1887.] Researches on the Spectra of Meteorites. 



149 



be increased at least twenty-fold : this would give ns 400 millions of 

 meteorites falling on the earth's surface daily. If we consider, how- 

 ever, only those visible to the naked eye, and if we assume that the 

 absolute velocity of the meteors in space is equal to that of comets 

 moving in parabolic orbits, Professor H. A. Newton has shown that 

 the average number of meteorites in the space that the earth traverses 

 is in each volume equal to the earth about 30,000. This gives us a 

 result in round numbers that the meteorites are distributed each 

 250 miles away from its neighbours.* 



If, then, these observations may be accepted' to be good for any 

 part of space, we may, and indeed must, expect celestial phenomena 

 which can be traced to meteorites in all parts of space. 



Further, we have the experience of our own system that these 

 meteors are apt to collect in groups. 



A comet, it is now generally accepted, is % swarm of meteors in 

 company. Such a swarm finally makes a continuous orbit by virtue 

 of arrested velocities ; impacts will break up large stones and will 

 produce new vapours in some cases, which will condense into small 

 meteoroids. 



A meteorite in space under any of the conditions indicated by the 

 comets, new stars, and such first-magnitude stars as a Orionis, will 

 evidently be subject to collisions, but only to a greater number of 

 collisions than those which must ordinarily occur if space is as full of 

 meteorites as Professor Newton's calculations, from observations made 

 on the earth, would naturally seem to indicate. 



The Velocity of Luminous Meteors. 



In spite of the difficulties which attend the observations necessary 

 to determine the velocity of meteors entering our atmosphere, many 

 observations have been made from which it may be gathered that the 

 velocity is rarely under 10 miles a second or over 40 or 50. It is 

 known that the velocities of some meteor-swarms are very different 

 from those of others. Professor Newton, our highest authority on 

 this subject, is prepared to consider that the average velocity may be 

 taken to be 30 miles a second. 



Result of Collisions. 

 If we take these velocities as representing what happens in other 

 regions of space, and assume the specific heat of the meteorites to be 

 0*10, the increase in their temperature when their motions are arrested 

 by impacts will be roughly as follows : — 



* Article " Meteorites," Professor Js T ewton, 1 Encyclopaedia Britannica,' 9th edition, 

 vol. 16. 



