90 The Realm of Nature CHAP. 



evanescent track of light. At certain times, particularly 

 about loth August and I3th November, this phenome- 

 non is so common that showers of shooting - stars are 

 seen. At those dates the Earth crosses the orbits of 

 two comets. The November shower is sometimes mar- 

 vellously magnificent, and the grandest displays recur 

 at intervals of about 33 years. The last is still re- 

 membered in 1866, and a similarly fine spectacle may be 

 looked forward to in 1899. Meteors are not falling stars, 

 for the stars are as numerous after a meteor shower as before. 

 They are produced by small solid bodies, on the average 

 perhaps as large as a pea, which enter the Earth's atmosphere 

 with enormous velocity. The energy of motion is converted 

 into heat by the friction of the air, and the solid is im- 

 mediately driven into vapour and vanishes, being con- 

 densed into fine invisible dust ( 1 6 1 , 277). Meteors usually 

 begin to glow at the height of about 80 miles above the 

 Earth's surface, and die out at the height of at least 50 

 miles. 



135. Meteorites. It has occasionally happened that 

 meteoric masses of considerable size, weighing several 

 pounds or even hundredweights, have fallen on the Earth, 

 and in about a dozen cases this has happened in the sight 

 of intelligent witnesses. Meteorites, as such masses are 

 termed, are of at least two classes, either metallic composed 

 mainly of iron and nickel, or stones resembling volcanic 

 rock, although frequently associated with minerals not 

 known in terrestrial rocks. They often contain carbon, 

 and almost always considerable quantities of various gases 

 absorbed in their pores. When a powdered meteorite is 

 heated in a tube from which the air has been exhausted, 

 and through which an electric current is passed, it glows 

 with a faint light, the spectrum of which is very like that of 

 comets, strongly confirming the meteoric theory of those 

 bodies ( 133). The close relation of meteors and comets 

 was proved very forcibly in 1861 when the Earth dashed 

 through the tail of a comet; again in 1872, and in 1885 

 when Biela's comet was calculated to cross the Earth's orbit 

 close to the Earth's place at the time. The only sign of the 



