Earthquakes occurring under the sea sometimes generate great waves which sweep in over the land destroying coast towns and shipping. These waves sometimes rise to a height of even a hundred feet above sea level. Ships have been carried by them a long distance inland and left high and dry. These waves, wrongly called tidal waves, have no connection with the wind.
132.Temperature of Ocean Waters.
Experiment 125. -Fill a flask of about 500 cc. with water. Press intothe mouth of the flask a cork through which a glass tube about 30 cm. longextends. The tube should be open at both ends and should not extend into the flask below the bottom of the cork. When the cork is pressed in, the water will be forced up into the tube for several centimeters. See that the cork is tight and that there are no bubbles of air in the flask or tube.
Now place the flask for fifteen or twenty minutes in a mixture of ice and water and carefully mark with a rubber band the point at which the water in the tube comes to rest. Take the flask out of the freezing mixture and notice immediately whether the water in the tube rises or falls. Continue for five or ten minutes to notice the action of the water in the tube. The volume of the water is not the least when it is at the temperature of melting ice, 32° F., but when it is a little above this temperature.
Unlike fresh water, which is densest at a little above freezing, sea water continues to decrease in volume and grows denser as it is cooled, until it reaches its freezing point at 28° F. Hence the cold water near the poles gradually sinks and creeps under the warmer water of lower latitudes maintaining a temperature of 32° to 35° on the bottom, even at the equator. This steady creep of cooled surface water along the bottom supplies the animals of the deep ocean floor with the air which they must have. Without it the water at great depths would have its air exhausted and all life would be destroyed.
At the surface of the ocean the temperature of the water varies in a general way with the latitude; it is over 80° at the tropics and about the freezing point at the poles. Near the poles and near the equator there is very little variation in the temperature of the surface water during the year, but in the intermediate latitudes the annual variation is considerable. Below the surface the effect of solar heat rapidly diminishes and at a depth of 300 ft. it is probable that the annual variation in temperature is nowhere more than 2° F. Below 600 ft. there is probably no annual change in temperature.
On the surface the daily average range of temperature is not more than 1° F. and the annual range does not exceed fifteen degrees, except where the same surface is washed at different seasons by currents of different character, and near the shore, where the heat of the land affects it. This contrast in temperature conditions between the ocean and the land is most marked. The life conditions in one are uniform and unvaried while in the other they are most changeable and are subject to extremes of temperature. That is why the land animals must be much more highly organized than those of the sea in order to survive these changeable conditions.
133.The Best-known Ocean Currents. -The ocean is a region ofnever-ceasing motion. At considerable depths its motion is very slow, but near the surface, where the prevailing winds can affect it, the movement is considerable. Circulating around each ocean there is a continuous drift of surface water extending to a depth of from 300 to 600 feet and vary- ing in rate from a few miles up to fifty or more miles a day. In fact these rotating currents are the chief natural basis for the division of the oce- anic area into six oceans, as our geographies generally divide them.
These currents circulate in the northern hemisphere in the direction in which the hands of a watch move and in the southern hemisphere in the opposite direction. In the centers of these rotating areas the water is nearly motionless and here are often found great masses of floating seaweed filled with a great variety of small animals. These accumulations of seaweed are called sargasso seas.
That these surface drifts have a definite direction of movement is indicated by observations made on the courses taken by a great number of wrecks. The direction of these movements has also been determined by throwing from ships in different parts of the ocean thousands of bottles in which had been placed the date and a record of the latitude and longitude of the ship. The places where the bottles came ashore showed the direction of the currents.
If the movement of the water is slow, ten or fifteen miles a day, it is called a drift; if faster, a current. The principal currents have been given names and have been most carefully charted. The warm current that flows northeastward off the southeast coast of North America is called the Gulf Stream. That off the east shore of Asia, which also flows northeast, is called the Kuro Siwo or Japan Current. The cold current off the east coast of Labrador flowing southeast is called the Labrador Current; and the cold current which flows northward off the west coast of South America is called the Humboldt Current. Other names are given to different parts of the ocean movement, but those mentioned here are the most important.
Where the ocean currents are unimpeded by the land, they flow in the direction of the prevailing winds. It has been found that the currents change their directions with a change in the direction of the prevailing wind, such as occurs in the Indian Ocean when the heat equator is farthest north of the earth"s equator. It appears that ocean currents are primarily a result of the wind circulation.
