134.Effects of Ocean Currents. -A knowledge of ocean currentsis of importance to mariners, as the course and speed of a vessel may be considerably affected by them. They also greatly affect the condi- tions of animal life in different parts of the sea. Not only do these cur- rents bring food to animals which have not the power of motion but they determine the area in which certain animals may live.
The Bermudas, 32° north of the equator, are coral reef formations, while the Galapagos, almost on the equator, are surrounded by too cold water to have any such reefs. At 68° north, near the LofotenCORAL FORMATIONS IN THE BERMUDAS.
Islands, are the great cod fisheries of Europe. On the western side of the Atlantic these fisheries are on the Grand Banks, latitude 45° north. Many other similar illustrations of the effects of these currents on the distribution of animal life might be cited.
The temperature of winds blowing from the sea is modified by these currents and greatly affects the habitability of the earth for man. Hammerfest at 71° north is a flourishing seaport, but there are no important settlements above 50° on the western side of the Atlantic. Alaska, the prevailing winds of which are warmed by blowing over the warm ocean, is a region which promises much for human habitation, while the region on the opposite side of the Pacific must remain almost destitute of human inhabitants. It should be noted that the effect of the warm ocean waters would be slight, except along the coast, were it not for the air movements.
135.Tides. -Probably the first thing that impresses us on visit-ing the seashore is the regular rising and falling of the water each day. These movements of the water are called tides. If we observe the tides for a few days, we find that there are two high and two low tides each day. As the tidal current comes in from the open ocean and the waterrises, it is called flood tide, and as it runs out or falls, ebb tide. When the tides change from flood to ebb or ebb to flood, there is a brief pe- riod of "slack water."HIGH TIDE IN THE BAY OF FUNDY.
If we observe closely, we shall see that the corresponding tides are nearly an hour later each day than they were the day before, and that the time required for the completion of two high and two low tides is nearly 25 hours. Continued observation will show, as Julius C?sar stated many centuries ago, that there isapparently a relation between the phases of the moon and the height of the tides. The greatest rise and fall of the water will be found to occur about the time of full and new moon.
The amount of rise and fall of the tides is not the same in different places. Upon shores of oceanic islands the difference between high and low water, or the range, as it is called, is not more than two or three feet. In funnelshaped bays where the tidal current is compressed as it moves in toward the bay head, therange is very much greater. In the Bay of
LOW TIDE AT THE SAME PLACE.
Fundy the range is sometimes as great as seventy feet.
Sometimes these compressed tidal currents are formed at the mouths of rivers and move up the rivers like a wall of surf several feet in height, endangering all vessels which are not securely moored. Such tidal surfs are called bores and are found in the Seine, Amazon and other rivers.
The tidal current as it sweeps between islands often forms eddies and whirlpools which make navigation very dangerous. An example of this is found at Hell Gate, N. Y., and at the famous Maelstrom off the coast of Norway. On the other hand in flat countries where the riversAN ATOLL IN THE MID-PACIFIC.
are shallow, ports which could not otherwise be reached are made accessible to ships of considerable burden at the time of high tide. At these places the time of leaving or making port changes each day with the time of high tide. A striking example of this is the port of Antwerp.
The tidal currents are also continually changing the waterin bays and harbors and thus keeping them from becoming stagnant and foul. They also bring food to many forms of shore life which have but little or no power of movement, such as clams and other shellfish, and by exposing some of these at the ebb give man a chance to acquire them readily for food.
THE VATERLAND.
One of the largest vessels afloat.
It has been found that the position of the sun, as well as that of the moon, affects the height of the tide. If the earth, moon and sun lie in nearly the same line, the tidal range is greatest. This is called springtide. When the sun and moon act at right angles upon the earth, the tidal range is least and this is called neap tide. The tidal undulations have been proved by astronomers to be due to the rotation of the earth and the gravitational attraction of the sun and moon upon its water envelop. The moon is much the more effective because it is nearer.
In the open sea the rise and the fall of the tides are of equal duration but at the head of bays the tide rises more rapidly than it falls so that low tide does not occur midway between two successive high tides.
136.Corals and Coral Islands. -In warm, clear, shallow areas ofthe sea are found small animals called corals. Great colonies of these are able by united action to build barriers capable of withstanding the waves and of raising submarine lands into islands. These reef-building corals cannot live at a greater depth than 20 fathoms (120 feet), or where the mean tempera-ture is lower than 68° F., or where the discharge of riv- ers makes the water fresh or muddy. As they are fixed animals and must get their food from the surround- ing water, they flourish best where the warm cur-rents flow past continually, bringing a fresh supply of food. In tropical seas many islands are fringed by reefs built by these animals.
In the tropical Pacific circular coral reefs are found nearly inclosing large shallow lagoons. Soundings outside of these reefs showPANAMA CANAL.
An example of man"s domination over nature.
that the sea rapidly sinksTwo great oceans artificially united.