Category Science

Why does the leaning Tower of Pisa not falls?

          Everybody knows that in the city of Pisa in Italy, there is a beautiful tower that ‘leans’. Very few people know the reason of its leaning. Every year thousands of people go there to see this wonder. It is made of white marble. The walls are four metres thick at its base. It has eight storeys and is 55.4 metres high and measures 15.8 metres round the bases. There is a stairway which leads to the top and has 300 steps. From its top one can have a magnificent view of the city and the sea which is ten kilometres away.

          Now the question arises: what makes this tower lean and why does it not fall? Plumb line from the top is five metres away from the base, i.e. it leans over by five metres. If we drop a ball from its top, it would hit the ground five metres away from its base. It was intended as a bell-tower for the cathedral which is nearby. Its construction was started in 1174 A.D and completed in 1350 A.D. When the construction started nobody thought that it would lean, but it started leaning after the third storey was completed. The foundations of the tower were laid in sand and this may explain why it leans. Since the tower started leaning, the plans of its construction were modified and then the tower was completed. Since 1918, yearly measurements have been made. They show that the leaning is increasing. During the last one hundred years the tower has leaned another 30 cms. 

          But why does it not fall even while leaning? According to science, anything will remain stable as long as the vertical line drawn from its centre of gravity passes through its base. The centre of gravity is that point where the whole mass of the body is supposed to be concentrated. Till today the vertical line from the centre of gravity has been falling within the base of this tower. That is why it has not fallen. It is believed that when the tower leans further and the line from its centre of gravity pass out of its base, it will fall down. However, by using different new methods and techniques, Engineers and Technologists are now trying to arrest its further leaning.

 

What is a mirage?

          During the summer season, travellers in desert often get an impression of the presence of a pool of water at a distant place. But on reaching there they do not find a single drop of water. The desert deers also get the similar impression. They run here and there in search of this illusory water and finally die of thirst. A similar phenomenon is observed when we drive on a concrete road on a hot summer day. It is simply an optional illusion and is called mirage. Do you know how is it caused?

          The mirage is caused by total internal reflection of light. During the summer, the earth’s surface becomes very hot. These heats up the air in its vicinity but the upper layers of the atmosphere remain cool. Since hot air is always lighter than cold one, the air near the earth’s surface becomes lighter than the air of the upper layers. In scientific terms the refractive index of hot air is less than that of cold air. As such the refractive index goes on increasing as we go above the earth’s surface up to a certain height. 

          In such circumstances when the light rays coming from trees and plants enter from a medium of higher refractive index to a medium of lower refractive index, they get deviated from their path. This phenomenon is called refraction of light. As the rays travel more distance of varying refractive index the angle of refraction goes or increasing and a stage comes when it becomes more than 90°. At that stage the refracted light goes back into the medium from which it had started. This is called the total internal reflection of light. As a result of this reflection, the images of the trees and plants appear inverted, giving an impression as if these images have been formed in water. This gives rise to the illusion of the presence of a pool of water. This is what is known as a mirage.

         Sometimes another kind of illusion producing the opposite effect takes place. Over water, a layer of cold air may lie under a layer of warm air. This causes the light to bend downward. For example, a ship beyond the horizon may appear to be lifted into the sky. 

What causes ocean currents?

          Most people think that the water in the oceans is still. But it is not so. It is always moving in regular patterns. This movement of water is called ocean currents. There are many kinds of currents. One kind is a ‘stream’. A stream is a current with distinct boundaries. Another kind is a ‘drift’. A drift does not have distinct boundaries. Do you know how ocean currents are produced?

          There are three main reasons for the occurrence of the ocean currents. (1) The density of sea water varies from place to place, because the salt content is not same at different places. Water flows from the regions of higher density to regions of lower density thus producing currents. (2) Sun’s rays fall on the surface of the sea at different angles and as such produce unequal heating. This generates ‘convection currents’ in the sea. (3) Winds blowing on the sea surface push water into current.

          Currents are also caused by the rotation of the earth. Generally, the earth’s rotation produces clockwise currents in the northern hemisphere and counter clockwise currents in the southern hemisphere.

          Amongst these, the Gulf Stream is the most important. The water of this stream is blue and warm. This stream flows from the Gulf of Mexico north to Canada like a river in the middle of the Atlantic Ocean. London and Paris are less cold, because of this stream. It keeps ports in Norway free of ice all year. In addition to these, Brazil current, Japan Current North Equatorial current, North Pacific current etc., are the main ocean currents of the world.

          A cool current like the Labrador Current, which flows from the Arctic to the Atlantic, may carry icebergs into the trans-Atlantic shipping lanes. Ocean currents are important to the plants and animals in the oceans. The movement of water brings them food and nutrients. It also helps them during migration. Currents also affect the climate of the land. Ocean currents move water from warm to cold areas and vice versa.

          The collective name for a complex system of ocean currents flowing in the environs of the equator in Atlantic, Pacific and Indian Ocean is known as ‘Equatorial Current’. The pacific north equatorial current is the part of clock-wise ocean current system with Japan Current, North Pacific current and California current. Similarly in Atlantic, the North Equatorial Current is the part of the clockwise system with Gulf Stream, and Canaries current.

 

What causes tides in the sea?

          The giant water waves which rise and fall periodically in the sea are called tides. Sometimes these tides are so high that they appear like high walls moving on the surface of the sea. Do you know what causes these tides?

          The main cause behind the formation of these tides in the sea is the force of attraction between the earth and the moon. We know that all the bodies in the universe attract one another with a force known as gravitational force. This force also acts between the earth and the moon. The moon is pulled towards the earth and the earth towards the moon. This force of gravitation acting between the earth and the moon is so strong that it can easily break a steel rod with a diameter of 400 kms. This force affects the solid part of the earth slightly but it causes rise and fall in the sea water noticeably.

           At any point in the sea, the water level keeps on rising for six hours and subsequently for next six hours it keeps on falling. This rise and fall of the water level is known as tide. The water level rises after every 12 hours 25 minutes. This means that during one day and one night, i.e., 24 hours and 50 minutes, every place in the sea experiences tides two times. Tides are formed both in the seas facing the moon as well as those on the other side. Since the moon comes to the opposite side of the earth after 12 hours and 25 minutes, tides are again formed on the same two spots.

          The force of attraction between the sun and the earth also affects the formation of tides but to a very small extent. When the sun, earth and moon come in a straight line, the earth experiences the maximum force of attraction. And the tide caused on such occasions is very large. It is known as the ‘spring tide’. The sea experiences such tides on the full moon and the new moon days. When the sun and moon are in a perpendicular direction, the earth experiences the minimum force of gravitation. The tide formed on such occasions is very small and is called the ‘neap tide’. Such tides occur between the full moon and the new moon days. Tides can be as high as 15 metres.

          Tides are useful in many ways. They bring many precious items of sea, on to the shore. The tidal power of the sea is also being used to generate electricity.  

Why does bread have pores?

          Bread is a popular food in almost all the countries of the world. Its ingredients vary from country to country. In most of the countries, however, bread is made from wheat or rye flour. In some other places it is also made from rice, barley, potatoes, peas and beans.

          According to the recorded history, the making of bread was started around 3000 B.C. in Egypt. Yeast was also discovered there only. Nowadays the common bread is made from dough prepared by kneading flour with water. A little yeast, sugar and salt are added to it. The yeast causes the dough to rise by forming bubbles of carbon dioxide gas in the dough. The dough is then formed into the shape of a loaf and baked in a loaf tin in the oven. In the process of baking the gaseous bubbles burst resulting in small pores inside the bread. It is mainly the yeast which gives the bread its taste and flavour. You can see these pores very easily in any bread piece.

          Cakes also have pores in them, but they are produced by the baking soda and not by the yeast. Baking soda, which is the mixture of tartaric acid and sodium bicarbonate, is used in the preparation of cakes. When this mixture is added to the dough and it is baked, carbon dioxide is produced. The bubbles of this gas burst on baking causing small pores. 

How does snowfall occur?

          Snow is formed of tiny crystals of frozen water that fall from the sky. Snow flakes have an infinite variety of shapes. They are usually hexagonal and may be flat, needle shaped or star shaped. No two snowflakes are exactly alike. Freshly fallen snow reflects about 95% of the sun’s heat back into the space. Do you know how snow fall takes place?

          Due to sun’s heat there is a continuous evaporation of water from the seas, rivers, lakes, ponds etc. Since water vapour is lighter than air, it goes up in the atmosphere and turns into cloud. We know that the temperature decreases as we go higher in the atmosphere. Since the capacity of air to hold water vapour decreases with the fall in its temperature at a certain height a large quantity of water vapour gets accumulated in the air. The air having excess of water vapours is said to be supersaturated. Under this condition the water vapours condense on the dust and smoke particles present in the air. On further cooling, they get converted into snow particles. These particles combine with each other to form crystals of snow. When the air cannot bear their weight, they fall down as snow flakes and go on accumulating on the mountains.

          Now the question arises: why does it snow on the mountains only? Why does it not fall in the plains too? In fact the possibility of snowfall at any place depends upon two factors – the altitude of the place above the sea-level and its distance from the equator. The higher the altitude of the place, the greater is the probability of snowfall there. Similarly the larger its distance from the equator, more are the chances of snowfall there. Even though the amount of snow formed in the atmosphere is very large, only a very small fraction of it falls down on the mountains in the form of snow. The remaining portion comes down as rains because while passing through hotter regions, it melts and turns into water. However, the temperature being low at the mountains, the snow does not melt into water there. It goes on piling up as snow and under the increasing weight of its accumulation layer after layer, it becomes harder and harder.

          Snowfall is very useful for us. When snow melts in summer, the melted water flows to rivers and is used for irrigation purposes. Fresh snow is very light and a good insulator, protecting the underlying plants from severe cold. Snow is a bad conductor of heat because of the air trapped between its particles. As such it acts as a blanket for the earth. In colder regions, people make houses of snow as it provides excellent protection against cold weather. In hilly areas, the roofs of the houses are made in a slanting shape, so that snow does not accumulate there. It saves the inhabitants from the adverse-effects of snow and also prevents the roofs from collapsing.