IV. Answer the following questions:

1. What are the four states of matter? 2. Can a liquid be changed into a gas? 3. Under what conditions does the change of a substance take place? 4. When is a solid changed into a liquid? 5. What substance exists in three possible states? 6. What is the difference between vapour and steam? 7. What is the freezing point of water? 8. At what temperature does melting take place? 9. What does the term "latent heat" mean? 10. What do we call evaporation? 11. What is the boiling point of water? 12. When does a liquid boil? 13. What does Fig. 15 illustrate?

V. Define the following terms:

boiling, evaporation, freezing, latent heat, boiling point

VI. Form six sentences using the following nouns quali- fied by adjectives:

Model: gradual change

When water is boiling one can observe a gradual change of water into vapour.

atmospheric pressure, usual state, liquid fuel, important effect, latent heat, open container, continuous motion

VII. Translate the following text:

The student probably remembers that evaporation may take place from the surface of ice. This is the fact which is familiar to us because we see it in the disappearance of snow in a dry east wind, though the temperature does not rise to the melting point. Evaporation also consumes heat, a fact which may be easily illustrated as follows: if you wet one hand before going out on a cold day in winter, you will feel that your wet hand is much colder than the other one. It is the absorption of heat from your hand that causes the cooling effect.

It is necessary to point out that the same number and kind of molecules that are to be found in, say, a kilogram of steam are also present in that very amount of water or ice. Why, then, do these different states exist and why have they such widely varying properties? For the simple reason that molecules move differently in each of these states.

VIII. Describe Fig. 15.

IX. Form statements which are not true to the fact. Ask your comrades to correct them.

X. Give a heading to each paragraph of the text. Exp tain why you have given such a heading.

_{XI. Retell the text.}

^{LESSON TWENTY-THREE}

TIDES AND TIDAL ELECTRIC STATIONS

The main purpose of this article is to give some idea of a natural source of electric energy as well as the existing methods, ways and means of its use.

Coal, oil, wind, water and atomic energy—these are our familiar sources of power, but what is tidal energy? The pe­riodic rise and fall of the ocean on a coast line is well known. In the open ocean the change in level amounts to only about 61 cm, or so. On the coasts of continents it is often much greater. At any rate, tides like wind will last for ever.

Utilization of tidal energy has been studied more or less extensively in a number of countries, such as the USSR, the USA, France, Canada, England, and others. Tidal forces are expected to make considerable contribution if not to present day, at least, to future electricity production. By intercepting ocean or sea water at high tide in a basin and by directing it back through turbines at low tide, one could create a kind of a working hydroelectric plant.

Although there are tides along all coasts of all conti­nents, not all the places possess the necessary qualities for the installation of an efficient, reliable and economical tidal electric station. The most important features are the amplitude of the tide, the surface mm * of the basin, and the size of the dam required. The higher the tide, the larger the area of the available basin, and the smaller the dam, the better the locality. Suitable places can be found, for instance, on the French coast and it is there that a big t.e.s. (tidal electric station) was constructed.

Г The Soviet Union's tidal power sources are among the greatest in the world. No wonder that several large tidal electric stations are planned here. Kislaya Bay on the Barents Sea was found to be a suitable place for the construction of the first experimental t.e.s.'At high tide the water from the Barents Sea flows in through an operating turbine, while at low tide the water is conducted back to the sea through that very turbine, making it work in both directions. Such a turbine makes it possible to use both high and low tides to the best advantage.

The Kislaya Bay station is an experimental project, its capacity being rather limited. But that is not the principal thing. What is important is the possibility of test­ing the design of the station under consideration. The re­sults obtained enable the designers to solve the main problems for several other powerful and superpowerful tidal plants to go into operation in future. One cannot help men­tioning in this connection the Belomorsky project of a t.e.s. We expect it to have a power capacity greater than that of Bratsk, the biggest hydroelectric station in the world.

^{Active Words and Expressions}

area, at any rate, to the best advantage, capacity, coast, dam, design, to direct, to enable, idea, a kind of, level, locality, no wonder, ocean, rise, tide, ways and means

_Exercises

/. Translate the following sentences:

1. The Soviet scientists do try to utilize the energy of the tides. 2. It is the Soviet Union that has great tidal power sources. 3. It is the Barents Sea where the first experimental t.e.s. was built. 4. It is the operating turbine that makes it possible to use high and low tides. 5. The results obtained did enable the designers to solve the problems of tidal elec­tric station construction. 6. The water does evaporate from the surface of the container.

//. Translate the following sentences and define the func­tion of the Infinitive:

1. To construct a tidal electric station one should take into consideration the locality requirements. 2. To find a suitable place for a tidal electric station is not an easy thing do. 3. Not all places are known to possess the necessary qualities for the efficient tidal electric station installation. 4. Our aim is to find a suitable place for the construction of a t.e.s. 5. The Belomorsky tidal electric station is expected to have a greater power capacity than that of the Bratsk hydroelectric plant. 6. The amplitude of the tide, the size of the dam are the most important features to be taken into consideration when constructing a t.e.s. 7. We know the oper­ation of a tidal electric station to depend upon the periodic rise and fall of the ocean.

///. Answer the following questions:

1. What is the subject of the present article? 2. What sources of power do you know? 3. What does the change in the ocean level amount to? 4. What countries study to the utilization of tidal energy? 5. Is it possible to build a tidal electric station at any coast? 6. What sea was chosen for the construction of the first t.e.s.? 7. What makes it pos­sible to use high and low tides to the best advantage? 8. What is the capacity of the Kislaya Bay station? 9. Is it neces­sary to construct tidal electric stations in our country?