, is that of the TIDES-a term applied to the periodic rising and falling of the waters, occasioned chiefly by the attraction of the moon, but partly also by that of the sun. In obedience to the universal law that "every particle in nature attracts every other particle with a force inversely as the square of the distance," the earth is attracted by the sun and moon, but more by the latter, in proportion to its greater proximity. Land

and water alike experience this attraction, but the particles of the latter being free to move among themselves, the mass of the ocean is drawn out beyond its normal circumference towards the attracting bodies. Had the earth been immovable as regards the sun and moon, this bulging out of the waters would have been stationary; but as she turns on her axis, meridian after meridian is brought directly opposite to the attracting force, and thus the rising of the waters becomes a great tidal wave or flow that travels round the globe. The moon, we have said, exercises the greater attraction (her attraction being to that of the sun as 100 to 38), but when the sun and moon are in conjunction, or in opposition (that is, at new and full moon), the sum of the two attractions will cause the greatest possible rise, known as spring-tides; and when the moon is in her quadratures (that is, at her first and last quarters), the sun's attraction, acting in a different direction, will diminish the lunar tide, and then we will have the least rise, or neap-tides. The following diagram may assist the comprehension of these phenomena:

Earth.

Moon.

s'

Here t being the nearest point of the earth's surface to the moon, the waters at that part are most attracted towards that luminary, and of course rise highest; while on the opposite side at t the earth is drawn, as it were, away from them, and they stand out nearly at the same height as those at t. But as the waters rise simultaneously at t t they are drawn away from e e', and as the earth turns round, each point on its surface will necessarily have two high-waters and two low-waters per day. In other words, the sea flows or rises as often as the moon in her apparent circuit passes the meridian, both the arc above and the arc below the horizon, and ebbs or falls as often as she passes the horizon, east and west. The solar day, however, being only 24 hours, and the lunar (owing to the moon's monthly course round the earth) being 24 hours, 54 minutes, it requires rather more than a rotation of the earth to bring the same meridian to the same position, relatively to the moon, as it had the preceding day. In other words, it requires more than 24 hours to bring the moon round to its vertical position over any given place, and thus the tides of one day are always about an hour later than they were on the preceding day. Again, had the moon been the sole attracting body, the tides would have risen always to the same height; but

the sun, exerting a simultaneous attraction either along with or against that of the moon, creates an alternate maximum and minimum of flow. Thus, when the sun is at S and S' his attraction is combined with that of the moon, and a higher tide is the result. When at S the darkened side of the moon will necessarily be towards the earth, and it is new moon; and when at S' the illuminated face of the moon will be towards the earth, and it is full moon; so that the higher or spring-tides take place alternately at new and full moon. On the other hand, when the moon is 90° from the sun's place (that is, when she is in her first and last quarters, or half moon), his attraction, being exerted at right angles, counteracts that of the moon, and the result is the lower or neap-tides the proportion of spring to neap being as 138 to 62, or nearly as 7 to 3. The height and time of the tides thus vary with the moon's age; and this being known, their recurrence and culmination at any given spot can be calculated with the greatest exactitude. The greatest tides occur, of course, when the luminaries are nearest and pass most vertically to the place of observation; and as each tide has only about six hours to flow and about six hours to ebb, the highest must necessarily be the swiftest, and the lowest the slowest.

130. Such, in general terms, is the theory of the tides; and had the surface of the globe been entirely covered with water, the tidal wave would have been regular and continuous from meridian to meridian, and, as a consequence, highest in the region of the equator, and gradually falling away towards either pole. But the continuity of the ocean being interrupted by land, and this land lying in a great measure meridionally, as well as being irregular in outline, and consisting in many parts of islands, the course of the tidal flow is obstructed, and deflected into various courses. Under the present arrangement of sea and land, these courses are, however, sufficiently persistent; and thus their directions, times, velocities, and heights, can be determined with accuracy for the purposes of navigation. The Southern Ocean, encircling the globe, and being comparatively uninterrupted by land, may be regarded as the area in which the tidal wave receives its great primary impulse. It is thence carried forward, deflecting itself northward into the Indian, Atlantic, and Pacific Oceans, where, uniting with the minor tide-waves generated in these expanses, it flows, rises, and subdivides, according to the outline of the coasts, the depth of water, and the obstruction of islands.

131. Notwithstanding the complications arising from these causes, there is still great regularity in the bi-diurnal flow and ebb

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of the tides; and by noting the times at which the same highwater reaches different parts of the coast, a series of lines connecting these points may be laid down so as to indicate the course of the tidal wave with great precision. Such a series of lines are termed co-tidal lines, or lines of simultaneous tide, and mark the progress of the summit of high-water from its origin in the Southern Ocean to its remotest ramifications in northern waters. We say northern waters, for though the primary and normal direction of the tidal wave is from east to west, in obedience to the apparent course of the sun and moon, yet, on entering the troughs of the Indian and Atlantic Oceans, it is compelled to assume a northerly course in accordance with the configuration of these seas. Thus the new or full moon high-water that passes Van Diemen's Land every morning at twelve, takes twelve hours to reach Ceylon, and thirteen to reach the Cape of Good Hope; in another twelve hours it has passed up the Atlantic, and arrived at Newfoundland; at the end of the third twelve it has rounded the north of Scotland, and is opposite to Aberdeen; at the fourth twelve, or at midnight of the second day, it is opposite the mouth of the Thames ; and it is "not till the morning of the third day that this wave fills the channel of the Thames, and wafts the merchandise of the world to the quays of the port of London." (See Map of Co-tidal Lines.)

132. The tides, we have said, may be regarded as taking their rise in the uninterrupted expanse of the Southern Ocean. As the wave proceeds westward, it is deflected northward broadly into the Indian Ocean, rapidly and deeply into the larger channel of the Atlantic, and slowly and feebly into the Pacific, where its course is obstructed by numerous islands and coral-reefs. The velocity of the tidal wave depends primarily on the conformation and depth of the ocean-proceeding with the greatest rapidity where the ocean is freest and deepest. As the co-tidal lines are laid down at hourly distances, they afford a pretty correct estimate of the tidal velocity—the wider the lines (that is, the greater the distance travelled over in one hour) the greater the speed, and the closer the lines the slower the rate of progress. In the free depths of the Southern Ocean this velocity may equal 1000 miles per hour, while in restricted seas like the North Sea the rate is scarcely a twentieth of that amount. As the tidal wave differs from a windwave in not being a mere undulation, but a wave of translation, its height in any sea will depend mainly on the configuration of the shore, the form of bottom, and the direction in which it is propelled. In the open expanse of the Southern Ocean the tidal wave rarely exceeds five or six feet, and in the Indian and Atlantic Oceans perhaps eight or ten; but in bays and gulfs opening

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