Category Science

How is the depth of the sea measured?

          Before the early part of the 20th century, the only known method of determining the depth of the sea at any point was by means of a cable. The ship stopped and a hemp line or wire with a weight at the end was lowered to the bottom. This was a slow, laborious task and not very accurate.

          Today man has developed innumerable instruments that help him to go deep into the seas and acquire more and more knowledge about their bottoms. Do you know how the depth of the sea is measured with the help of modern instruments?

          The equipment which is used to measure the depth of the sea is called a ‘fathometer’. It is installed on a ship. It produces sound waves having frequencies of more than 20,000 hertz. These are called ultrasonic waves. They cannot be heard by human ears. These waves are transmitted inside the sea. They are reflected back by the bottom of the sea. The reflected waves are received by a receiver and the total time taken by them to travel from the sea surface to the bottom and back to the surface is measured. Half of this time multiplied by the velocity of sound in sea water gives the depth of the sea at the point of the experiment. Thus the depth of the sea can be measured at any point. This technique is known as echo sounding or echo ranging. 

          This technique has been used to measure the depths of various seas and the deepest spot in each of the seas has been located using this technique. The Pacific Ocean is the deepest ocean. The average depth of this ocean is 4,282 metres (14,048 feet). The deepest spot in this ocean is located near Guam and has a depth of 10,668 metres (35,000 feet). Guam is an island in the Pacific Ocean. It is about 5,000 kms (300 miles) to the west of the Hawaii islands. A more recent survey located a spot with a depth of 10,924 metres (35,839 ft.) — as per a data obtained by Japan Maritime Safety Agency, using a narrow multi-beam echo sounder. Using depth as a criterion, the Indian Ocean occupies the second place. Its average depth is 3963 metres (13,002 feet). The Atlantic Ocean, coming third, has an average depth of 3926 metres (12,880 feet). Amongst the shallow seas, the Baltic Sea occupies the first place. Its average depth is 55 metres (1 80 feet).

            Studies of the oceanic crust have revealed that they are also as uneven as the surface of the earth. They are full of the skeletons of the aquatic animals as well as big and small sea plants. Volcanic ash is also found there.

          The most modern method of studying the oceanic crust is to use the laser beams. The laser beams are transmitted into the water of the ocean and the time taken by them from the surface to the bottom and back is measured. On multiplying the velocity of speed of light with this time, one gets twice the distance of the bottom of the sea. This method is very accurate. 

What is fire?

          A material that burns combines quickly with oxygen in the air. Burning, then, is a rapid process of oxidation. Most materials that burn are made up of mainly carbon and hydrogen. Combustion is another name for burning.

          In 17th century combustion was explained by other theory. Antoine Lavoisier, a French Chemist, discovered in 1774, that oxygen is essential for combustion. Some substances combine with oxygen extremely slowly. In that case no light and very little heat is produced. Rusting of iron is an example of such a slow oxidation.

          For a fuel to burn, certain conditions must be met. First, a substance that can burn must be present. Second, a good supply of oxygen must be present. Third, the fuel must be treated until it reaches its ignition temperature. The ignition temperature is also called the kindling point.

          Some substances like paper have low kindling points while other fuels like coal must be heated considerably before they can burn. Some substances like white phosphorus have a very low ignition point that is why it is kept under water.

          It would be almost impossible to count the number of ways in which fire is used. Fire provides light and cooks the food. Fire is used to heat water to make steam. Steam is used to run umpteen kinds of engines. Fire is used in industries for separating metals from their ores. Fire is also used in automobiles, boats and airplanes.

          When fire is under control it might be called man’s best friend. But once it gets out of control it is undoubtedly his worst enemy. Every year fires kill thousands of lives. It can destroy buildings and forests in large a proportion if it gets out of control.

 

What is smoke?

          Whenever wood, coal or some other substance burns, smoke comes out. Smoke is produced due to the incomplete combustion of fuels. If complete combustion of fuels takes place, no smoke will be produced. In fact smoke is a colloid of solid particles in a gaseous form.

          Most of the fuels contain carbon, hydrogen, oxygen, nitrogen and small quantities of sulphur. When any fuel burns, we get carbon dioxide, water vapour, nitrogen and traces of sulphur dioxide. For the complete combustion of fuels a large quantity of oxygen is required because burning is a process of oxidation. Due to the lack of oxygen, fuels do not burn completely. As a result of this smoke is produced. Smoke mainly consists of carbon dioxide, water vapour and carbon particles. When the number of carbon particles present in the smoke is large, it appears gray or dark. These very carbon particles get deposited on the walls of the chimneys. This deposit is called soot.

          Smoke is the biggest pollutant of air and is very harmful from the health point of view. Today it has become a major problem in the cities. If it does not get scattered by wind, the atmosphere of the cities becomes highly foggy. Smoke is particularly harmful for the heart and lungs and is the cause of many diseases. It is also harmful to the eyes.

          However, in certain respects, smoke is useful also. It is used to protect the orchards from cold. In wars, smoke helps in camouflaging. It is also helpful in rain formation by enabling the water vapour to condense on its particles. 

How are icebergs formed?

          In the Southern Hemisphere, the Antarctic ice sheet overflows its land support to form shelves of ice on the sea; huge pieces, as much as 200 km. across, break off to form icebergs. In Northern Hemisphere icebergs are generally not over 150 metres across. However most icebergs are from some 20 glaciers on the west coast of Greenland. When glaciers, drifting from mountains and valleys, reach the sea, they got broken into big pieces by sea waves and tides. These big pieces of ice are called icebergs. However there are some glaciers which do not break even after floating for long distances in the sea and appear like mountains of ice.

          The sizes of the different icebergs vary. The smallest iceberg measures 5 to 6 metres in length and breadth but the bigger ones might be hundreds of metres long. Some icebergs with lengths and breadths of half a kilometre have been seen floating in the oceans. They float in sea water because ice is lighter than water. Their one-tenth part remains above water and the remaining under water. For example, if a 50 metre high iceberg is seen above water, then 450 metres of it will be under water. These icebergs contain huge quantities of snow. Some of them may contain 200,000,000 tons of ice. As an iceberg floats, some of the ice melts and pieces break off. Eventually, it completely disappears.

          Because of their huge weight and volume, the icebergs do not move in the sea on their own. They are pushed ahead by the sea waves. They are very dangerous for ships. A collision with them can cause a ship wreck. Though they can be detected with the help of modern instruments, yet mishaps do take place occasionally. On 14th April 1912, a ship named Titanic was broken to pieces when it collided with an iceberg. This ship was sailing to New York; 1513 passengers, who were aboard this ship lost their lives.

          A similar accident took place on January 30, 1959 when the ship Hans Hedtoft collided with an iceberg in southern Greenland and was wrecked. Now the United States and other countries have formed an International Ice Patrol. The patrol uses ships, planes and radars to locate icebergs.

 

Why does the size of the moon appear to change?

          On the full moon day, the moon appears like a shining disc but it goes on waning till it disappears on the new moon night. And then it again starts increasing in size and becomes a full bright disc, on the full moon day. Changes in the moon’s size are called the ‘phases’ of the moon. Do you know why it occurs? 

          The fact is that the moon neither increases nor decreases in size. Its size simply appears to be changing because it changes its position with respect to the sun. We all know that the moon is the only satellite of the earth and revolves round the earth. It travels around the earth about 384,400 km away. This orbit takes 27 days and eight hours. It is illuminated by the sunlight. Only one side of the moon faces the earth. We do not see its other side. When the moon comes in between the earth and the sun, its bright side is not visible to people on the earth and only its dark side faces the earth. As a result the moon cannot be seen. This is the new moon day. As the position of the moon changes due to its motion from east to west with respect to earth, some illuminated part of its surface is visible to us. A week after the new moon, almost half of the illuminated lunar disc becomes visible to us. The bright part of the moon’s surface goes on increasing gradually and on the full moon day the whole disc, seen from the earth, is fully illuminated. On this day the earth is in between the sun and the moon. In the next fifteen days the bright portion of the lunar surface goes on decreasing and on the fifteenth day it disappears completely. In short, the changes in the apparent shape of the moon depend upon how much of its illuminated surface is visible to us.

          The cycle continues like this and we see the different phases of the moon. On the full moon day it appears just after the sunset and disappears only at the sunrise. The time between new moon phases is called the synodic month.

 

Why does wind blow?

          We know that air is the mixture of oxygen, nitrogen, carbon dioxide, water vapour and dust particles. Air in motion is called the wind. Do you know how the wind blows?

          When any place on the earth gets heated by the sun’s rays, the air of that place also gets heated. Due to this heating the air expands and hence its density decreases or it becomes lighter. The hot air, due to this lightness, goes up in the atmosphere. And this causes a decrease in atmospheric pressure in that area. Under such a condition air from high pressure cold regions rushes to that place to create an equilibrium. This movement of air is called the blowing of wind.

          In the areas adjoining the sea, the earth becomes hot during the day. Due to this air becomes lighter and goes up in the atmosphere. To restore balance, cold air from the sea blows towards the land. During the night, a reverse movement takes place, that is, the earth becomes colder than the sea water and as such the air moves from the land towards the sea.

          It is very hot in the regions surrounding the equator. It creates an equatorial belt of low pressure. So there is a constant upward movement of hot air from these regions. This hot air flows out to the north and south.

          The rotation of the earth in its axis considerably affects the directions of the winds. The westerly winds are the direct result of the earth’s rotation from the west to the east. The spinning of the earth makes all in the northern hemisphere deflect towards the right and in the southern hemisphere, towards the left. Superimposed on the general wind systems are local winds. These are caused by temperature differentials associated with topographical features such as mountains and coastal belts. The presence of mountains also influences the wind direction. The mountains obstruct the winds and change their directions. The speed and direction of wind is measured by an instrument called an anemometer.