METEORITES, METEORS, AND SHOOTING-STARS. 735 



the highest repute for good personal work in adding to human 

 knowledge. In presence of this host of speculations, it will not, I 

 hope, he a useless waste of your time to inquire what we may reason- 

 ably believe to be probably true. And if I shall have no new hy- 

 potheses to give to you, I offer as my excuse that nearly all possible 

 ones have been already put forth. This Association exists, it is true, 

 for the advancement of science, but science may be advanced by re- 

 jecting bad hypotheses as well as by framing good ones. I begin with 

 a few propositions about which there is now practical unanimity 

 among men of science. Such propositions need only be stated. The 

 numbers that are to be given express quantities that are open to re- 

 vision and moderate changes : 



1. The luminous meteor-tracks are in the upper part of the earth's 

 atmosphere. Few, if any, appear at a height greater than one hun- 

 dred miles, and few are seen below a height of thirty miles from the 

 earth's surface, except in rare cases, when stones and irons fall to the 

 ground. All these meteor-tracks are caused by bodies which come 

 into the air from without. 



2. The velocities of the meteors in the air are comparable with 

 that of the earth in its orbit about the sun. It is not easy to deter- 

 mine the exact values of those velocities, yet they may be roughly 

 stated as from fifty to two hundred and fifty times the velocity of 

 sound in the air, or of a cannon-ball. 



3. It is a necessary consequence of these velocities that the meteors 

 move about the sun and not about the earth as the controlling body. 



4. There are four comets relating to four periodic star-showers 

 that have occurred on the dates of April 20th, August 10th, November 

 14th, and November 27th. The meteroids which have given us any 

 one of these star-showers constitute a group, each individual of which 

 moves in a path which is like that of the corresponding comet. The 

 bodies are, however, now too far from one another to influence appre- 

 ciably each other's motions. 



5. The ordinary shooting-stars in their appearance and phenomena 

 do not differ essentially from the individuals in star-showers. 



6. The meteorites of different falls differ from one another in their 

 chemical compositions, in their mineral forms, and in their tenacity. 

 Yet through all these differences they have peculiar common proper- 

 ties which distinguish them entirely from all terrestrial rocks. 



7. The most delicate researches have failed to detect any trace of 

 organic life in meteorites. 



These propositions have practically universal acceptance among 

 scientific men. We go on to consider others which have been re- 

 ceived with hesitation, or in some cases have been denied. 



With a great degree of confidence we may believe that shooting- 

 stars are solid bodies. As we see them they are discrete bodies, sepa- 

 rated even in prolific star-showers by large distances one from another. 



