According to the progressive wave theory, in the Southern Ocean, where there is a complete absence of land, two gigantic tidal waves are produced by the tide-generating forces i.e. the sun and moon.

One of these tidal waves, due to the rotation of the earth, moves from east to west along with the moon, though lagging a little behind it.

The other tidal wave on the other side of the earth also travels in the same way from east to west. These waves are known by the term ‘primary waves’. The time taken by these tidal waves to make one round of earth is 24 hours, 50 minutes.

Thus half a round takes 12 hours, 25 minutes. The crests of these waves are called ‘high tide’ and the troughs are known as ‘Low tide’.

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Since these waves surround the entire earth, at any particular latitude, the length of the wave (the distance from crest to crest) is half the circumference of the globe at that latitude. It is to be stated that the tidal wave travels round the earth at its own proper rate independently of the sun and moon.

As stated earlier, the rate depends on the depth of the water. However, the difference between the wave’s independent rate and that forced upon it by the two celestial bodies will create complexities.

If the earth were completely covered by water, there would have been a smooth travel of the tide around the globe from east to west. But the great land masses of the continents of South America, Africa and Australia stretching from north to south present obstructions in its progress.

In the Antarctic Ocean, there being no land mass, the tides follow the moon freely and travel round the globe from east to west. They are like two long but low waves. As the wave passes over the South Atlantic, a secondary wave or a branch wave produced from the primary wave travels up the Atlantic Ocean from south to north.

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Similar branch waves are set up in the Pacific and the Indian Oceans, and in these oceans too they move from south to north. Thus, it is evident that it is in fact the progressive wave generated in the Southern Ocean around the Antarctica which produces tides in other oceans.

In their onward journey towards the north, further sub-branches are set up by the secondary waves so that tides are produced in the marginal seas and gulfs of these oceans. The Atlantic Pacific and Indian Oceans may be considered as big and open gulfs of the Southern Ocean.

As the waves move up from south to north, much of their energy is dissipa­ted. However, it is due to these branch waves that tides are generated in the oceans, seas and other adjoining water bodies.

According to the protagonists of the progressive wave theory, the impulse for producing these tides comes from the primary wave of the Southern Ocean.

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From the above discussion it is evident that all the tidal waves originally move northward from the Southern Ocean. The crests of these waves produce high tides.

The lines showing the occurrence of high tides at various places at the same time are called the co-tidal lines. When the crest of such a wave happens to be at the coast, the high tide is produced. But when the trough lies at the coast, it is known as the low tide.

The progress of tides in the Atlantic Ocean by means of the co-tidal lines. From this map it is amply clear that the Atlantic tidal wave travels with higher velocity in the deep waters of the middle part of the ocean than in the shallower water along the shore.

As the tidal wave travels towards the north, its curvature increases. When the wave’ reaches Europe its alignment is from north to south and the wave seems to approach from the west.

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On the western side of the Atlantic, even though their trend is from north to south, the wave approaches the coast from the east.

It is interesting to note that if at any place the high tide produced by the tidal wave from one end coincides with the low tide due to the tidal wave from the other end, the two neutralise each other with the result that there is no tide at all.

Another important point to remember is that with the onward movement of the wave towards the north, the so-called age of the tide increases.