116 THE UNIVERSE A VAST SYSTEM OF PARTS stones, preceded by four terrific explosions, occurred at Johnstown, Colorado, on the afternoon of July 6, 1924. Many more such showers of meteorites have been recorded. The largest meteorite fall known in modern times fell in Central Siberia and completely devastated an area of over 1000 square miles. It fell on June 30, 1908, in the primeval forest of Siberia in the Province of Yenissei. It was seen at a distance of 4000 miles. Nearly 50 miles away people were burned by the heat. Trees over a wide area, as well as a herd of 1500 domesticated reindeer, were destroyed. The shock Fig. 32. Meteor crater from the air near Winslow, Arizona. (From the Yerkes Observatory', reprinted by permission of the Chicago University Press.) produced by the meteorite was registered by a seismograph at Irkutsk, 600 miles distant, while the air-compression wave was registered by a barograph in the same city. did not fall in a densely populated area. It is fortunate, indeed, that this meteorite The Origin of Tektites Is Still a Mystery. Many years ago the peasants of Bohemia occasionally plowed up curious pieces of rough green glass in deposits which were laid down before man learned how to make glass. More recently thousands of similar small glassy objects called "tektites" were discovered in Australia, Tasmania, British Borneo, China, Texas, Sweden, and other parts of the world. At one time these tektites were thought to be of volcanic origin, but most of the tektite areas are free of volcanic activities. Another theory is that these glassy objects are meteorites that melted when they reached the earth's atmosphere.
COMETS AND METEORS 117 STUDY QUESTIONS 1. How did the ancients consider comets? 2. What causes the tail of a comet? 3. What is the position of the comet's tail with reference to the sun? What causes this? 4. What is the generally accepted idea of the composition of a comet? 5. How large are meteors? 6. Why do meteors get hot? 7. How far from the earth do meteors usually appear and disappear? 8. What are the three types of meteorites? Of what is each composed? 9. Give some examples to show the force of the impact when a meteor collides with the earth. 10. W^hy do meteoric showers appear periodically? 11. What information may be gained from the observation of the trails of meteors? 12. What does the fact that certain meteorite swarms follow the paths of disintegrated comets suggest? 13. The following hypotheses concerning the origin of meteorites have been suggested: (1) they were thrown up by volcanoes; (2) they were produced by volcanoes on the moon; (3) they are of the same origin as meteors. Suggest some possible observations that might be made to prove the truth or untruth of each hypothesis.