17. Translate the sentences paying attention to the participles and participle clauses:

1. In fact the Sun is a variable star, its variation following an 11-year cycle.

2. The laboratory study of plasma had been started long before 1929, many important discoveries in the field of gas dis­charge phenomena having been made in the 1800's.

3. Whereas solar infra-red radiation can be observed from a high-flying balloon as explained earlier, most ultraviolet and all X-radiation is absorbed far above the balloon ceiling.

4. Since the nuclear rocket is capable of producing greater thrust per pound of propellant, it can produce thrust equal to that of the chemical rocket while using less propellant.

5. With the maximum cycle temperature being limited by the source, the designer must determine how close he wishes to bring the boiling temperature to that maximum condition.

6. In atomic and nuclear processes energies are usually expressed in terms of electron volts, 1 electron volt being the energy gained by 1 electron being acted on by a voltage of 1 volt.

7. Observations made using artificial sodium glows are of great interest to the investigation of atmospheric structure.

8. The ions thus formed will be accelerated in the direction towards the cathode.

9. Ohm supposed that wires presented a certain "resistance" to the passage of current, long or thin wires presenting more resis­tance than short or thick ones. The smaller the resistance, the greater the current.

10. When used in a space vehicle in which the orientation re­lative to the solar vector changes, the efficiency curve will change with aspect.

11. Using the value of the solar constant, it is found that the Sun is sending out energy at the rate of 9 hp per square centi­metre of surface.

12. Having an equatorial diameter of 74,000 mi, Saturn is sur­rounded by great rings lying in the plane of its equator.

13. A man pulled by the Earth with a force of 160 Ib is said to weigh 160 Ib.

14. Interplanetary spacecraft using nuclear propulsion are expected to be reduced in weight in comparison with nuclear rockets.

15. The word plasma was first used to denote a gas in which an important fraction of the molecules are dissociated into ions and electrons, the gas as a whole remaining electrically neutral.

16. Newton believed that the acceleration of the Moon in its orbit differed in no essential respect from the acceleration of parti­cles acted on by the Earth's gravities.

17. The electromagnetic wave theory of radiation, based on the work of Maxwell and Hertz, was so successful that we must try to apply it to the spectrum just described.

18. Radiation goes through space and when absorbed, it agitates the molecules of the absorbing material so that it is converted into heat energy.

19. The reader is familiar with the great technical importance of electron emission, as applied in radio tubes.

20. Molecules and atoms are in a continuous state of motion, with an average velocity depending on the temperature.

21. Having seen how the electrons in the ionosphere are pro­duced by solar radiation, let us ask how the different layers affect the travel of radio waves in the different frequency ranges.

22. This radiation, being little absorbed, penetrates to the D-region and there joins with the X-rays in providing ionization.

23. The alkali atoms have the lowest energies of ionization, indicating that their electronic structures change to a rare-gas structure.

24. The emissive type of the photoelectric cell as used in telegraph and television systems is similar in appearance to the ordinary thermoionic valve.

25. A screen material of a different category from those so far discussed is white crystalline potassium chloride: this substance darkens when bombarded with electrons.

26. Launched on Friday afternoon, Mars 3 like its predecessor Mars 2, weighs just under five tons and is said to be on the same trajectory around the Sun.

27. As seen through the telescope, Mars presents a rather smooth surface.

28. Having challenged the established notion of a fixed Earth, Copernicus put the Sun, instead of the Earth, at the centre of the solar system.

29. A particular flight considered to be representative of early manned missions has been selected as the basis of this study.

30. With 40,000 or more computers being used now, it is obvi­ous that it is necessary to assess their strengths and weaknesses to help make decisions about their use.