ASTRONOMY AND COSMIC THEORIES. 101 



once a year. Then a number of these small bodies, 

 many of them perhaps no longer than pebbles or grains 

 of sand, coming into our atmosphere, became heated and 

 even vaporized by the friction due to their rapid planet- 

 ary motion, and appeared to us as shooting stars; while 

 larger masses, whose exterior alone became heated, either 

 exploded or fell entire as meteorites. The exceptional 

 displays of the November meteors at intervals of about 

 thirty-three years is due to the fact that the stream is 

 much denser in this part of its orbit, where the meteoric 

 matter may be slowly aggregating to form a planetary 

 body. 



A large number of such meteor streams have now 

 been observed; but the most remarkable discovery is 

 that in some cases, and probably in all, comets form a 

 part of such meteoric streams. This has been proved by 

 showing that the orbits and times of revolution of cer- 

 tain comets coincide exactly with those of meteor 

 streams as independently observed. Thus, Tempel's 

 comet, seen in 1866, coincides with the November 

 meteors, or Leonids; Biela's comet, with the Androm- 

 eda meteors; while the bright comet of 1862 coincides 

 with the August Perseids. Seventy such cases of the 

 association of comets and meteor streams are now 

 known; and Professor Lockyer has completed the proof 

 of the connection by showing that, when fragments of 

 meteoric stones are intensely heated in a vacuum, they 

 afford a spectrum closely resembling those of comets. 

 Some meteors are visible every fine night, and it has 

 been calculated by Professor Newton of Yale College 

 that seven and a half millions enter the earth's atmos- 

 phere every day; and if we add to these the much 



