528 1'HILOSOPHICAL TRANSACTIONS. [ANNO 1784. 



explanation of the nature of these vapours, nor of the manner in which they 

 can be raised up through air so extremely rare ; nor, supposing them so raised, 

 does he account for their regular arrangement in a straight and equable line of 

 such prodigious extent, or for their continuing to burn in such highly rarefied 

 air. indeed, it is very difficult to conceive, how vapours could be prevented, in 

 those regions where there is in a manner no pressure, from spreading out 

 on all sides in consequence of their natural elasticity, and instantly losing that 

 degree of density which seems necessary for inflammation. Besides, it is to be 

 expected, that such trains would sometimes take fire in the middle, and so pre- 

 sent the phenomenon of '2 meteors at the same time, receding from each other 

 in a direct line. 



These difficulties have induced other philosophers to relinquish Dr. Halley's 

 hypothesis, and propose, instead of it, one of a very opposite nature, that 

 meteors are permanent solid bodies, not raised up from the earth, but revolving 

 round it in very eccentric orbits; or, in other words, that they are terrestrial 

 comets. The objections to this opinion however seem equally great. Most 

 observers describe the meteors, not as looking like solid bodies, but rather like 

 a fine luminous matter, perpetually changing its shape and appearance. Of this 

 many defenders of the opinion are so sensible, that they suppose the revolving 

 body gets a coat or atmosphere of electricity, by means of which it becomes 

 luminous ; but whoever carefully peruses the various accounts of fire-balls, and 

 especially ours of the 18th of August when it divided, will perceive that their 

 phenomena do not correspond with the idea of a solid nucleus enveloped in a sub- 

 tile fluid, any more than with the conjecture of another learned gentleman, that 

 they become luminous by means of a contained fluid, which occasionally explodes 

 through the thick solid outer shell. 



A strong objection to this hypothesis of permanent revolving bodies, is derived 

 from the great number of them there must be to answer all the appearances. 

 Such a regular gradation is observed, from those large meteors which strike all 

 beholders with astonishment, and occur but rarely, down to the minute fires 

 called shooting stars, which are seen without being regarded in great numbers 

 every clear night, that it seems impossible to draw any line of distinction between 

 them, or deny that they are all of the same name. But such a crowd of revolving 

 bodies could scarcely fail to announce their existence by some other means than 

 merely a luminous train in the night; as, for instance, by meeting or justling 

 sometimes near the earth, or by falling to the earth in consequence of various 

 accidents; at least we might expect they would be seen in the day-time, either 

 with the naked eye, or by telescopes, by some of the numerous observers who 

 are constantly examining the heavens. 



Another argument of great weight against the hypothesis that fire-balls are 



