NORTHERN LIGHTS— EVE 159 



become nearer to the equator, and they disappear Tvhen at the lowest 

 attained latitudes. 



These relatively cool, dark whirlpools reveal magnetic properties 

 discovered by Hale through the Zeeman effect, and they may perhaps 

 be compared with the "lows" or cyclones which so often bring storm, 

 rain, and flood. The periodicity of sunspots, auroras, magnetic storms 

 on the earth, and changing radio phenomena has been found to hold 

 good for the fluctuations of the white polar caps on the planet Mars 

 and even for a cycle of ring growths in the great and ancient trees 

 of western America. 



METEORS 



Most people are familiar with shooting stars or meteors and many 

 have seen in their lives dozens or hundreds of them; yet it always 

 comes as a surprise to learn that no less than 20 million of them 

 every day plunge into our atmosphere with velocities ranging up to 

 130 mile« a second. Sometimes these visitors are but the size of a 

 pin's head, and at other times they are large enough to pierce the 

 atmosphere and reach the earth. The famous Arizona crater may 

 liave been formed long ago by a giant meteor; the crater is 1,400 

 yards wide and more than 500 feet deep. In 1908, a great meteor, 

 estimated to weigh 130 tons, fell in Siberia and devastated by its 

 great heat hundreds of square miles of country. The elevation of 

 most frequent meteoric displays is about 40-60 miles above the earth. 

 It is somewhere in this region that the temperature rises according 

 to the theory of the reflection of sound waves, to which already 

 reference has been made. Sometimes the meteors are of iron, some- 

 times of stone, and it is not easy to understand how they become 

 red or white hot when rushing through cool air, how indeed they 

 acquire more heat from the bombardment of molecules than is 

 carried away by them. However, the luminosity of meteors occurs in 

 rarefied air at heights of 100 to 30 miles above the earth. An experi- 

 ment in the laboratory of a similar character would be difficult to 

 make, because our projectiles achieve a speed of a few thousand feet 

 a second as contrasted with meteors having velocities of many miles 

 a second. 



MOTHER-OF-PEARL AND NOCTILUCENT CLOUDS 



There occur rarely and at great elevations iridescent clouds, as 

 remarkable for their beauty as for their height. They are generally 

 observed over regions of low barometric pressure and it is probable 

 that the clouds are formed of supercooled water vapor.* St0rmer 

 and his coworkers have measured the altitudes of many of these 



* S. Chapman, Nature, vol. 129, p. 497, Apr. 2, 1932. 



